Male mating tactics in the shrimp Palaemonetes pugio (Decapoda, caridea):: Precopulatory mate guarding vs. pure searching

The guarding of females approaching a limited period of sexual receptivity is a common mating tactic of males. In many decapod crustaceans, such as the shrimp Palaemonetes pugio, females can only copulate during a short period after a reproductive molt. It has been predicted that mate guarding by males (pre-copula) evolves in such species if sex ratios are not highly female-biased and if males can detect the molt stage of the female. The mating tactics of males were investigated in P. pugio. Time-lapse video observations were made on interactions among two males, a pre-molt,female, and an inter-molt female (20 replicates). There was no evidence that males recognized a pre-molt female until 24 h before its molt. Significant numbers of male contacts with pre-molt females occurred 1 h before and after the female molt. Copulation took place within 1-3 min of the molt. No behavior commonly associated with mate guarding in decapods was observed - no clasping, agonistic behavior, or close association. It is concluded that the male's mating tactic is pure searching, wherein males haphazardly contact many females in order to find a receptive one. The high encounter rate in nature of these very mobile, aggregated shrimps is proposed as the factor responsible for the evolution of pure searching. It is hypothesized that pure searching is the male tactic of the many species of decapod shrimps with small males, sexually monomorphic cheliped weapons, and aggregated populations.