Phylum: Mollusca
Class: Bivalvia
Order: Filibranchia
Family: Mytilidae

Genus/species: Mytilus californianus

GENERAL CHARACTERISTICS: California mussels are relatively large mussels, with a blue-black shell with strong radial ribs and irregular growth lines. Maximum size is about 13 cm (5 in). The shell surface is often worn and eroded.


DISTRIBUTION/HABITAT:  California mussels form extensive beds, commonly mixed with gooseneck barnacles (Pollicipes polymerus). Attached firmly to the substrate by tough proteinaceous byssal threads, mussels are able to thrive in the most surf-swept areas. Mussel beds break the force of waves and collect organic debris, and so provide shelter and food for other organisms. Worms, chitons, snails, clams, isopods, crabs, and sea cucumbers are among the invertebrates found in mussel beds.

The upper limit of the mussel beds is set primarily by physical factors, particularly time out of water. The lower limit is set in part by the predation of the ochre star (Pisaster ochraceus), which practices vertical foraging, moving up and down with the tides. Mussels range from low intertidal to 40 m (131 ft) deep and are common in surf-swept, rocky areas, and are found from Alaska to southern Baja California.


DIET IN THE WILD: Mussels filter fine organic detritus and living plankton from sea water. A mussel filters 1.8 l to 2.8 l (2 to 3 qts) of water an hour.


PREDATORS: California mussels are favored food items of seas stars (especially ochre stars), crabs, predatory snails, shorebirds, sea otters, and humans. Sea otters have devastated formerly extensive mussel beds in Monterey Bay.


REPRODUCTION: Mussels are either male or female. During breeding they broadcast sperm or eggs into the sea where fertilization occurs. They spawn November to May. Mussel larvae are free-swimming for about four weeks. After settling and attachment they grow to full size in about three years.



  • The California mussel attaches to rocks by fibers called byssal threads. These threads are produced as a liquid by the byssal gland. The liquid runs down a groove formed by the foot. When the foot pulls back, exposing the liquid to sea water, the liquid solidifies into a thread.
  • A large mussel moves by breaking old threads, then attaching new ones to another spot; a small mussel creeps around on its foot.
  • By filtering algae spores from the water, mussels may limit algae growing around mussel beds.
  • Coastal areas are at risk from development and pollution. Mussels are also at risk because many of their homes are in danger of being ‘loved too much’ by too many tide pool visitors.


California Academy of Sciences Docent Guide 2015


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Encyclopedia of Life

Monterey Bay Aquarium…