DISTRIBUTION AND GROWTH OF THE KEYHOLE LIMPET FlSSURELLA BARBADENSIS GMELIN
JONET WARD The Bel/airs Research Institute of McGill University, St. James, Barbados, West Indies
ABSTRACT The vertical and horizontal distributions of F. barbadensis on the rocky shores of Barbados are described. Growth rates are obtained and the effects of growth and environmental variation on some of the structural features of the shell are examined. The results are compared with the growth, distribution, and shell variations of temperate climate limpets.
INTRODUCTION The genus Fissurella is found throughout temperate and tropical seas. Many species live below low tide level while others inhabit deep water or the intertidal zone (Pilsbry, 1890; Farfante, 1943; Zuniga, 1951; Gauld & Buchanan, 1959; Warmke & Abbott, 1961). F. barbadensis ranges from southeast Florida and Bermuda to the West Indies and from Mexico to British Guiana (Farfante, 1943). Lewis (1960) describes this species in Barbados living Iowan the shore from mean low water to mean sea level. There appear to be no published studies on the growth of fissurellids. However, growth rates of various Patellacea have been obtained for Patella vulgata (Russell, 1909; Orton, 1928), Acmaea dorsuosa (Abe, 1932), Patel/oida conulus (Hamai, 1937), and Patina pellucida (Graham & Fretter, 1947). The effects of growth and variation in environment on shell structure have been examined in several limpets. In A. limatula dif- ferent intertidal habitats produce variations in shell weight, in the volume of the extra-visceral space between the shell and the soft parts, in the heart rate, and in the condition of the gonads (Segal, 1956a, b). The shell height in Patella vulgata (Orton, 1928; Moore, 1934) and Acmaea dorsuosa (Abe, 1932) is greater in dry, more exposed, habitats than in damp ones. The shells are also thicker in a dry habitat. An attempt has been made to compare the tropical species, F. barbadensis Gmelin, with temperate limpets in respect to distribution, growth, and the effects of different environments on some of the structural features of the shell. The work was supported by a grant from the National Research Council of Canada. The author is very grateful for the suggestions and encourage- ment given by Dr. J. B. Lewis of the Bellairs Research Institute. My thanks also go to Dr. G. T. Ward, of the Brace Research Institute of Mc- Gill University, for discussions on some of the mathematical procedures. 300 Bulletin of Marine Science [17(2)
MATERIALS AND METHODS Distribution.-The density of F. barbadensis was measured at 12 stations around the island of Barbados by counting the specimens in areas 1 m square. The average daily period of exposure to the air, allowing for wave action, was calculated geometrically from the wave amplitude and the mean tidal height as given by Lewis (1960). Direct observation confirmed these calculations. The wave amplitude was measured by recording the maximum and minimum wave heights on a vertical scale on several days of average sea conditions. Absolute Growth.-The absolute growth rates were studied over a period from June 1964 to November 1965 using three methods: 1. A group of measured limpets was placed on the concrete piles of a wharf along the west coast of Barbados. This area had previously been cleared of all other members of the species. Every month the limpets were remeasured. A small number of specimens of various sizes was used in this experiment so that individual growth rates could be obtained. 2. Other specimens were kept in aquaria with running sea water and mea- sured at monthly intervals. They fed on algae which grew on the rocks provided and on the side of the aquaria. The data from 1 and 2 (above) were employed to obtain a relation be- tween the length of the limpet and its age as follows: The data were first plotted as the regression of length at time (t + 1) on length at time t using the equation
Lt+1 = mLt + i where m is the slope of the line and i is the vertical intercept (Walford, 1946; Taylor, 1959). The von Bertalanffy growth equation (1938) was then used to represent the growth curve Lt = Loo [1 - e-K(t-to)] where Loo = i/(1-m) is the asymptotic length and the constant K is given by K = (-logem). By this means, the age of any specimen of length Lt is estimated as t' = (t - to). The accuracy of the estimation depends upon the accuracy of the value of to in the von Bertalanffy equation. In the case of the limpets Patelloida conulus (Hamai, 1937) and Acmaea dorsuosa (Abe, 1932), the age, t, can be determined independently from the annual rings on the shell and the constant, to, the theoretical age at length zero, thus evaluated. The age of F. barbadensis cannot be obtained from annual rings because of the relatively steady growth of this species throughout the year and the 1967] Ward: Distribution and Growth of Fissurella 301
