Moss Landing Marine P. 0. !Jox 223 . Moss Calif. 95039
THE SPAWNING CYCLE AND JUVENILE GROWTH RATE OF THE GAPER CLAM, TRESUS NUTALLI, OF ELKHORN SLOUGH, CALIFORNIA
A thesis submitted to the faculty of San Francisco State College in partial fulfillment of the requirements for the degree Master of Arts
by
LAURENCE L. LAURENT
San Francisco, California
June, 1971 ACKNOWLEDGMENTS
I am deeply indebted to several people for their help in bringing this paper into existence. To Dr. James
Nybakken of the Moss Landing Marine Laboratories a very special thanks is owed for his guidance, advice, help and concern given throughout the duration of this study.
To Dr. Robert Beeman of San Francisco State College go my regard and gratitude for several reasons: for sparking the initial excitement of Marine Biology in undergraduate courses, for his ready accessibility to questions, complaints and the general trials and tribu lations of student life, and, not least of all, for his friendship. To Pat Clark, a fellow graduate student who joined me late in the study, go my thanks for taking on the juvenile sampling and for his much needed help with the statistical end of things; I would also like to express my gratitude to the California Department of Fish and Game, Marine Resources, for their financial support of this study through grant number S-1556.
To my wife Sandra I owe much that can't be expressed. On top of all the support, love, encourage- ment and understanding she has given over the five years of our marriage, she is now preparing to give me the
iii greatest gift of all this July, 1971--a child. It is to the two of them that this paper is dedicated. May the child have the opportunity to be as fortunate as I feel.
iv TABLE OF CONTENTS Page
ACKNOWLEDGMENTS. iii
LIST OF FIGURES. 0 • • vi
LIST OF TABLES ix
INTRODUCTION . . . . . 1
METHODS AND MATERIALS ... 7
Spawning Cycle Study • 7
Juvenile Growth Study •. 8
RESULTS OF THE SPAWNING CYCLE STUDY. 13 Description of Gonadal Condition •. 14
Adult Size and Sex Ratio • 34
DISCUSSION OF SPAWNING STUDY RESULTS . 36
RESULTS OF JUVENILE GROWTH RATE STUDY .. 39 DISCUSSION OF THE JUVENILE GROWTH RATE STUDY . 50
SUMMARY •. 52
LITERATURE CITED 54
v LIST OF FIGURES
Figure Page
1. Map of Study Area. . . . 3
2. Gonad of Female from 4 February, 1970. 16
3. Gonad of Female from 4 February, 1970. 16
4. Gonad of Male from 4 February, 1970. 17
5. Gonad of Female from 17 February, 1970 . 17
6. Gonad of Female from 17 February, 1970 18
7. Gonad of Female from 5 March, 1970 18
8. Gonad of Male from 5 March, 1970 . 19 9. Gonad of Female from 7 April, 1970 . 19 10. Gonad of Male from 7 April, 1970 . 20
11. Gonad of Female from 27 April, 1970. 20
12. Gonad of Male from 27 April, 1970. 21 13. Gonad of Female from 6 May, 1970 . 21 14. Gonad of Male from 6 May, 1970 . . 22 15. Gonad of Female from 6 May, 1970 . 22 16. Gonad of Male ( ? ) from 6 May, 1970 . 23
17. Gonad of Female from 24 May, 1970. 23
18. Gonad of Male from 24 May, 1970. 24 19. Gonad of Female from 19 June, 1970 . . 24 20. Gonad of Male from 19 June, 1970 . 25 21. Gonad of Clam, Sex Unknown, from 21 July, 1970. 25 vi Figure Page
22. Gonad of Clam, Sex Unknown, from 21 July, 1970. 26
23. Gonad of Female from 18 August, 1970 . 26
24. Gonad of Female from 18 August, 1970 . 27
25. Gonad of Female from 15 October, 1970. 27
26. Gonad of Male from 15 October, 1970 28
27. Gonad of Female from 13 November, 1970 . 28
28. Gonad of Male from 13 November, 1970 . 29
29. Gonad of Female from 12 December, 1970 . 29
30. Gonad of Male from 12 December, 1970 . 30
31. Gonad of Female from 9 January, 1971 . 30
32. Gonad of Female from 9 January, 1971 • 31
33. Gonad of Male from 9 January, 1971 • 31
34. Gonad of Female from 26 January, 1971. 32
35. Gonad of Male from 26 January, 1971. 32
36. Correlation of the generalized repro ductive condition with the relative abundance of juveniles in the 4.0 mm shell length size range as found in the sampling throughout the year • 38
37. Frequency distribution of juveniles sampled on 24 May, 1970. 41
38. Frequency distribution of juveniles sampled on 5 June, 1970 41
39. Frequency distribution of juveniles sampled on 17 June, 1970 • 42
40. Frequency distribution of juveniles sampled on 1 July, 1970. 42
41. Frequency distribution of juveniles sampled on 21 July, 1970 . 43
vii Figure Page
42. Frequency distribution of juveniles
sampled on 18 August 9 1970. 4J
4J. Frequency distribution of juveniles sampled on 12 December, 1970. 44
44. Frequency distribution of juveniles sampled on 9 January, 1971. 44 45. Frequency distribution of juveniles sampled on 27 January, 1971 . 45 46. Frequency distribution of juveniles sampled on 8 February, 1971 . 45 47. Frequency distribution of juveniles sampled on 23 February, 1971. 46
viii LIST OF TABLES
Table Page
l. Summary of Individual Shell Length Changes and Average Individual and Class Growth Rates of Juvenile Tresus nuttalli in Controlled Conditions. • 47
2. Summary of the Average Changes in Shell Length of Three Groups of Juveniles in Controlled Conditions. . 48
ix INTRODUCTION
This paper reports on the spawning cycle of the
clam Tresus nuttalli (=Schizothaerus nuttalli Conrad,
1837) and the growth rates of their juveniles. A member
of the family Mactridae, T. nuttalli is known by many
common names, among them "gaper clam," "horseneck clam,"
and "bigneck clam." The entire study was performed on
clams of Elkhorn Slough (Fig. 1), 36°48'35", 121°47'05",
at the California State Colleges' Moss Landing Marine
Laboratory at Moss Landing, California from January,
1970 to February, 1971.
