Predator density, not structure, influences intraspecific competition in the mud crab Dyspanopeus sayi

Structured habitats can offer prey refuge by reducing the encounter rates between predators and prey. However, structured habitats can also reduce predator-predator encounters, thereby dampening intraspecific interactions and increasing overall prey consumption. In this study, we tested whether intraspecific competition among Sayi mud crabs Dyspanopeus sayi foraging on mussel Mytilus edulis prey was modified by the density of mimicked eelgrass Zostera marina shoots. Predation was measured in 3 predator densities (1, 3, or 6 predators) and 1 of 5 levels of shoot density. We found no impact of Z. marina on intraspecific competition, regardless of predator density. Intraspecific competition was weak; per capita crab consumption decreased as predator density increased and became significantly reduced when predator density increased from 1 to 6. However, there was no evidence of non-additive predator density effects when predator abundance doubled from 3 to 6 predators. Our results also indicate that eelgrass structure may not offer the sessile prey M. edulis refuge when one or more D. sayi predators are present. Further exploration is needed to understand whether the lack of a structure effect is due to our use of sessile rather than mobile prey as used in other studies. Predator density and not structural density influenced intraspecific competition among D. sayi predators when they were foraging for M. edulis in mimicked Z. marina shoots.