Mytilus californianus (Conrad, 1837) California Mussel
Synonyms:  None
Phylum Mollusca  Class Bivalvia   Subclass Pteriomorpha    Order Mytiloida     Family Mytilidae
	Mytilus californianus:  Very small individuals. Royal Palm State Beach, Los Angeles
	(Photo by: Dave Cowles, April 4, 1993)

Description:  This mussel shell has a thick profile and the anterior end (umbo) is sharply pointed.  It has strong radial ribs and irregular transverse growth lines.  Parts of shell are often eroded.  The periostracum is usually heavy and blue-black.  The shell interior is blue-gray, may be slightly iridescent.  Attach to rocks by byssal threads.

How to Distinguish from Similar Species:  The most similar local mussel species is the blue or bay mussel, Mytilus trossulus.  M. trossulus has a smoother shell and is often wider than M. californianus for its length.  Mature M. californianus are distinguished especially by the strong radial ribs and irregular transverse growth lines, which M. trossulus does not have.  M. galloprovincialis lives in the southern end of M californianus' range (from central CA south), and is nearly indistinguishable from M. trossulus.  Mixed clusters of M. californianus and M. galloprovincialis can be found on the open coast in southern areas.  M. trossulus seldom has much presence in the northern wave-exposed coasts, so exposed mussel beds are mostly M. californianus.

Geographical Range:  Aleutian Islands to Baja California

Depth Range:  Intertidal to 24 m

Habitat:  Common in intertidal zone 3 (lower midlittoral).  Less common subtidally.  Clings to rocks in wave-exposed areas, especially on the open coast.

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)

A study done on the related species Mytilus edulus (from the Atlantic) found that the movements of individual mussels correspond to a Levy walk pattern rather than a ballistic or Brownian pattern.  This resulted in creating clusters with small-scale crowding, along with larger-scale dispersion.  In this pattern mussels maximized the number of mussels immediately close to them (which reduced their chance of being dislodged by wave action) while at the same time minimized the number of mussels in the general area (which would increase competition for food).  [de Jager, Monique, Franz J. Weissing, Peter M. J. Herman, Bart A. Nolet, and Johan van de Koppel, 2011.  Levy Walks Evolve Through Interaction Between Movement and Environmental Complexity.   Science 332 pp 1551-1553.  A commentary on the article is on page 1514 of the same issue]

References:
Dichotomous Keys:

General References:
Morris et al., (1980)

Scientific Articles:
Carrington, Emily, Gretchen M. Moeser, Sean B. Thompson, Laura C. Coutts, and Carrie A. Craig, 2008.  Mussel attachment on rocky shores:  the effect of flow on byssus production.  Integrative and Comparative Biology 48:6 pp 801-807

Gosling, Elizabeth, 1992.  The Mussel Mytilus:  Ecology, Physiology, Genetics and Culture.  Developments in Aquaculture and Fisheries Science 25.  Elsevier.  563 pp.

López-Duarte, Paola C., Henry S. Carson, Geoffrey S. Cook, F. Joel Fodrie, Bonnie J. Becker, Claudio DiBacco, and Lisa A. Levin, 2012.  What controls connectivity?  An empirical, multi-species approach.  Integrative and Comparative Biology 52:4 pp. 511-524

Wootton, J. Timothy, 2002.  Mechanisms of successional dynamics: consumers and the rise and fall of species dominance.  Ecological Research 17 pp. 249-260

Zweibel, Jennifer A. and Markes E. Johnson, 1995.  Late Pleistocene mytilid and petricolid bivalves from the open rocky shores of Pacific Baja California (Mexico):  Unusual preservation of macrofossils.  Journal of Coastal Research 11:3 pp 704-716

M. californianus often dominates the lower intertidal zone on the open coast.  The seastar Pisaster ochraceous is an important predator.
Photo by Dave Cowles near Goodman Creek, 7-2004

A cluster of M. californianus on rocks at Little Corona del Mar, Ca.  Note how they cover much of large areas of the lower intertidal.  Photo by Dave Cowles, March 2005.

In areas exposed to very heavy surf, mussels may be most common in cracks or depressions where they have not been swept off by logs (above).  In other spots, they may be abundant on exposed rock faces but the aggregations often have an abrupt lower edge, probably due to predation by Pisaster ochraceous (below).  Photos by Dave Cowles, Sept 2005 at Shi Shi beach.

California mussels and the goosneck barnacle Mitella polymerus often compete for space in exposed areas of the lower intertidal.  Mytilus usually eventually wins unless it is removed by seastars.
Photo by Dave Cowles, September 2005 at Shi shi beach, WA

This species can grow very large.  Photo by Dave Cowles, Little Corona del Mar, CA March 2005

Individuals hold onto the substrate or to one another by byssal threads, which they secrete by a gland on their foot.  Photo by Dave Cowles, March 2003

When mussels cluster so tightly together they often attach to one another, making a several-mussel-thick pile of animals called a druse. Parts of the druse can then be torn off by waves and cast ashore. Photo at Kalaloch Beach #4 by Dave Cowles, July 2022

On mussels such as Mytilus californianus, only the posterior adductor scar is large.  In this photo anterior is to the left and posterior is to the right.
The pallial line is marked by the border between the whitish and dark nacre.  The oval-shaped, shiny scar near the pallial line on the right is the scar
for the posterior adductor muscle.
 

Mussel recruitment looked very good at Kalaloch in 2008.  This view shows an intertidal rock surface with many small mussels (about 1.5-2 cm long, probably first-year individuals) with many more smaller brown individuals (less than 1 cm long, probably newer recruits) packed tightly among them.  Such surfaces were common in summer 2008.  Photo by Dave Cowles, July 2008

A number of low boulders scattered across the sand at Beach #4 had abundant recruits again in 2024, as this photo shows. Most of these individuals are not likely to survive to adulthood due to sand scouring, possible burial under the sand as the beach shifts, predation by Nucella snails, as seen here, or by seastars. Photo by Dave Cowles, July 2024

Pisaster ochraceus is famous as a keystone predator on Mytilus californianus mussels. According to the studies that support the idea of Pisaster ochraceous as a keystone predator, the mussel Mytilus californianus competes for space on the intertidal rocks with barnacles such as Balanus glandula, Semibalanus cariosus, and Pollicipes polymerus. Mytilus californianus is a superior competitor for space and excludes the other species from the rocks unless Pisaster ochraceus eats enough of the mussel to make room for the other species. However, on the Olympic Peninsula coast including Shi Shi Beach which is just a few miles from where the keystone predator experiments took place, the situation is more complicated than that. The photos below, taken at Shi Shi Beach July 2024, show that, rather than being excluded by Mytilus californianus, mussels and even Fucus disticus algae can often grow on them instead and live in abundance covering the mussel. Pisaster ochraceus were also present but not in high numbers.

Clusters of young barnacles heavily encrusting Mytilus californianus

The array also includes bigger barnacles.

Even several species of algae are piling on, attached to the mussels.