Life history adaptations of the fingernail clams Sphaerium occidentale and Musculium securis to ephemeral habitats

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Abstract
Life histories of the fingernail clams S. occidentale and M. securis from a temporary pond are examined to show adaptations to ephemeral habitats. M. securis ceases growth upon birth and only newborn occur in the dry pond; in S. occidentale all length classes are omnipresent. Aestivation is not obligatory in the life cycle of either species. Both grow and reproduce regardless of aestivation experience. Life history strategies of both clams counteract environmentally imposed, density-independent mortality pressures. S. occidentale has slower growth and greater longevity than M. securis which must complete its life cycle before the pond dries. During aestivation, broods continue development in S. occidentale but not in M. securis. If water is present in fall, both opportunistically use this period for development. Both produce large numbers of small newborn and exhibit multivoltine and iteroparous strategies, although M. securis releases consecutive broods only during a single spring birth period and only if the aquatic season is extended; S. occidentale releases consecutive broods during a spring birth period, in autumn if water is present, and during a 2nd year. A mixed tactic, according to the stochastic theory, has been adopted by both for survival under highly variable environmental stresses.
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