Autumnal vs Spring Hatching in the Fairy Shrimp Siphonophanes grubii (Dybowski) (Crustacea, Anostraca): Diversified Bet-Hedging Strategy?

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Abstract
Polyphenism, as opposed to polymorphism, refers to the coexistence of several distinct phenotypes having a common genotype. Poly-phenism can be selected for in unpredictable environments. Here we document and analyse a case of diphenism in the north-European fairy shrimp Siphonophanes grubii (Dybowski), in relation to the temporary and unpredictable nature of its habitat. The active part of this species' life cycle usually consists of a single, short-lived, spring cohort. Here we report field observations on autumnal hatching and on a long-lived, overwintering cohort; we show that the winter cohort runs the risk of total failure. due to the pond freezing entirely or drying up during winter. If, however, environmental conditions allow winter survival, animals reach a larger size, reproduce for a longer time, and display higher fecundity, than do animals from the spring cohort. Laboratory experiments support the theory that these differences are purely phenotypic and dependent on temperature. Using an analytical model adapted from Cohen (1966), we propose that the coexistence of both a winter and a spring cohort in the same ponds can be interpreted as a diversified bet-hedging strategy.

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