Tresus nuttalli and Tresus capax (Gould, 1950),
a northern species, are wide-ranging, economically
important Pacific coast game clams, but little is known
of their life histories. Some of the previous studies
of the two species concern the change in shell morphology
with growth (Pohlo, 1964), the autecology ofT. capax and
T. nuttalli in Humboldt Bay (Stout, 1967), maximum burrowing depths of T. nuttalli (Armstrong, 1965) and
the distribution of T. capax and T. nuttalli in the
Washington state area (Pearse, 1965). The only spawning
cycle study to date was conducted on T. capax in Humboldt
Bay (Machell, 1968).
1 2
Tresus have been reported from Baja California to Southern Alaska; T. capax is not found south of
Humboldt Bay, California and T. nuttalli is not found north of northern Washington waters (Swan and Finucane,
1952). Both species occur subtidally to depths ap- proaching 100 feet and intertidally to about the +1.0 foot level in calm bodies of marine water such as embay- ments, sloughs and estuaries (Fitch, 1953). T. nuttalli is the largest recent American clam; shells with lengths as great as 250 mm have been reported (Nicol, 1964). As juveniles, they are rather active burrowers as they live in the more turbulent upper layers of substrate (Pohlo,
1964), but as adults they inhabit a permanent burrow which may exceed three feet in depth (stout, 1967). As with most bivalves, Tresus are filter feeders, feeding from suspended detrital and algal particles (MacGinitie,
1935) in the lower layers of water through a greatly extensible siphon.
Living at considerable depths, the adult Tresus have few natural enemies. Perhaps the most important predators, besides man, are bottom feeding elasmobranchs.
Skates and rays have been reported to feed on Tresus
(stout, 1967) and stomachs ofLeopard Sharks, Triakis semifasciatus Girard, 1854, have yielded long partially digested siphons that appear to belong to Tresus (per- sonal observation). Sea otters, Enhydra lutris nereis 3
){ I
MONTEREY BAY
shows location of Elkhorn Slough in c of Monterey Bay. Area within the square is enlarged in Figure lB to show study area. 4
(Merriam, 1923), have been observed eating what appeared to be Tresus in Monterey Bay, California, by Fish and
Game biologists (Paul Wild, personal communication), but these sightings remain unconfirmed. As a juvenile,
Tresus is as susceptible to predation as other bivalve members of the upper substrata, especially to the Moon
Snail, Polinices lewisi (Gould, 1847) (Stout, 1967 and personal observation).
The genus also serves as host to other inverte brates in various commensal and parasitic relationships.
MacGinitie (1953) found that pea crabs (Pinnixa ~.) live commensally in the mantle cavity of Tresus nuttalli;
Pearse (1965) reported that the crab lives under the visceral skirt (an epithelial extension of the inner palp) of T. capax and feeds on mucus-bound food strings produced by the clam. Stout (1967) reported that in
Humboldt Bay, where T. nuttalli and T. capax are sym patric, the pea crab is found only in the mantle cavity of T. capax and the rate of occurrence approaches 100 percent. Further south, where T. capax does not occur, the pea crab will reside within T. nuttalli. Of the clams collected from Elkhorn Slough for this study, about
13 percent of them (20/151) were inhabited by the pea crab. MacGinitie (1935) also reported that T. nuttalli in Elkhorn Slough were widely infested with the cysts of a tapeworm, Echeneibothrium maculatum (van Beneden), 5 which were found in the visceral mass, pedal muscles, mantle and ctenidia of the clam. A nemertean worm,
Malacobdella grossa (Muller, 1776), mentioned by Ricketts,
Calvin, and Hedgpeth (1968) to occur commensally in the mantle cavities of T. nuttalli and many other pelecypods, was found in but one adult Tresus during thie entire study.
More recently, another relationship was dis- covered by Stout (1970). While studying Tresus nuttalli in Humboldt Bay, he found 50 species of invertebrates, representing ten phyla and eight species of algae, growing onthe clams' siphonal plates. Although few in vertebrates and algae were identified, a similar community was independently found to exist on the si phonal plates of T. nuttalli in Elkhorn Slough. This siphonal plate community may represent one of the most dense and diverse aggregation of animals in the slough environment.
In his 1935 study, MacGinitie warned of the heavy clamming pressures on the several species of game clams in Elkhorn Slough which might lead to their possible local extinction. For some bivalves once counted as common or abundant (Protothaca staminea, Saxidomus nuttalli and Clinocardium nuttalli) at the time of his study, MacGinitie's warning seems to have been partially correct; these species appear to be much reduced now in
Elkhorn Slough (personal observations). However, Tresus 6 nuttalli has been as much subjected to heavy clamming pressures as these other species yet it has been able to maintain a dense population in the slough. This study provides a probable answer to this seeming paradox. METHODS AND ~~TERIALS
Spawning Cycle Study
Beginning in late January, 1970, and proceeding
through February, 1971, a minimum of six and a maximum
of ten adults were collected during each series of day
light tides lower than -0.7 feet from a mudflat adjacent
to the central harbor area of Elkhorn Slough (Figure lB).
