SOME ASPECTS OF THE BIOLOGY OF FISSURELLA BARBADENSIS (GMELIJ) SOME ASPECTS OF THE BIOLOGY OF FISSURELLA BARBADENSIS (GMELIN) by
~ thesis subaitted to the Faculty et Graduate Studiea aad Reaearch ia par tial tultilaeat ot the requireaeats tor the degree ot Master ot Scieace.
Zoology Departaeat, McGill Uaiversity, Moatreal. ACKNOWLEDGEMENTS
I am indebted to my husband, Dr. G.T. Ward of the Brace Research Institute, for valuable discussions concerning some of the mathematical procedures and to Dr. J.B. Lewis of the Bellairs Research Institute for his advice and encouragement. MY thanks are also due to Dr. M. Goldstein of the Botany Department of McGill University for his assistance in identification of algae and to Dra. T.E. Bowman and M.L. Jones of the u.s. National Museum, Washington, for identifying the commensale. I am also very grateful to Dra. D.M. Steven and Joan Marsden of the Zoology Department of McGill University for their co-operation and advice and to Mr. J.W. Pollock of the same department for photographing the slides of the digestive tract and gonad stages. I would like to thank those etudents and friends who have saved so much time by helping with the measuring of specimens. My thanks also go to Mrs. Ian Macintyre who typed this thesis so patiently. This study was supported by a grant from the National Research Council of Canada to Dr. J.B. Lewis.
ii PREFACE
The keyhole limpet, Fissurella barbadensis (Gmelin) 1 is a diotocardian prosobranch gastropod belonging to the most primitive group, the Zeugobranchia. It is closely related to such genera as Diodora, Puncturella and Lucapinella. F. barbadensis ranges from S.E. Florida and Bermuda to the West Indies and from Mexico to British Guiana (Farfante 1943). The anatomy of this species has been described by Fischer {1857), Pilsbry {1890) 1 Farfante (1943) 1 Abbott (1958) and others whi1e eco1ogica1 observations have been recorded by Wi1lcox (1905b), 01msted (1917) and Lewis (1960). The work was carried out at the Bellaire Research Institute of McGill University in Barbados, West Indies. The purpose of the study was to compare some aspects of the biology of r. barbadensis with those of temperate forma. The resulta are part of a larger project to examine the differences between tropical and temperate marine animals.
iii TABLE OF CONTENTS Page ACKNOWLEDG:WENTS ...... ii PREFACE ...... iii. LIST OF TABLES •••••·•••••••••••••••···············•••••••• rl LIST OF ILLUSTRATIONS ••••••••••••••••••••••••••••••••••••• vii.
PART I. DISTRIBUTION AND GROWTH
Introduction •••••••••••••••••••••••••••••••••••••• 1 Materi.als and Methods ••••••••••••••••••••••••••••• 4 Resulta Horizontal and Vertical Distribution •••••••••••• 8 Absolute Growth and Specifie Growth Rates ••••••• 12 Structural Changes with Growth •••••••••••••••••• 15 Structural Changes with Horizontal Distribution.. 15 Structural Changes with Vertical Distribution ••• 17 Discussion •••••••••••••••••••••••••••••••••••••••• 50 II. FEEDING AND DIGESTION
Introduction •••••••••••••••••••••••••••••••••••••• 58 Materials and Methods ••••••••••••••••••••••••••••• 60 Resulta ••••••••••••••••••••••••••••••••••••••••••• 62 Discussion •••••••••••••••••••••••••••••••••••••••• 65 III. RISTOLOGY OF THE DIGESTIVE TRACT
Introduction •••••••••••••••••••••••••••••••••••••• 69 Materials and Methods ••••••••••••••••••••••••••••• 69 Resulta Oral Tube and Buccal Cavity...... 70 Oesophagus •••••••••••••••••••••••••••••••••••••• 73 Stomach and Digestive Gland ••••••••••••••••••••• 75 Intestine ••••••••••••••••••••••••••••••••••••••• 78 Abbreviations Used in Figs. 31-37 ••••••••••••••••• 81 Discussion •••••••••••••••••••••••••••••······••••• 87
iv Table of Contents continued Page PART IV. BREEDING CYCLE
Introduction •••••••••••••••••••••••••••••••••••••• 91 Materials and Methods ...... 93 Results ...... 94 Gonad Stages: Neuter ...... 96 Developing ...... 96 Spawning •••••••••••••••••••••••••• 98 Abbreviations Used in Figs. 39-57 ••••••••••••••••• 104 Discussion •••••••••••••••••••••••••••••••••••••••• 119 V. BEHA VI OUR
Introduction •••••••••••••••••••••••••••••••••••••• 121 Materials and Methods ••••••••••••••••••••••••••••• 124 Results ••••••••••••••••••••••••••••••••••••••••••• 124 Discussion •••••••••••••••••••••••••••••••••••••••• 126
VI. CONCLUSIONS ••••••••••••••••••••••••••••••••••••••••• 128
SUMMARY ...... 133 LITERATURE CITED ...... 135 LIST OF TABLES
Table Page
l. The Vertical Intertidal Zones of the Rocky Shores in Barbados, after Lewis (1960) ••• 8 2. Absolute Growth Rates of F. barbadensis Based on Individual Measurements •••••••••••••• 12 3. Statistics on Structural Changes with Growth •••••••••••••••••••••••••••••••••• 16 4. Statistics on Structural Changes with Vertical Distribution ••••••••••••••••••• 18 5. Statistics on Structural Changes with Horizontal and Vertical Distribution •••••••••••••••••••••••••••• 19 6. Statistics on Structural Changes with Vertical Distribution ••••••••••••••••••• 21 7. Feeding Rates ofF. barbadensis ••••••••••••• 63 8. pB of Digestive Tract ofF. barbadensis ••••• 64
Resulta of the Eazyme Tests ••••••••••••••••• 10. Distribution of Gonad Stages •••••••••••••••• 101
Ti LIS~ OF ILLUSTRATIONS Figure Page 1. Map of Barbadoa Showiag Aaaual Wia4 Directioa aad Deaait7 of Fiaaurella Populatioaa at 12 atatioaa ••••••••••••••••••••••••••••••• 22 2. Six Mea•a Bay Collectiag Area at Heaa Low Water Spriaga ••••••••••••••••••••••••••••• 23 Balf Mooa Fort Col1ectiag Area at Meaa Low Water Spriaga ••••••••••••••••••••••••••••• 23 4. »eep Water Harbour C.llectiag Area at Meu. Low Water Spriaga ••••••••••••••••••••••••• 24 5. Oiatiaa Collectias Area at Heu. Low Water Spriaga ••••••••••••••••••••••••••••••••••• 24 6. Bathaheba Collectiag Area at Meu. Low Water Spriaga ••••••••••••••••••••••••••••••••••• 25 The Leagtà of Iadiridual F. barbadeaaia froa Jul7 1964 to Jul7 1965 •••••••••••••••••••• 26 a. Specifie Growth Batea of Iadividual r. bar iadeaaia froa Wharf aad Aquariua •••••••••• 27 Leagth Diatributioa of Populatioaa of l• barbadeaaia froa Meathl7 Traaaecta, Juae 1964 to Juae 1965 •••••••••••••••••••• 28 10. Size Frequeac7 Diatributioa of Populatioaa of F. bar~adeaaia froa Moat&l7 Traaaecta, Juae 1964 to Juae 1965 •••••••••••••••••••• 29 11. Câaage ia the Relative Beigàt of Shel1a wità Growth ••••••••••••••••••••••••••••••• 30 12. CJaaage ia tàe Basal Sllape of Sll.ella w1 th Growth •••••••••••••••••••••••••••••••••••• 31 13. The Relatioa Betweea Relative Beight aad Bize of Shella from Differeat Borizoatal Locatioaa ••••••••••••••••••••••••••••••••• 32
't'ii List of Illustrations continued
Figure Page 14. The Relation Between Basal Shape and Size of Shells from Different Horizontal Locations ••••••••••••••••••••••••••••••••••• 33 The Relation Between Relative Height and Size in High and Low Level Shells from Six Men's Ba7 ••••••••••••••••••••••••••••••••••••••••• 16. The Relation Between Basal Shape and Size in High and Low Level Shells from Six Men's Bay • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 35 17. The Relation Between Relative Height and Size in High and Low Level Shells from Ralf Moon Fort •••••••••••••••••••••••••••••••••••••••• 36 18. The Relation Between Basal Shape and Size in High and Low Level Shells from Ralf Moon Fort •••••••••••••••••••••••••••••••••••••••• 37 19. The Relation Between Relative Height and Size in High and Low Level Shells from Deep Water Harbour ••••••••••••••••••••••••••••••••••••• 38 20. The Relation Between Basal Shape and Size in High and Low Level Shells from Deep Water Harbour ••••••••••••••••••••••••••••••••••••• 39 21. The Relation Between Relative Height and Size in High and Low Level Shells from Oistins ••••••••••••••••••••••••••••••••••••• 4o 22. The Relation Between Basal Shape and Size in High and Low Level Shells from Oistins ••••••••••• 41 The Relation Between the Shell Weight and the Wet Weight of Soft Parts in High and Low Level Shells from Six Men's Bay ••••••••••••••••••• 42 24. The Relation Between the Shell Weight and the Wet Weight of Soft Parts in High and Low Level Shells from Oistins ••••••••••••••••••••••••• 25. The Relation Between the Volume Under the Shell and the Wet Weight of Soft Parts in High and Low Level Shells from Six Men's Bay • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 44
viii List of Illustrations continued
Figure Page
~. The Relation Between the Volume Under the Shell and the Wet Weight of Soft Parts in High and Low Level Shells from Oistins •••••••••••• 45 The Relation Between the Extra-visceral Space and the Wet Weight of Soft Parts in High and Low Level Shells from Six Men 1 s Bay •••••••••••••• 28. The Relation Between the Extra-visceral Space and the Wet Weight of Soft Parts in High and Low Level Shells from Oistina •••••••••••••••••••• 47 29. The Relation Between Shell Thickness and Shell Length in High and Low Level Shells from Six Men's Bay •••••••••••••••••••••••••••••••• 48 30. The Relation Between Shell Thickneas and Shell Length in High and Low Level Shells from Oistins •••••••••••••••••••••••••••••••••••••• 49 31. Longitudinal Section Through the Dorsal Wall of the Buccal Cavity •••••••••••••••••••••••••••• 82 32. Cross Section Through the Buccal Region •••••••••• 82 33. Longitudinal Section Through Dorsa-lateral Wall of Mid-oesophagus •••••••••••••••••••••••••••• 83 34. Longitudinal Section Through the Oesophageal Gland •••••••••••••••••••••••••••••••••••••••• 83 Longitudinal Section Through a Region of Abundant Gland Cella in the Oesophageal Gland ••••••••• 84 Longitudinal Section Through the Gastric Shield in the Main Part of the Stomach •••••••••••••• 84
Cross Section Through Region 2 of the Intestine •••••••••••••••••••••••••••••••••••• 85 38. Diagram to Show the Four Regions of the Intestine •••••••••••••••••••••••••••••••••••• 86 39. Section Through Neuter Gonad ••••••••••••••••••••• 105
40. Section Through ~~e Gonad Stage I Developing ••••••••••••••••••••••••••••••••••• 105
ix List of Illustrations continued
Figure Page 41. Section Through Male Gonad Stage II DeYeloping ••••••••••••••••••••••••••••••••••• 106 42. Section Through Male Gonad Stage III Developing ••••••••••••••••••••••••·•••••••••• 106 43. Section Through Male Gonad Stage IV Developing ••••••••••••••••••••••••••••••••••• 107 44. Section Through Male Gonad Stage V Developing ••••••••••••••••••••••••••••••••••• 107 45. Section Through Male Gonad Stage IV Spawning ••••••••••••••••••••••••••••••••••••• 108 46. Section Through Male Gonad Stage III Spawning ••••••••••••••••••••••••••••••••••••• 108 47. Section Through Male Gonad Stage II Spawning ••••••••••••••••••••••••••••••••••••• 109 48. Section Through Male Gonad Stage I Spawning ••••••••••••••••••••••••••••••••••••• 109 49. Section Through Female Gonad Stage I Developing ••••••••••••••••••••••••••••••••••• llO 50. Section Through Female Gonad Stage II Developing ••••••••••••••••••••••••••••••••••• llO 51. Section Through Female Gonad Stage III Deve~oping ••••••••••••••••••••••••••••••••••• lll 52. Section Through Female Gonad Stage IV Developing ••••••••••••••••••••••••••••••••••• lll 53. Section Through Female Gonad Stage V Developing ••••••••••••••••••••••••••••••••••• 112 54. Section Through Female Gonad Stage IV Spawning ••••••••••••••••••••••••••••·•••••••• 112 55. Section Through Female Gonad Stage III Spawning ••••••••••••••••••••••••••••••••••••• 113 56. Section Through Female Gonad Stage II Spawning ••••••••••••••••••••••••••••••••••••• 113 List of Illustrations continued
Figure Page 57. Section Through Female Gonad Stage I Spawning ••••••••••••••••••••••••••••••••••••• 114 58. Seasonal Variation in Breeding Activity from June 1964 to June 1965 Showing Percentages of Neuter, Developing and Spawning Specimens •••••••••••••••••••••••••••••••••••• 115 59. Seasonal Variation in the Mean Gonad Size from June 1964 to June 1965•••••••••••••••••••••••• 116 60. Seasonal Variation in Developmental Stages of the Breeding Cycle from June 1964 to June 1965 •••••••••••••••••••••••••••••••••••• 117 61. Proportions of Male, Female and Neuter Specimens at Different Shell Lengths ••••••••••••••••••• 118 PAR! I.
DISTRIBUTION AND GROWTR
Introduction The genus Fissurella is distributed throughout temperate and tropical seas. Many species live below 1ow tide level and a few in deep water and in the intertidal zone (Pilsbry 1890; Farfante 1943; Zuniga 1951; Warmke and .A.bbott 1961). Gauld and Buchanan (1959) record F. nubecula living on the rocky shores in Ghana in the lithothamnia zone, the lowest of the intertidal zones reaching to just above mean 1ow water springs. Lewis (1960) has described !• barbadensis in Barbados living low on the shore from mean 1ow water to mean sea level. He found this species abundant on beach rock platforms on the leeward coast.
Many other limpets also live intertidally. The vertical distribution of Patel1a in Britain ranges from above mean low water springs to be1ow mean high water springs, variations occurring with the particu1ar habitat and species concerned (Orton 1929; Eslick 194o; Evans 1947; Das and Seshappa 1947). .A.cmaea is distributed from below mean low water springs to above mean high water springs but, again, the range varies with species and location (Abe 1931; Shotwell 1950; Segal 1956b). Wave action is probably the most important factor influencing the distribution of intertidal animale; yet, as pointed out by Lewis
l 2
(1964), its measurement is one of the most difficult problems for the biologist, being complicated mainly by the shore topography. The effects of successive periods of emersion and submersion on intertidal flora and fauna and the importance of the splash-line at any tidal level have been studied by numerous workers (Colman 1933; Delf 1942; Stephenson 1942; Doty 1946; Southward and Orton 1954; Southward 1958; Hodgkin 1960) and well reviewed by Doty (1957). Experimenta of Doty and Archer (1950) indicate that tide factors are paramount in controlling the vertical range of intertidal animaJs physiologically and sometimes also physically. Absolute and relative growth and allometric relations have
been examined in a variety of molluscs using linear measurements, weight and volume. This work is reYiewed by Wilbur and Owen (1964). The growth rate of a species bas been shown to be affected by age, gonad maturity, temperature, food supply and other environmental factors (Fraser 1931; Hamai 1937; Moore l938a; Thompson 1942; Fox and Coe 1943; Bullock 1955; Dehnel 1955; Prosser 1955; Leighton and Boolootian 1963; Williams 1964a,b). In Britain, Patella vulgata attains a length of about 20
to 25 mm. at the end of the first year and reac~es 38 to 48 mm. in the second year (Russell 1909; Orton 1928b). Acmaea dorsuosa grows 6 mm. in the first year and is 8 to 12 mm. long after two years and 11 to 18 mm. after three years (Abe 1932). One year old Patelloida conu1us is about 3 mm. long. This species attains 5 mm. after two years and 6 mm. after three years {Hamai 1937). Patina pe11ucida grows to 10 mm. in the first year after which most of the animals die (Graham and Fretter 1947). The effects of variation in horizontal and vertical distri bution in a given species have been examined in several groups of molluscs. In Acmaea limatula variation in intertidal height causes a change in the rate of heart beat. condition of the gonade and in the shell weight relative to the wet weight of the soft parts (Segal l956a.b). Similar phenomena occur in Mftilus californianus. In this species, differences in intertidal height can be compared with variations in populations from different latitudes. Lower intertidal Mftilus acts like higher latitude individuals (Rao l953a; Segal et al. 1953). The weight of the shell for any given weight of soft parts is a function of the number of hours per day that the mussel is under water (Rao l953b). Comparable intertidal and latitudinal differences may occur in Acmaea. Ramai (1937) shows a difference in the growth curves of
Patelloida grata and P. conulus between damp and dry habitats. Both localities show an increase in relative height with length but in damp habitats the relative growth is more intense and has a lower ratio in the young animale. In Patella vulgata (Orton l928b; Moore
1934) and in Acmaea dorsuoea (Abe 1932) shell height is greater in dry, more exposed, habitats than in damp ones where there is lese emersion. Abe (1932) also found that growth rates in A. dorsuosa were s1ower and the shell thickened more quickly in a dry habitat. The rate of growth of Patella vulgata is apparently greater in algal covered areas than in wave-beaten places although both habitats support large populations (Southward and Orton 1954). Shells of this species from flat stones and rocks at low tide levels have a regular outline with a large number of small fine ribs while those 4
from rough weathered granite rock are heavier and thicker with an irregular outline and 12 to 14 very prominent ribs between the small
ones (Russell 1907). Populations of P. vulgata from high shore levels have been found to differ from low level populations in metabolic rate, rate
of water loss under desiccating conditions and in the ability to withstand this loss of body water. P. aspera, which is limited to low levels exposed to wave action, is not able to acclimate as well as P. vulgata to high intertidal habitats (Davies 1965). An attempt has been made to compare F. barbadensis with these temperate forme by studying the distribution and growth and the effects of different habitats on some of the physical characteristics of the species.
