The two valves are roughly triangular or bluntly oblong with rounded umbones near the anterior end.

The annual growth lines are clear and there is a fine sculpturing of concentric grooves and ridges.

[8] M. modiolus is found growing on hard substrates including shells and stones and the byssus threads of other mussels.

Juveniles growing on byssus threads are more likely to survive than free living individuals and this results in the formation of cold-water reefs of mussels.

The species is tolerant of low levels of oxygen and of a diminished quantity of the phytoplankton on which they feed.