THE BREEDING ECOLOGY OF CANCER GRACILIS (CRUSTACEA, DECAPODA, CANCRIDAE) AND THE MATING SYSTEMS OF CANCRID CRABS

Brachyuran crabs in the family Cancridae are found in a variety of hab itats, ranging from 'open' sandy or muddy grounds to 'structured' rock y bottoms rich in refuge space. Cancel gracilis Dana inhabits shallow open habitats in the north-east Pacific Ocean. Females in all reproduc tive conditions aggregate in relatively small areas, where mating acti vity appears most intense. Maximum mating activity was observed toward s the end of the spawning/hatching season (August). Females play an ac tive role in mating, often initiating interactions with the prospectiv e male partner. Mating in the field was non-assortative with respect t o size. There was no indication of direct male 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 sper matophores in the vas deferens but did not react to receptive females. Sperm competition is likely because sperm can be retained across moul ts and multiple copulations are possible within a receptive period. Fe male 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-p roduced sperm plugs and post-copulatory mate guarding. Results are dis cussed in the context of the 'structured-to-open' habitat gradient, wh ich may underlay much of the diversity in cancrid reproductive ecology . We argue that, by comparison with C. magister (a larger size sympatr ic species also found in open habitats), (1) reproductive asynchrony a nd female gregariousness in C. gracilis increase the environmental pot ential for polygyny, resulting in a female defence polygynous mating s ystem, and (2) reproductive asynchrony inflates the operational sex ra tio, leading to stronger sexual selection.