Materials and Methode The density of F. barbadensis in different habitats was determined by counting the specimens in an area one metre square. The vertical zones used were the same as those described by Lewis (1960). Wave amplitude was determined by a series of measure ments taken on several days of average sea conditions at representa tive collecting areas. The measurements were made by recording maximum and minimum wave heights on a vertical scale. The average daily period of emersion, allowing for wave action, was calculated geometrically from the wave amplitude and the mean tidal height
(as given by Lewis, 1960). Direct observations were used at some habitats to confirm these calculations. Three methode were used to determine the growth rates of the 5 species. From June 1964 to June 1965 a monthly samp1e of approximate1y 100 1impets was co1lected along transect 1ines perpendicu1ar to the shore in an area of beach rock at Six Men•s Bay. All sizes of animale above 6 mm. long were represented in the samp1e from their upper 1imit in the pink zone to their lower limit in the surf zone. The length, width and height of each animal were measured with vernier calipers to the nearest 0.5 mm. All animale were replaced within the same transect area from which they had been co1lected to ensure that any particular aize group was not depleted from the population. The transect was made in a different position each month. Specimens be1ow 6 mm. long were not included in the transects as they are easily overlooked and would therefore not give a re presentative sample of the small aize group. Other specimens were kept in aquaria with running sea water and measured at monthly intervals. They fed on algae on the rocks provided and on the aide of the aquarium. Another group of measured limpets was placed on the concrete piles of a wharf along the l~eward coast. This area had previously been cleared of all other members of the species. Every month the limpets were remeasured. A small number of specimeas of various sizes was used in this experiment so that individual growth rates could be obtained. All attempts to mark the shells were unsuccessful. In the course of the experiment from 40% to 75% of the limpets were lost off the piles during most months. To determine changes in body dimensions, weight and volume with distribution and growth, 837 specimens were collected from various 6 parts of the islaad. The leagth, width aad height of the liapets were aeasured with calipers to the aeareat 0.5 aa. lt ia appareat evea to a caaual observer that the ahell is roughly the shape of aa elliptical cone. Siace a criterioa of aize was aeeded, this shape waa chosea. The voluae of aa elliptical
coae, in coaaoa with maay other three diaensioaal objecta, ia aerely a constant times the product of the three leadiag diaeaaioaa. la thia caae the leadiag diaeaaioaa are the major and aiaor diaaetera of the baae and the height of the ahell. The voluae is expressed aa ~ .n.2.B where B is the height and Dais the geoaetrical aeaa diaaeter of the shell base, that is the square root of the product of the leagth aad the breadth. la fact the shells are not exactly the shape of an elliptical coae as they are soaewhat flat- teaed at the apex due to the apical hole aad have the aaterior end alightly tapered. Bowever, theae differences would have a negligible effect oa the volume. For coaveaieace the aize waa calculated aa Da 2 .H and to obtaia the voluae this value must be multiplied by ~ 2 the constaat ~· Usiag Da .B aa a basia, the iatercompariaoa of voluaes will be accurate providiag that the shells are of the saae shape whether they are exact elliptical coaes or aot. The specifie growth rate was calculated uaiag the toraula, 2(x._1 - x.> xa+l + x. 2 where x. is the aize (Da .B) at tiae ta' aad Xa+l is the new aize at tiae ta+l (Baaai 19,7). Meathly iatervals were used. The ahape of the ahell is expreased as the relative height B/Dm. The shape of the base is expressed as L/B = leagth/breadth 7
of the shell. The shell thickness was measured by a vernier gauge to 0.1 mm. using an area about 2 mm. posterior to the apical hole and dorsal to the large shell ribs.
The shell and soft parts were separated b~ cutting the muscles of attachment. Both parts were damp dried on paper towelling and weighed to the nearest 0.001 gm. The volume under the shell was measured to the nearest 0.01 cc. by inverting the shell in a plasti cine eup. Water, to which soap had been added to reduce surface tension, was introduced through a 1 cc. graduated pipette until the meniscus was level with the edge of the shell. The mean value of three readings was used. The size of the extra-visceral space was calculated on the assumption that the specifie gravity of the soft parts of all ani mals was the same and equal to 1.00. The volume of the space was obtained by subtracting the volume of soft parts from the volume under the shell. Lines were fitted to the plotted points by the method of least squares. T-tests were used to determine whether the differences between the mean points and the differences between the slopes of the lines were statistically significant. The statistical calculations were based on the methods of Stanley (1963). The number of contri
buting specimens is marked beside each plotted point. A class 2 interval of 500 mm.3 (Dm .H) was used in the analysis of the size data, one of 2 mm. was used in analysing the length data and one of 0.1 ga. was used for the wet weight measurements. In the first two cases a class number was used to facilitate calculations. 8
Resulta Horizontal and Vertical Distribution The density of F. barbadensis, measured at twelve stations around the island, is maximua along the leeward, West, coast (Fig. l) and more specifically on the horizontal platform of beach rock at Six Men's Bay (Fig. 2). Relatively few speciaens inhabit the South East and East coasts which receive heavier seas (see wind rose in Fig. 1). In habitats where F. barbadensis is scarce other keyhole limpets auch as F. nodosa and Diodora spp. become more abundant.
A relatively sheltered environment is obviously more favourable to F. barbadensis.
Lewis (1960) has already described the different vertical zones and their principal fauna at various stations around the rocky shores of Barbados. Table 1 shows a suamary of some of the zones and their intertidal heights which will be used in the fol- lowing account.
TABLE 1
THE VERTICAL INTERTIDAL ZONES OF THE ROCKY SHORES IN BARBADOS, AFTER LEWIS (1960)
Zone Range Tidal height in feet aboTe zero datua
Surf Mean low water springs - mean low 0 - 1.2 water
Pink Mean low water - just below mean 1.2 - 2.8 high water
Green Just below mean high water - 6 2.8 - 3.8 inches above mean high water 9
The distribution of F. barmadensis at four se~ected stations is as follows: Six Men•s Bay (Fig. 2). This station is situated on the West coast and is exposed to moderate sea conditions. The area consista of horizonta~ beach rock platforms and large boulders. The average amplitude of the waves breaking on this shelf is about 6 inches. F. barbadensis has a vertical range from below mean low water to mean high water and inhabite an area from below the surf zone, through the surf and pink zones and into the green zone on one high level boulder. The greatest density occurs in the pink zone where up to 34 animale may be found per square metre. A limpet living in the pink zone, one foot above mean low water, has an average emersion without receiving wave action of 9.5 out of 24 hours. The upper pink and green zones are dry at low tide on a sunny day. The large algae supported by the pink zone include Chaetomorpha, Hypnaea, ChladoEhora, Enteromorpha and Ceramiua. At this station most of the limpets do not use the shelter provided by algae, fissures in the rocks or tidal pools as protection from wave action.
Balf Moon Fort (Fig. 3). This is a cliff station on the West coast exposed to moderate sea conditions. The average ampli tude of the breaking waves at this location is about 10 inches. Fissurella inhabite the region from below mean low water to just below mean high water. This range includes an area frem belo• the surf zone into the pink zone, the latter having the maximum density at this station of up to 10 specimens per square metre. A limpet living in the pink zone, one foot above mean low water, has an 10
average emersion time of 8 out of 24 hours without receiving any wave action. None of the inhabited zones dry out at 1ow tide. The
surf and pink zones support the 1arge a1gae Chaetomorpha, ~' Hypnaea and Dict:opteris. The limpets use the a1gae and fissures in the surf zone for protection from the waves.
Deep Water Harbour (Fig. 4). Situated on the South West coast, this bou1der station is subject to moderately exposed sea conditions with an average wave amplitude of about 10 inches. Fissure1la is found from below mean low water to mean high water or from below the surf zone up to the green zone. The maximum density, reaching up to 14 anima1s per square metre, is found in the surf and pink zones. None of the inhabited zones dry out at low tide and limpets living one foot above mean low water are never free from wave action. The surf and pink zones support Chaetomorpha, ]!!!, Ceramium, Chladophora and B;ropsis. Only the sheltered sides of the large boulders are inhabited by Fissurella,thus giving protection from the pounding waves.
Oistins (Fig. 5). This cliff station is situated on the South West coast and is subject to moderately exposed sea conditions. The average amplitude of the waves breaking at this station is about 10 inches. Fissure1la inhabite the surf, pink and green zones from mean low water to mean high water. The maximum density of up to 7 specimens per square metre is found in the surf and pink zones. The average emersion time without wave action for a limpet living one foot above mean low water is 8 out of every 24 hours. The upper pink and green zones dry out at low tide on a sunny day. 11
Chaetomorpha1 ~ and Dictyota are supported by the surf and pink zones and the limpets use the algae and crevices in the rocks as protection from the waves.
At stations on the East and South East coasts F. barbadenais is limited to the pink zone where the amplitude of the waves during average sea conditions reaches 18 inches. This zone is therefore subjected to continuous wetting. The limpets find protection in rock pools, crevices, the sheltered aides of boulders and in the algal covering (Fig. 6). It is interesting to notice the high density of limpets at St. Lawrence on the South West coast where the bay is protected by a rocky bank offshore. The amplitude of the waves in the bay is reduced to about 2 inches at low tide, rising to about 8 inches at high tide during average sea conditions. The pink and green zones dry out at low tide. In one region at this station the number of large specimens, 25 to 35 mm. in length, is greater than at any other station on the island. These resulta support the view that F. barbadensis is more suited to a sheltered environment. Seasonal changes in distribution due to temperature changes, winter storms or breeding populations were not observed. The mean monthly air and sea temperatures range from 25 0 to 28 0 C throughout the year (Sailing Directions for the West lndies 1949) while the coastal waters range from 25.5 0 to 28 .50 a (Lewis 1960). The winter swells (Donn and McGuinness 1960) which occur periodically and laat only a few days may cause some depletion in the limpet population.
Fissurella maintains a scattered distribution on the rocks and1 12 eTen where the density is comparatively high, feeding and resting animals were rarely observed to touch each other. There was no indication of the limpets voluntarily either avoiding or seeking the sunlight. Some specimens inhabit algal covered areas and crevices in the rocks but the majority are exposed directly to the sun, including those animals living in the green zone where the deaiccation factors are greater than lower on the shore.
Absolute Growth and Specifie Growth Rates The growth of individual limpets living on the wharf is shown in Figs. ? and 8. fhe specifie rate of growth is highest for small 2 specimens and drops sharply as the animale reach Dm .B = 500 mm.3 (Fig. 8), equivalent to an absolute size of 131 ma. 3 The absolute 2 growth of animals, expressed volumetrically as n. .H, is shown in Table 2.
TABLE 2
ABSOLUTE GROWTH RATES OF F • BARBA.DENSIS BASED ON INDIVIDUAL MEASUREMENTS
Size, Dm 2 .H, range Approximate Growth in ma.3 per month, A (Da2.B), in mm.3
200 - 500 200 500 - 1,000 310 1,000 - 1,500 390 1,500 - 2,000 430 2,000 - 2,500 460 2,500 - 3,000 490 The nuaber of ~impets is too sma~~ to show any seasonal changes in growth rates (Fig. 7) but it is apparent that the growth curves do not level off in mature animals. A limpet of 10 to 15 mm. shell length grows about 2 mm. per month, one 15 to 20 mm. long grows about 1.5 mm. per month while a 20 to 25 mm. specimen increases in length about 1 mm. per month. From the growth rates obtained it is estimated that an indi- vidual Fissurella lives from one to two years and usually attains 3 2 ) a length of 25 to 30 mm., or a size of 3 1 000 to 7,000 mm. (Dm .H • No specimens were found over 35 mm. long at any time during this
In the aquarium only small specimens ahowed a significant increase in size. When the growth is plotted, as in Figs. 7 and 8, the values lie in the same range as that for the liapets living on the wharf. However, these animale developed very prominent ribs on the new areas of shell growth from the time that they were brought into the aquarium, possibly due to the lack of wave action. The aize distribution of samples from the monthly transects is shown in Figs. 9 and 10. The presence of two or more modes in many of the frequeney distribution eurves indicates that more than one age class is present in the population. Although distinct age groups are not always clearly defined, probably because of the continuous breeding season (Part IV and Fig. 58), nevertheless monthly increases in modal values are apparent.
In Fig. 91 lines fitted approximately to the modes are super imposed on the frequency distribution curves to show the growth rates of two age classes within the population. Thua, in June of 14
1964 the modal value lies at 18 mm. In August it has increased to 21.5 mm. and it reaches 22.5 ma. in September and 24 mm. in November. The increase in length of 6 mm., from 18 to 24 mm., is achieved in a period of five months. The average growth rate is therefore 1.2 mm. per aonth which agrees c1oae1y with the rates derived from 1aboratory and wharf specimens (Fig. 7). Similarly, a 1ine has been drawn from the 17 mm. mode of the December frequency distribution curve. The modal value increases to about 22 mm. in March and 23 mm. in June. Therefore in six months there is an increase in length of 6 am., from 17 to 23 mm., giving an average growth rate of 1 mm. per month. Again, this growth is simi1ar to the rates obtained from laboratory and wharf liœpets. Lines have also been drawn approximately through the modes of the size frequency distribution curves in Fig. 10. A modal value of 750 mm. 3 in June, 1964, increases to 2,250 mm. 3 in October 3 and 3 1 250 mm. in December. ~he increase over six months, from 750 to 3,250 mm. 3 , is 2,500 mm. 3 or an average growth of about 420 mm. 3 per month. This growth rate agrees closely with the rates obtained from the growth of individual specimens living in the laboratory and on the whart.(Table 2). The lower modal value of 250 mm. 3 in October increases to 3 3 750 mm. in December, 1,750 mm. in February and 21 750 mm.3 in 3 April. The growth of 2,500 mm. , from 250 to 2 1 750 ma.3, over six months gives an average growth of approximately 420 mm.3 per month. This growth rate is close to the rates obtained from wharf and laboratory specimens. 15
It must be reaeabered that the figures givea above, both for the absolute volumetrie aize aad for the absolute voluaetric growth rates, are in teras of the criteria defined in the aethods aad must n be multiplied by the factor of ~ ia order to obtaia approxima- tions to the real values. The statistics for the following resulta are ahowa ia Tables 3 to 6 aad are indicated by a reference auaber ia the text.
Structural Chaages With Growth A general iacrease ia the relative height of the shell occurs with growth (Ref. 1; Fig. 11). This change is accoapaaied by a geaeral broadening reaultiag ia a aere circular basal shape (Fig. 12) aad by a thickeaiag of the shell (Ref. 2; Figa. 29, 30). The wet weight of the soft parts of the aaimals ia equivaleat to the aize since the deasity is assuaed to be uaity. As the aaiœals ia- creaae ia size the ahells become heavier (Figs. 23, 24) with a larger volume uader the shell aad extra-visceral &pace betweea the shell aad the soft parts (Figa. 25-28). These chaages occur at differeat rates accordiag to the horizontal aad vertical distributioa ot the limpet.
Structural Changes With Horizontal Distribution Speciaeaa froa four stations were used to deteraiae whether the shape of the shell was affected by the horizontal distributioa. The resulta are showa in Figs. 13 and 14. The shells at Oistias are relatively higher spired per givea aize thaa those at the other three stations. The rate of iacrease is aot sigaificantly greater at Oistias thaa at Six Mea's Bay (Ref. 7). The ateep chaage ia ratio 16 with iacreasing size at Ha1f Mooa Fort is probab1y becauae of the aaal1 auaber of 1impets ava11ab1e for co1lectioa.
TABLE 3 STATISTICS ON STRUCTURAL CHANGES WITH GBOWTH
y Probability Fiducial liaits Ref. N -x by x Ne. claas about b{x for 95~ auabers cer aiaty
1 697 4.67 0.389 +0.00464 b x differa Upper lirait = slgaificaatly 0.00487 frea zere, Lewer lillit = P< 0.01 0.00441
2 697 4.67 1.456 -0.00555 !{x differa Upper liait = gnificaat1y -0.00490 fr•• zero, Lewer limit = P< 0.01 -0.00620
.A.bbreviatioas: N = auaber of speciaeas ia Allple byx = regressiea coefficient of 1 Oll x
Sàe1la fr•• the statioaa oa the South West coast have a more circular base per givea aize thaa those froa the West coast habitats {Fig. 14). The high rate of ratio decreaae at Ralf Mooa Fort is probably due te the aaall auaber of apeciaeas at this station. At Oistiaa the ahe1ls are geaeral1y ae thicker per givea leagth thaa at Six Mea•a Bay (Figs. 29, 30). The weight of the shell, the voluae uader the ahell aad the extra-visceral apace ef aniaala of equal wet weight are geaerally greater at Oistias thaa at Six Men's (Fige. 23-28). 17
Structural Changes With Vertical Distribution Since the mean tidal range in Barbados is 2.3 feet there is little physical separation of populations living at different intertidal heights in most areas. At Six Men's Bay high level animale taken from the upper limita of the pink zone were compared with low level animale living at the edge of the surf zone. High level shells are significantly larger than those from the lower levels (Ref. 3). They are generally higher spired than shells of similar size located at the low levels although there is no significant difference between the rate of change in relative height with increasing size of the high and low level shells (Ref. 8; Fig. 15). Shells from the two levels do not differ generally in basal shape but the rate of decrease of the L/B ratio is more intense at the low level (Ref. 9; Fig. 16). High level animale at the Half Moon Fort cliff station were collected from the pink zone while the low level animale were obtained from the upper edge of the surf zone. Shells from the high level are significantly larger (Ref. 4) but do not generally differ in relative height from low level shells of the same size, although the rate of increase in relative height is greater in shells from the high level (Ref. 10; Fig. 17). Low level shells do not differ in basal shape from high level shells and there is no difference in the rate of ratio decrease (Re~ll; Fig. 18). At the Deep Water Harbour boulder station high level shells were collected from the upper pink and lower green zones while low level animals were obtained from the surf and lower pink zones. No significant aize difference (Ref. 5) or difference in relative 18
height or in the rate of change ia shape (Ref. 12) is fouad between high and low level shells (Fig. 19). There is generally ao differeace between the basal shape of ahella ia the two habitats although the decrease in relative breadth with iacreaaing aize ia more pronounced ia high level shells (Ref. 13), probably due to iaaufficient high level aaimala in the aample (Fig. 20). These resulta indicate that the enviroamental factors affecting the limpets are aot aufficiently different in the high and low level habitats at this station to cause demoaatratable changea ia the aspecta of the ahell shape that have beea atudied.