Collecting was done by shovel. The clams were brought
to the laboratory where they were weighed on a spring
type gram scale and their shell lengths (greatest poste rior-anterior dimension) and widths (greatest lateral measurement) taken by vernier calipers. These measure- ments were taken to: 1) determine if a sexual size dimorphism exists and, 2) to attempt to determine the minimum size at sexual maturity. After measurement, the soft parts were removed from the shells and fixed in Bouin's seawater solution.
To determine the spawning condition of the adults, gonadal blocks were dissected from the dorsa-posterior area of the fixed body, dehydrated and infiltrated by dioxane-paraffin method (Galigher, 1964), embedded in paraffin, sectioned, stained and then examined micro- scopically for evidence of sexual condition. This method
7 8 is a common one, having been used by other workers to determine the spawning cycle of other clams (Pfitzenmeyer,
1965 and Calabrese, 1970). Sectioning was done at seven microns on a Spencer A-0 820 rotary microtome. Staining of the thin sections was by standard hematoxylin and eosin procedures (Humason, 1969). Photomicrographs of the gonad sections were taken with a Nikon SUR-KE micro scope fitted with an EFM semi-automatic camera attachment using Kodak Plus-X 35 mm black and white film.
Juvenile Growth Study
Although there have been estimates made about the growth rates of Tresus (MacGinitie, 1935; Marriage,
1954), no definitive growth studies have been made. Un like some other bivalves, the growth rates ofT. nuttalli cannot be determined from growth rings on its shell since it is essentially a smooth-shelled clam. In such a situation, there are two possible methods of determining growth; by plotting the size frequency distributions of a population in successive samples through time or by repetitive measurements of a population either in laboratory conditions or preferably in a controlled area in the original environment. When this study was initi- ated, the latter possiblity was rejected because the facilities for maintaining a population of clams in the laboratory were not available and because there appeared to be no way to maintain a control population of clams 9 in the environment safe from public disruptions. Thus,
the method of population sampling was chosen.
Wilbur and Owen (1964) offer advice to those attempting to determine growth rates from population sampling by size frequency distributions. They warn that the method is adequate only under those conditions of seasonal growth that create well-defined size classes,
that possible errors may arise from poor representation of a year or size class, that difficulty of estimation of the age of the youngest year or size class may occur in the absense of other information and that continuous spawning over several months would give increasing size ranges of juveniles. Although this advice is certainly well-taken, there is no way of predetermining whether any of these conditions occur when, as in this situation, the information does not exist. While recognizing the limitations, population sampling of the juveniles, with the objective of following size classes in size frequency graphs, was begun in early May, 1970 and continued until
February, 1971.
Population samples of the juveniles were taken from the same mudflat and under the same conditions as the adult samples. The sample areas for juveniles ranged from quarter meter square to meter square areas and were excavated to a depth of 10 em. The sample depth of 10 em was chosen because the juveniles of interest 10
(from 4.0 mm to about 20.0 mm in shell length) were not
found at greater depths during this study. The substrate within the sample area was then passed through sieves in
the field to collect small clams for later separation,
identification, counting and measurement in the labora
tory. At first, the samples were passed through standard
geologic sieves with a final mesh opening of 2.0 mm.
This mesh size proved to be the smallest practical open
ing since anything smaller collected extremely large
amounts of detritus. Later, a larger six inch deep
sieve box with a 2.0 mm mesh was substituted for the
geologic sieves to obviate possible loss of the sample by 'washing overboard' and to facilitate the sifting of
larger volumes of substrate at one time. After the
juvenile Tresus had been isolated from the samples in
the laboratory, their shell lengths were taken by vernier
caliper to the nearest 0.1 mm.
As sampling proceeded, it became clear that the frequency distributions of the size classes were not as distinct as was hoped. In an attempt to remedy this,
a method devised by Harding (1949) to analyze polymodal frequency distributions was attempted. This method involved plotting the juvenile shell lengths of each
sample group on probability scale graph paper. On this
type of paper, a percentage scale on the bottom axis
runs from 0.01 percent on the left to 99.99 percent on 11 the right. At the top axis, the scale is repeated in the opposite direction. The percentage scale is crowded in the middle of the graph and more widely spaced at either side. The result is that any normally distributed population plotted gives a straight line, but frequency distributions composed of several component age groups give curved lines. These curved lines may then be resolved arithmetically into straight line components which represent the age groups in the population. The point on the graph where a straight line component inter sects the 50 percent probability level is the mean; the standard deviation is indicated by the points where the line crosses the 15.87 percent and the 84.13 percent probability levels. Williams (1964) has shown that this method works well in a growth study of polymodal distri butions of an English periwinkle, Littorina littorea
(L • ) . Cassie (1952) made some improvements in Harding's analysis.
