Life-history characteristics of a stream population of the freshwater clam Sphaerium striatinum Lamarck (Bivalvia: Pisidiidae)

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Abstract
Life-history characteristics of a population of Sphaerium striatinum in Little Four Mile Creek, OH, were studied from August 1977 to April 1980. Two generations of clams were produced each year, with major recruitment occurring in April – early July and August–October. Individuals born during either birth period lived for about 1 year. Minimum size at birth was 4.0 mm, and only adults with a shell length > 10.0 mm contributed to recruitment in the population. Examination of seasonal size distributions of embryos revealed an average embryonic development rate of 0.32 mm∙week−1 during April–October; the rate was lower during other times of the year. The annual selection ratio (average number of young born per adult) was 10.49:1 (7.68:1, April–July; 3.81:1, August–October); embryonic mortality was 97.5%. These results are compared with published values for a lake population of S. striatinum and indicate that the differences observed in life history characteristics for these populations apparently do not conform to expectations based on r- and K- selection theory. Instead the differences can best be explained by a "bet-hedging" theory dependent on stochastic rather than deterministic processes. Life-history characteristics of other pisidiid clams are compared with results from this study and discussed in relation to the adaptive plasticity of freshwater molluscs.
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