Rhythms and Reproduction

Abstract

Abstract Reproductive rhythms can be found in numerous crustacean species. This chapter reviews the temporal scales of rhythms and how these rhythms are entrained and maintained by external cues and endogenous clocks. The occurrence and synchrony of rhythms vary along latitudinal and depth gradients, which may depend on the availability of zeitgebers (e.g., temperature and photoperiod), changing selective pressures such as predation risk, and variability in larval development rates that affect the timing and synchrony of reproductive rhythms. Commonly observed rhythms are reproductive migrations and synchronized larval release, which are often timed to reduce predation risk for newly hatched larvae. In crustaceans, reproductive rhythms rarely evolve under pure density-dependent selection for synchrony. Pure density dependence is common in marine broadcast-spawning invertebrates like corals, which rely on accumulation of gametes in time and space to ensure fertilization. Instead, (density-independent) selection for synchrony with environmental cycles that track changes in factors affecting fitness such as energy expenditure, predation risk, or food availability seems to be the rule, although some exceptions may exist. In contrast to natural selection, the possible contribution of sexual selection on reproductive rhythms has rarely been considered. Selection for enhanced mating possibilities should favor reproductive synchrony, but deviations from synchrony will affect the operational sex ratio and influence sexual selection. Finally, the chapter discusses the possibility of sexual conflict over reproductive timing between males and females and explores circumstances under which synchronous reproductive rhythms might be abandoned.