3
<141m2
< 201m 2 <71m2
o 2 4 MILES I I I II
FIGURE1. Map of Barbados showing density of Fissurella barbadensis at 12 stations and the annual wind direction. Wind rose based on 50-year means (U. S. Navy, 1949). ], Half Moon Fort; 2, Six Men's Bay; 3, Paynes Bay; 4, Deep Water Harbour; 5, St. Lawrence; 6, Oistins Bay; 7, Silver Sands; 8, Long Bay; 9, Conset Bay; 10, Bath; 11, Bathsheba; 12, River Bay. accuracy of the estimated age, t', therefore depends upon the accuracy of a separate estimate of to inferred from the study of specimens in the labo- ratory. From observations on newly settled limpets in the aquaria it has been determined that to is probably negligible and is unlikely to be greater than two weeks. 3. From June 1964 to June 1965, a monthly sample of approximately 100 limpets was collected along transect lines perpendicular to the shore in an area of beach rock at Six Men's Bay (Fig. 1). All sizes of animals above 6 mm long were represented in the sample from their upper limit in the pink zone to their lower limit in the surf zone. The length, width, and height of each animal were measured to the nearest 0.5 mm with vernier calipers. All animals were replaced within the same transect area from which they had been collected to ensure that any particular size group was not depleted from the population. The transect was made in a different position each month. Specimens below 6 mm long were not in- cluded in the samples because they are easily overlooked. 302 Bulletin of Marine Science fl7(2)
TABLE 1 THE VERTICAL INTERTIDAL ZONES OF THE ROCKY SHORES IN BARBADOS, AFTER LEWIS (1960)
Tidal height above zero datum Zone Range (ft) (m) Surf Mean low water springs-mean low water 0-1.2 0-0.37 Pink Mean low water-just below mean high water 1.2-2.8 0.37-0.85 Green Just below mean high water-6 inches above mean high water 2.8-3.8 0.85-1.16
Relative Growth.- The horizontal beach rock platforms at Six Men's Bay were selected as a sheltered area and the cliff station at Oistins (Fig. 1) was chosen as an exposed area to study the variations in shell dimensions, weight, and volume with distribution and growth. At each station high level specimens were collected from the upper pink and lower green zones and the low level animals were obtained from the lower pink and upper surf zones. These zones have already been described by Lewis (1960) and are summarized in Table 1. The shell of Fissurella is approximately the shape of an elliptical cone. The volume of an elliptical cone is a constant, 7T/12, times the product of the three leading dimensions which, in this case, are the major and minor diameters of the base and the height of the shell. The volume is thus ex- pressed as [17T2 . D2H] where H is the height and D is the geometrical mean diameter of the shell base. The shells are not exactly the shape of an elliptical cone, as they are somewhat flattened at the apex due to the apical hole and have the anterior end slightly tapered; the differences, however, would have a negligible effect on the volume. For convenience, the con- stant, 7T/12, was omitted from the calculations. The values for D~H must therefore be multiplied by the constant to obtain the true volumes. The ratio H/D is used as an index to show the variation in the relative height of the shell. The shell length/breadth, L/B, ratio is used to indi- cate the variation in the shape of the base of the shell. The shell thickness was measured by a vernier gauge to 0.1 mm using a point about 2 mm posterior to the apical hole. The shell and soft parts were separated by cutting the muscles of at- tachment. Both parts were damp dried on paper towelling and weighed to the nearest 0.001 g. The volume under the shell was measured to the nearest 0.01 cc by inverting the shell in a plasticine cup. Water, to which soap had been added to reduce surface tension, was introduced through a 1967J Ward: Distribution and Growth of Fissurella 303
+ W HAR F 30 o AQUARIUM
25 E E
.•...+ 20
UJ e>
5
o o 5 10 15 20 25 30 LENGTH AT AGE t (mm) FIGURE 2. Growth data for individual F. harhadensis.
