The Opportunity for Post-Copulatory Sexual Selection in the Ectoparasitic Pea Crab, Dissodactylus primitivus (Brachyura: Pinnotheridae)

Pea crabs, Dissodactylus primitivus, inhabit multiple echinoid (heart urchin) hosts. Male and female crabs move among hosts in search for mates, and both sexes mate multiple times, creating opportunities for post-copulatory sexual selection. For such selection to occur, only a fraction of the males who succeed in mating can also succeed in siring progeny. Jossart et al. 2014 used 4 microsatellite loci to document parentage and mating frequencies of both sexes in D. primitivus. From these data we identified the mean and variance in female offspring numbers, as well as the proportions of the female population that were gravid and not bearing offspring. We next identified the proportions of the male population who had (1) mated and sired offspring, (2) mated but failed to sire offspring, and (3) failed to mate altogether. We used these results to estimate the opportunity for selection on males and females in terms of mate numbers and offspring numbers, and estimated the sex difference in the opportunity for selection (i.e., the opportunity for sexual selection) using both forms of data. We then partitioned the total variance in male fitness into pre- and post-copulatory components and identified the fraction of the total opportunity for selection occurring in each context. Our results show that the opportunity for selection on each sex was of similar magnitude (0.69-0.98), consistent with this polyandrogynous mating system. We also found that 37% of the total opportunity for sexual selection on males occurred within the context of post-copulatory sexual selection. However, the fraction of the total opportunity for selection that was due to sexual selection, estimated using both mate numbers and offspring numbers, was 9% and 23% respectively. Thus, we further reduced our estimate of the opportunity for post-copulatory sexual selection in D. primitivus to less than 10% of the total opportunity for selection (0.37 of 0.09 and 0.23 = 0.03 and 0.09). Our results provide the first estimate of the maximum possible strength of post-copulatory sexual selection in crustaceans using this approach.