Late in the present study, a method to maintain a control population of clams in the environment was devised. The method consisted of placing measured juveniles in sand in polyethylene buckets and mounting the buckets by brackets to a submerged wooden post im- planted in the slough floor. The point where the buckets were mounted is never exposed by tides and therefore protected from most forms of unwanted human interference. 12
The protection from natural predators also seemed good
since the buckets were suspended two to three feet from
the slough floor, thus being well removed from such animals as Polinices. To test th1s method, five buckets
containing a total of 62 juveniles ranging from 4.2 mm
to 50.9 mm were implaced with the use of SCUBA gear on
the 5th and 11th of February, 1971. The buckets were recovered on the 24th and 26th of March, 1971. RESULTS OF THE SPAWNING CYCLE STUDY
Gonadal sections performed on 125 clams in a
twelve month period beginning February, 1970 and ending
January, 1971, have yielded evidence of the state of
sexual maturity of Tresus nuttalli throughout the study period, Ropes and Stickney (1965), in their study of the spawning cycle of Mya arenaria, divided the stages of gonadal condition into five artificial phases: (1) inactive, (2) active, (3) ripe, (4) partially spawned and (5) spent. Because this scheme is useful and be cause standardization is desirable for related studies, these phases will be used to describe the gonadal con dition of T. nuttalli. Although either the male or female gonads can be used to define sexual maturity, the female gonad, because of the much greater size of its product, is much easier to interpret for reproductive readiness.
The inactive phase in Tresus nuttalli is charac terized by ovaries and testes so collapsed and reduced as to make it impossible to distinguish sexes and, at times, even difficult to locate the gonads among the digestive gland and connective tissues (Figure 22). The active phase is distinguished by the appearance of
13 gametogenic activity on the periphery of the follicle walls and sexes now become distinct. In the early stages
of the female active phase (Figure 24), the follicles
are composed of developing oocytes and in the latter
stages of oocytes and mature ova which fill the lumen of
the follicles (Figure 3). In both sexes, as maturation
proceeds, the follicle walls swell until, in the ripe
phase, neighboring follicles displace intervening di
gestive diverticulae and connective tissue until the
follicles come into contact with one another (Figure 9).
The partially spawned phase is typified by partially
evacuated follicles whose walls remain distended, espe
cially in the female, for some time after the gametes have been spawned (Figures 2 and 32). The partially
spawned phase is often difficult to distinguish from the
early active phase if the follicle walls do not remain distended; however, the presence of phagocytes within
the follicle lumen (Figures 3 and 27) indicates that
spawning has occurred (Galtsoff, 1964) and is useful in differentiating the two phases. The spent phase is identified by completely evacuated follicles whose walls have collapsed (Figure 21).
Description of Gonadal Condition
4 February, 1970. The follicles of the females appear to be in mid-active (Figure 2) to partially spawned phases (Figure 3) on this date. The only male 15
collected appears to be in an active phase (Figure 4).
17 February, 1970. The females of this date appear to be in late active (Figures 5 and 6) to partially
spawned phases (not pictured).
5 March, 1970. The females of this period appear uniformly ripe (Figure 7) as do the males (Figure 8).
The exteriors of the follicle walls are in contact with
other follicles and the interior of some female follicles
show fewer gametocytes around the periphery, indicating
the cessation of gametogenesis.
7 April, 1970. The ripe phases of 5 March con
tinue for the females (Figure 9) and males (Figure 10) of this date.
27 April, 1970. Follicular ripeness seems to reach a peak during this period (Figures ll and 12) and female gametogenesis seems to be completed.
6 May, 1970. Ova and sperm still crowd the female and male follicles in some, (Figures lJ and 14), but in others (Figures 15 and 16) spawning has occurred.
24 May, 1970. Both the follicles of the females
(Figure 17) and the males (Figure 18) appear much reduced from the r~pe phase, but gametogenesis is again evident in this early active phase.
19 June, 1970. Males and females of this period once again are in a phase somewhat resembling the active phase of 17 February. In the females (Figure 19), oocytes 00
0
Figure 2. Female from 4 February. This female shows a late active phase of oogenesis. Mature ova (o) crowd the follicle while oocytes (oo) are in various stages of development. The distended follicle walls are beginning to displace the digestive diverticula (DD). (200X)
Figure 3. Female from 4 February. Partially spawned phase. A few mature ova (o) remain within the still distended follicle. O~cytes on the follicle walls are apparently being phagocytized (P). (200X) 17
Figure 4. Male from 4 February. Although difficult to interpret, this male appears to be in a mid-active phase. Spermatocytes (s) are located in the thickened periphery of the follicle and spermatozoa (s) are probably in the lightly-colored area in the center of the follicle. (lOOX)
00
Figure 5. Female from 17 February. Late active phase. Mature ova are beginning to crowd the follicle, but gametogenesis continues as evidenced by the many oocytes (oo). {220X) 18
Figure 6. Female from 17 February. Late active phase. The light colored sphere within the oocytes and ova is the nucelus (N) and the dark-stained body within the nucleus is the amphinucleous (A). (200X)
Figure 7. Female from 5 March. Ripe phase. Follicle walls (F) contact one another. (lOOX) 19
s
Figure 8. Male from 5 March. Ripe phase. The follicle is greatly distended and the spermatozoa (s) occupy an increasingly larger area. (200X)
DD
Figure 9. Female from 7 April. Ripe phase. Only a small amount of digestive diverticula (DD) is evident due to follicle crowding. (lOOX) 20
Figure 10. Male from 7 April. Ripe phase. (lOOX)
Figure 11. Female from 27 April. Ripe phase; gameto genic activity along follicle wall is essentially halted. (lOOX) 21
Figure 12. Male from 27 April. Ripe phase. (lOOX)
Figure 13. Female from 6 May. Ripe phase. (lOOX) 22
Figure 14. Male from 6 May. Ripe phase. (lOOX)
Figure 15. Female from 6 May. Spawning appears to have taken place and the follicles are once again lined with oocytes. ( lOOX) 23
