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The Spawning Cycle and Juvenile Growth Rate of the Gaper Clam, Tresus Nutalli, of Elkhorn Slough, California

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The Spawning Cycle and Juvenile Growth Rate of the Gaper Clam, Tresus Nutalli, of Elkhorn Slough, California 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.
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