TABLE 4 STATISTICS ON STRUCTURAL CHANGES WITH VERTICAL DISTRIBUTION
Reference Iatertidal B Probability Nuaber leve1
Bigh 31 2585.45 3734817 P 4 Bigh 39 1001.55 434171 0.02>P>O.Ol Low 33 654.12 314610 High 36 979.82 603452 5 Low 67 1363.98 1054308 0.10>P>0.05 6 Bigh 34 1318.69 511349 Lew 46 1158.15 641203 0.50>P>0.10 Abbre"t'iatioaa: N = auaber of çecimeaa ia aamp1e SD = ataadard de"t'iatioa The 1ower green aad upper piak zoaea at Oistiaa were iahabited by the high 1eYel aaiaala at this cliff atatioa while 11apeta froa low levela 1ived ia the lower pink aad surf zoaea. High le"t'e1 ahella 19 are not significant1y 1arger than low level shells (Ref. 6) but they are higher spired per given aize and the rate of increase in relative height is greater {Ref. 14; Fig. 21). TABLE 5 STATISTICS ON STRUCTURAL CHANGES WITH HORIZONTAL AND VERTICAL DISTRIBUTION 2 2 Ref. Habitat N b b SD SD Probability yx xy x y No. (class nos.) Ois tins 149 +0.00708 +13.7 5.923 0.007950 P>0.50 7 Six Men's 223 +0.00644 +89.3 9.700 0.0006992 8 High 73 +0.00660 +69.6 9.216 0.06363 0.50>P>0.10 Low 71 +0.00561 +112 4.872 o.ooo2484 High 73 ... o.oo67o -24.7 9.216 o.oo2496 o.05>P>O.o2 9 Low 71 ..o.Ol24 -31.6 4.872 0.001848 10 High 39 +0.0159 +49.1 1.729 0.0005733 P 11 High 39 -0.0354 -17.5 1.729 0.003854 0.50>P>O.l0 Low 33 -o.o441 -16.4 1.254 0.003387 High 12 36 +0.00884 +78.7 2.410 0.001825 P>0.50 Low 67 +0.0101 +56.1 4.220 0.0007646 13 High 36 -0.0169 -22.3 2.410 0.001825 P The low level shells geaerally do aot differ in basal shape from high level shells and the rate of chaage ia shape is aot sigaificaatly more intense in shells from the low level {Ref. 15; Fig. 22). Liapets from the green zone at Six Mea•s Bay were compared with those liviag low ia the piak zoae for differences ia water holding capacity. The rate of iacrease ia shell weight with iacrease ia aize is greater in high leve! shells (Ref. 18; Fig. 23) but for a givea wet weight of soft parts these shells are generally no heavier thaa low leve! shells. High leve! shells have a larger volume uader the shell (Ref. 19) and a larger extra-visceral space (Ref. 20) thaa low level shells of similar wet weight (Figs. 25, 27). High leve! shells of givea leagth are thiaaer, although the rate of iacreasiag thickaess is no less (Ref. 16), thaa those from the low leve! (Fig. 29). Similar data were obtained from Oistins where high level aai aals were collected from the greea aad upper piak zoaes and lew leve! speciaeas from the lower piak aad surf zoaes. The rate of iacrease of shell weight is greater in low leve! shells (Ref. 21) but the two groups ia general show no difference ia shell weight per given wet weight of soft parts (Fig. 24). The volume under the shell (Ref. 22) and the extra-visceral space (Ret. 23) also show ao difference betweea high aad low levels for a givea wet woight of soft parts (Figs. 26, 28). The high leve! ahells are ao thiaaer per givea leagth thaa those from the low leve! (Ref. 17; Fig. 30). The shells collected from Oistias aad the low leve! shells from Six Mea's Bay have saoothed shell ribs while high leve!, green zoae, shells at Six Mea's show more proaiaeat ribs than at the other areas. Ia the surf aad piak zones at all habitats the ahells are 2l covered with the large algae inhabiting these zones and those in the pink zone are usually also coated with coralline algae. T.A.BLE 6 STATISTICS ON STRUCTURAL CHANGES WITB VERTICAL DISTRIBUTION 2 2 Ref. Inter- N b b SD SD Probability No. tidal y x xy x 7 1evel 18 Bigh 35 +1.73 +0.564 0.1054 0.3228 P 22 High 30 +1.56 +0.627 0.03466 o.o8631 Low 40 +1.64 +0.605 0.05778 0.1567 O.lO>P>0.05 High 23 30 +0.571 +1.52 0.03466 0.01301 Low 40 +0.638 +1.51 0.05778 0.02445 0.50>P>O.l0 Abbreviations: N = number of specimens in sample byx = regression coefficient of y on x bxy = regression coefficient of x oa y SD = standard deviation 22 1 HALF MOON FORT 2 SIX MEN'S BAY 0 5 10 °/o 3 PAYNES BAY L...J.....J 4 DEEP WATER HARBOUR 5 ST LAWRENCE 6 OISTINS BAY 7 SI LVER SAN OS 8 LONG BAY 9 CON SET BAY 10 BATH , BATHSHEBA <1/m~ 12 RIVER BAY < 10/m~ <1/m~ < 20/m~ :a. <1/m. < 20/m~ <7/m~ < 1/m~ 0 2 4 MILES 1 1 1 FIG. 1. Map ot Barbados showing annual wind direction and density of Fissure11a populations at 12 stations. Wind rose based on 50 year means (Sai1ing directions tor the West Indies, 1949). 23 pink zone FIG. 2. Six Men's Bay cellecting area at mean low water springs. FIG. 3. Half Moon Fort collecting area at mean low water springs. 24 FIG. 4. Deep Water Harbour collect ing area at mean low water springs. FIG. 5. Oistins collecting area at mean low water springs. 25 FIG. 6. Bathsheba collecting area at mean low water springs. Note rock pools and algae providing shelter. e e 30------~~----~--,--.--~--.--r--~~--, -WHARF 25 1- ...... --~ ~ ------AQ.UAAIUM . -2: :E r ~ ~ / • • - 20 % .... zC!) ,_ ...... &.LI .... , .r 1\) ~Jt~ /tt" _. 1 0'\ .... " .... l5 r .1 " &.LI ~ % 41) 10 s~~--~--~--~--~~--~--~--~--~~~~--~ JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL 1964 1965 FIG. 7. The 1ength of 1ndividua1 F. barbadensis from Ju1y 1964 to Ju1y 1965. 27 +=WHARF o •AQUARIUM 0 1.0 ....lLI 0.8 c( cr :I:.... ~ 0.6 0 a: (!) u CL: ü Q4 ~ + + + 0.2 + + + 0 0 1000 2000 3000 4000 SIZE (Dm~H) MM.1 FIG. 8. Specifie growth rates of individual F. barbadenais from wharf and aquarium. N N 00 00 e e ____. ____. JUN JUN bar F. F. MAY MAY 1965. 1965. of of 5 5 APR APR June June •f. •f. 196 196 to to MAR MAR 1964 1964 FEB FEB Oi)4o Oi)4o populations populations of of JAN JAN June June DEC DEC ______ ___;, ___;, NOV NOV DISTRIBUTION DISTRIBUTION transects, transects, distribution distribution OCT OCT OF OF SEP SEP 1964 1964 monthly monthly Length Length SCALE SCALE AUG AUG 9· 9· from from -:------_:_=:__~~~:.___-:---- JUL JUL FIG. FIG. ______ JUN JUN ....._ ....._ r--r· r--r· badenais badenais s.o s.o 18.0 18.0 12.0 12.0 36.0 36.0 24.0 24.0 30.0 30.0 MM. MM. LENGTH LENGTH e e ~ e JUN monthly from MAY APR 19G5 barbadensis MAR F. of FEB JAN populations of DEC Of. NOV 1 DISTRIBUTION 1 2030 distribution 1 10 1965. OF OCT 1 0 June SCALE SEP to 1964 frequençy 1964 AUG S~ze June 10. JUL ~ 1 FIG. JUN transeets, 0 10 70 90 not 50 80 60 20 40 120t 30 100 • 100 ...... - SIZE rm~H e e .48 .46 + {S) .44 .42 H + {St.) ~40 + ('1) . 381 ~ + (t3o) ~ .36 + (1) .34 0LL...__~~-:::--:::~~;Q4o'o~~~~~~~o;;c;-;; 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 SIZE (Om~H)MM~ 1 2 3 4 5 6 7 8 FIG. 11. Change in the relative height, H/Dm, of shells with growth. The fiducial limite for 95% certainty about the calculated Y values are shown. The numbers in brackets denote the number of specimens measured in each size group. Y = 0.3672 + o.oo464x. e e u------~----~~----~------~--~--~~~------~~ + (-.3) 1.5 L +(S') ('1.) -8 ...... + (cf.') (1."1) ... (•'1.) ... lill + ('Ja) (to)~ ...,_- --t:....:.. -;,______- + (S) + (5) 1.4 + ------~-(4.o) + tu) ~ 1.3 +b> 1.2~~~~~~~~---L--~--~--~--~--~~--~--~------~--- e 5oo 1ooo 1soo2ooo 2soo3ooo 3soo4ooo 45oo 5ooos5oo sooo 65oo 1ooo75oo aooo ssoo SlZE (Dm ~H)MM~ 1 2 3 4 5 ' 7 8 g 10 11 12 13 14 15 16 17 CLASS NUMBER FIG. 12. Change in the basal shape, L/B, of shella with growth. The fiducial limita for 95% certainty about the calculated Y values are shown. The numbers in brackets denote the number of specimens measured in each aize group. Y = 1.4820 + o.00555X. N N VI VI 1 1 8500 8500 e e +(&) +(&) X(l X(l 17 17 rtt:,M•s rtt:,M•s 8000 8000 = = (1) (1) MM&ouP. MM&ouP. + + dif 16 16 + + Y Y SI~ SI~ D.W. D.W. ~ ~ 7500 7500 from from +(t) +(t) x(J) x(J) 15 15 0.3942 0.3942 specimens specimens Fort: Fort: = = 1000 1000 X(2) X(2) Y Y of of 14 14 shells shells Moon Moon 6500 6500 (1) (1) of of x x 13 13 number number Ha1f Ha1f , , Oistins: Oistins: aize aize 6000 6000 OlSTIN$ OlSTIN$ {'1.) {'1.) --- ...... ~ ~ the the ...... 12 12 and and 5500 5500 ,_, ,_, ,_, ,_, . . - 11 11 denote denote o.oo644X. o.oo644X. H/Dm, H/Dm, + + 0.00526X. 0.00526X. &000 &000 + + 10 10 4500 4500 (.a) (.a) (2.) (2.) 0.3650 0.3650 brackets brackets height, height, f+t~ f+t~ ------:--- x x +(s') +(s') 0 0 ~ ~ 9 9 = = 0.3444 0.3444 in in = = Y Y 4000 4000 UMBER UMBER Y Y N N +(S') +(S') relative relative (Dm~H) (Dm~H) 3500 3500 numbers numbers Men's: Men's: tLASS tLASS } } Six Six Harbour: Harbour: The The 51ZE 51ZE 3000 3000 ...... /~ /~ between between ~~ ~~ . . Water Water 2500 2500 \ \ - group. group. 5 5 6 7 8 8A'f 8A'f fQIIQ' fQIIQ' Deep Deep re1ation re1ation ~- 2000 2000 locations. locations. aize aize 4 4 MM~Uit MM~Uit ....., ....., "Uii "Uii The The ""· ""· ...... 0\STtMI 0\STtMI 1>. 1>. 11'111. 11'111. ~" ~" 1500 1500 (1.4) (1.4) __ __ each each • • "' "' :: :: z z 0 0 x x 0 0 (!) (!) 13. 13. + + O.Ol32X. O.Ol32X. in in + + horizontal horizontal 1000 1000 2 2 3 FIG. FIG. 500 500 l~ l~ 1 1 ferent ferent measured measured 0.3294 0.3294 0.00708X. 0.00708X. + + 0 0 e e .32------~~~~~--~---L---L--~--~--~--~--~--~--~--~------·441- ~ ~ (1) (1) 8500 8500 l l e e )( )( OUR OUR +W +W 17 17 FORT FORT 8A 8A UR UR •s •s (l) (l) MARI MARI 8000 8000 + + - MOOK MOOK 16 16 W. W. MARIO MARIO D. D. MEN MEN W. W. IST\NS IST\NS 7500 7500 0 0 different different measured measured O. O. HALF HALF suc. suc. 15 15 • • • • • • • • 0.00948X. 0.00948X. 1.5970 1.5970 0 0 lit lit • • + + = = - 7000 7000 (2.) (2.) from from Y Y 14 14 'l 'l (1) (1) specimens specimens (1) (1) 6500 6500 1.4571 1.4571 x x 0 0 ahells ahells 13 13 Fort: Fort: = = of of of of Y Y {l) {l) 6000 6000 Moon Moon 12 12 0 0 ...... o.._(2.) ...... o.._(2.) OISTINt OISTINt "· "· size size number number (3) (3) (1\0. (1\0. (':1.) (':1.) 5500 5500 Half Half x x • • 11 11 the the Oistins: Oistins: and and 3 3 (?) (?) lt) lt) 5000 5000 0 0 x x 10 10 MM. L/B, L/B, denote denote 4500 4500 (S) (S) 0.00957X. 0.00957X. 9 9 + + NUMBER NUMBER - 0.00680X. 0.00680X. shape, shape, Om~H) Om~H) - ( ( (S) (S) 4000 4000 8 8 + + brackets brackets basal basal 1.534o 1.534o CLASS CLASS in in {!If.) {!If.) <•> <•> 3500 3500 = = ~1\\11101 ~1\\11101 SIZE SIZE 1.4451 1.4451 7 7 x x e e ...... = = Y Y Y Y (l?) (l?) (t)a. (t)a. \1'1)"- t•> t•> 3000 3000 ' ' between between FORT FORT 6 6 • • 'X. 'X. e e numbers numbers • • Men•a: Men•a: t'S') t'S') 1 1 2500 2500 The The MooN MooN 5 5 - + + Six Six Harbour: Harbour: relation relation ('11) ('11) KALF KALF 2000 2000 ------ 4 4 0 0 Water Water The The )1"1~·- group. group. .. .. 1500 1500 locations. locations. 3 3 0( 0( Deep Deep 14. 14. aize aize 1000 1000 2 2 e;•.J"'-. e;•.J"'-. FIG. FIG. each each 500 500 horizontal horizontal in in 0.0409X. 0.0409X. 1 1 E) E) 1 1 1 1 ~ ~ 0 0 e e 1.2----~--~~~~--~--~--~--~--~---L--~--~--~--~--~--~~ 1.2----~--~~~~--~--~--~--~--~---L--~--~--~--~--~--~~ 1.6--~~~--~--~--~--~--~--~--~--~~--~--~--~--~--~~ 1.6--~~~--~--~--~--~--~--~--~--~~--~--~--~--~--~~ 1.31 1.31 1.4 1.4 1.5 1.5 8 8 .!:. .!:. 4=" \H e 7500 +(1) 15 and level: 7000 number LEWL +(1) 14 , high .. Low the au 6500 in 13 LII\15L aize 6000 denote +(•) 12 LOW and o.oo66ox. + 5500 ---- 11 +(•) B/Dm, brackets (l) 5000 0 in 0.3769 "'lt.) 10 = -- Y 4500 MM~ height, +{S) numbers NUMBER 4000 ~l ---- level; +(1) The _... (Dm~.H) relative 3500 (0) High -- 0 +(tl CLASS Bay. SIZE 3000 between 6 7 8 9 +(Q O{J) -- L. group. Men's 2500 (o,) r.> LaveL L&VIi s Six aize _.J relation LOW Hte-H 2000 trom :: : ol'll ~Il each The 0 + -- in o.oo561x. (11) 1500 + 0 +{n) shells 15. '") -- 1000 0 FIG. level 0.3688 specimens .,..,.. = 500 lt.•) (1) 1 2 3 4 ot __ low Y + 0 .50 .48 .46 .44 .36~~~~--~--~--~--~~-L--~--~--~--~--~--~--~-- .42 .40 .3& Dn J:l e e e + • MHi'M Lli\IEL 0 • l.OW UiVEL 0 (2.0) + {1\) ---- (1.,) 0 (3) 0 h) o--..._-- + (") ~~ ...._---.... 0 (t.) ~) + ('&.) 06'J) 0; +('1) + {\) +(u) + {\) ..._...._ (l) Ml&l LEV&L ~ +(11) ...._...._ 1.4 LoW Ll'\IC'L --- -t-(1) +h) 1.3L--'--__.______.--:-:--:-::--:-::-::-:::~~;:w~i05c~tsoo~~~;c;oo7!0 500 100 0 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 SIZE (Dm ':H) MM~ 1 2 3 4 5 (, 1 8 9 10 11 12 13 14 15 CLASS NUMBER FIG. 16. The relation between basal shape, L/B, and size in high and low level shells from Six Men's Bay. The aumbers in brackets denote the nuaber of specimens in each aize group. High level: Y = 1.5474 - o.oo670X. Low level: Y = 1.5492 - O.Ol24X. 'gt e aum aad 0.3375 aize • The Y H/Dm, each Fort. ia 3500 level: +(1) height, Mooa Low (t) 3000 + LE.VEL Ralf 3 specimens LiVEL. (\) 2500 relative MM. HI&H of 0 BER from LOW O.Ol59X. •)H. .H) + 2000 2 NUM -(2.),;"" / betweea aumber shells (Om / L&.VE.L 1.1!'1&1.. 1500 (~ the 0.3218 -1- ~") = L.OW H\eH level CLASS • • SIZE Y 1000 (tt.) + relation 0 0 2 3 4 5 6 7 denote low The 500 aad level: 1 ~~~ 17. 0 high brackets Righ -4-o .46 .44 .42 • .38 .3& .321 .34 ia ia FIG. H 0.00931X. o-;' group. size + bers e \,N \,N -..J -..J e e ia ia High High in in size size and and 0.0441X. 0.0441X. group. group. - numbers numbers 3500 3500 L/B, L/B, size size The The +(1)1 +(1)1 7 7 L5VEL L5VEL l..liVfit. l..liVfit. 1.6124 1.6124 (t) (t) 3000 3000 = = each each 6 6 ...... shape, shape, LOW LOW HIGH HIGH Fort. Fort. Y Y c c "' "' in in 2500 2500 0 0 + + MM~ MM~ 0{1) 0{1) basal basal Moon Moon ) ) level: level: '' '' 2000 2000 2:H 2:H NUMBER NUMBER o(1) o(1) Ralf Ralf - specimens specimens Low Low (Dm (Dm between between 1500 1500 ·-"' ·-"' of of from from ...... CLASS CLASS SIZE SIZE (4) (4) 1000 1000 \l"rJ~) \l"rJ~) 0 0 number number 0.0354X. 0.0354X. shells shells relation relation - 500 500 the the The The 1 1 2 3 4 5 level level r---,...---,----r---.----r---- 0 0 1 1 1 1 1.5770 1.5770 18. 18. 1.7 1.7 1.5 1.5 = = denote denote low low 1.4 1.4 1.6 1.6 1.3 1.3 Y Y and and FIG. FIG. L L B B - level: level: brackets brackets high high e e e e . 48 ~~~~~~~~~~' X. : HIGH LEVEL .46 o : lo'W LEVEL oCil 0 (1) .44 LOW LEVEL / / .42 / / H / Dm .40 / Ml&-H LEVE.L 0 (15) (~) / 0(2) .3S o FIG. 19. The relation between relative height, H/Dm, and size in high and low level shells from Deep Water Harbour. The numbers in bra ckets denote the number of specimens in each size group. High level: Y = 0.3257 + 0.00884X. Low level: Y = 0.3314 + O.OlOlX. \0 \H e in 0.00680X. (o) 5000 size - 10 group. numbera _., LE'IIEL and L&Yt:L. >l 4500 ~u- EL. The 9 V size J!L_ 1.~~71 LOW HI&H li. L/B, : = : 4000 &K + 0 each Y +(1) o(o) !4l in shape, Harbour. 3500 (1) 7 8 0 +hl level: MM~ (1) 3000 Water basal ____ G Low specimens NUMBER -o Deep ~1) 2500 of ____JL.....-____JL.....-____JI...,______Ji...,___L..____, 5 (Dm~H) o~ + between _ from O.Ol69X. 2000 o CLASS number - SIZE - _ 500 , ~~~ the relation shella ____,~...______J 0 ,±.!:! - 1.~710 _ The = 1000 {1,) level + denote Y 20. low (i) (13) 500 0 1 2 3 4 + and FIG. level: 1 ~~~~~~~~~~ 1 ...______,~,..______, 0 1 brackets 2 1.3 1.5 1.4 1.6 1. in high High B l - e e e + :: MIGH LE\IEL 0:: LOW L.EVEL .S> 0 l2.) Hl&H LEVEL. + (3) .48 0 h) _....-../" .46 0 ('-) ---,..,..... __.- LOW LE.VIL 0(1) + ---('l) ,..,..... --- .44 H Dm 0 {1) +(9) / .42 0 (1) + l~>).. b~6•l ~·}- 0 --- -- 0 ('l.l.) _.- > ~ (S) g .40 _....---- ...- o( 0 (•) o (n) 1'1.) .38 .36~--L---~--~~~--~~~--~--~--~------~~ 0 500 10 0 0 1500 20 0 0 2 500 3000 350 0 4000 4500 5000 5500 6000 6500 SIZE ( Dm2..H) MM~ 1 2 3 4 5 6 7 8 9 10 11 12 13 CLASS NUMBER FIG. 21. The relation between relative height, H/Dm, and aize in high and low level shells from Oistins. The numbers in brackets denote the number of spec~ensin each aize group. High level: Y = 0.3965 + O.Ol08X. Low level: Y = o.3834 + o.o0711x. e e 1.6 .---,--.---.---..---,---r--,----r------,...------r----r---r--.. 0 (J,) + • HIGH 1..&1/liL. 0 "' '-OW UVU. 1.5 l 0 (') +(<\)- -- {, - - 0 (1.'1.) -- 0 ,;) 8 --. 0 (2.) 0 (12.) --. -- + (2.) -.,.Jll__ 1.4 ~ 0 ('1) 0 {1) ~) ----+(2.) --. ------. -- o {to) 0 (\) HlGH.. LiVEL L0\.1-- I.E\IEL ~ 1.3 1.2 ~_...... __...... __.___.____.___.___.___.___.___.___.___._____. 0 SOO 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 GOOO 6500 2 SIZE (Dm .H) MM~ 2 3 4 5 6 7 8 9 l 0 11 12 13 CLASS NUM BER FIG. 22. The relation between basal shape, L/B, and aize in high and low level shells from Oiatins. The numbers in bracketa denote the number of speci mens in each size group. High level: Y z 1.4369 - 0.00799X. Low level: Y = 1.4742 - O.Ol05X. 42 2.2 2.0 1.8 1.6 - 1.4 .... 12 ~ :i 1.0 Lù J: Vl 0.8 0 0.6 + 0.4 0.2 0 ~--~--~----L---~--~----L---~--~ 0 0.2 0.4 o. 6 0.8 1.0 1.2 1.4 WET WT. OF SOFT PARTS (GM.) FIG. 23. The relation between the shell weight and the wet weight of soft parts in high and low level ahells from Six Men'e Bay. Each point representa one individual. High level: Y • -0.013 + 1.73X. Low level: Y • 0.120 + 1.37X. 43 2.2 LOW LEVEL 2.0 0 1.8 0 • 1.6 0 • -. 1.4 HI&H LIVIL. ~ (.!) -.,...: 1.2 ~ -l -l 1.0 LLJ J: 1/') 0.8 0.6 0.4 0.2 0 ~--~--~--~------_.--~ 0 O. 2 O.it O. 6 0.8 1.0 WET WT. OF SOFT PARTS (GM.) FIG. 24. The relation between the shell weight and the wet weight of soft parta in high and low level shells from Oistins. Each point representa one individual. Bigh level: Y • 0.012 + 1.89X. Low level: Y = -0.057 + 2.21X. 44 2.2 tti&H LiVEL 2.0 1.8 1.6 -u . u 1.4 -w ~ / LOW LEVU :::> 1.2 -J 0 > 1.0 -J ..J UJ :t: U1 0.8 0.6 0.4 0.2 0 0 0.2 0.4 o. 6 0.8 1.0 1. 2 1.4 WET WT. OF SOFT PARTS (GM. ) FIG. 25. The relation between the volume under the ahell and the wet weight of soft parts in high and low level shells from Six Men'a Bay. Each point representa one individual. High level: Y = o.oo4 + 1.53X. Low level: Y = 0.058 + l.l?X. 45 LOW LEVEL •j 1.6 1 1.4 .1 . ;· -u. 1.2 HI&H LEVI u -lJJ 1.0 ~ :J ...J 0 > 0.8 ...J ...J lJJ 0.6 :J: V) 0.4 0.2 0 ~--~--~--~----._--~--~ 0 0.2 0.4 0.6 0.8 1.0 WET WT. OF SOFT PARTS (GM.) FIG. 26. The relation between the volume under the shell and the wet weight of soft parts in high and low 1evel shel1s from Oistins. Each point represente one individual. Bigh 1eve1: Y = 0.010 + 1.56~ Low level: Y = -0.025 + 1.64X. 46 .80 IUQ.M lEVI.L + .70 -u ... u + -w .60 u ~ U) .50 ..J + <( a: + w .40 u U) + + -> .30 1 • < + a: 0 0 _.. LOW LEYIL ~ + +' ,..,... + 0 .,. 0 >< .20 lt w + ' ++ ,..,... 0 o.._..eY .,...,. 0 00 • .1 0 0 + 0 0 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 WET WT. OF SOFT PARTS (G M.) FIG. 27. The relation between the extra-visceral space and the wet weight of soft parts in high and low level shells from Six Men's Bay. Each point representa one individual. High level: Y • 0.0073 + 0.52~ Low level: Y • 0.0538 + 0.175X. 47 .70 . 0 - LOW L.~VU/ ~ .60 u ./ -LlJ u .50 •••+ 1 ~ • 0 tl) .. + 1 HI.M L1iV5L ...J . 40 <( 0 a:: + LlJ • u . 30 0 tl) + > -1 0 <( .20 ....a:: )1( + LlJ . 10 0 0 0.2 0.4 0.6 0.8 1.0 WET WT. OF SOFT PARTS (GM.) FIG. 28. The relation between the extra vi•ceral •pace and the wet weight of soft parts in high and low level shella from Oistina. Each point representa oae individual. High level: Y = 0.0089 + 0.571X. Low level: Y = -0.0195 + o.638X. e e 1.2 + - 0 :::r :::r ...... 1.0 0 0 (/') 0 0 0 oe LOW lE.VeL Vl w 0.8 0 0+----- z .-- ;:,c: 0 __.9 -tr 0 0 HIGH LEVEL u ~ - 0.6 + .,.g..-o-$+ •+ élo + + :x: ~ 1- __. ~.~ --- ++ 0 ...J 0.4 + w Oi ct 0.2 1-+ 0 10 14 18 22 26 30 34 SHELL LENG TH {MM.) 2 3 4 5 6 7 8 9 10 11 12 13 CLAS S NUMBER FIG. 29. The relation between shell thickness aad shell length in high and low level shells from Six Men's Bay. Each point repre senta one individual. High level: Y = 0.2334 + 0.049?X. Low level: Y = 0.3303 + o.o512X. e e + : IU&W LS'4&L o • LOW LI&V&L 0 . 