Environmental change and the evolutionary history of the periwinkle ( Littorina littorea) in North America.

Abstract

Carcinus maenas was first found N of Cape Cod in 1905, and reached its present northern limit in Nova Scotia in the 1950s. Comparisons of museum samples of periwinkles collected before and after the arrival of the green crab revealed that the frequency of repair (number of breakage-induced scars per shell) increased significantly in the 10-14mm and 25-35mm size classes but not in the 2 intervening classes. There was no corresponding increase in shell thickness despite the fact that thickness and the incidence of repair are correlated positively. The lack of an adaptive response to increasing levels of unsuccessful shell breakage through time is unlikely to be the result of genetic or phenotypic impoverishment. Instead, extensive gene flow between areas of high and low predation may account for the stability of shell thickness through time. S of Cape Cod, where frequencies of repair are higher yet shells are thinner than N of the Cape, the widespread tendency for high summer temperatures to be associated with fast growth and thin shells may account for the absence of well-developed armor.-from Author