Brachyuran crabs in the family Cancridae are found in a variety of habitats, ranging from ‘open’ sandy or muddy grounds to ‘structured’ rocky bottoms rich in refuge space. Cancer gracilis Dana inhabits shallow open habitats in the north-east Pacific Ocean. Females in all reproductive conditions aggregate in relatively small areas, where mating activity appears most intense. Maximum mating activity was observed towards the end of the spawning/hatching season (August). Females play an active role in mating, often initiating interactions with the prospective male partner. Mating in the field was non-assortative with respect to size. There was no indication of direct mate choice by either males or females; resolution of male-male competitive interactions was size-based. Males below minimum mating size (77 mm carapace width) had spermatophores in the vas deferens but did not react to receptive females.

Sperm competition is likely because sperm can be retained across moults and multiple copulations are possible within a receptive period. Female spermathecae are of the ‘ventral’ type (contrary to what has been indicated for cancrids), and during intercourse are beyond the reach of male gonopods. Mechanisms to exclude competing sperm include male-produced sperm plugs and post-copulatory mate guarding.

Results are discussed in the context of the ‘structured-to-open’ habitat gradient, which may underlay much of the diversity in cancrid reproductive ecology. We argue that, by comparison with C. magister (a larger size sympatric species also found in open habitats), (1) reproductive asynchrony and female gregariousness in C. gracilis increase the environmental potential for polygyny, resulting in a female defence polygynous mating system, and (2) reproductive asynchrony inflates the operational sex ratio, leading to stronger sexual selection.