Figure 16. Male (?) from 6 May. Spawning has occurred and digestive diverticulae once again surround the follicles. ( lOOX)
Figure 17. Female from 24 May. Active phase. o;)genesis has begun again as shown by the heavy oocytic activity at the follicular periphery. (200X) Figure 18. Male from 24 May. Active phase. The fol licle appears more dist~nded than those of the previous date. (lOOX)
Figure 19. Female from 19 June. Late active phase. (lOOX) 25
Figure 20. Male from 19 June. Active phase. (lOOX)
Figure 21. Clam from 21 July. Spent phase. The col lapsed follicles are completely empty of gametes and are now in a state of contraction. (lOOX) 26
F
Figure 22. Clam from 21 July. Inactive phase. The follicles (F) in this phase are completely contracted and displaced by digestive diverticula. (lOOX)
Figure 23. Female from 18 August. Mid-active phase. Oogenesis is occurring again. (lOOX) 27
Figure 24. Female from 18 August. Early active phase. Primary o8cytes line the follicle wall. (200X)
Figure 25. Female from 15 October. Late active phase. (lOOX) 28
Figure 26. Male from 16 October. Active phase. (lOOX)
Figure 27. Female from 13 November. Partially spawned phase. The follicle is much reduced and what may be phagocytes (P) can be seen within the lumen. (lOOX) 29
Figure 28. Male from 13 November. Partially spawned phase. The follicles are much reduced from those of the previous month. (lOOX)
Figure 29. Female from 12 December. Late active phase. oHgenesis is again evident and the lumina are full of ova. (lOOX) JO
Figure JO. Male from 12 December. Late active phase. Follicles are again expanding and filling with sperma tozoa. ( lOOX)
Figure Jl. Female from 9 January, 1971. Mid-active phase. ( lOOX) Jl
Figure J2. Female from 9 January. Partially spawned phase. ( 200X)
Figure JJ. Male from 9 January. Partially spawned phase (?). (lOOX 32
Figure 34. Female from 26 January. Late active or early ripe phase. (lOOX)
Figure 35. Male from 26 January. Active phase. (lOOX) 33 of varying stages and mature ova are beginning to distend the follicle walls. The males (Figure 20) appear to be in an active phase.
21 July, 1970. The clams of this period appear spent (Figure 21) and/or inactive (Figure 22). Sexuality is impossible to determine during these phases.
18 August, 1970. The inactive phase of the previous sampling date continues to this date for most of the clams (not pictured). Two clams demonstrated what appears to be an early active phase (Figure 24) while only one proved to be in a mid-active phase (Fig ure 23).
September, 1970. No adults were collected during the month due to a lack of tides low enough to expose the mudflats.
15 October, 1970. All clams from this date again appear to be in the mid-active phase of develop ment (Figures 25 and 26).
13 November, 1970. The follicles of the females of this date, (Figure 27), while not appearing spawned, are much reduced from those of the previous sampling.
The only evidence that spawning may have occurred is the presence of what may be phagocytes in the follicle lumina (Figure 27). The male condition (Figure 28) appears to be in a partially spawned state.
12 December, 1970. The females of this period J4
(Figure 29) again appear to be in an active phase with
o~genesis evident along the walls and with mature ova
distending the follicles. The males (Figure JO) also appear to be in an active phase.
9 January, 1971. At this date, the females
sampled show a range of sexual states from mid-active
(Figure 31) to partially spawned (Figure 32). The males are typified by the partially spawned phase (Figure JJ).
26 January, 1971. The female condition at this
sampling appears to be in the late active phase (Figure
J4) with rounded, mature ova beginning to fill the follicles. The males of this date appear to be in the late active phase also (Figure 35).
Adult Size and Sex Ratio
There appears to be no sexual size dimorphism.
Of the clams sampled, the females were slightly, but not significantly, smaller than the males in length and width; the females had an average shell length of 120.4 mm and an average width of 54.0 mm while the males averaged
124.1 mm in length and 54.8 mm in width.
The sex ratio of Tresus nuttalli, based on 107 individuals (the sex of 18 clams could not be distin- guished), is 50:50 (54 females, 53 males). No hermaph- rodites were found. Although the size at which the onset of sexual maturity occurs was not determined, the smallest distinguishable female had a shell length of 75.0 mm and 35 demonstrated approximately the same degree of follicle development as larger (102.2 mm to 123.5 mm) females of the same period. In the same manner, sections of the two smallest males sampled, with shell lengths of 70.0 mm and 91.0 mm, have shown ripe-appearing testes. DISCUSSION OF SPAWNING STUDY RESULTS
Histologic study of the gonadal sections of
Tresus nuttalli has yielded adequate data on the clam's spawning cycle, but total reliance must not be placed on this alone. This rather subjective information must be correlated with the presence of either larvae in the plankton or settled juveniles. The reason this corre- lation is important is reflected in a study done on the spawning of T. capax in Humboldt Bay (Machell, 1968) on the basis of gonad examination alone. The results of that study pointed to a spawning period beginning in
January and ending in late March. However, a study done the same year on the autecology of T. capax and T. nut talli in Humboldt Bay (stout, 1967) showed that the early juveniles of T. capax (4.0 mm to 10.0 mm in shell length) were present from April to late September. If the growth rates of juvenile T. capax are similar to what the growth rates of juvenile T. nuttalli appear to be (see Juvenile Growth Rate section of this paper), the presence of juvenile T. capax in September indicates that spawning probably occurs well after the end of
March, perhaps until June. This statement is also based on the assumption that larval life of T. capax is com-
36 37
parable to other temperate bivalves so far studied; that
is, about a maximum of thirty days (Loosanoff, 1963).
The presence of juveniles in the 4.0 mm to 4.9 mm
size range throughout the sampling period has been cor
related with a subjective rating of the majority of the
adult females' sexual condition in a month's sampling
as determined by histologic examination (Figure 36). It
should be remembered that probably thirty to sixty days have passed since the juveniles in this size range were
spawned.
