This chapter reviews sex determination and sex allocation strategies among crustaceans with different sexual systems (gonochorism, sequential and simultaneous hermaphroditism, and androdioecy), from the perspective of evolutionary ecology. The discussion includes genetic, environmental, and cytoplasmic sex determination in free-living and parasitic crustaceans, timing and frequency of sex change especially in partial protandry, the effects of mating group size on resource allocation by simultaneous hermaphrodites, and sex ratio and determination in androdioecious crustaceans. The fascinating diversity of crustacean reproduction stimulated theoretical biologists to construct models to explain them, and empirical biologists attempted to test hypotheses derived from those models. This review clearly shows that the interaction between theoretical and empirical studies has facilitated understanding of the evolutionary conditions of diverse sexual strategies among crustaceans. Since sexual strategies often interact with other aspects of adaptive strategies such as life history, integrating different aspects into both theoretical and empirical studies will provide further understandings into crustacean sexual systems. In addition, the authors point out the potential of phylogenetic comparative analyses using natural history data as a tool to understand the tempo and mode of evolution of sex allocation strategies.
