Influence of biogenic habitat on the recruitment and distribution of a subtidal xanthid crab

Biogenic habitat can structure marine communities by serving as both a complex substrate and food source for adult and recruiting organisms. We investigated the role played by biogenic habitats (Mytilus edulis and Crepidula spp. beds) in influencing the subtidal distribution (kilometers scale) of the mud crab Dyspanopeus sayi in Narragansett Bay, New England (USA). In field surveys, D. sayi were 1 to 2 orders of magnitude more abundant on M. edulis and Crepidula spp. beds than at sites lacking these habitats, and a laboratory experiment confirmed that D. sayi will consume both of these species. In a field-based, substrate-choice experiment, modules containing rocks had higher D. sayi recruit densities than those containing M. edulis or Crepidula spp. This indicates that structural complexity of those biogenic substrates, rather than their availability as prey, is the primary factor influencing patterns of D. sayi recruitment. Densities of recruit and adult D. sayi on recruitment modules were higher at bare sites than at sites with biogenic habitat, where modules represented islands of structure in otherwise simple habitat. This suggests small-scale habitat selection by D. sayi. Adult D. sayi occupied modules indiscriminately at bare sites, but exhibited a preference for modules containing mussels at sites with biogenic habitats. The importance of structure for adult and recruit D. sayi, reinforced by adults' response to prey presence, likely explains this organism's association with hard, biogenic habitats in the studied system.