THE EFFECT OF PEA CRABS ON PREDATION OF HOST MUSSELS BY SEA STARS

Pea crabs parasitize a large proportion of mussels in Narragansett Bay , Rhode Island. Large mussels (>50 mm shell length) are more heavily p arasitized than smaller mussels. Previous work on the energetics of se a star feeding behavior showed that although large mussels provide lar ge sea stars (>100 mm radius) with the maximum rate of energy intake, these sea stars prefer to feed on smaller mussels. It was hypothesized that large sea stars avoid large mussels because the pea crabs interf ere with the feeding behavior of the sea star. The behavior of pea cra bs during predatory attacks was observed directly. Another group of se a stars were allowed to feed on a choice of parasitized and parasite-f ree mussels. Pea crabs do not mechanically interfere with sea star fee ding. Pea crabs were always successful in abandoning their host during attacks, and were not eaten by the sea star. However, sea stars did p refer unparasitized mussels in selection experiments. Once a host muss el has been attacked, pea crabs do not physically defend their host.