Presence of juveniles, nearly constant through
out the year, suggests that spawning is nearly continuous
throughout the year. However, both the gonadal condition and abundance of juveniles from month to month indicates
that Tresus nuttalli has a bimodal spawning period with peaks of spawning intensity from about April to June and from November to about February. 100 (j) Qo ro~ Spent H s 80 s 0 Partially ..::!" Spawned ~ ·rl
60 (I) (j) .-l ·rl Ripe ):! (j) > 40 ::J """)
Ct-; Active 0 I \ (j) I Qo \ 20 ro I \ .j.:l \ p \ I \ (]) Inactiv I () \ I I / 'Ill H 1111...__ II -.... _..... _ I (j) . . . p., ,.0 H H :>-. ~ .-l Qo p.. .j.:l ? () p ,.0 (]) ro p.. ro ::J ::J ::J (j) () 0 (j) ro (j) ~ :::8 Figure 36. Correlation of the generalized reproductive condition (solid line) with the percent abundance of juveniles (dotted line) in the 4.0 mm to the 4.9 mm shell length size range as found in \...J the population sampling. co RESULTS OF JUVENILE GROWTH RATE STUDY From 4 May, 1970 to 13 November, 1970, eleven samples of juvenile populations were taken. Of the eleven samples, five could not be considered in the growth analysis due to either poor sampling technique or low juvenile representation. The six samples used in the growth analysis represent the time from late May to mid-August, a period of 84 days. The number of juveniles in each sample ranged from 23 to 56, averaging 38.5 individuals. Frequency distributions of each sample group are depicted in figure 37 through 42 by a histo gram. The class interval is one mm (4.0 mm class = 4.0- 4.9 mm). Figures 43 through 47 are histograms of the size distributions of juveniles sampled during December, 1970 and January and February, 1971. These latter sample groups were not considered with the earlier sample groups in the growth analysis since they are from different spawning periods, but are offered here as evidence that a winter spawning did occur. Using Harding's (1949) method of polymodal analysis on a population sampled over a period of time, the growth rates can be calculated by following the mean lengths of the initial size class through time and run- 39 40 ning a regression analysis on the resultant plotted means versus time. However, when the mean shell lengths of the juvenile Tresus are plotted against time between sample dates, no one clear-cut progression of a size class can be seen. This is due to the phenomenon of nearly continuous spawning. In this case, polymodal analysis is valueless. Except for the results of another experiment, nothing more concerning the growth rates of juvenile Tresus could now be presented. As has been mentioned, five containers of 62 pre-measured juveniles were placed in the slough in early February, 1971 to see if main tenance of a population in such a manner was feasible. When the containers were recovered, some loss of clams had occurred, but 36 juveniles were still alive in the buckets. Of these 36, twenty individuals in three of the containers displayed a red growth ring near the middle of their shells. Measurement of the shells for length, disclosed that each red ring corresponded exactly to an initial length measurement of a clam. This allowed for individual measurement of growth. Apparently, the juveniles had somehow incorporated some form of ferric hydroxide, which exists as a problem in the running sea water system of the laboratory, into their shells during their four day stay in the laboratory. Table 1 summarizes the initial length, recovery length, total length in crease and the resultant growth rate of each individual juvenile. 20 ,....-- 19 19 r-- 18 18 17 17 16 16 (J) 15 15 r-1 ro 14 14 ::i 13 13 '0 ·ri :> 12 12 ·ri 11 11 '0 ~ 10 10 ·ri 9 ,-- 9 Ii-I 0 8' ,-- 8 I-- 7 7 (J) ' H 6 i-- 6 Figure 37. Distribution of the juve Figure 38. Distribution of the niles sampled on 24 May 1970. juveniles sampled on 5 June 1970. 5 5- 4 4 3 3 2 2 1 C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! C! "' Figure 39. Distribution of the Figure 40. Distribution of the juveniles sampled on 17 June 1970. juveniles sampled on 1 July 1970. 5 5 4 4 3 3 2 2 1 ~~~~~~~~~~ 0~~~~~~~ ~~~~~0 ~ ~ ~ ~ ' Figure 39. Distribution of the Figure 40. Distribution of the juveniles sampled on 17 June 1970. juveniles sampled on 1 July 1970. 9 ~ 8 7 - ----., 6 5 4 - 3 3 -;-- 2 2 - 1 - n f I I I I ~0~~0~~~0~~~~~~~0 \Dt'--00())0.- Figure 41. Distribution of the Figure 42. Distribution of the juveniles sampled on 21 July 1970. juveniles sampled on 18 August 1970. 9 8 ..--- j- 8 7 7 6 6 5 5 4 r-- 4 3 3 2 2 tJ 1 q q q q q q q q q q q q q q q ['.. ~ U'"l \0 ['.. ,.... ('() ~ U'"l \0 00 0\ ,....0 .... N ~ Figure 43. Frequency distribution of Figure 44. Frequency distribution juveniles sampled on 12 December 1970. of juveniles sampled on 9 January 1971. q I I []] Q n D [b n q q 0 0 0 q q q q Tlmq q q q q q q q q q q q q q tf) 1.0 r.....: oO 0'\ 0 N 1.0 ['-. C() 0'\ 0 .