SHELL SELECTION AND AGGRESSIVE BEHAVIOR IN TWO SYMPATRIC SPECIES OF HERMIT CRABS

Two species of hermit crab, Pagurus acadianus and P. pubescens are distributed sympatrically in offshore waters of Frenchman Bay, Maine. Laboratory and field investigations indicate that smaller sized specimens of P. acadianus have a preference for Littorina shells over Thais shells whereas P. pubescens individuals enter shells of either Thais or Littorina depending on the relative abundance of each type. Larger P. acadianus individuals which usually exceed the size range of P. pubescens principally occupy shells of Buccinum and Polinices. P. acadianus shows a strong preference for mollusc shells colonized by Hydractinia echinata.

Isolation increases the level of aggression in P. acadianus individuals, but the low levels of agonistic behavior found in tests with P. pubescens crabs appear not to be affected by changes in crab density. In both species, subjects in shells of too small a size by volume showed a high degree of dominance in trials over crabs occupying normal or large shells; a factor which can be considered an adaptation to accommodate individual growth.

It is concluded that behavior related to shell selection in the two species of crabs is sufficiently different to allow co-existence of the two species on the basis of resource partitioning, except in situations where smaller sized Littorina shells without Hydractinia colonies constitute a single resource base. However, there may be considerable intra-specific competition between larger specimens of P. acadianus for limited supplies of Buccinum shells and between crabs of all sizes located in appropriate sized shells and those which have outgrown their domicile shells.