BIOLOGICAL VERSUS ENVIRONMENTAL SOURCES OF MORPHOLOGICAL VARIATION IN ESTUARINE POPULATIONS OF LITTORINA LITTOREA (LINNE, 1758) IN NEW ENGLAND

Biological and environmental sources of morphological variation were evaluated in populations of Littorina littorea from two estuaries in New England. Intraspecific morphological groups were characterized using linear discriminant function analysis on a suite of sixteen morphometric variables relating to length, width, height, weight, and recent growth history. Subsequent characterizations were based on a multivariate generalization of the allometric growth model. The parameters of each model were derived from the eigenvectors resulting from principal component analyses using within-group covariance matrices of a log transformed subset of seven variables.;In each estuary clinal trends exist that explain variable (depending on the intraspecific group) and usually small (< 50%) tendencies in each unique morphological group.;Three independent trends in morphological variation were identified that correspond to size, weight, and shape. These trends jointly explain more than 98% of the total variation observed. The significance of each trend among the observed intraspecific sources of variation varied between Rhode Island and Maine. Populations in Point Judith Pond, Rhode Island, were composed of three groups: normal snails, nonparasitized snails with elevated haemolymph carotenoid concentrations (= brown-footed), and snails parasitized by the digenean Cryptocotyle lingua. Normal and brown-footed snails were characterized by identical size and weight trends but unique shape trends. Parasitized snails possessed unique size and shape trends but were identical to normal and brown-footed snails in their weight trend. Populations from the Sheepscot River, Maine, were composed of the same three intraspecific groups as well as a group parasitized by the digenean Renicola roscovita which was pooled with those parasitized by Cryptocotyle lingua. An additional stratification of samples according to tidal height was possible in Maine. The size, weight and shape trends differ between high and low tide marks. At the high tide mark normal and brown-footed snails were characterized by identical size, weight and shape trends. Parasitized snails at the high tide mark differed from normal and brown-footed snails in size and shape trends but had the same weight trend. At the low tide mark normal and brown-footed snails possessed the same size and weight trends but differed in their shape trends. Parasitized snails at the low tide mark differed from normal and brown-footed snails in their size, weight and shape trends.