Enlarged lantern size in similar-sized, sympatric, sibling species of Strongylocentrotid sea urchins: From phenotypic accommodation to functional adaptation for durophagy

Abstract

Strongylocentrotus droebachiensis is a well known omnivorous sea urchin with an unrivalled capacity to destroy North Atlantic kelp forests. S. pallidus is a lesser known, morphologically similar, and closely related species with no record of destructive grazing, despite its larger lantern size. I quantify the lantern size of both species using bivariate allometric analysis, and test the hypothesis that enlarged lantern size facilitates durophagy, the consumption of hard prey, by measuring the feeding capacity of urchins with different lantern sizes when offered a hard-shelled prey, the blue mussel, Mytilus edulis. The results suggest that S. droebachiensis has a limited capacity for durophagous feeding irrespective of lantern size, whereas in S. pallidus the ability to exploit hard shelled prey is positively related to lantern size. This is apparently the first evidence of a relationship between trophic morphology and diet in regular sea urchins. The hypothesis of systematic latitudinal variation in the lantern size of S. pallidus is reappraised and rejected. S. droebachiensis had larger gonads than S. pallidus in field samples, confirming that its small lantern is not impeding nutrient acquisition in shallow habitats. © 2007 The Author(s).