1.4 t + 00 -~ 1.2 0 0 0 ~ -Cl) Cl) 1.0 1- 0 LLI z ... x •• u o.sl N\H LW&L+ ~ ~ 00 00 0 -x .... -J + 0 0 -J 0.6 + w r ~AlO x ++ 0 + 1 -$ Cl) 0.4 0.2 0 8 12 16 20 24 28 SHELL LENGTH (MM.) 1 2 3 4 5 6 7 8 9 10 C LASS NUMBER FIG. 30. The relation between ahell thickness and shell length in high and low level shells from Oistins. Each point representa one individual. High level: Y = 0.1439 + 0.0989X. Low level: Y = 0.0558 + O.l03X. 50 Discussion F. barbadensis is most abundant in relatively sheltered regions where the largest specimens are to be found. The vertical range of the species is also greater in sheltered areas. These distributions may be affected by some of the following factors, assuming that mortality due to predators is similar at all the habitats. Feeding movements may not be as restricted by wave action and there may be more available food in relatively sheltered environments. Limpets collected at low tide from exposed regions do not have full stomachs like those collected from more sheltered areas, although the type of food consumed is apparently the same at all habitats (see Part II). Natural mortality from the physical force of the waves may be lower in sheltered regions. The ability to survive in the green zone in these areas indicates that the factors causing desiccation during emersion may be lees than in the exposed habitats. The drying rate is possibly lower because of the greater relative humidity and reduced air velocity in sheltered environments (see wind rose Fig. 1). The vertical distribution of some species of Patella in Britain is similar to that of F. barbadensis. However, the mean tidal height in many localities in Britain is 12 to 20 feet (Brown's Nautical Almanac 1965) whereas in Barbados it is only 2.3 feet (Lewis 1960). The difference in tidal height indicates that in areas of similar wave amplitude a specimen living, for example, at mean tide level in Britain will not receive as much splash during emersion as a specimen living at a similar tidal level in Barbados. Wave action is favourable to Patella intermedia while the amount and type of plant cover affects the abundance of P. vulgata which 51 iahabita both expoaed aad algal covered aheltered habitats (Southward aad Ortoa 1954). The upper limit of the vertical raage of P. TUlgata may be raiaed by ahade, algal cover aad aplaaà (Evaaa 1947). It appears that theae factors do aot affect the diatributioa of ...r • barbadenaia ia the aame way, aince the vertical raage of tAis apecies is reduced iD aore exposed habitats where there ia greater aplaah. The distribution of P. vulsata, therefore, seema to be iafluenoed aere by factors affecting deaiccation thaa by the degree of wave actioD. Fretter aad Graham (1962) auggeat that it ia impossible for aa7 zeugobraach liapet with aa apical aperture to iahabit the aame high tide levels as P. TUlgata aad that all zeugobraDcha are fouad at or below low water level, where the deaiccatioa factors are at a aiaimum. Certainly many speciaeas of F. barbadeasia do iahabit the region aear low water level ia areas where the aubstrate rarel7 driea out. However, considerable auabera of theae limpeta, including seme of the largeat apeciaena, live just belo• aeaa hish water 1 along with Acaaea jaaaiceaais1 where the roCks dry out at low tide. The evidence auggests that F. barbadeaaia ia able to withataad a greater emersion tiae thaa British keyàole limpets. This abilitJ m&J be due either to physiological differences or to milder drying condition& in aheltered Barbados habitat&. Fiaaurella (=Diedora) aubecula iD Ghana ia most abuadaat ia aoderatel1 exposed eaviroaaenta and lesa ao in expoaed habitats. Its vertical range ia lesa thaa that of F. barbadeaaia and it ia liaited to parts of the shore that are continually wet (Gauld and Buehaaaa 1959). fhe vertical distribution of F. aubecula ia therefore aiailar to that of F. barbadenaia in exposed eavironments. It aay 52 be restricted compared with that of F. barbadensis in sheltered habitats because of a lower relative humidity and greater air velocity ia Ghaaa than in Barbados causing a higher drying rate of the limpets duriag emersion. Grouping of populations, as fouad in Acaaea (Abe 1931) in Patella whea algae are pleatiful (Lewis 1954) and ia other iatertidal gastrepoda (Moultoa 1962), is not evident in F. barbadensis. Rough aeas aad low temperatures cause specimens of Acmaea dorsuosa to scat ter from their respective groups aad move higher or lower oa the shore during the wiater moaths (Abe 1931). Patella vulgata moves to higher levels ia autuaa as the temperature falls and down in the spring to avoid desiccatioa (Lewis 1954). The temperature difference during the winter aoaths in Barbados is very amall and does not appear to cause any movement of the Fissurella population. The absolute growth in some species can be measured in terms of the distaace between the annual rings for most of the life spaa. This method has been used to describe the growth curves of some limpets including Patelleida conulus, Patella vulgata (Hamai 1937) and Acmaea dorsuoaa (Abe 1932). Growth curves using linear measurements have also been described for Patella vulgata (Russell 1909; Orton 1928b). The relatively steady growth of F. barbadensia throughout the year does not cause the formation of distinct annual rings and therefore the duration of life of the species cannet be determined by this method. It is estimated from the growth rates that F. barbadensis reaches a length of 21 to 23 mm. in one year and 28 to 30 mm. in two years. Therefore this limpet attaias a similar size in one year as P. vulgata. However the latter aniaal grows 53 considerably faster in the second year than F. barbadensis (Russell 1909; Orton 1928b). The growth rates in these two species are far greater than in Acmaea dorsuosa (Abe 1932), Patelloida conulus (Ramai 1937) or Patina pellucida (Graham and Fretter 1947). In his review of molluscan life spans Comfort (1957) has sug gested that in wild populations of annual and biennial species there is no plateau at the end of the life cycle and no relative accumulation of large size groups. Death occurs while growth is still in progress and is not purely size dependent. In pluriennial molluscs death in some populations is independant of age while in other wild populations a definite senescence occurs and mortality during the "plateau of vigour" is very low. In F. barbadensis the number of large size groups is relatively small and no accumulation occurs. There appears to be no plateau in the graphs plotted in Fig. 7 suggesting that death occurs during the growth period. In Patella the growth rate and final age have been found in versely related (Comfort 1957) and in Venus mercenaria Hopkins (1930) suggests that large size is incompatible with long life. Haliotis rufescens is not sexua~ly mature for the first 13 years of life (Bonnot 194o). Patella vulgata and Acmaea dorsuosa live 10 to 20 years (Comfort 1957) and Patina pel1ucida (Graham and Fretter 1947) and Haliotis tuberculata (Stephenson 1924) live 1 to 2 years. From the growth rates obtained it is estimated that F. barbadensis also lives for 1 to 2 years. Farfante (1943) records that F. barbadensis grows to 4o mm. in length and an average sized specimen in the Bahamas is 32 mm. long. In Barbados very few members of this species reach over 30 mm. and not one was measured over 35 mm. long. These resulta may iDdieate that northern populations attain a greater size than southern popu lations of the same species. Possibly southern populations grow more quickly than northern populations at first but reach a smaller size and have a shorter life span, as has been found in the razor clam Siliqua patula (Weymouth et al. 1931). Similar resulta have been obtained for My& arenaria and other molluscs (Newcombe and Kesler 1936; Thorson 1936). The growth rate of Acmaea dorsuosa is greater in a damp environment and the shells do not thicken as quickly as in a dry habitat (Abe 1932). Although the growth rates ofF. barbadensis in these two environments were not compared with each other, specimens from a dry habitat reach a larger size but do not have thicker shells than those from damp areas. Both F. barbadensis and Patella vulgata (Southward and Orton 1954) grow to a larger size in more sheltered regions and have more prominent ribs in areas of reduced wave action. On the West coast of Barbados, high level shells are larger than those from lower levels whereas on the South West coast no intertidal aize difference is apparent. Larger sizes of P. vulgata are most numerous at higher levels (Das and Seshappa 1947) while low level Acmaea scabra and A. limatula are larger than those at high levels (Segal et al. 1953). In P. vulgata (Russell 1909), Patelloida conulus (Ramai 1937) and Acmaea spp. (Shotwell 1950) the shells become relatively broader with increasing size as in F. barbadensis. The basal shape of the shell of F. barbadensis is not altered by the vertical distribution 55 at any one habitat but the West coast shells appear to be more elongate than those from the South West coast (Fig. 14). Russell (1907) fouad that in P. vu1gata the relative breadth of high 1eve1 shel1s is slightly greater at almost all sizes than that of 1ow 1eve1 she11s. He a1so found that shel1s from exposed habitats are relatively narrower than those from she1tered regions. Davis and Fleure (1903), however, report that exposed she1ls are relatively broader, as has been found in Fissurella she1ls. Shells become relatively higher with increasing aize in Acmaea spp. (Shotwell 1950), Patelloida oonu1us (Hamai 1937) and Patella vu1gata (Russell 1909) as in F. barbadensis. The relative height ofF. barbadensis shells from high level habitats is greater than from 1ow levels at Six Men's Bay (Fig. 15) and at Oistins (Fig. 21). High level specimens are exposed to more drying during emersion than low level limpets. There is a greater difference between high and low level habitats at these two stations than at Half Moon Fort and Deep Water Harbour where the substrate never dries out. No difference in the relative height of the shel1s from high and low levels is apparent in the last two regions (Figs. 17, 19). High 1evel P. vulgata shells are relatively higher per given size than shells from low levels (Russell 1907). These specimens were co1- lected in Britain from high and low water marks respectively and therefore the emersion times differ significantly at the two leve1s as at Six Men's Bay and Oistins. The greater relative height of shel1s at Oistins and Six Men's Bay than at Half Moon Fort and Deep Water Harbour (Fig. 13) may be related to the greater emersion times at high 1evels in the first two regions. Russell (1907) and Davis and Fleure (1903) found exposed shells of P. vulgata to be flatter than sheltered shells of the same length but there is no definite trend apparent from the data on F. barbadensis. Fissurella shells from the relatively exposed Oistins station are of similar weight and thickness to those from the more sheltered habitat at Six Men's Bay (Figs. 23, 24, 29, 30), whereas exposed Patella shells are thicker and heavier than those from sheltered areas (Russell 1907). A greater difference in !• barbadensis shells might have been found if the exposed shells had been collected from the East or South East coasts. The scarcity of this species prevented collection in these regions. Orton (1928b) feels that wave action plays only a minor role in controlling the shell height of P. vulgata which is governed mainly by factors causing desiccation of the animal. Drying conditions also appear to affect the shell height of F. barbadensis. Moore (1936) found that in Purpura lapillus the diet rather than the wave action altered the shape of the shell but in F. barbadensis the diet is similar in all habitats (see Part II). High level A. limatula shells have a greater shell weight and volume under the shell per unit wet weight of soft parts than shells from low levels (Segal 1956a). F. barbadensis shows no difference in shell weight for a given wet weight of soft parts at different intertidal heights (Figs. 23, 24} but high level shells from Six Men's Bay have a larger volume under the shell per unit wet weight (Fig. 25) as in A. limatula. High level Fissurella shells from Six Men's are thinner per given length than low level shells (Fig. 29), contrary to the trend observed in Acmaea (Abe 1932; Segal 1956a). 57 In the case of F. barbadensis high level shells possess a greater volume under the shell than low level shells of equal weight (Figs. 23, 25), while the reverse trend occurs in A. limatula. The amaller absolute aize of the soft parts per given shell weight in specimens of A. limatula from high levels more than compenaates for the smaller Tolume enclosed by these shells (Segal 1956a). Therefore both !• barbadensia from Six Men'a Bay and A. limatula show a larger extra visceral space in high level shells. Specimens from both intertidal levels at Oistins (Fig. 28) have an extra-visceral space of similar aize to high level limpets from Six Men's Bay (Fig. 27), possibly because the dessication factors are greater at the more exposed Oistins habitat. The per centage of water lost per unit time from the shell during emersion is probably lesa in specimens from Oistins and high level animals from Six Men's Bay than in low level animals from the latter habitat because of the larger extra-visceral space. Rowever, the Oistins and high level Six Men's Bay limpets may have to withstand longer or more severe desiccation, requiring additional water. Segal and Dehnel (1962) have shown that during emersion the extra-visceral water in A. limatula seems to serve an osmoregulatory function and is also a temperature buffer. The ability to regulate the body temperature by evaporative cooling in F. barbadensis has been sug gested by Lewis (1963). PART II. FEEDING AND DIGESTION Introduction A variety of food is ingested by the Zeugobranchia and Patel lacea. The principal diet of Haliotis tuberculata (Crofts 1929) 1 H. cracherodii (Leighton and Boolootian 1963) 1 Acmaea spp. and Patella spp. is algae while that of the fissurellids Emarginula reticulata and Diodora (~issurella) apertura is mainly eponges with the possible addition of detritus (Graham 1955; Fretter and Graham 1962). Boutan (1886) mentions the herbivorous diet of some species of Fissurella and identifies debris of diatoms and small algae in the digestive tract. The rate of feeding in Patella vu1gata has been ca1cu1ated by Moore (1938b), and by Southward (1964) who found a cyclic relationship between intertidal 1impets and algae which is stabilized mainly by the wave action. These limpets, in conjunction with other gastropode, are thought to be responsible for a con siderable amount of rock erosion (Southward 1964). Wi11cox (1905a) has suggested that the ingested rock particles may aid in pulverizing the food. Little information is available on the digestive enzymes of the diotocardians. Those that have been isolated appear to have an optimum pH of about 5 to 6, the same pH as the part of the digestive 59 tract where they are located (Yonge 1925). In Patella vulgata no enzymes have been found in the salivary glands (Graham 1932) and it is suggested that these glands are used only for lubrication in all diotocardians (Fretter and Graham 1962). Enzymes for protein, carbo hydrate and fat digestion have been recorded from the oesophageal and digestive glands of Raliotis and Patella by several workers (Fretter and Graham 1962). Intracellular digestion of food may take place in the digestive gland cella. Although Graham (1932) found no evidence of digestive enzymes in this gland in Patella, Ro~en (1937) has demonstrated a proteinase which must be activated by a reducing agent and is inhibited by iodoacetic acid, indicating that the enzyme may be an intracellular cathepsin. Dodgson and Spencer (1954) have located sulphatases in Patella which possibly aid in the digestion of sulphated polysaccharides in the algae. A rich source of ~ - glucuronides have been recorded from the digestive gland of P. vulgata. This gland also contains p - glucuronidase and many other simple glucosidases (Marsh and Levvy 1958). The enzymes , - D - fucosidase and ~ - D - galactosidase have also been isolated from the digestive gland of this species (Levvy and McAllan 1963). Seasonal variations in the blood glucose concentration and the glycogen concentration in the digestive gland and foot of Patella have been demonstrated by Barry and Munday (1959). In the lamel libranch Lasaea rubra, which lives at high tide level, a tidal periodicity is imposed on its feeding and is reflected in the action of the crystalline style and of the digestive gland (Morton 1956). 60 Materials and Methods Specimens of F. barbadensis were observed, collected and examined at various hours of the day and heights of tide in different parts of the island to determine when feeding occurred, the type ot food ingeeted and digested and the effect of different habitats on the food and feeding times. The length of time necessary for food to reach the different parts of the gut was ascertained by allowing the limpets to feed on an algal covered rock that had previously been etained with Nile blue sulphate. The specimens were dissected at intervals and the position of the stained algae in the gut was noted. Starved limpets were also dissected at intervals to establish the rate of food pas sage through the gut when feeding is suspended. Indicator paper was used to determine the pB of the digestive tract and of the mixtures of extract and subetrate used in the enzyme tests. Extracts of various parts of the gut were prepared by grinding them with a little clean sand and distilled water. The mixture was centrifuged and the supernatant fluid wae used as the extract. The extract and substrate were covered with a few drops of toluene before incubation. Boiled extracts were ueed as controle. Except in the experiment to detect proteases, incubation was continued for 3 days. Tests were made every 2 to 3 hours for the first 8 to 10 hours and lesa frequently for the remaining period. To test for the presence of proteases a 10% solution of gelatin was incubated with the extract. When cooled to 0° C after 20 to 24 hours the gelatin solidified between 24 and 25° C in the controle 61 aad ia those mixtures where the gelatin was not digested. The presence of proteases was indicated by the lack of solidification of the mixture at 0° c. To test for lipases olive oil stained to saturatioa with Sudan III was emulsified in a Waring bleader with bile salta (George 1952). After dilutioa the largest drops were pinkish under the microscope aad the smallest were colourless. Tea drops of 0.2N NaOH were added to the extract. Nile blue sulphate diluted 1:10,000 was used as a couaterstain. The presence of fatty acids caused the droplets to change from pink to blue as the Nile blue sulphate reached them. la a second test for the presence of lipases, fresh and boiled milk diluted 1:50 was used as a substrate. To 10 ml. of this diluted milk one drop of 0.04% bromthymol blue was added as iadicator and one drop of 0.2N NaOH was used to tura the milk a pale blue (Agrawal 1963). The presence of lipases caused the blue solution to turn yellow. A 1% starch solution was used as substrate to test for the presence of amylase. The iodiae test was employed to iadicate the breakdowa of starch. C~iaical test papers and Beaedict's solutioa were used to demonstrate the presence of glucose. Invertase was demoastrated using a 5% sucrose solution and 1% cellobiose was used to show cellobiase. Grouad filter paper acted as a substrate tor cellulase. Solutions of grouad Y!!! and 5~ agar agar were used to establish the presence of enzymes capable of digesting more complex polysaccharides. Cliaical test papers aad Benedict ' s solution. were employed to demonstrate the monosaccharide breakdowa products. 62 Resulta Fissurella feeds by browsing on algal covered rocks. The method of feeding and the pattera produced are similar to those described for Acmaea and Patella. When observiag a specimen on the glass of aa aquarium the trail marking the iagested algae is seen as a series of joined arcs that are not always aearly parallel due to the ir regular forward movemeat of the limpet as it swings its head from aide to aide. The width of the trail is equal to the width of the aouth. Along with the algae Fissurella iagests sandgrains, rock particles, diatoms and occasionally Foraaiaifera, Radiolaria1 small crustaceans and bivalve molluscs. All the coasuaed material except the Green and Blue-green algae appears to be passed through the gut uachaaged. There was little differeace ia the types of algae iagested by limpets from different intertidal levels and horizontal habitats. Most of the algae ia the gut belong to the Blue-greeas and Lzagbya sp. is the most common. Other genera include Oscillatoria, Phoraidiua and Aaacystis. The most abuadaat Green alga is Chloro chytrium sp. (Taylor 1960), endophytic maialy in Lyagbya. Ulothrix, Cladophora aad Purcursaria are also fouad and occasionally Y!!!