In polyandrous mating systems, females mate multiple times and males have evolved adaptations for sperm competition which increase the number and fitness of their offspring. Mate guarding is a widespread monopolization strategy in groups where female receptivity is temporally restricted and often associated with the molt. Precopulatory guarding occurs in branchipods, copepods, peracarids and decapods. Postcopulatory guarding is notable in numerous brachyurans with males protecting females until her exoskeleton has hardened. During copulation, male success in fertilization depends on an effective sperm transfer mechanism, the precise placement of ejaculates closest to where female gametes are fertilized. Male copulatory systems are highly diverse and strongly adapted to these tasks, especially the structures that interact with the female genital ducts. The elaborate tips of brachyuran gonopods are supposed to act in the displacement, possibly even in the removal of rival sperm masses; however, sperm removal is only evident in crayfish: males eat spermatophores previously deposited by other males. During copulation of several crustacean groups, males transfer secretions that harden and form a sealant. These sperm plugs, plaques and gel layers may protect their own sperm, prevent remating or seal off rival sperm from the site of fertilization. Several groups of isopods and decapods have internal insemination, elaborate sperm storage organs and some exhibit internal fertilization. The intensity of sperm competition increases with the latency between the processes of insemination and fertilization. This chapter gives on overview on mate guarding, male sealants and the anatomical adaptations to sperm competition in crustaceans. We also briefly discuss the consequences of multiple matings for the genetic diversity of broods, i.e., single vs. multiple paternities. There is still a lack of data for many crustacean groups. Moreover, it is often hard to assess how successful a male strategy to ensure paternity actually is as many studies focus on either the behavioral, anatomical, or molecular aspects, while comprehensive multi-level studies on crustacean sperm competition are virtually absent from the literature.
Sperm Competition in Humans: Mate Guarding Behavior Negatively Correlates with Ejaculate Quality