1 cc graduated pipette until the meniscus was level with the edge of the shell. The mean value of three readings was obtained. The size of the extra-visceral space was calculated on the assumption that the specific gravity of the soft parts of all animals was the same and equal to 1.00 (Segal, 1956a). The volume of the space was then obtained by subtracting the volume of soft parts from the volume under the shell. The shell weight, volume under the shell, and extra-visceral (E- V) space were plotted against the wet weight of soft parts (W). Straight lines were fitted to the data by the method of least squares. T- tests were used to determine whether the differences between the slopes of lines and between mean points were statistically significant at the 0.05 level. 304 Bulletin of Marine Science [17(2)
\001'-000\- -...-....-.....-.....-.. OOON"","\O N<'1<'1<'1<'1 --- '-' "--' '-' '-'"
...-..,-...-..-.....-.. 1'-0\_<'11/) NN<'1<'1<'1 --- '-" '-' '-' "--"
.-....-...-...... -.....-.. \OooON""," --NNN '-' '-' '-' '-" '-" 1-011-'0-0--0:.....• )oooooi ...-...-...--- 1/)1'-0\-<'1 'l""""l...... -l...... -4NN '-'" '-" '-' '-' '-"
,.....,...... -....-....-...0 N "","\000------"-' '-" '-' '-" ~I--4HHH
,..... "'-""-"'''-'''''-'- <'11/)1/)0\- '-" '-" '-" '-" '-" ~ ~-,q_.[ ~ ~ ~ ,...... -: .-: 1967] Ward: Distribution and Growth of Fissurella 305
E 30 E
:r l- I.!) 20 Z W ...J
...J ...J 10 W :r III
6 12 18 24 AGE t'(monthsl
FIGURE 3. The relation between the shell length and the age of F. barbadensis.
RESULTS Horizontal and Vertical Distribution.-F. barbadensis is most abundant along the leeward, west coast especially on the horizontal platform of beach rock at Six Men's Bay (Fig. 1). Relatively few specimens are found on the east and southeast coasts which receive heavier seas. In these more exposed habitats other keyhole limpets, such as F. nodosa and Diodora spp., are more abundant than in sheltered regions. At several stations on the west and southwest coasts F. barbadensis has a vertical range from below the surf zone to the green zone (Table 1). Some specimens are therefore continually submerged. Others in the upper
I-+-l o 40 % 36 E .§. 30 :r 6 24- z w ...J 18
...J ...J w 12 :r III 6 J J A SON D J F M A M J FIGURE 4. The length distribution of populations of F. barbadensis from monthly transects, June 1964 to June 1965. 306 Bulletin of Marine Science [17(2)
+ HIGH LEVEL o LOW LEVEL 1.6 20 11 o + _ 17 3 -0 _ o 2 ~ L 1.5 ---- __ ,--.6+ 2 8 o 5H 5 2 1 o __.1 + + + + B 7 ~7 B ---- _ + 12 11 1 + 1.4 11 ------+ 1 1 + 1.3 o 10 20 30 40 50 60 70 2 -2 3 o Hxl0 (mm)
FIGURE 5. The relation belween the basal shape, LIB, and the volume, D2H, of high and low level shells from the sheltered region. Solid line-high level; broken line-low level. Numerals indicate numbers of specimens. pink and green zones may be exposed on flat rock surfaces for an average of lO hours out of every 24 hours without receiving any wave action, as at Six Men's Bay, or they may receive continuous wetting at stations where the substrate never dries out, as at the Deep Water Harbour. At anyone station, the density of F. barbadensis is greatest in the upper surf and pink zones. The relatively high density of limpets at St. Lawrence Bay on the south- west coast is probably due to a rocky bank offshore which protects the bay, particularly at low tide, from the heavy surf. In one region at this station the number of large specimens, 25 to 35 mm long, is greater than anywhere else on the island. On the east and southeast coasts F. barbadensis is limited to the pink zone. This zone is continually subject to wave action and the species finds protection in rock pools and crevices, on the sheltered side of boulders and in the algal covering. Populations of some temperate limpets migrate up and down the shore or scatter and regroup at certain seasons of the year. These movements have been related to temperature changes and winter storms (Abe, 1931; Lewis, 1954). Seasonal movements have not been observed in F. bar- badensis which maintains a scattered distribution on the rocks. Most spec- imens are exposed directly to the sunlight and give no indication of seek- ing the shade. A few inhabit crevices in the rocks or areas covered with ]967] Ward: Distribution and Growth of Fissurella 307