--! N m tf) 1.0 ['-. C() 0'\ """ .--! .--! .--! .--! ...-! .--! ...... """ ...... Figure 45. Frequency distribution Figure 46. Frequency distribution of juveniles sampled on 27 January of juveniles sampled on 8 February 1971. 1971. :i I I [ko rn '=! 0 '=! 0 '=! '=! '=! '=! q '=! ['... ,.... -:1< lfl \0 00 0'1 0 ,....C\1 ,....ct"l Figure 47. Frequency distribution of juveniles sampled on 23 February 1971. 47 TABLE 1 Summary of Individual Shell Length Changes and Average Individual and Class Growth Rates of Tresus nuttalli Juveniles in Controlled Conditions (/] ..c: :>. -1-' ctl r-l ~ '-.....Y-iQ (1j 0 :>. <1) 0 ;:J H r-l H (/] 0.0 7.3 mm 18.2 mm 10.9 mm/43 days 0.25 mm/day 7.4 mm 18.8 mm 11.4 mm/43 days 0.27 mm/day 7.6 mm 20.4 mm 12.8 mm/43 days 0.30 mm/day 7-7 mm 19.2 mm 11.5 mm/43 days 0.27 mm/day 8.5 mm 20.7 mm 12.2 mm/43 days 0.28 mm/day 0.26 mm/ day 8.6 mm 19.2 mm 10.6 mm/43 days 0.25 mm/day 8.7 mm 20.0 mm 11.3 mm/43 days 0.26 mm/day 9.4 mm 19.0 mm 9.6 mm/43 days 0.22 mm/day 10.6 mm 21.3 mm 10.7 mm/43 days 0.25 mm/day 10.7 mm 23.2 mm 12.5 mm/47 days 0.27 mm/day 11.7 mm 24.2 mm 12.5 mm/47 days 0.27 mm/day 12.2 mm 25.1 mm 12.9 mm/47 days 0.27 mm/day 0.25 mm/ day 12.4 mm 25.2 mm 12.8 mm/47 days 0.27 mm/day 12.9 mm 21.9 mm 9.0 mm/43 days 0.21 mm/day 14.5 mm 25.0 mm 9.5 mm/43 days 0.22 mm/day mm 31.0 mm 11.5 days 0.27 19.5 mm/43 mm/day]- 0.25 mm/ 23.4 mm 34.o mm 10.6 mm/47 days 0.23 mm/day day 35.8 mm 44.0 mm 8.2 mm/49 days 0.17 mm/day} 36.9 mm 44.3 mm 7.4 mm/49 days 0.15 mm/day 0.14 mm/ day 39.5 mm 44.4 mm 4.9 mm/49 days 0.10 mm/day 48 Even when the red rings on the juveniles' shells are not used as an indication of growth, it is possible to obtain an idea of growth rates by comparing the aver- age initial shell length of the juveniles in each con- tainer to the average shell length at recovery. Table 2 summarizes this data from three of the five containers. TABLE 2 Summary of the Average Changes in Shell Length of Three Groups of Juveniles in Controlled Conditions f:tO r-1 I ~ r-1 s ·r-l ..c:zoaJ ::J :s: H r.JJ'-.....H aJ :>. :>. aJt; ~ r-1 r-1 aJ H H aJ aJ rJl aJ ·r-l cO cO f:tO,.C: aJ aJ f:tO ..c: f:tDro'H f:tO,.C: cO ·r-l (!) ·r-l cO-I-l :> (!) :> cO -\-) cO aJ 0 cO-I-l -\-) -\-) aJ f:UJ -\-) H f:tO 0 (!) f:tO o H f:tO H H rJl H :S: aJ !=l ·r-l N ~ ·r-l aJ ~ () N ~ () (!) ~ CD u H :>. (!) 0-\-) 0 ~ ·r-l ro ~ :> (!) (!) ·r-l cO (!) :> aJ :> ~ aJ cO :> H ro C) Hr.JJP:: H~H p:;r.np:; p:;~H ~H..OO ~t;p:; A 9.8 mm- 13.4 mm 23.2 mm- 26.3 mm 12.9 mm/ 0.28 mm/ 23.4 mm 34.0 mm 47 days day B 4.2 mm- 7.3 mm 15.8 mm- 17.7 mm 10.4 mm/ 0.22 mm/ 8.9 mm 21.0 mm 47 days day c 6.0 mm- 9.4 mm 18.2 mm- 21.2 mm 11.8 mm/ 0.27 mm/ 19.5 mm 31.0 mm 43 days day 49 If the growth rate for the smaller juveniles, between 4.0 and about 20.0 mm, can be assumed to be nearly constant at an average of .25 mm/day, it may be estimated that a 20.0 mm clam is about three months old; that is, about three months have passed since the clam metamorphosed from a planktonic larva to a settling spat. DISCUSSION OF THE JUVENILE GROWTH RATE STUDY There are several possible reasons why Harding's growth analysis did not work in this situation. The first and most obvious possibility is that the juvenile sample sizes were just not large enough to accurately permit sorting out the size classes. Another reason is that a particular size class may have been poorly repre- sented in a sample due to low spawning success and/or high larval and juvenile mortality rates. However, the most important reason is that continuous spawning over an extended period has caused a constant recruitment of juveniles. When this latter possibility occurs, the juvenile population would appear static, or near-static, in its size distribution. That is, as the members, say, of the 4.0 mm class grew into the 5.0 mm class, their position would be filled by more recently spawned juveniles reaching the 4.0 mm class, a situation which Figures 37 through 40 show happened. The fact that juveniles in the 4.0 mm class were found in eight of the ten sample months from May to February (even the samples without 4.0 mm class juveniles, from the months of October and November, contained juveniles in the 5.0, 6.0 and 7.0 mm classes) suggests 50 51 that spawning is nearly continuous. The adult gonadal condition also suggests that continuous spawning could occur since gametogenic activity reflecting active to ripe phases has been shown in all sample months except in July and August when most of the clams appeared to be sexually quiescent. Although recruitment is nearly continuous, juvenile sampling indicates that there are two peaks in juvenile abundance (Figure 36) and these peaks closely approximate the time of maximum adult reproductive readiness. The phenomenon of nearly continuous spawning coupled with early rapid growth of the juveniles is the most probable answer to the question of how Tresus nut talli has been able to maintain a dense population in Elkhorn Slough while being subject to heavy predation by humans. During the course of this study it was observed that other game clam species (Protothaca, Clinocardium and Saxidomus) once counted as common or abundant by MacGinitie (1935) appear to have decreased in numbers. That this may be so suggests that these species may be less successful reproductively than T. nuttalli and per haps further study will show them to possess shorter, possibly unimodal, spawning cycles or slow growth rates. 