• The sheaths of both L:asbya aad Chlorochytriua are appareatly not digested and are passed out ia the faeces. Feediag occurs periodically throughout the day and aight. It is interrupted by emersion whea ao wave actioa is received, although some limpets will contiaue to feed uatil the substrate begins to dry out. Fissurella may start to move aad browse as sooa as the first wave of the risiag tide washes over the shell. Dissectioa of freshly collected speciaeas reveals the buccal and oesophageal regions of the gut to contain little or no food while the stomach and in- testine of limpets from the West and South West coasts are always full, regardless of the time of day or height of tide. The main part of the stomach of specimens collected at low tide from the East coast was only partly filled with food while the style sac and intestine were packed with faecal material. The times for food to reach the different parts of the digestive tract are shown in Table 7. TABLE 7 FEEDING BA TES OF F. BARBADENSIS Humber of animale in which food reached: Time from start of Oeso Main Style First Second Fourth Total number fee ding phagus sto- sac Inte Inte Inte of animale (hours) mach stine stine stine 0.5 l 5 6 1.0 4 4 8 2.0 5 3 8 3.0 3 2 3 8 4.0 2 2 4 8 6.0 2 6 8 s.o l 7 8 10.0 12 12 The regions of the intestine are shown in Fig. 38, Part III. Food passes quickly through the buccal and oesophageal regions to the stomach where it is held for digestion lesa than two hours during continuous feeding. In starved specimens the food moves much more slowly through the digestive tract and after 10 hours the stomach still contains partly digested algae. The gut is virtually empty of faecal material after 24 hours in starved animale. These resulta indicate that Fissurella feeds sufficiently under natural conditions to keep the stomach at least partly filled and the 64 intestine full at all times. If feeding is inhibited by emersion the rate of movement of material through the digestive tract is apparently reduced. The pH of the digestive tract and of the incubated mixture is shown in Table 8. The different parts of the gut have approximately the same pH which varies from 6 to 6.5. The pH of the incubated mixtures was 6.5. TABLE 8 pH OF DIGESTIVE TRACT OF F. BARBADENSIS Region pH of digestive pH of extract fluid and substrate Salivary glands 6 - 6.5 6.5 Anterior oesophagus 6 - 6.5 Oesophageal gland 6 - 6.5 6.5 Posterior oesophagus 6 - 6.5 Stomach - main part 6 - 6.5 6.5 Stomach - style sac 6 - 6.5 6.5 Digestive gland 6 - 6.5 6.5 Intestine 6 - 6.5 The results of the enzyme tests are shown in Table 9. Most of the enzymes detected are present in the digestive gland. Only an amylase was located in the oesophageal gland and no enzymes were recorded from the salivary glands. TABLE 9 RESULTS OF THE ENZYME TESTS Substrate Enzrae Sali Oeso Mai• part Style Dige yaey phageal of stoaach sac stive glaads gland gland Protei• 10% gelati• Protease -· +· Lipid Olive oil Lipase Milk Lipase Carbohrdrate 1% starch Amylase + + 5% sucrose Invertase + 1% cellobiose Cellobiase + Cellulose Cellulase 5% agar agar lli! • - = no enzymes detected + = eazyaes detected Discussion F. barbadensis probablJ contributes to rock erosion like the limpets living in temperate waters, since particles of rock are frequently fouad in the digestive tract. During feeding Fissurella, like Patella vulgata (Southward 1964), moves forwards along the substrate while Acmaea tessulata moves backwards (Willcox 1905a). The algal diet of F. barbadensis is similar to that of other Fissure11a spp., Baliotis, Acmaea and Patella, whereas the diet of the fissurellids Diodora apertura and Emarginula reticu1ata is mainl7 carnivorous. Both P. vulgata (Southward 1964) and F. barbadensis appear to show no selectioa of food ingested in the 1aboratory. It 66 is possible that little selection occurs in Fissurella in nature since a variety of material is ingested including Y!!! which did not appear to be digested by the tissue extracts in the enzyme tests. The intertidal snails Littorina planaxis and L. scutulata re quire between ~ and 6 hours for food to pass through the digestive tract. These species feed sufficiently to keep the gut full at all times (North 1954). Similarly F. barbadensis has an adequate food intake, when submerged or exposed on a damp substrate, to maintain a full or partly filled stomach constantly. Small quantities of enzymes present in the gut may not have been detected because the methode employed lacked the required sensitivity. However the absence of enzymes in the salivary glands would agree with the hypothesis that these glands are used for lubrication only in diotocardians. An amylase has been found in the oesophageal gland of P. vulgata (Graham 1932) as in F. barbadensis. In 1888 Haller recorded that the stomach of Diodora apertura has a crystalline style at certain times. Graham (1939) has found no indication of a style in this species but describes a mucous rod in the style sac that is the first stage in the formation of the faecal rod. A similar rod is found in the style sac of Emarginula reticulata (Graham 1939), Patella vulgata (Graham 1932) and~· barbadensis. Green and Blue-green algae have been identified in the gut of F. barbadensis. Cellulose has been demonstrated in the cell walls of some of the Greens while in others it may be replaced by, or associated with, xylan or mannan (Kreger 1962). There is no con clusive evidence that cellulose occurs in the Blue-green algae. 6? The undigested sheaths found in the intestine of Fissurella suggest that cellulose and other algal cell wall components cannot be di gested. In the enzyme tests there were no positive resulta for cellulose digestion. Cellobiose digestion in Fissurella indicates that a ~ - glucosidase is available to act on the ~ - D - glucoside units. Mucilages form a large part of the organic constituents of algae and perform a wide variety of functions. In some species of the Green algae these polysaccharide mucilages yield a variety of simple sugars, sulphuric acid esters and uronic acid on hydrolysis, while other species contain acid resistant polyuronide groups which are difficult to hydrolyse. Mucilage hydrolysis in the Blue-green algae yields various simple sugars and sometimes sulphuric, galacturonic and glucuronic acids (O'Colla 1962). Agar is commonly found in the Red algae but the enzyme tests show negative resulta for digestion of this mucilage. Red algae in the stomach and intestine of Fissurella indicate that they are probably passed through the gut unchanged. Sucrose, stored as a reserve substance, bas been found in considerable quantities in several of the Green algae (Meeuse 1962) and an enzyme bas been demonstrated in Fissurella that is capable of digesting it. Sucrose hydrolysis may be carried out either by a ~ - fructofuranosidase or yeast invertase, optimum pH 4.5, or by an ~ - glucosidase or taka invertase, optimum pH 6 to 8 (Barrington 1962). Apart from measuring the pH of the digestive gland as 6 to 6.5 in Fissurella, no attempt was made to identify the enzyme or enzymes concerned in the hydrolysis. 68 The reserve products of the Green algae are usually true starches while Cyanophyte starch appears to be the same as the amJlopectin portion of higher plant starches {Meeuse 1962). Amy lases have been located in both the oesophageal and digestive glands of Fissurella suggesting that some of these algal reserves may be readily digested. From the enzyme tests, Fissurella does not appear to be able to digest fats, unlike Patella and Haliotis {Fretter and Graham 1962), although fat accumulation is common in all types of algae and lipide play a significant role in the cell metabolism. The protein component of algae is possibly digested by a protease which has been located in the digestive gland of the limpet. Protein digestion has also been recorded in Patella and Haliotis (Fretter and Graham 1962). EART III. BISTOLOGY OF THE DIGESTIVE TRACT Introduction Anatomical and histological features of the digestive tract of the Zeugobranchia have been reported by several authors (Fischer 185?; Boutan 1886; Bourne 1910; Crofts 1929; Odhner 1932; Graham 1939, 1949; Fretter and Graham 1962). The guts of Patella and Acmaea have been described by Davis and Fleure {1903), Graham (1932, 1949), Runham (1961), Fretter and Graham (1962) and other workers. The digestive tract of F. barbadensis is similar in many respects to that of other zeugobranchs, and to Patella and Acmaea. The following account describes only the differences between !• barbadensis and these other species. Materials and Methods Three specimens were preserved whole in Bouin's solution. They were dehydrated in alcohol, cleared in benzene, embedded in paraffin and sectioned at 8 ~ • The sections were stained using one of the following techniques: Erlich 1 s Baematoxylin and Eosin Y (BE) Mallory's Triple (MT) Periodic acid - Schiff (PAS) (McManus) (Pearse 1961) 69 70 Azure A - Eosin B (Li~lie, 1954) at pH 5.0 Protargo~ - S (Bodian 1937). Another specimen was fixed in 10% aqueous buffered form~in, embedded in paraffin and sectioned at 8~ • These sections were stained for iron with Perls Ferrocyanide Reaction (after Lison and Bunting) (Pearse 196~). Fresh specimens were dissected and examined for ciliary cur- rents and to confirm histologie~ details in the stained sections. Vital stains such as methylene blue and neutral red were employed. Fatty substances were demonstrated using 2% osmium tetroxide. Resulta Oral Tube and Buccal Cavity The muscu~ar ~ip surrounding the horse-shoe shaped mouth is lined by a cuticular epithelium of columnar cella, 13 ~ high. The ce~ls have elongate nuclei and lie on a thin basement membrane under- ~ain by circular and oblique muscle fibres. The longitudinal folds of the oral tube terminate at the outer lip which therefore has a ridged appearance. The cuticular co~umnar cella forming the epithe~ium of the oral tube and the anterior end of the buccal cavity (Fig. 3~) are 23 ~ tall with elongate nuclei (n) lying in the central or basal part and fine granules (ai) packed in the apical region of the ce~l. The cytoplasm is p~e-staining. The apical granules appear brown in PAS and dark green in azure A. They could not be identified in the slides stained with Mallory's triple (MTJ. Qoblet-shaped cella (p) are scattered through the epithelium. They appear vacuolated 71 in fresh tissue, often showing fine granules in the vacuoles, but etain heavily in many techniques including haematoxylin, azure A, PAS and protargol. They are purple in haematoxylin, magenta and reticulate in PAS, blue and reticulate often with fine pink granules in azure A and pale blue and reticulate in MT. The spherical nuclei lie at the base of the cell and have prominent nucleoli. The oral tube epithelium lies on a thin basement membrane surrounded by longitudinal muscle and a thick outer wall of circular muscle. Both muscle layera are interwoven with oblique fibres. At the anterior end of the buccal cavity (Fig. 31) the basement membrane (bm) is supported by a thick connective tissue layer (ct) and outer muscle coat s. The inner lips are covered by cuticular columnar cella, 13~ tall, supported by a thick basement membrane and circular muscle fibres. The goblet celle in the roof of the buccal cavity become more abundant at the base of the jaw and in this region the basement membrane and cuticle are thicker. The epithelium of the ventro lateral walls is similar to that lining the dorsal wall except that the cella are 27 fL high. In the buccal pouches the columnar celle are the same height but the cuticle is very thin and is absent in parts of the epithelium. Behind the jaws the epithelium of the dorsal wall is ciliated and two dorsal folds (Fig. 32, d) originate on the dorso-lateral wall, each one single, and pass back into the oesophagus. The dorsal food channel and the folds are richly ciliated. The cella in the dorsal food channel are 25 ~ high and have elongate reticulate nuclei with prominent nucleoli in the basal half of the cell. The cilia are 72 7 to 10 ~ long with prominent basal granules. Varioue sizee of granulee occur in the cella, moetly in the apical part. They appear brown in PAS and potassium ferrocyanide and green in azure A. They cannot be identified in MT although with this etain the apical part of the cell appeare deneer than the proximal part. Goblet celle are interspersed between the ciliated cella. They etain the same as those deecribed in the oral tube. The epithelium ie supported by a very thin layer of connective tissue, circular and outer longitudinal muscle layera. The roof of the radula sac is lined with columnar cella, 20 to 25 .f" in height, with basal nuclei and apical granules staining green in azure A. The epithelium is covered by a cuticle and is eupported by longitudinal and oblique muscle fibres. The radula teeth are impregnated with iron in those sections stained with Perle ferrocyanide reaction. A fine nerve network lies beneath the epithelium throughout the buccal region. The nerve fibres are branches of nerves (Fig. 32, 1) from the labial, buccal and cerebral ganglia. The buccal caecum (Fig. 321 b) is lined by the epithelium covering the buccal maas but the celle are secretory. They are about 10 ~ tall and the dark-staining nucleus lies at the base. The apical part of the cell is vacuolated and contains fine gran ules which appear purple in haematoxylin and PAS, with and without diastase, dark green or blue in azure A and negatively in MT. Proximally, the cytoplaem stains darkly in haematoxylin and a red reticular network is apparent in MT. Connective tissue lies be neath the epithelium. 73 The paired salivary glands lie mainly above the dorsal wall of the anterior oesophagus but they also extend into the roof of the buccal cavity. They are small acinous glands formed of tubules (Fig. 32, s} and each opens by a duct (sd) into the buccal cavity (be) lateral to the anterior end of the dorsal folds (d). In the living animal the glands are cream coloured. The ciliated columnar cella are 15 jL tall with basal spherical nuclei and prominent nucleoli. The epithelium of the ducts is the same as that of the tubules. Some of these cella contain granules near the apical surface which stain PAS negative, dark green or blue in azure A and red in MT. Goblet-shaped cella with broad bases and narrow necks are scattered between the gland cells. They have elongate nuclei centrally placed in the cell. The vacuolated cytoplasm stains PAS positive, blue in azure A and pale blue-green in MT. The tubules are surrounded by connective tissue which is more obvious near the ducts. Oesophagus The ciliated dorsal folds and dorsal food channel are con tinuous with those in the buccal cavity. The floor of the oeso phagus, dorsal to the anterior end of the radula sac, is lined with ciliated columnar cella interspersed with goblet cella, as on the dorsal folds. Behind this region, but anterior to the posterior end of the salivary glands, two ciliated ventral folds arise and unite as they pass back into the mid-oesophagus. These folds are lined by ciliated columnar cella, 15 to 20 )4 tall, with basal nuclei and apical granules which stain in the same way as those on the dorsal folds. The cilia are 5 to 7 JL long with prominent basal granules. They beat in the direction of the stomach. The epithelium lies on connective tissue and on a band of circular muscle. Goblet cells are scattered in the epithelium. The lining of the lateral pouches is the same as that of the dorsal and ventral folds. Just anterior to the oesophageal gland there are many more goblet celle in the epithelium of the oesophagus than more anteriorly. The oesophageal gland is pink in the living animal and abounds in mucous. The epithelium lining the dorsal folds and food channel and the tip of the mid-ventral fold is the same as in the anterior oesophagus. For a short distance lateral to the dorsal folds the ciliated cella (Fig. 33,ci) are 38 Jl tall with basal nuclei (n) and apical granules (ai) staining green in azure A. The villi which form the lateral pouches are composed of two rows of epithelial cells separated by connective tissue (Fig. 34, ct). The ciliated columnar cella are low, 15 to 20 ~ tall, with elongate pale staining nuclei (n) and prominent nucleoli. The cytoplasm stains lightly and contains granules (a) which are yellowish-green in the living cell, PAS negative, red in ~, green in azure A and blue in potassium ferrocyanide. The cilia (ci) are 5 ~ tall with prominent basal granules (Fig. 35, bg). They beat towards the dorsal or ventral folds from where the secretion is directed into the dorsal food channel. Between these ciliated cella are goblet-shaped cells (p) which are much more numerous in some areas (Fig. 35) than in others (Fig. 34). They are very pale in fresh material but they etain positively and appear reticulate in PAS, with and without diastase digestion, blue with fine pink granules and reticulate in azure A 75 and pale blue and reticulate in MT. These gland cella have basal spherical nuclei (Fig. ~5, n) with prominent nucleoli. They appear aimilar to the goblet-shaped cella in the anterior parts of the digestive tract. There was no indication of any fatty substances in this gland from the tissue stained with osmium tetroxide. The posterior oesophagus is a straight narrow tube lined by ten low longitudinal folds of uniform height formed by the epi thelium. The cella are ciliated columnar, 20 ~ high, with elongate centrally placed nuclei and apical granules staining pale green in azure A. Occasional goblet cella are scattered through the epi thelium which is supported by inner circular and outer longitudinal muscle coats. Posteriorly, the oesophagus lies on the right aide of the foot. It entera the stomach ventrally on a papilla. The latter is formed of the convoluted stomach epithelium and is sup ported by connective tissue and a thin wall of muscle. A thin layer of nerve fibres lies beneath the epithelial cella of the oesophagus. These fibres are branches of nerves from the buccal, pleural and branchial ganglia. Stomach and Digestive Gland In the living animal the cella lining the apex of the oeso phageal papilla are 25 to ~ ]L tall and covered with short dense cilia. The nucleus is oval-shaped and centrally placed. These ciliated cella are interspersed with a few large PAS-positive goblet cella. Lateral to the apex of the papilla the ciliated cella contaiu ~pical granules which are brown in the unstained living cella and also in potassium ferrocyanide. 