A Field Study of Intraspecific Competition for Food in Hermit Crabs (Pagurus bernhardus)

Abstract

A tethered, frame-mounted video camera deployed on the sea-bed was used to observe the competitive interactions that occurred between hermit crabs,Pagurus bernhardus,that were attracted to food patches (dead dragonets,Callionymus lyra) of differing size.Hermit crab numbers on the small food patch ceased increasingc. 20min after the camera arrived on the sea-bed, whilst numbers on the large patch increased throughout the experiment. The number of observed aggressive interactions increased with increasing hermit crab density, but was generally highest on the small patch. The probability of a hermit crab being able to feed increased with size for each of three size-groups on the small patch, whereas on the large patch, both large and medium-sized hermit crabs were equally likely to feed. Small and medium-sized hermit crabs had a higher probability of being able to feed on the large patch than the small patch. As the density of hermit crabs around a patch increased, the proportion of small individuals actively feeding decreased. The size-frequency distribution of hermit crabs on the large patch was significantly different from that on the small patch, with the latter being skewed towards larger individuals. These results suggest that the intensity of competition increases both with increasing numbers of hermit crabs and decreasing size of food resource. Large hermit crabs were more successful at feeding than smaller crabs when competition was more intense.