52 Sl.JMMARY A twelve month study to determine the spawning cycle of adult clams, Tresus nuttallis and the growth rates of their juveniles was conducted in Elkhorn Slough, California. The spawning study was accomplished by histological inspection of the gonads of clams collected during each series of tides lower than -0.7 feet. An attempt was made to determine growth rates of the juve- niles by two methods: frequency distribution analysis of population samples taken during every low tide series and by maintenance of premeasured juveniles in a con trolled situation under natural conditions. The study revealed that the spawning cycle of Tresus nuttalli extends through most of the year with the exception of July and August when a majority of clams appeared sexually inactive. Correlation of gonadal maturity with juvenile abundance suggests that there are winter peaks of spawning. No sexual size dimorphism was found and the clams proved to be strictly dioecious. Determination of juvenile growth rates by prob ability paper analysis proved unsuccessful due to prolonged spawning periods. However, a study of the growth of juveniles in a controlled situation revealed 53 that juveniles with shell lengths between 4.o mm and about 20.0 mm grow at an average rate near 0.25 mm per day. LITERATURE CITED Armstrong, Lee J. 1965. Burrowing limitations in pele- cypoda. Veliger 7(3):195-200. Calabrese, Anthony. 1970. Reproductive cycle of the Coot Clam, Mulinia lateralis (Say) in Long Island Sound. Veliger 12(3):265-269. Cassie, c. M. 1952. Some uses of probability paper in the analysis of size frequency distributions. Australian J. of Marine and Freshwater Res. 5(3): 513-522. Fitch~ J. E. 1953. Common marine bivalves of California. California Dep. of Fish and Game Fish Bull. 90. 102 p. Galigher, A., and E. N. Kozloff. 1964. Essentials of practical microtechnique. Lea and Febiger, Phila- delphia. 484 p. Galtsoff, P. S. 1964. The American oyster, Crassostrea virginica Gmelin. U.S. Fish and Wildlife Service Fishery Bull. 64. Harding, J. P. 1949. The use of probability paper for the graphical analysis of polymodal frequency dis- tributions. J. of the Marine Biol. Ass. of the U.K. 28(2):141-153. 55 Humason, Gretchen. 1969. Animal tissue techniques. W. H. Freeman and Co., San Francisco. 569 p. Loosanoff, Victor L., and Harry C. Davis. 1963. Rearing of bivalve molluscs, p. 1-136. In Advances in Marine Biology (Vol. 1). Academic Press, London and New York. 410 p. MacGinitie, George E. 1935. Ecological aspects of a California marine estuary. Amer. Midland Naturalist 16(5):629-765. Machell, John R. 1968. The reproductive cycle of the clam, Tresus capax, Family Mactridae. Unpublished master's thesis, Humboldt State College, Arcata, California. 26 p. Marriage, Lloyd D. 1954. The bay clams of Oregon. Fish Commission of Oregon Fishery Bull. 20. 90 p. Nicol, David. 1964. An essay on the size of marine pele- cypods. J. of Paleontol. 38(1):968-974. Pearse, Jack B. 1965. On the distribution of Tresus capax and Tresus nuttalli in the waters of Puget Sound and the San Juan Archipelago. Veliger 7(3): 166-170. Pfitzenmeyer, H. T. 1965. Annual cycle of gametogenesis of the soft-shelled clam, Mya arenaria, at Solomons, Maryland. Chesapeake Sci. 6(1):52-59. Pohlo, Ross H. 1964. Ontogenetic changes of form and mode of life in Tresus nuttalli. Malacologia 1(3): 321-330. Ricketts, E., Jack Calvin and Joel Hedgpeth. 1968. Be- tween Pacific tides. Fourth edition. Stanford Univ. Press, Stanford, California. 614 p. Ropes, J., and A. P. Stickney. 1965. Reproductive cycle of Mya arenaria in New England. Biol. Bull. (Woods Hole). 128(2):315-327. Stout, William E. 1967. A study of the autecology of the Horseneck Clams, Tresus nuttalli and Tresus capax, in South Humboldt Bay, California. Unpub lished master's thesis, Humboldt State College, Arcata, California. 42 p. 1970. Some associates of Tresus nuttalli (Conrad, 1837) from Humboldt Bay, California. Veliger 13(1):67-70. Swan, E. F. and J. H. Finucane. 1952. Observations on the genus Schizothaerus. Nautilis 66(1):19-26. Wilbur, Karl M., and Garth Owen. 1964. Growth, p. 211- 244. In Physiology of the Mollusca (Vol. 1). Academic Press, New York. 473 p. Williams, E. E. 1964. The growth and distribution of Littorina littorea (L.) on a rocky shore in Wales. J. of Anim. Ecol. 33(2):413-432.