+ HIGH LEVEL o LOW LEVEL 1 2 + + .48 5 +
!:!. .44 ---- D 11 .••.... -- + --0-- ---2 7 _- -- , +1 .40 o -3-- 02 01 20 _tf"" 11 --- 11 + ---- 012 8 + 1 017 .36 o 10 20 30 40 50 60 70 D2H x 10 -2 (mm3) FIGURE 6. The relation between the relative height, HID, and the volume, D~H, of high and low level shens from the sheltered region. Solid line-high level; broken line-low level. Numerals indicate numbers of specimens. large algae such as Chaetomorpha, Hypnaea, CLadophora, Enteromorpha, Ceramium, VLva, Bryopsis, Dictyopteris, and Dictyota. Absolute Growth.-The results of the growth of limpets from the wharf and aquaria are shown in Figures 2 and 3. From Figure 2: m = 0.882; i = 4; Leo = 33.9; K = 0.126. The approximate relationship between shell length and age is plotted in Figure 3. An 18-mm limpet is about 6 months old and one of 26.5 mm is about 1 year old. At 30 mm it is about 18 months and few limpets exceed this length in Barbados. No specimen over 35 mm was found at any time during the study. The length distribution of samples from the monthly transects is shown in Figure 4. The presence of two or more modes in many of the frequency distribution curves 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 (Ward, 1966), never- theless monthly increases in modal values are apparent. Two main groups of limpets appear to have entered the population during the 13-month period, one attaining a length of 20 mm in August and the other the same length in January. Lines fitted approximately to the modes are super- imposed on the frequency distribution curves in Figure 4 to show the growth rates of two age classes within the population. The growth rates obtained agree closely with the rates derived from the laboratory and wharf specimens. 308 Bulletin of Marine Science [17(2) , 4-; L-o :.-." 0....• 0~.D >,'_ c 0 0 ....• ....• 0 ....• 0 •..•"Oll) \r) \r) \r) .- ll) 0 0- 0 0 1\ 0 "=li:.Dol •..•~ 0 0 0 0 0 0 0.. 0 0 olUll) 1\ 1\ V V V V V 1\ .D<.e.D 1\ 0·- ll) 0.. 0.. 0.. 0.. 0.. 0.. •..•CU 0 0.. 0.. p....~~ \r) 0
NN N <'0<'0 ' V"> •...o ""- 0 N 0 'O~.o 0 0 ;>.._ c 0 0- 0 0 - 0 •..•'O~ :,:::+-,(1) C!- A A A A V"> 0- -0 -0 0 0 0 0 0 .-.0"'c •..•~ p., p., p., p., "'o~ V V V V .0<1::.0 A A A A A 0·- ~ p., p., p., p., p., •...co 0 0 V"> 0 ~ bOr:: V"> 0 V"> "Vi Q) ~:I: 0 0 0 0 3: 0 0:: 0 '00- '00•••.• ..... >Ll _ V"> - 0\- t-' Relative Growth.-Structural variations in the shell of F. barbadensis with growth and distribution are expressed in terms of rates. The results of the comparisons between specimens from exposed and sheltered environments and from high and low shore levels are shown in Table 2. The equations of the lines and the statistics are given in Table 3. From Table 2 it may be seen that in the sheltered habitat the LIB ratio decreases with increas- ing calculated volume (D2H) of the shells at a greater rate at low than at high shore levels (Fig. 5). In the exposed region the rate of increase in the HID ratio with increasing calculated volume is greater at high than at low shore levels and exposed shells increase in thickness with increasing shell length at a greater rate than sheltered shells. The rate of increase in shell weight, volume under the shell, and extra-visceral space with increas- ing wet weight of soft parts is greater in limpets from the exposed habitat than in those from the sheltered region and is also greater in high level shells from the sheltered area than in those from low levels (Figs. 7-9). From the samples collected, general comparisons can be made between TABLE 5 STATISTICS ON ABSOLUTE DIFFERENCES IN CALCULATED VOLUMES OF SAMPLES OF F. barbadensis Probability of Intertidal Mean volume significant difference Region level N (mm') SD' between means HIGH 31 2585.45 3734817 Sheltered P 0.01 LOW 39 928.94 625275 < Exposed HIGH 34 1318.69 511349 0.50> P > 0.10 LOW 46 1158.15 641203 N = number of specimens in sample. SD = standard deviation. 