76 At the base of the papilla the epithelium increases in height and is 30 to 4o y- • The cilia are replaced by a thin cuticle but the cella are similar to those on the papilla and have brown apical granules. This cuticularized area is the edge of the gastric shield. It is greenish-brown in the living animal with a sheen over the surface caused by the cuticle. The height of the epithelium of the gastric shield varies from 30 to 60 ~ while the cuticle (Fig. 36, c) varies from 15 to 150 p- , the thickest part covering only a relatively small area. Leaving the region of the oesophageal papilla the cuticular cella of the shield are packed with granules (ab) which are dark green in the living cella, brown and a few blue in potassium ferrocyanide and green in azure A. These granules appear brown in PAS and MT. The nuclei (n) lie at the base of the cella and the epithelium is underlain by a very thin basement membrane and by circular and oblique muscle coats (m). The anteriorly directed folds of the posterior ·sorting area are lined wi th cilia ted columnar cella, 15 to 25 y high, wi th central nuclei and apical granules which are brown in the living cell and very pale green in azure A. The folds are formed by con nective tissue and the region is supported by a thin coat of circular muscle which surrounds the whole stomach. The epithelium of the style sac is composed of tall, narrow and very densely ciliated cella. The cilia are 12 r in length and have prominent basal granules. The cytoplasm is denser in the apical part of the cell. The cella are 30 to 4o J tall rising to 90 ~ on the major typhlosole and on the prominent fold which divides the style sac into upper and lower parts. In the intestinal groove 77 the cella are 45 to 50 ~ high. The oval-shaped nucleus lies in the lower half of the cell and some apical granules can be located which etain green in azure A and negatively in PAS and MT. The ducta from the digestive gland open into the atomach in the region of the oesophageal papilla. The low ciliated longitudinal folda of the posterior sorting area extend into the large collecting ducts where they fade out. The epithelium of the ducta ia composed of ciliated columnar cella, 15 to 25 fL tall, with basal apherical nuclei and prominent nucleoli. Apical granules, which form large aggregations, etain green-brown in fresh tissue, orange-red in PAS, negatively in MT and green in azure A. They appear as fine blue granules in potassium ferrocyanide. Goblet cella, acattered be tween the ciliated cella, etain as those described in the anterior parts of the gut. The epithelium of the ducta lies on a thin layer of muscle. The most common type of tubule cell is 15 ~ high and lobed apically. Spherules of secretion appear to be formed from the distal surface in both fresh and fixed tissue. The cytoplasm is dense at the apical surface while in the rest of the cell it is highly vacuolated. The nucleus is pale-staining and lies at the base of the cell. The cytoplasm contains granules which may be single or united in large groups and which etain red-brown in fresh tissue. Granules staining pale PAS positive, with and without diastase, negatively in MT and green in azure A are scattered through the cytoplasm, singly and in groups, and are abundant near the distal lobed surfaces. Large deposits that lie at the base of the cell etain blue with potassium ferrocyanide. 78 The second, less common, type of cell, triangular in section and occurring mainly in the crypts of the tubules, stains darkly in all the techniques used. The spherical nucleus lies at the base of the cell and has a dark-staining nucleolus. The cell is filled with fine granules which are particularly dense in the narrow neck of the cell adjacent to the duct of the tubule. Tbese granules stain PAS positive, with and without diastase digestion, green and occasionally red in azure A and negatively in MT. In Perla ferro cyanide reaction the granules in the apical part of the cell stain blue. Fatty substances could not be identified in the cells of the digestive gland using osmium tetroxide. The tubules are surrounded by thin connective tissue, containing occasional amoebocytes, and are bathed by blood circulating within the haemocoel. The nerves innervating the stomach and digestive glands originate from the visceral ganglia. Intestine The digestive tract is 2.5 times the length of the body. The intestine extends posteriorly from the style sac and forma a large loop in the visceral mass before passing dorsally to form a smaller loop which terminates at the anus. Histologically the intestine may be divided into four regions (Fig. 38): the first lying ventral to the style sac; the second from the posterior end of the body anteri orly to complete the first and part of the second loop; the third extending from the first half of the second loop through the heart; and the fourth short region from the heart to the anus. 79 Ia the first region the folds of the style sac merge with the longitudinal folda of the intestine which are uaiform in height. The ciliated columaar cella are narrow and 30f high. The tall, 7f , cilia have prominent basal granules with fine fibriles ex tending from them and conTerging towards the basal part of the cell. The elongate aucleus lies in the lower third of the cell with a promiaent aucleolus. Granules in the apical region are greenish brown in fresh tissue and appear pale brown in PAS. They stain pale green ia MT, dark and light green ia azure A and blue in potassium ferrocyaaide. Maay PAS-positive narrow goblet cella are dispersed between the ciliated cella. Throughout the intestine, a thin basement membraae and circular and longitudinal muscle coats (Fig. 37, m) surrouad the epithelium. The longitudinal folds are supported by connective tissue (ct). Nerve fibres, which are branches of aerves from the Tisceral ganglia, ianerTate the epi thelial cella. Ia the second aad loagest region of the intestine the longi tudinal folds disappear. The ciliated columaar cella are 25f high aad have elongate auclei (Fig. 37, a) situated in the lower third of the cell with promiaent aucleoli. The cilia are Br loag with proainent basal granules (bg). Granules (a) ia the apical half of the cell are auch more numerous thaa in the first regioa and appear browa in PAS. They etain pale green in MT, 4ark green ia azure A and blue in potassium ferrocyanide. There are only a few PAS-positive goblet celle in this region. The third region of the intestine passes through the ventricle of the heart. The epithelium of this region is 12f in height 80 and has long cilia that are nearly as tall as the celle. Elongate nuclei lie in the lower third of the celle and there are very few apical granules. The latter appear brown in PAS and etain blue or brown in potassium ferrocyanide and pale green in MT and azure A. PAS-positive goblet celle are absent from the region and the muscle wall is very thin. Longitudinal folds appear towards the end of this part of the intestine. The longitudinal folds in the fourth region are larger than in region three and are supported by connective tissue. The cells are 15 to 20 r tall and have short 5 }'- cilia with prominent basal granules. Oval-shaped nuclei lie in the lower third of the celle. The apical cytoplasmic granules are no more numerous than in the third region and appear pale brown in PAS and potassium ferrocyanide. They etain pale green in MT and azure A. Large PAS-positive goblet cells with basal spherical nuclei are commonly found. The epi thelium is surrounded by thick layers of circular, oblique and longi tudinal muscle. This muscle is replaced by connective tissue at the anus. 81 Abbreviations Used in Figs. 31-3? a = apical granule staining blue in Perls ferrocyanide reaction ab = apical granule staining brown in Perls ferrocyanide reaction ai = apical granule b = buccal caecum be = buccal cavity bg = basal granule of cilium bm = basement membrane c = cuticle ci = cilium ct = connective tissue d = dorsal fold f = faecal material 1 = labial commissure m = muscle n = nucleus p = PAS-positive gland cell ra = radula s = salivary gland sd = duct of salivary gland 82 FIG. 31. Longitudinal section through the dorsal wall of the buccal cavity. PAS. x 570. FIG. 32. Cross section through the buccal regioa. Lillie's azure A. x 30. FIG. 33. Longitudinal section through dorso-lateral wall of mid-oesophagus. Lillie's azure A. x 570. FIG. 34. Longitudinal section through the oesophageal gland. Perls ferrocyanide. x 570. 84 FIG. 35. Longitudinal section through a re gion of abundant gland cells in the oesophageal gland. HE. x 570. FIG. 36. Longitudinal section through the gastric shield in the main part of the stomach. Perla ferrocyanide. x 570. 85 FIG. 37. Cross section through re gion 2 of the intestine. Perls ferrocy anide. x 570. 0'\ 0'\ ()0 ()0 e e 3 3 4 4 2 2 ON ON FOOT FOOT HEART HEART ~~~I~N ~~~I~N REGION REGION REGI REGI OF OF EDGE EDGE intestine. intestine. the the \ \ \ \ \ of of """""-"t----- "----- regions regions _ _ J----+-1---+--- ...... ...... o o 1 1 lli lli four four ...... 1 1 \ \ 1 1 ...... "' "' , , ...... 1 1 1 1 ' ' / / "' "' ...... .; .; the the "' "' ',"" ',"" .; .; ...... (\ (\ ...... 1 1 \ \ ' ' \',,___, \',,___, 1 1 show show 11: 11: to to Diagram Diagram 1 1 38. 38. ------,.L..----+ ------,.L..----+ ----+-----1- FIG. FIG. ----+----++ ----+----++ 1 1 NOS NOS LA LA SAC SAC G G STOMACH STOMACH REGION REGION OESOPHAGEAL OESOPHAGEAL STYLE STYLE O~~~~EH~~~S O~~~~EH~~~S e e 87 Discussion The apical granules that occur in nearly all the ciliated columnar celle of the digestive tract appear to be similar to those described by Graham (1932) as pigment granules in Patella vulgata. Their green or brown colour in fresh tissue in F. barbadensis is certainly responsible for the colour of the gut epithelium. These granules are also mentioned in Diodora apertura by Boutan (1886). Newbigin (1898) has suggested that the pigment in Patella is pro duced metabolically from the chlorophyll in the algae consumed. In the oesophageal gland, gastric shield, digestive gland ducts and most of the intestine of Fissurella these apical granules stain blue in Perle ferrocyanide reaction, indicating the presence of iron (Pearse 1961). The apical granules in the main part of the stomach and digestive gland ducts in Haliotis tuberculata (Crofts 1929) and in the intestine of Diodora (Fretter and Graham 1962) also stain positively for iron. The bases of the radula teeth in Patella become impregnated with iron after the radula has been secreted (Runham 1961). Iron is also present in the radula teeth of Fissurella. Both types of digestive gland cell in Haliotis (Crofts 1929) and Fissurella con tain iron-positive granules. Those granules in the more common type of cell may be absorbed along with the partly digested food since iron is an essential element in algal metabolism (Wiessner 1962) and would therefore be readily ingested. The staining reactions of the goblet-shaped gland celle which occur throughout the digestive tract suggest that a mucin-like material is secreted such as a neutral mucopolysaccharide, a 88 mucoprotein or a glycoprotein (Pantin 1948; Lillie 1954; Pearse 1961). This substance is probably used for lubrication. The goblet cella are abundant at the base of the jaw in Fissurella as in Diodora and Haliotis (Fretter and Graham 1962). The buccal caecum, which is characteristic of members of the Fissurellidae, conforma with the description and figures given by Fretter and Graham (1962). Histologically the salivary glands appear to be solely for lubrication since the only gland cella are the goblet-shaped cella similar to those occurring in other parts of the digestive tract and no enzymes were detected in the enzyme tests (see Part II). The glands are similar to those of Haliotis (Crofts 1929) and Diodora (Fretter and Graham 1962) although in the latter genus cella con taining secretory granules are also found. In the oesophageal gland only one type of gland cell bas been identified, the large goblet-shaped cell which bas similar staining properties to cella of the same shape found throughout the digestive tract. The large amount of mucous associated with this gland in the living animal and the staining reactions of the gland cella suggest that these cella produce a mucin-like substance. An amylase bas been located in the oesophageal gland (see Part II) and it is possible therefore that the goblet cella have a double function in this region. Mucous cella have not been recorded from the oeso phageal gland of Patella. The gross anatomy of the stomach of F. barbadensis and direction of the ciliary currents are very similar to those of Diodora apertura (Graham 1939, 1949). The two types of digestive gland cell correspond to those described for other diotocardian prosobranchs. The en zymes for protein and carbohydrate digestion (see Part II) are probably located in the more common type of cell although the celle in the crypte also appear to be able to secrete some substances as the necks are packed with granules. Absorption and intracellular digestion seem to occur in the more common type of cell as in Baliotis, Patella and Acmaea (Crofts 1929; Graham 1932; Fretter and Graham 1962). PAS-positive substances correspond to the distri bution of acid or alkaline phosphatases in the digestive glands of Patella and Fissurella and a measure of functional dependance between these two groups of substances has been suggested by Genesi (1955). In tests to identify the presence of fatty substances in the oesophageal and digestive glands of F. barbadensis negative resulte have been obtained using both osmium tetroxide and extracts incu bated with milk and olive oil (see Part II). Fatty droplets have been demonstrated between the vacuoles at the base of the more common type of digestive gland cell in Patella (Graham 1932) and in both types of cell in Haliotis (Crofts 1929). At the beginning of the intestine the typhlosoles fuse across the intestinal groove in Haliotis and Scissurella forming a siphoa which rune parallel to the intestine, rejoining it near the anus. There is no siphon in Fissurella, Diodora or Emarginula (Fretter and Graham 1962). The Fissurellidae have a much shorter intestine than the Patellacea. In Diodora, Puncturella and Fissurella the short intestine may be correlated with the second shell opening and the low littoral habitat and therefore little or no storage of faecal material is required. The intestine of Fissurella forma two 90 loops, as in Emarginula, while in Diodora it forms only one loop and in Patella six loops. The gut of Diodora apertura is 4 times the length of the body (Graham 1949) while in F. pustula (Fischer 1857) and F. barbadensis it is 2.5 times the body length. In Patella vulgata the intestine is 8 times the length of the shell (Fretter and Graham 1962). The intestine of Diodora, as described by Gabe (Fretter and Graham 1962), is similar to that of Fissurella. The ciliated epi thelial cella are tall near the style sac and low within the heart of both limpets. Iron granules are numerous in the region from the style sac to the heart and absent from the heart to the anus in Diodora, similar to the condition found in Fissurella. The epi thelium of the anal region in Diodora contains gland cell~as in Fissurella, and also alkaline phosphatase. The muscular coats of the intestine in both genera become thinner within the heart and thicker again near the anus. The long intestinal cilia within the ventricle of Fissurella are probably responsible for moving the faecal material towards the anus with little assistance from the thin muscle wall. Graham (19~9) has described five histologically different regions of the mid-gut of Patella, the first being equi valent to the style sac of other diotocardians. The hind gut of Patella has longitudinal folds and contains mucous cells but very few pigment granules and is therefore similar to the equivalent region in Fissurella. PART IV. BREEDING CYCLE Introduction Reproductive cycles in Zeugobranchia and Patellacea have been described for Haliotis spp. (Crofts 1929; Bonnot 1940; Leighton and Boolootian 1963; Boolootian et al. 1962), Acmaea spp. (Fritehman 1961a,b,c 1 1962), Pate11a vulgata (Orton et al. 1956) and P. depressa (Orton and Southward 1961). Spawning in these species is usual1y seasonal and may be inf1uenced by one or more factors ineluding air and sea temperature changes, availability of food and mechanical stimulation from rough seas. In Aemaea scabra spawning may be related to tidal action or to the lunar cycle (Fritchman 196lc). Haliotis rufescens is subtidal and apparent1y spawns throughout the year. It is suggested (Boo1ootian et al. 1962) that the seasonal variation in food supply does not influence B. rufescens while it does affect the intertidal B. craeherodii whieh has a spawning period in 1ate spring and summer. The land snail Limieolaria martensiana living near the equator also has a eontinuous breeding season. Peaks of spawning occur in January-February and July and they are apparently associated with two annua1 wet and dry seasons. These two spawning peaks allow the young animale to feed during the wettest months (Owen 1964). 91 92 In Diodora apertura (=Fissurella reticulata) the eggs are shed from the anterior end of the mantle cavity of the female and are spread by the foot in gelatinous masses on a previously aelected sur face. Boutan {1886) records that the adhesive substance comes from an accessory gland in the urinogenital duct. Fretter and Graham (1962) auggest that the shell, which surrounds the gelatinous coat of the ovum in the ovary, swells when the egg is released and may be the only cause of adhesion. The egg laying process takes two to three hours and fertilization occurs after the eggs are laid. The apermatozoa are liberated through the apical hole of the male (Boutan 1886). The trochophore and veliger stages are passed through in the ge1atinous material and the young emerge as miniature adults. In Fissurella (=Diodora) nubecula the eggs are fertilized within the ovary and the spermatozoa are grouped into spermatophores {Medem 1945). Fissurella barbadensis and Acmaea jamaicensis have free swim- ming trochophores and the pelagie 1ife lasts for two to three days (Lewis 1960). In Haliotis tuberculata (Crofts 1937, 1955), Patella spp. (Smith 1935), PatiDa pellucida (Lebour 1937) and Acmaea virginea (Boutan 1899) the eggs are re1eased singly into the p1ankton and there is a free swimming trochophore for a short period. The arctic species A. rubella is viviparous (Thorson 1935). Large very yolky eggs are produced and the young develop without a larval stage (Thorson 1936). In A. tessulata the eggs are surrounded by mucous secreted by the foot and a type of copulation occurs (Willcox 1905a). Only one type of spermatozoon is found in Fiasurella, Haliotis and Patella (Tuzet 1930) and four gamones facilitating external fertilization 93 have been located in these genera (Medem 1945). Sex reversa! occurs in 12% of a population of F. nubecula {Bacci 1947) and in at least 90% of a Patella vulgata population (Orton 1920, 1928a, 1946). Nearly all P. vulgata specimens below 10 mm. in shell length are neuter; from 16 to 25 mm. specimens are 90% male; at 40 mm. the proportion of sexes is equal and at 60 mm. 60 to 70% are female (Orton et al. 1956). P. coerulea shows a 92% aex reversa! (Bacci 194?) while in P. aspera and P. depressa it is rare or absent (Dodd 1956). Hermaphrodites occur occasionally in P. vulgata, P. aspera and P. depressa. Materials and Methode From June 1964 to June 1965 a random sample of thirty five specimens was collected at the Deep Water Harbour at bimonthly intervals. The length, width and height of each shell was measured to 0.5 mm. with vernier calipers and the gonad was examined by cut ting away the posterior part of the foot from the shell and re flecting the foot anteriorly. The sex, size and position of the gonad and the stage of deTelopment or spawning were noted by obser vation under a dissecting microscope. The virgin gonads, whose sex could not be determined under high magnification, were termed neuter. High power magnification was necessary to define the aex of the first developing gonad stage. The grouping of stages is based on the scheme used by Orton, Southward and Dodd (1956) in Patella vulgata. Each stage is partly determined by the size of the gonad relative to the aize of the limpet. The mean gonad size is calculated by multiplying the number 94 of individuals in each gonad developing and spawning stage by the number allotted to the stage. The values obtained are added together and divided by the total number of specimens in the sample, that is by thirty five. The 95% confidence limita for the mean gonad size are plotted in Fig. 59. Samples of each stage of gonad development were preserved in Bouin's solution, dehydrated in alcohol, cleared in benzene and sectioned at 8 ~ • The sections were stained with Ehrlich's haematoxylin and eosin Y (HE) and the microscopie structure of the gonad was examined. Resulta The majority of limpets below 12 mm. in length were found to have virgin gonade and after the October collections specimens be low 11 mm. were not included in the samples. This explains the small number of neuter animale below 12 mm. in Fig. 61. Specimens begin to show gonad development when they are between 8.5 and 18 mm. long, the average size being about 12 mm. in both sexes. Once de velopment has commenced the gonad never returns to a resting state during the remaining life span, as indicated by the lack of any neuter stages in specimens above 18 mm. long (Fig. 61). The re productive celle were identified as follows. Spermatogonia. These are pale-staining cells, 5 fA in diameter, with a little cytoplasm. The small centrally placed nucleus, about 2 ~ in diameter, has granular chromatin and a pro minent nucleolus. The cells are supported by the connective tissue base of the seminiferous tubules and may be found in the gonade of all mature limpets throughout development and spawning. 