Phenotypical Improvement of Morphological Defences in the Mussel Mytilus edulis Induced by Exposure to the Predator Asterias rubens

Abstract

Blue mussels, Mytilus edulis, were cultured in field enclosures in close vicinity of, and in absence of its predator, the starfish Asterias rubens. After four weeks, the morphology differed, such that predator-exposed mussels were significantly smaller in outer size (shell length, height and width), but had significantly larger posterior adductor muscle, thicker shell, and more meat per shell volume. These morphological changes suggested an improvement of the defences, which was confirmed in a predation assay where starfish needed significantly longer time to open predator-exposed mussels. The adaptive value was also demonstrated by a significantly larger survival of predator-exposed mussels in an assay where starfish foraged on a mixed population of exposed and control mussels. The experiment shows that predator-induced morphological plasticity exists in M. edulis. Such phenotypic plasticity may explain some of the morphological variation that is found in the field.