1967] Ward: Distribution and Growth of Fissurella 311 TABLE 6 COMPARISON OF SHELL STRUCTURE IN DIFFERENT SPECIES Dependent Independent variable variable Habitat Species Rate of decrease in: Exposed Sheltered LIB I II P. vulgata I I F. barbadensis High Low I I P. vulgata, F. barbadensis (exposed) LIB Ratio remains P. conulus, A. dorsuosa constant (high and low) Rate of increase in: Exposed Sheltered HID I II P. vulgata I I F. barbadensis High Low II I A. dorsuosa, P. vulgata F. barbadensis (exposed) Rate of increase in: High Low Shell weight W II I A. Umatula, F. barbadensis (sheltered) Volume under shell W II I A. Umatula, F. barbadensis (below 1.0 cc) (sheltered) E-V space W II I A. Umatula, F. barhadensis (sheltered) II = greater order of magnitude. I = lesser order of magnitude. the absolute values of the structure of shells from different environments. The results are shown in Table 4. Figures 5 to 10 illustrate the results for the sheltered region. Table 4 shows that the LIB ratio does not alter with shore level but is generally greater in the sheltered than in the exposed habitat. The HID ratio however is greatest in high level exposed shells, less in low level exposed and high level sheltered shells, and lowest in low level shells from the sheltered region. Exposed shells show no difference in thickness, weight, volume, and extra-visceral space between the two shore levels. Shell thickness of low level sheltered shells is similar to those in the exposed region. Shell weight is less at both shore levels in the sheltered habitat than in the exposed area. High level sheltered shells and exposed shells have similar values for volume under the shell and extra-visceral space. The calculated volume, D2H, of high level shells in the sample collected from the sheltered environment is significantly greater than that of low level shells from the same region. In the exposed habitat no difference was found between the volume of the shells at the two levels (Table 5). 312 Bulletin of Marine Science [17(2) 7 8 2.2 2.0 2.0 u ~ 1.8 -J E -J 01 w 1.6 J: 1.6 l- VI J: 1.4 C) 0:: W w 0 1.2 z 1.2 ~ ::> -J 1.0 -J w W ~ J: 0.8 ::> 0.8 VI -J 0 0.6 > 0.4 Q4 0.2 0.4 0.8 1.2 OJ. 0.8 1.2 WE T WEIGHT OF 50 F T PARTS { gm} FIGURES 7 AND 8. 7, The relation between the shell weight and the wet weight of soft parts of high and low level shells from the sheltered region. Solid line-high level; broken line-low level. Numerals indicate numbers of specimens. 8, The relation between the volume under the shell and the wet weight of soft parts of high and low level shells from the sheltered region. Solid line-high level; broken line-low level. Numerals indicate numbers of specimens. DISCUSSION F. barbadensis is most abundant in relatively sheltered regions where the largest specimens are found. The vertical range is also greater in more sheltered areas. It is apparent from the large numbers of specimens living just below mean high water level that this species can exist under condi- tions of considerable desiccation and is able to withstand longer exposure to the air than British keyhole limpets whose distribution is limited to low water level and below (Fretter & Graham, 1962). However, the mean tidal range in many localities in Britain is 12 to 20 feet whereas in Bar- bados it is only 2.4 feet (Brown's Nautical Almanac, 1965). The dif- ference in tidal height indicates that in areas of similar wave amplitude an animal living, for example, at mean tide level in Britain will not receive as much splash during exposure to the air as one living at a similar tidal 1967] Ward: Distribution and Growth of Fissurella 313 9 10 + 1 1.2 + 0.7 1 u ~ 0.6 E 1.0 UJ u E « a.. 0.5 U) Lf) Lf) 9 8 ..J UJ 0.8 0 « z 2 / Q: 0.4 ::.: 0 /0 2 UJ u /6 + u / Lf) :I: I- ;; 0.3 0.6 5/ I ..J /J o 0 ..J « UJ / Q: 22...... - /7 I- 0.2 + :I: x /" Lf) 1 UJ 4i!f'" o 0 3 2 0.4 9 ~{9 0.1 4 yo 2 + 5 4 3 1 0.2 0 0.4 0.8 1.2 10 15 20 25 30 WET WT OF SOFT PARTS (gm) SHELL LENGTH (mm) FIGURES 9 AND 10. 