Northern Horsemussel, Modiolus modiolus

The Northern Horsemussel, Modiolus modiolus, is a species of marine bivalve mollusk belonging to the family Mytilidae.

It is a large mussel growing to 9 inches long though 4 inches is the more typical size. The shell is a purplish or dark blue and robust with horny protuberances when young. The two valves are roughly triangular or bluntly oblong with rounded umbones close to the anterior end. The annual growth lines are clear and there is a fine sculpturing of concentric grooves and ridges. The interior of the shell is white with a broad pallial line, large anterior adductor muscle scar and smaller posterior adductor muscle scar. The body is a deep orange color and the mantle is unfrilled. The shell is firmly attached to the substrate by byssus threads.

This mussel can be found along the Atlantic coast of North America, from the Arctic Ocean to Florida, and along the Pacific coast, from the Arctic Ocean to California. It can also be found on the European seaboard of the Atlantic Ocean from the United Kingdom northwards. It’s seen from low tide mark to depths of 50 meters in British waters and 80 meters off of the coast of Nova Scotia.

The survival rates of the young individuals are low but by the time they reach 4 centimeters long, at an age of four years, the individuals are too large and tough to be predated upon by starfish, crabs, and the whelk Buccinum undatum. The juveniles growing on byssus threads are more likely to survive than the free living individuals and this results in the formation of cold water reefs of mussels.

Image Caption: Modiolus modiolus, the northern horsemussel. Credit: Georges Jansoone/Wikipedia