A lateral plate dine, sexual dimorphism, and phenotypic variation in the black-spotted stickleback, Gasterosteus wheatlandi

Abstract

We examined samples of Gasterosteus wheatlandi collected in coastal marine waters from northern Maine to southern New York. Gasterosteus wheatlandi exhibited remarkable variation in lateral plate number, including phenotypes similar to the low, partial, and complete morphs described in Gasterosteus aculeatus. Partials and completes, however, seemed to represent a continuum, thus we could only recognize two discreet phenotypes, low and partial–complete. The low phenotype predominated strongly north of Cape Cod and was rare or absent south of it. Within both phenotypes mean plate number increased toward the southern end of the range; thus, both lateral plate number and lateral plate phenotype exhibited a pronounced step-cline. We found significant sexual dimorphism among localities for total lateral plates and for a suite of other morphological characters. Both sexes had higher mean vertebral counts north of Cape Cod, a trend which ran counter to the cline in lateral plates. The pervasive sexual dimorphism in G. wheatlandi, and the fact that latitudinal variation in lateral plates ran counter to the trend in G. aculeatus, suggest that different processes may be responsible for maintaining the geographical variation in these two species.