Size-dependent competition in a gregarious porcelain crab Petrolisthes cinctipes (Anomura: Porcellanidae)

Studies of competition in benthic marine organisms have focused on sessile filter-feeders and mobile grazers, to the neglect of mobile filter-feeders, including porcelain crabs. The present study demonstrates intraspecific competition in Petrolisthes cinctipes (Randall) (Porcellanidae: Anomura). P. cinctipes settles gregariously and lives in high-density, multi-size aggregations. Studies of gregarious species often identify the benefits of aggregation without quantifying the concomitant costs; this study quantifies these costs. In field and laboratory experiments, P. cinctipes growth rates declined with increasing conspecific density and this decline was more severe for smaller individuals. P. cinctipes also fed less frequently at high density, and this effect was more pronounced for smaller individuals. This indicates that size-dependent agonistic interactions in P. cinctipes have effects on feeding rate and, ultimately, on growth rate. Resource levels were manipulated in laboratory experiments by using filtered and unfiltered seawater. The effect of resource level also depended on size: smaller crabs were more sensitive to changes in food level. Thus, conspecific density and food availability both affected growth rate in a size-dependent manner. Therefore, the costs of aggregation are more extreme for recruits than for adult crabs; recruits suffer more from competition, but may also benefit more if conspecific cueing results in settlement to sites with higher food availability.