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Abstract

On the Atlantic coast of North America, Littorina littorea (L.), the European periwinkle, has been introduced in historical times. The shells of this gastropod are an important resource for the hermit crab Pagurus longicarpus (Say). In Long Island Sound, collections of empty shells available to P. longicarpus show that while small shells consist mostly of native species, large shells consist mostly of Littorina littorea. In addition to this quantitative shift in shell availability, tests with male Pagurus longicarpus show a qualitative, i.e. independent of shell size, shift in shell preference: small-sized males prefer native shells while large-sized specimens prefer Littorina littorea shells. Further, field collections of Pagurus longicarpus show that shell utilization patterns of males exhibit a similar ontogenetic response. (Females do not attain the large sizes of males and generally do not utilize Littorina littorea shells.) The male shift in shell utilization corresponds to the shift in shell preference and may precede the shift in shell availability. These data suggest that the century-old arrival of L. littorea to Long Island Sound has modified the shell utilization and preference patterns of Pagurus longicarpus.