9, The relation between the extra-visceral space and the wet weight of soft parts of high and low level shells from the sheltered region. Solid line-high level; broken line-low level. Numerals indicate numbers of specimens. 10, The relation between the shell thickness and the shell length of high and low level shells from the sheltered region. Solid line-high level; broken line-low level. Numerals indicate numbers of specimens. level in Barbados. Unlike F. barbadensis, F. nubecula in Ghana is re- stricted to parts of the shore that are continually wet (Gauld & Buchanan, 1959). The growth curve indicates that F. barbadensis reaches a length of about 26.5 mm in 1 year. This is slightly more than the length attained by Patella vulgata in Britain during the first year. The latter limpet grows consider- ably faster in the second year than F. barbadensis and has a life span of 10 to 20 years (Russell, 1909; Orton, 1928; Comfort, 1957). Farfante (1943) records that F. barbadensis grows to 40 mm in length and an average sized specimen in the Bahamas is 32 mm long. In Barbados very few specimens of this species reach over 30 mm and not one was measured over 35 mm long. These results may indicate that northern populations attain a greater size than southern populations of the same 314 Bulletin of Marine Science [17(2) TABLE 7 COMPARISON OF SHELL STRUCTURE IN DIFFERENT SPECIES Dependent Independent variable variable Habitat Species LIB Exposed Sheltered II 1 P. vulgala 1 11 F. barbadensis High Low II I A. dorsuosa I I P. vulgala, F. barbadensis HID Exposed Sheltered II I F. barbadensis, P. vulgala (below 30,000 mm:l, D2H) High Low II I A. dorsuosa, P. vulgala, F. barbadensis Shell thickness L Exposed Sheltered II I P. vulgata 1 1 F. barbadensis High Low II 1 A. limalula, A. dorsuosa ] 1 F. barbadensis (exposed) I II F. barbadensis (sheltered) Shell weight W High Low II I A. Umatula I I F. barbadensi.l' Volume under shell W n I A. limalula, F. harbadensis (sheltered) Volume under shell Shell I II A.limatula weight II 1 F. harbadensis (sheltered) E-V space W II I A.limatula, F. barbadensis (sheltered) ------II = greater order of magnitude. I = lesser order of magnitude. 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 results have been obtained for Mya arenaria and other mollusks (Newcombe & Kessler, 1936; Thorson, 1936). The significant results presented in this paper between the slopes of the regression lines include the rates of change of ratios with the calculated volume of the shells in different environments. These results have been compared with those obtained by other workers for various Patellacea by 1967] Ward: Distribution and Growth of Fissurella 315 recalculating their data in terms of ratios and calculated volumes. The shell shape of these species is convergent with that of F. barbadensis al- though they are not closely related. Abe's data (1932) have been used for Acmaea dorsuosa, Hamai's data (1937) for Patelloida conulus and Russell's data (1907) for Patella vulgata. The comparison of results is shown in Table 6. The results of Segal (1956a) using A. limatula have been compared with those obtained for F. barbadensis in regard to the rate of change of shell weight, volume under the shell, and extra-visceral space in Table 6. Patella vulgata usually grows to a larger size in more sheltered regions (Russell, 1907) like F. barbadensis. Generally, larger sizes of P. vulgata are more numerous at high shore levels (Das & Seshappa, 1947) and this is also true of F. barbadensis in sheltered areas. However, low level Acmaea spp. are larger than those from high levels (Abe, 1932; Shotwell, 1950; Segal et al., 1953). The results obtained for the absolute differences in the LIB and HID ratios of F. barbadensis from different environments may be compared with those for other species using the data of other workers recalculated in terms of these ratios and the calculated volumes. Abe's data (1932) have been used for A. dorsuosa and Russell's data (1907) for P. vIIlgata. The comparison is shown in Table 7. Orton (1928) reports that wave action plays only a minor role in controlling the shell