Biology/Natural History: A very common inhabitant of the lower midlittoral (zone 3), where it often dominates the substrate in waveswept areas (picture). Specializes in living on large boulders and bedrock. Can move slowly from place to place by systematically breaking and remaking byssal threads. A filter feeder, filters 2-3 liters/hour. Spawns all year but spawning peaks in July and December in CA. A favorite prey of the seastar Pisaster ochraceous (picture). Small mussels are eaten by seabirds and by the oyster drill snails Nucella emarginata,Ceratostoma nuttali, and Roperia poulsoni, although it is less vulnerable to predation by snails than is M. trossulus (Wootton, 2002). In central CA parasitic isopods are often in the mantle cavity, as is also a pycnogonid and a pea crab. May become poisonous in summer months through ingestion of dinoflagellates, especially Gonyaulax catanella (causes paralytic shellfish poisoning).

Although this species may experience high flow in the intertidal environment due to wave action, byssal thread production seems to be limited to flows of < 50 cm/s. Mussel aggregations sharply reduce water flow within them and make possible the production of byssal threads (Carrington et al., 2008)