Mating systems in caridean shrimp (Decapoda: Caridea) and their evolutionary consequences for sexual dimorphism and reproductive biology

In this paper we review functional and evolutionary relationships among mating systems of caridean shrimp and specific traits such as general biology/ecology, sexual systems, behavior and morphology. Four mating systems are described based on reports from available literature, and a fifth system is recognized but published information is insufficient to describe it in detail. ‘Monogamy’ occurs in many species inhabiting monopolizable refuges or hosts, especially when environmental conditions restrict the probability of intraspecific interactions. In contrast, free-living species experience higher encounter rates and males can dominate or search. In ‘neighborhoods of dominance’ mating systems, large males have higher reproductive success since they perform better in fights for receptive females. In ‘pure searching’ mating systems, small and agile males do better because they search more efficiently for mates within the population. The fourth mating system is ‘search & attend’ occurring in solitary symbionts, which experience variable ecological and demographic environments: depending on environmental conditions and ontogenetic stages it may either be profitable for males to search or to attend hosts with sexually attractive females. Sexual systems of caridean shrimp are characterized by their high diversity and intraspecific plasticity, including gonochorism and different forms of protandric or simultaneous hermaphroditism. The identified mating systems partially explained this diversity: In monogamous species, low encounter rates and lack of sexual dimorphism favors simultaneous hermaphroditism but gonochory usually occurs among these species probably because mates are not strictly faithful. Species with neighborhoods of dominance mating are gonochoristic because both sexes benefit from being large. Pure searching species have a wide opportunity for the evolution of protandry since small males benefit while the opposite is true for females. In species with search & attend the situation is variable, and only some males benefit by acquiring large size while others do better by searching or changing sex. In contrast to other crustaceans free-living shrimp do not exhibit precopulatory mate guarding and they are relatively unaggressive. The characteristic and efficient escape behavior of shrimp using their contractile abdomen might be one important trait that has constrained the evolution of such behaviors. Our review indicates that the mating systems of caridean shrimp depend both on environmental and on biological characteristics.