height of P. vulgata. He suggests that the main factors governing the height are those causing desiccation of the animal. Drying conditions also appear to affect the shell height of F. barbadensis. Shell thickness, weight, volume, and extra-visceral space of F. bar- badensis are compared with the results obtained for A. limatula (Segal, 1956a), A. dorsuosa (Abe, 1932), and P. vulgata (Russell, 1907) in Table 7. High level F. barbadensis shells of equal weight from the shel- tered habitat enclose a larger volume than low level shells. In A. limatula the reverse trend occurs but the smaller absolute size of the soft parts per given shell weight in this species from high levels more than compensates for the smaller volume of fluid held by these shells (Segal, 1956a). Both F. barbadensis from the sheltered region and A. limatula, therefore, show a larger extra-visceral space in high level shells. Segal & Dehnel (1962) have shown that during exposure to the air the extra-visceral water in A. limatula seems to serve an osmoregulatory function and is also a tempera- ture buffer. The ability to regulate the body temperature by evaporative cooling in F. barbadensis has been suggested by Lewis (1963). Structural variations in the shell of F. barbadensis with growth and dis- tribution are therefore generally similar to those of Patella and Acmaea al- though they seem to be less pronounced in F. barbadensis. This is prob- ably due to the smaller mean tidal range in Barbados than in the regions 316 Bulletin of Marine Science [17(2) inhabited by the other species and possibly also to the relatively small dif- ference between the exposed and sheltered habitats selected in Barbados. SUMARIO DISTRIBUCI6N Y CRECIMIENTO DE LA LEPA Fissurella barbadensis GMELIN La distribuci6n de Fissure/la barbadensis alrededor de las costas rocosas de Barbados esta intluenciada por la acci6n de las olas. Un ambiente relativamente protegido es mas favorable que uno con exposici6n con- siderable a las olas. Las proporciones de crecimiento indican que una lepa de 18 mm tiene unos 6 meses de edad y que un ejemplar de 26.5 tiene alrededor de 1 ano. En un habitat protegido las conchas en el nivel bajo de la costa se vuelven relativamente mas anchas en una proporci6n mayor, con el aumento de su volumen, que las conchas en el nivel alto de la costa. En una regi6n expuesta las conchas en el nivel alto se vuelven relativamente mas altas con el aumento de volumen, en una proporci6n mayor que las conchas del nivel bajo. Las conchas expuestas aumentan en grosor en una proporci6n mayor, con el aumento de la longitud de la concha, que las conchas protegidas. La proporci6n en el aumento del peso de la concha, volumen bajo la concha y espacio extra-visceral con el aumento del peso hUmedo de las partes blandas es mayor en lepas procedentes de un habitat expuesto que en aquellas procedentes de regiones protegidas y es tambien mayor en conchas protegidas procedentes de los niveles altos de la costa que en aquellas procedentes de los niveles bajos. Las conchas protegidas procedentes de los niveles altos son significativamente mayores que las conchas procedentes de los niveles bajos de la misma regi6n. Los resultados son comparados con la distribuci6n, crecimiento y variaciones en la concha de lepas de dimas templados. REFERENCES ABE, N. 1931. Ecological observations on Acmaea dorsuosa Gould. Sci. Rep. T6huku imp. Univ. Fourth Ser., 6: 403-427. 1932. The age and growth of the limpet (Acmaea dorsl/osa Gould). Sci. Rep. T6huku imp. Univ. Fourth Ser., 7: 347-363. BERTALANFFY, L. VON 1938. A quantitative theory of organic growth. Hum. BioI., 10: 181-213. BROWN'S NAUTICAL ALMANAC 1965. Brown's nautical almanac. Brown, Son and Ferguson, Ltd., Glasgow, 942 pp. COMFORT, A. 1957. The duration of life in molluscs. Proc. malac. Soc. Lond., 32: 219- 241. DAS, S. M. AND G. SESHAPPA 1947. A contribution to the biology of Patella: on population distribution and sex proportions in Patella vulgata Linnaeus at Cullercoats, Eng- land. Proc. zool. 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