An effective sperm competition avoidance strategy in crabs drives genetic monogamy despite evidence of polyandry

For polyandrous species where females have sperm storage structures, males develop several strategies to avoid sperm competition and thus to maximize the number of eggs fertilized. On the other hand, females may receive several benefits from multiple paternity (indirect and directly), and a potential sexual conflict can arise. This research describes the mating systems of an exploited crab species (Metacarcinus edwardsii), integrating (1) the individual level by assessing the mating behavior in a scenario of potential polyandry, (2) the organ level by examining histological sections of seminal receptacles from localities with scenarios of contrasting sex ratios, and (3) the genetic level by measuring the number of parents involved in egg clutches. We found that females can mate with multiple males under experimental conditions. Further, in all localities, we found histological evidences that sperm receptacles stored ejaculates from more than one male. However, contrary to expectations, genetic analysis revealed high probability of single male paternity of all progeny in each egg clutch. In this mating system, males compete to be the single male that mates with a receptive female, investing energy in guarding behavior and foregoing opportunities to mate with other females, all in order to ensure their paternity. However, females benefit from multiple mating (or potential for it) by prolonged guarding behavior, protecting them from predation after molt (soft-shelled period). The mating system of M. edwardsii can be defined as polygamous (where both sexes can mate multiple times) with genetic monogamy.