The Fat Horsemussel (Modiolus capax) is a species of large, saltwater mussel in the family Mytilidae. It is commonly found along the Pacific coast of North America, from California to Peru, in sandy or muddy substrates in shallow to moderately deep waters. Thick, elongated, and somewhat oval with a smooth, glossy surface. The color varies from reddish-brown to dark purple. Filter feeder, primarily consuming plankton and organic particles from the water.