THE PEA CRAB, PINNOTHERES-PISUM (LINNAEUS, 1767), AND ITS ASSOCIATIONWITH THE COMMON MUSSEL, MYTILUS-EDULIS (LINNAEUS, 1758), IN THE SOLENT (UK)

Pea crabs (Pinnotheres pisum (Linnaeus)) from 5366 mussels (Mytilus ed ulis (Linnaeus)) were collected from two sites in the Solent, southern England, between 1972 and 1974 in order to determine the relationship s between pea crab sex, size and occupancy, mussel size, time of year and position on beach. Larger mussels were more likely to be occupied by larger, female crabs, but pea crabs will occupy smaller mussels if fewer hosts are available. Mussel occupancy tends to increase from hig h to low water where food is more readily available. Pea crabs do not occupy mussels at random; there were more male/female pairs and fewer single males and pairs of females than expected. Possible mechanisms t hat explain these results are discussed. Berried females were found be tween April and October with a peak of 70% in berry in June and July. Larger females came into berry earlier, and may have been a year older , than smaller females. The association with mussels is best described as amensalism for male pea crabs and parasitism for females.