95 Spermatocytes. The cella are 3 to 5 y in diameter with a dark-staining spherical nucleus, Zjk in diameter, containing scat tered chromatin. The cytoplasm is granular. Spermatids. At first these Z ~ cells have scattered chromatin and a granular cytoplasm. Later they have a dark-staining nucleus with densely packed chromatin. Spermatozoa. Only one type of spermatozoon is found. The head is about 4 ~ long. Oogonia. These are 6 ~ cells, each with a pale-staining oval-shaped nucleus containing scattered chromatin and a nucleolus. There is very little cytoplasm and the cell boundaries are often hard to define. The cells are embedded in the connective tissue tra- beculae, usually in groups, and they may be found in the gonads of all mature specimens throughout development and spawning. Small oocytes. The celle have a diameter ranging from 7 to 4o f • They are dark-staining with a large nucleus, 5 to 17 ~ in diameter, scattered chromatin and a prominent nucleolus. Medium oocytes. The diameter of these cells reaches 80 fk , including the gelatinous coat which surrounds the eell. The nu cleus has a diameter of up to 27 )A with reticular chromatin and a prominent nueleolus. Large oocytes. These cells are from 80 to 180 ~ in diameter, including the gelatinous coat which increases in thickness as the cell becomes larger and swells on contact with sea water. Both the male and female gonads are enveloped in connective tissue and a layer of muscle surrounded by the pallial epithelium. The gonad stages were identified by the following criteria. Neuter {Stage 0) At this stage the sex can only be determined after the gonad has been sectioned if it is already beginning to show development {Fig. 39). The gonad is a thin elongate structure lying between the visceral mass and the foot. It is situated just posterior to the end of region l of the intestine {Fig. 38) and is invisible to the naked eye. The covering membrane is mottled brown or green and the gonadal tissue is translucent or beige. In neuter gonade just prior to development {Fig. 39), the oogonia {og) and spermatogonia are embedded in connective tissue (ct). Developing Stage I. The sex can be determined under low magnitication by the colour of the gonad. Growth has occurred but the gonad does not reach to the periphery of the visceral maas. The anterior end is adjacent to the end of region l of the intestine. The testis {Fig. 4o) is brown with some white mottling due to the ripe sperm atozoa in the seminiferous tubules. The latter are well developed at this stage. ~everal layera ot spermatogonia (sg) are supported by the connective tissue base of the tubules {ct). The tubule walls are also formed ot spermatocytes (sc), spermatids (at) and a few spermatozoa (sz). The ovary (Fig. 49) is translucent to beige, often slightly green from the developing eggs. Groups of oogonia (og), small oocytes {so) and a few medium oocytes are attached to the connective tissue trabeculae (ct) which are beginning to form. 97 Stage II. The sex can be recognised with the naked eye. The gonad reaches, and may extend just over, the posterior edge of the visceral maas. The testis (Fig. 41) is brown with more white mot tling in the tubules than in stage I. Spermatocytes (sc) and spermatids (st) form most of the tubule walls. Active spermatozoa (sz) can be seen under low magnification with their tails extending into the cavities of the tubules. In the ovary (Fig. 50) the eggs are pale olive green or bright green and there is a noticeable size variation. The yellow trabeculae (ct) extend through the gonad. Numberous small oocytes (so) are attached to the trabeculae along with the oogonia. There are more medium oocytes (mo) than in stage I and a few large oocytes (lo). Stage III. The gonad forms a thick extension to the posterior end of the visceral mass in both sexes. It lies adjacent to, or slightly overlapping, the end of region 1 of the intestine. The testis (Fig. 42) is brown or beige with white centres to the tu bules. The proportion of active spermatozoa (sz) is higher than in stage II. In the ovary (Fig. 51) there is a large range in egg size with considerable numbers of small (so) and medium (mo) oocytes and more large oocytes {lo) than in stage II. Stage IV. The gonad bas increased in thickness and extends dorsally. The anterior end overlaps the end of region 1 of the intestine. The amount of connective tissue {ct) in both sexes is reduced. The male gonad (Fig. 43) is beige, beige-pink or pale beige and is full and loose with numerous ripe spermatozoa (sz) and spermatids (st). There are a few spermatocytes {sc) and spermatogonia (sg) at the periphery of the tubules. The yellow 98 trabeculae in the ovary (Fig. 52) are masked by the green eggs which are more uniform in size than in stage III. Medium (mo) and large (lo) oocytes are predominant although considerable numbers of small oocytes (so) are attached to the connective tissue with a few oogonia (og). Stage v. The volume of the gonad is further increased with maximum extension posteriorly and dorsally. The testis (Fig. 44) is cream or white, sometimes tinged with pink or beige, and the amount of connective tissue (ct) is lesa than in stage IV. Sperma tozoa (sz) are abundant but considerable numbers of developmental stages are still present. The trabeculae (ct) in the female gonad (Fig. 53) are very thin and most eggs are mature, fully grown and loosely packed (lo). Numbers of small oocytes (so) and a few medium oocytes (mo) are still attached to the trabeculae. Spawning Stage IV. The aize and position of the gonad are the same as in stage IV developing. The tubules in the testis (Fig. 45) are beginning to break down. Spermatocytes (sc) are not as abundant as in stage v. The ovary (Fig. 54) is composed mainly of large oocytes (lo) but a few developmental stages are present. Gaps (g) occur between the eggs. Stage III. The gonad is the same size and occupies the same position as in stage III developing. It is one to two thirds of the full size at this stage. The tubules (Fig. 46) are still formed predominantly of spermatozoa (sz) and the developmental stages are relatively fewer than in the previous stage. In the ovary 99 (Fig. 55) more gaps are apparent between the mature eggs (lo) as spawning advances. There are a few developmental stages, as in stage IV spawning. Stage II. The gonad size and position are the same as in stage II developing. It is now approximately one third of the full size and the amount of connective tissue (ct) has increased in both sexes. The testis (Fig. 47) is composed mainly of spermatozoa (sz) but there are also numbers of spermatids (st) and spermatocytes (sc). The tubules have completely broken down. In the female gonad (Fig. 56) there are a few large (lo) and medium (mo) oocytes and oogonia (og) and numerous small oocytes (so). Stage I. The size and position of the gonad are the same as in stage I developing. It is almost fully discharged and has a large amount of connective tissue (ct) in both sexes. In the testis (Fig. 48) there are some spermatozoa (sz) and numbers of developmental stages. The female gonad contains small oocytes (so) and a few oogonia (og), medium oocytes (mo) and large oocytes (lo). Reproductive activity is summarized in Table 10 and Figs. 58 to 60. Fig. 60 shows the percentage of developing animals in Fig. 58 in more detail. From Fig. 58 it may be seen that spawning ani mals are found through nearly the whole year with peaks of spawning occurring in October-November, March, April and May. It seems probable that the marked fluctuations in spawning specimens be tween March and May are due to the small size of the samples and the sampling frequency. If the samples were considered on a monthly basis the spawning peaks would be greatly smoothed. lOO The mean gonad size for all specimens in each sample is shown in Fig. 59. The 95% confidence liaits do not vary greatly throughout the year, thus indicating a relatively constant range about the mean aize of the gonad. The fluctuations are not as marked between March and May in Fig. 59 as in Fig. 58. This again suggests that, if monthly samples were considered 1 a smoother curve would be formed. The re duced mean gonad sizes in aid-December and early January and in April and early May are mainly the result of the relatively large numbers of young developmental stages (Fig. 60; Table 10). The peaks of mean gonad size in mid-November 1 early December, early February and mid-March are due principally to the high percentages of ripe (stage V developing) specimens (Fig. 60) but in mid-November and mid-March the peaks are also due to a large number of spawning specimens (Fig. 58). During periods of decreased spawning (Fig. 58) the percentage of ripe animals (stage V developing) is reduced while the percentage of young developmental stages is increased (Fig. 60). At the peaks of spawning, developmental stages IV and V are relatively abundant and there are few limpets with gonads in young stages of development. Specimens below 11 mm. were not included in collections made after October, since most of these aniaals were found to have neuter gonads (Fig. 61). Consequently the numbers of neuter limpets collected after this time are reduced (Fig. 58). It is apparent from Table 10 that developing stages I and II and the spawning stages are passed through more quickly in males than in females. Out of all the samples collected about 18% of the male and 27% of the female limpets were found to be spawning. e e TABLE 10 DISTRIBUTION OF GONADSTAGES Date Neuter Male Female No. in samp1e Deve1oping Spawning Deve1oping Spawning 1964 (0) I II III IV v IV III II I I II III IV v IV III II I 3 Jun. -- 2 4 8 1 - - 1 -- 7 6 2 2 - 1 - 1 35 18 Jun. 5 1 4 4 4 1 - 1 1 3 4 5 1 1 35 - - - - - ....., 2 Jul. 3 1 4 4 6 3 2 1 1 2 1 1 2 1 2 1 35 - - - ~ 15 Jul. 4 1 - 2 8 1 1 1 2 - 3 - 3 1 4 2 1 - 1 35 3 Aug. 2 - 1 - 3 7 2 - 1 - 2 1 6 6 1 2 1 - - 35 17 Aug. 3 - - 4 6 3 - - 1 - 1 1 3 5 1 1 4 2 - 35 2 Sept. 1 - - 1 9 4 - 2 1 - -- 2 5 3 1 2 2 1 35 18 Sept. 4 - - 1 6 6 - 2 1 1 - -- 4 4 2 3 - 1 35 2 Oct. 3 - - - 13 1 1 - - 1 - - - 4 7 1 1 2 1 35 19 Oct. 1 - -- 7 3 4 1 3 --- - 5 6 1 2 2 - 35 2 Nov. 3 - - 1 8 3 1 - 1 -- - - - 5 7 5 - 1 35 18 Nov. - - - - 4 9 6 3 - -- - - 2 6 1 3 - 1 35 2 Dec. - - - 3 10 5 1 1 - - - - - 2 10 2 1 - - 35 18 Dec. - - - 8 8 ------1 9 6 - - 2 1 - 35 e e Table 10 continued Date Neuter Male Female No. in sample Developing Spawning Developing Spawn.ing 1965 (0) I II III IV v IV III II I I II III IV v IV III II I 6 Jan. -- 2 10 7 ------7 8 1 - - - -- 35 18 Jan. - - - 4 11 1 - - - 1 - 1 3 6 6 - - 1 1 35 1 Feb. - - - 2 6 8 - 1 - - - -- 6 10 1 1 - - 35 .... 15 Feb. 1 1 5 8 4 1 - 2 2 7 2 1 1 35 0 ------N 2 Mar. 1 - - 4 9 4 2 ------5 5 3 1 1 - 35 18 Mar. - - - 1 5 8 4 - 1 - - - - 2 5 5 2 1 1 35 6 Apr. 1 - 2 10 6 - - - - - 2 5 9 ------35 19 Apr. 3 - 1 4 3 3 3 2 3 1 2 1 1 4 1 - 1 2 - 35 3 May 3 - - 2 6 - 1 1 -- 2 4 11 3 1 1 - - - 35 18 May 2 - - 1 8 5 5 2 - -- -- 1 3 2 5 1 - 35 2 Jun. 2 - 4 5 4 - 1 2 - 2 1 1 4 1 - 1 4 3 - 35 17 Jun. 1 - - 3 6 5 3 2 4 - - 4 4 2 - - 1 - - 35 103 The proportions of the two sexes in limpets of all lengths over 12 mm., as shawn in Fig. 61, are approximately equal. Lesa than 5% more males than females were collected throughout the year. A chi square test (chi square = 2.120) shows that, for one degree of freedom, there is a probability of 16.5% that the differences between the observed proportions and equal numbers of both sexes could have arisen by chance. Therefore these differences are probably not real. There was no indication of any change of sex or hermaphroditism. 104 AbbreTiatioaa Uaed ia Figa. 39-57 ct = coaaectiYe tiaa•• g = gap foraed by ahe4 ...... lo = large oocyte • = auacle Ill aedi1111 oocyte -og = oogoaiua pe = pallial epithelium ac = apermatoc;rte ag = aperaatogeai1111 •• = aaall ooc,-te at • aper-ticl az = aperaatozooa 105 ct FIG. 39. Section through neuter gonad. Er lich's haematoxylin and eosin Y. x 570. FIG. 4o. Section through male gonad stage I developing. HE. x 570. 106 FIG. 41. Section through male gonad stage II deve1oping. HE. x 312. FIG. 42. Section through male gonad stage III developing. HE. x 570. 107 FIG. 43. Section through male gonad stage IV developing. HE. x 312. FIG. 44. Section through male gonad stage V developing. HE. x 312. 108 FIG. 45. Section through ma1e gonad stage IV spawning. HE. x 312. FIG. 46. Section through male gonad stage III spawning. HE. x 312. 109 FIG. 47. Section through male gonad stage II spawning. HE. x 570. FIG. 48. Section through male gonad stage I spawning. HE. x 312. 110 FIG. 49. Section through female gonad stage I deve1oping. HE. x 312. FIG. ,50. Section through female gonad stage II developing. HE. x 120. 111 FIG. 51. Section through female gonad stage III developing. HE. x 120. FIG. 52. Section through female gonad stage IV developing. HE. x 120. 112 FIG. 53. Section through fema1e gonad stage V developing. HE. x 120. • ~ mo FIG. 54. Section through female gonad stage IV spawning. HE. x 120. 113 FIG. 55. Section through female gonad stage III spawning. HE. x 120. FIG. 56. Section through fema1e gonad stage II spawning. HE. x 120. 114 FIG. 57. Section through female gonad stage I spawning. HE. x 120. J:: \.n e . . JUN . to . . . . . . . . . . . . MAY specimens. 1964 SPAWNING June APR '/, 1965 spawming from MAR CJ and FEB activity JAN deve1oping DEVELOPING DEC breeding •Jo • in neuter, . . NOV . •• . . • of • . " '0 . . • . . • • • . . • • " . . . OCT . . • . . • • . . ... variation . . • • . . • • . . . 1964 • SEP . . .. • ' NEUTER t • • percentages • . "/o • . . " • . f' AUG Seaaonal . . . . . . • .. 1.. . . . . . D . . showing 58. . . . JUL FIG. 1965 JUN June 0 80 20 60 40 100 e % 0\ 0\ ...... ...... 1 1 to to e e JUN JUN - 1964 1964 MAY MAY June June point. point. APR APR from from each each 1965 1965 MAR MAR size size about about FEB FEB gonad gonad JAN JAN p1otted p1otted mean mean are are the the DEC DEC in in NOV NOV 1imits 1imits OCT OCT variation variation confidence confidence SEP SEP 1964 1964 95% 95% Seasonal Seasonal AUG AUG The The 59. 59. JUL JUL - 1 1 FIG. FIG. 1965. 1965. JUN JUN June June 1 1 Il Il v v Ill Ill IV IV 0 0 <( <( z z (.!) (.!) c c <( <( Cl) Cl) (.!) (.!) 1- 1.1.1 1.1.1 e e ..., ..., ..., ..., -...1 -...1 e e v v JUN JUN breeding breeding MAY MAY sTAGE sTAGE the the o o APR APR of of 1965 1965 1v 1v MAR MAR stages stages sTAGE sTAGE FEB FEB EL::d EL::d JAN JAN 111 111 developmental developmental DEC DEC sTAGE sTAGE in in NOV NOV o o 1965. 1965. n n OCT OCT June June variation variation to to 1964 1964 SEP SEP sTAGE sTAGE 1964 1964 rrn rrn Seaaonal Seaaonal AUG AUG 1 1 June June 60. 60. JUL JUL from from sTAGE sTAGE FIG. FIG. JUN JUN ,,,,, ,,,,, cycle cycle 0 0 20 20 80 80 60 60 40 40 100 100 .,, .,, e e e e 6 0::: --- NEUTER UJ (\ ········ MALE m : ':. :::i: 1 :. ..+...... - FEMALE : :t--•" =>tnZz ~ ~ ....JLJJ 4 ca::::i: .....- out-UJ Il.. ; 1 : : ctn j -..;. zu_ ~0 2 ~ (!) {; L&.. 0 / ,1~ ' ... ~ ~, ~ 0 / +-;r. \ ~ ...... ~1 ()) 0 - 0 5 10 15 20 25 30 LENGTH (MM.) FIG. 61. Proportions of male, female and neuter specimens at different shell lengths. Total number of animals = 880. 119 Discussion The position of the gonad in F. barbadensis is similar to that in Acmaea spp. (Fritchman 196la) and Patella spp. (Orton et al. 1956; Orton and Southward 1961) but a fully ripe gonad of P. vulgata seems to be relatively larger than a ripe gonad ofF. barbadensis. The developmental and spawning stages also appear similar in P. vulgata and F. barbadensis. However, in the spawning male gonads of P. vulgata the non-germinal tissue becomes darker in colour as spawning advances causing the colour of the gonad to become darker. In F. barbadensis the male gonads remain cream or white until spawning is completed. Spawning is interrupted by periode of development in P. vulgata (Orton et al. 1956). This phenomenon may also occur in F. barbadensis since a large number of immature stages are found in discharging male and female gonads (Figs. 47, 48, 56, 5?). Alternatively, in the absence of a resting period, the presence of young developmental stages in spawning specimens may be merely a result of the continuous reproductive cycle of this species. The resulta suggest that F. barbadensis spawns at least twice a year. However, correlation between the appearance of small speci mens in the monthly transects and the peaks of spawning was difficult due to the almost continuous spawning period of the limpet. Acmaea jamaicensis also appears to have a prolonged breeding season in Barbados (Lewis 1960). In the more northern forma, !• scabra spawns throughout the year while many other Acmaea species are seasonal spawners. Fritchman (196lc, 1962) has noticed that both in A. limatula, in which spawning occurs only once a year but the resting period is very brief, and in A. scabra the turgor that 120 is characteristic of the ripe gonad of many limpets is not developed. Patella depressa has a very short resting phase between spawning and redeve1opment. This is because the gonade remain in the post spawning and spent stages until the rise in temperature in spring which causes redevelopment (Orton and Southward 1961). In A. 1imatu1a the brief resting period is due mainly to the slow redevelopment of the gonade (Fritchman 1962). Spermatogenesis in F. barbadensis appears similar to that de scribed for Trochus spp. by Tuzet (1930). The eggs of Diodora apertura are yellow and 14o ~ in diameter when they are shed. They are therefore slightly smal1er than the green eggs of F. barbadensis which reach about 180 ~ in diameter. The mature oocytes of E• vulgata are a1so green and the colour may be due to'chromoprotein Y' extracted from the female gonade of this species (Goodwin and Taha 1950). Unlike F. nubecula (Bacci 1947), there was no evidence of hermaphrodites or protandrous animale in F. barbadensis. Clustering, or pairing of the two sexes, during spawning was not observed in F. barbadensis, as in Patina pellucida, Patella coerulea and P. lusitanica (Fretter and Graham 1962). PART V. BEHAVIOUR Introduction Some aapects of the behaviour of F. barbadensis have been mentioned in the preceding chapters auch as the lack of clustering, feeding movements and the apparent indifference to bright sunlight. The homing ability of limpets has been a subject of considerable study for many years. Early observations have been reviewed by Villee and Groody (1940) and by Thorpe (1956). The "homes" are definite positions on the rocks where the limpets remain without movement. The surface of the home may be worn away by the limpet leaving a scar which may differ in colour from the surrounding sur faces. The scar is approximately the same shape as the shell of the owner. Russell (1907) notes the lack of agreement in the literature as to the time that Patella moves in relation to the tidal cycle. Apparently P. vulgata will feed in shaded or damp places during periode of emersion provided that there is no danger of desic cation (Orton 1929) while the chief movement occurs when the lim pets are submerged. Homing has been recorded in large individuals, while below 20 mm. the animals are more active and do not have definite homes. 