Lamarck, 1819
Description:
Shell brittle, oval, subtriangular, or pear-shaped, umbones prominent, pointed, and slightly curved ventrally. Posterior to umbones ventral margin is slightly concave and both shell edges form a flattened area. Sculpture of fine concentric lines, growth stages clear.
This species is readily confused with Mytilus edulis and many authorities have considered that only one species is present in British waters. The most useful distinguishing features of M. galloprovincialis are the beaked, downturned umbones, the degree of flattening of the ventral margin and the colour of the mantle edge. However, both species of Mytilus display great variation in shell morphology through the interaction of ontogenetic and environmental factors, and old specimens may be almost impossible to identify with certainty. As mixed populations of both species are found over much of the northern range of M. galloprovincialis , the possibility of hybridization may add a further complication.
Colour:
Colour blue to deep purplish black, periostracum light brown to blue-black; without the rays of colour typical of Mytilus edulis . Inner surfaces white beneath umbones, becoming bluer and darker posteriorly.
Animal:
Mantle edge of live specimens typically purple.
Habitat:
Intertidal.
Distribution:
South-west England, South Wales, and southern and western Ireland. From Cherbourg it extends westward along the coasts of France and south to the Mediterranean. Probably not in the North Sea.