12l. 122 Homing has not been observed by Villee and Groody (1940) in Acmaea disitalis, A. scabra, A. persona, A. pelta or A. scutum and these species have not been seen to move when uncovered by the tide. However Hewatt (1940) did observe homing behaviour in A. scabra under different environmental conditions and the home scar of this species is also mentioned by Fritchman (196lc). Large specimens apparently remain in the home for long periode, as has been re- corded for Patella. The scar of A. scabra is not sunken like that of Patella but may be recognised by its pale green colour (Hewatt 194o). Movement in this species begins after the waves of the rising tide wash over the shell several times and the animal re- turns to its home before emersion occurs. These limpets always retrace their paths, as has also been noted for Patelloida saccharina (Abe 1942). A. scabra above 14 mm. in shell length moves only when submerged, while small specimens are more active and may move during emersion. However, these small limpets usually inhabit low shore levels where the substrate is damp for most of the exposed period • • Large specimens of A. dorsuosa have homes but these are changed periodically (Abe 1931), as are the homes of P. vul&ata (Fretter and Graham 1962), while A. digitalis appears to have a home range but does not return frequently to the same spot (Frank 1964). No evidence of homing has been found in A. tessulata (Willcox 1905a) or in Haliotis tuberculata which is a very active species (Stephenson 1924). Willcox (l905b) records the limited homing powers of F. barbadensis in Bermuda. An individual that had been moved more than three inches from its soar was not observed to home. Littorina 1ittorea takes periodic feeding excursions simi1ar to those of limpets and tends to remain at the same intertidal leve1 that is adopted in the first year (Allen 1963). The feeding path is in the form of a 1oop and the species has given evidence for a 1ight-compass response (Newell 1958a,b). Limpets probably use the o1factory sense for homing which functions mainly through the margi nal pallia1 tentacles (Davis 1895). The outgoing trail may be re traced or there may be some form of memory of their immediate en vironment. Patel1a vu1gata in dry surroundings responds to splashing with sea water by raising the front of the she11 and extending the head and cephalic tentacles. If the stimulation is repeated most of the animale begin to wander (Arnold 1957). Sp1ashing with fresh or low salinity water causes withdrawal and clamping down of the shell onto the substrate. The salinity receptors are in the cephalic tentac1es and edge of the mantle. These responses are related to the chloride ion concentration, not to a variation in pH, osmotic pressure or minor ions, although the calcium ion concentration appears to be important (Arnold 1959). There is an increasing tolerance to low salinities in populations from higher shore 1evels. Arnold suggests that these responses enable high level animale to take advantage of spray, humidity or light rain to prolong the feeding period during emersion. Diodora apertura is intolerant of brackish waters of salinities below 21 parts per thousand (Fretter and Graham 1962). Materials and Methode Homing behaviour was examined at different tidal heights on the horizontal beach rock platforms at Six Men's Bay. Observations on responses to different salinities and on the sensory regions of the limpet were made in the laboratory. Specimens collected from low shore levels were placed on a dry slide and sti mulated by allowing one drop of water from a pipette to fall onto the shell. The height that the front of the shell was raised after stimulation was measured directly since no mechanical recording de vice was available. Resulta All specimens of F. barbadensis, except some below 10 mm. long, have a slightly sunken greenish coloured scar. As the rising tide washes over the shells many specimens become active and start to browse, while others remain quiescent for longer periode. Excursions were observed up to six inches from the home scar. If a limpet ia moved a few inches from the scar, which is just submerged by the rising tide, it will either remain where it is placed or, more pro bably, it will move off on a feeding trip. This is particularly true of small specimens while some of the larger animale may return home. By the time that the falling tide has exposed the limpets most of them have returned to their homes, into which the shells fit exactly. Others return while their scar is stlllsubject to wave action and a few continue their excursions until the substrate be gins to dry out. A limpet removed from its scar at this time usually 125 homes readily within ten minutes, provided that it is not moved away more than an inch. Some specimens will home from greater distances over a longer period, while others will merely stay where they are placed. Small animale do not home as quickly as larger ones. Lim pets moved backwards just out of their scars will turn through 360 degrees and return to their homes within fifteen minutes. The length of time an individual remains in any one home is unknown but speci mens have been observed in the same scars for several weeks. No fixed orientation was observed with respect to gravity or light either on the shore or in the laboratory. A sudden change in light intensity often causes movement. Specimens brought into the laboratory at low tide will crawl out of the water and adhere to the top of their container. Later many will settle along the water line with the apical hole either just submerged or just exposed. A quiescent limpet that is splashed with sea water, either in its natural environment or in the laboratory, will raise the front edge of the shell and move the head and cephalic tentacles. Con tinuous stimulation will cause movement. If fresh water is dropped onto the shell there is an immediate withdrawal of the head and the shell is clamped down tightly onto the substrate. Subsequent sti mulation with sea water causes a raising of the shell but the response is delayed. When waters of different salinity are used the responses are negative for those containing less than 50% sea water. Mixtures containing between 50 and 80% sea water pro duce both negative and positive responses. With 90% sea water the front of the shell is raised approximately 1 mm. from the substrate while lOO% sea water causes the shell to be lifted 2 to 4 mm. 126 Specimens were washed quick1y with fresh water after each stimu1ation. When sea water is dropped onto different parts of an inverted limpet there is no change in activity, a1though when the head is sti mu1ated the foot and 1ater the head and cepha1ic tentac1es stretch out towards the pipette. If fresh water is used on the head or the edge of the mant1e the specimen responds immediate1y with a contra ction of the mant1e and foot and withdrawal of the head. Similar stimu1ation on other areas with fresh water causes no response. The grack1es, Qpiscalus lugubris, may be natural enemies of F. barbadensis. These birds frequent the rocks at Six Men•s Bay just as the fa1ling or rising tide exposes or submerges the 1impets. At this time Fissure1la is often moving and is therefore much more easily removed from the rocks than at low tide when these animals are usually fitted neat1y in their scars. Commensale have been found 1iving within the mantle cavity of Fissurella. These inc1ude the po1ychaetes Nereidae Pseudonereis gallapagensis (Kinberg), Ceratonereis sp. and Ophe1iidae Polyophthalmus pictus (Dujardin), and the amphipod Cymadusa filosa (Savigny). Discussion Heming appears to occur in all size groups in Fissure11a, un like Patel1a, although it is more obvious in specimens over 10 mm. The resulta agree with those of Willcox (l905b) for this species, in that short feeding trips are usual1y taken often not more than two inches from the edge of the scar. Olmsted (191?) observed that Fissurella can move backwards without reversing the normal direction of the muscular contractions of the foot, although he suggests that 127 this reverse locomotion is probably seld~m used in nature, as turning usually takes place. The limpets do move backwards when fitting into their scars but only for approximately 1 mm., after which turning occurs. Abe (1931) bas demonstrated that when specimens of Acmaea dorsuosa are placed in an aquarium they form a line along the water surface, some partly submerged and others just exposed or just be low the water level. These observations are similar to those ob tained for F. barbadensis. Bowever, in A. dorsuosa all specimens orient with the head downwards, while no definite position was adopted by Fissurella. The responses of Fissurella to splashing with sea water and fresh water are similar to those obtained by Arnold (1957) in Patella and the salinity receptors seem to be situated in the same positions in both limpets. Low water Patella appears to be rather more tol erant of diluted sea water than Fissurella, which was also collected from low shore levels for these experimenta. It is obviously ad vantageous for an intertidal gastropod to be sensitive to fresh water in areas of heavy tropical showers. PART VI. CONCLUSIONS A number of aspects of the biology of Fissurella barbadensia reflect the conditions of its tropical environment. Unfortunately the shortage of literature on related tropical intertidal gastropods makes comparison with these forma difficult. The distribution is influenced by wave action. A relatively sheltered environment is more favourable than one where the exposure to waves is considerable and shelter must be sought. It is apparent from those specimens living just below mean high water level that this species can exist under conditions of considerable desiccation. It seems to be able to withstand a greater emersion time than tem perate keyhole limpets, whose distribution is limited to low water level and below. F. nubecula in Ghana is also restricted to the parts of the shore that are continually wet. The growth of F. barbadensis in the first year is similar to that of Patella vulgata in Britain. Both species attain a length ot between 20 and 25 mm. However, P. vulgata lives tor 10 to 20 years and reaches a much larger aize than F. barbadensis, which is thought to have a life span of 1 to 2 years. Patina pellucida lives for 1 to 2 years but only attains a length of 10 mm. after the first year. Acmaea dorsuosa grows to 6 mm. in the tiret year but, like 128 129 P. vulgata, this species lives for 10 to 20 years. Frequency di stribution curves obtained from random samples of F. barbadensis do not always show distinct age classes, probably because of the almost continuous spawning period. Variations in shell structure with growth and distribution have been examined in several limpets. Large shells of F. barbadensis, Acmaea spp., P. vulgata and Patelloida conulus are relatively broader and higher than small shells. In F. barbadensis the rela tive breadth, shell weight, volume under the shell and the extra visceral space are generally greater in more exposed habitats than in sheltered regions. P. vulgata and F. barbadensis grow to a larger aize in more sheltered environments. Large shell sizes are more common at high shore levels in both these species, while Acmaea limatula and A. scabra are larger at low shore levels. High level P. vulgata shells are relatively slightly broader than those from low levels. However, in F. barbadensis the basal shape of the shells is not affected by the vertical distribution. F. barbadensis shells from high levels at two stations in Barbados are relatively higher than low level shells of the same aize. Si milar resulta have been obtained for P. vulgata. A. limatula shells from high shore levels are heavier and have a greater volume and extra-visceral space per given wet weight of soft parts than shells from low levels. F. barbadensis shells from different intertidal heights show no difference in weight but high level shells from one West coast station show a greater volume and extra-visceral space per unit wet weight of soft parts than low level shells. High level shells from this station are 130 relatively thinner than those from low levels, while in A. limatula the reverse trend occurs. F. barbadensis feeds in a similar manner to Patella vulgata and Acmaea tessulata. These forma, in common with Haliotis spp., all consume algae as their principal diet, while Diodora apertura and Emarginula reticulata are mainly carnivorous. F. barbadensis on the West and South West coast& of Barbados feeds sufficiently to maintain a full stomach at all times, while feeding is apparently restricted under more exposed conditions. There are indications that the starch, sucrose and protein components of the consumed algae are digested. An amylase bas been recorded from the oesophageal glands ofF. barbadensis and P. vulgata and both these animale, in common with other zeugobranchs, appear to have salivary glands solely for lubrication. Protein digestion oc ours in the digestive glands of Haliotis, Patella and Fissurella but no positive indications of fat digestion have been obtained in Fissurella, unlike the resulta obtained for the other two genera. The histology of the digestive tract of F. barbadensis and closely related forma appears to be similar. This similarity is most marked between Diodora and rissurella, where the stomach and intestinal regions seem to be nearly identical. Food is probably moved in the digestive tract by both ciliary and muscular action except in regions, such as within the ventricle of the heart, where the muscular walls are very thin. The unicellular gland cella found throughout the gut appear to produce a mucin-like substance to bind the food pirticles and act as a lubricant during feeding. Additional supplies of lubricating 131 material are probably produced by the salivary glands. Digestion apparently begins in the oesophagus where enzyme and mucous secretions, which may be produced by the same cella of the oesophageal gland, are conveyed by cilia to the dorsal food channel. Further digestion occurs in the main part of the stomach where en zymes, produced by one or both types of digestive gland cell, are mixed with the food by ciliary and muscular action. Particle sort ing probably takes place in the posterior sorting area, the sub stances to be absorbed being conveyed by the ciliated longitudinal folds to the digestive gland. The undigested food passes into the style sac where it is compacted into a mucous bound rod, as in the cases of Diodora, Emarginula and Patella, before being conveyed into the intestine where further binding occurs. Granules which appear to be rich in iron are found in the oesophageal and digestive glands, in the stomach and in the intestine of Fissurella. A similar, al though perhaps not so extensive, distribution of iron granules is recorded in Diodora and Haliotis. Seasonal spawning occurs in most Zeugobranchia and Patellacea from temperate regions. These animals include Patella vulgata, P. depressa (Pennant), P. aspera, Diodora apertura and many Haliotis and Acmaea species. Continuous spawning is recorded in A. scabra and H. rufescens and it may also occur in the tropical species F. barbadensis and A. jamaicensis. A. limatula and P. depressa have a very brief resting phase between spawning and redevelopment, while in F. barbadensis there is no resting period. Sex reversal has been recorded in P. vulgata, P. coerulea and F. nubecula, while no evi dence was found for protandry or hermaphroditism in F. barbadensis. 132 It would be interesting to compare the breeding cycle of !• barbadensis in Barbados with the cycle at the northern limite of the range of this species, where specimens apparently grow to a larger aize. Possibly the gonad turgor would be greater at these northern limita, as has been found in some Acmaea species. The intense reproductive activity of F. barbadensis in Barbados may be the cause of the apparently lower growth rates and smaller final size. The behaviour of F. barbadensis does not differ greatly from that of temperate forma. However, exceptions occur in the seasonal cycles of movement associated with seme northern limpets and in the absence of a heming ability in many Acmaea species. Clustering or pairing was never observed in Fissurella, while in northern limpets grouping may occur when food is plentiful, during relatively high temperatures and calm seas or during spawning. A more complete account of the adaptation of F. barbadensis to tropical as compared with temperate environments will be obtained when further studies have been made on this and closely related species in tropical regions. SUMMARY 1) The distribution of Fissurella barbadensis around the rocky shores of Barbados is influenced by wave action. A re1ative1y sheltered environment is more favourable than one with considerable exposure to waves. 2) Limpets between 10 and 15 mm. shell length grow about 2 mm. per month, those between 15 and 20 mm. grow about 1.5 mm. per month, while specimens from 20 to 25 mm. increase approximately 1 mm. per month. When the product of the three leading dimensions is used as a measurement of size, a limpet between 200 and 500 mm. 3 grows about 200 mm. 3 per month. Specimens between 1,000 and 1,500 mm. 3 3 grow approximately 390 mm. per month and those between 2 1 000 and 2,500 mm. 3 increase about 460 mm.3 in a month. 3) Large shells are relatively broader and higher than small shel1s. The relative breadth, shell weight, volume under the shell and extra-visceral space between the shell and the soft parts are generally greater in more exposed habitats than in sheltered regions. Shells from high shore levels are usually relatively higher and may be relatively thinner than shells from low leve1s. High leve1 lim pets from one habitat were found to have a significantly greater volume under the shell and extra-visceral space than 1ow level animals of similar wet weights of soft parts. 133 134 4) The principal diet is algae and the intake is sufficient in most habitats to maintain a full stomach at all times. Food passes through the digestive tract in approximately 8 hours. 5) An amylase is located in the oesophageal gland and a protease, invertase, cellobiase and an amylase are found in the digestive gland. 6) The histology of the digestive tract is described and related to the resulta of the en~e tests. 7) Examination of the gonads of specimens collected at bimonthly intervals indicates that almost continuous spawning occurs in this species. 8) F. barbadensis has the ability to home and is sensitive to diluted sea water and to fresh water. Clustering or pairing was not observed. 9) Most of these aspects of the biology of F. barbadensis are similar to those of at least some of the temperate limpets. The principal difference occurs in the almost continuous spawning of this species compared with the seasonal spawning of most northern forms. F. barbadensis appears to be able to inhabit higher inter tidal levels than other keyhole limpets. Similarities in first year growth, structural changes in the shell with growth and distribution, diet, feeding methods and behaviour are evident between Patella vulgata and F. barbadensis. The digestive tract histology is similar in Diodora and Fissurella. Enzymes for fat digestion have not been located in F. barbadensis, unlike the re sulta obtained for Patella and Haliotis, but carbohydrate and protein digestion take place in all these animals. LITERATURE CITED Abbott, R. T. 1958. The marine mollusks of Grand Cayman Island, B.W.I. Monographs Acad. Nat. Sei. Phila. No. 11, l-138. 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