Mytilus californianus

Geographic Range

Mytilus californianus, the coast mussel, is found along the coast of the eastern Pacific Ocean, from southern Baja California, Mexico, and to southeastern Alaska. (Dehnel, 1956; Haderlie and Abbott, 1980)


The coast mussel is a marine species that makes its home on submerged, hard surfaces within the middle intertidal zone to subtidal areas up to 24 meters in depth. They favor surf zones and attach themselves to exposed rocks, pilings, or other conspecifics in those areas. (Dehnel, 1956; Haderlie and Abbott, 1980)

  • Range depth
    24 (high) m
    78.74 (high) ft

Physical Description

Coast mussels are bivalve mollusks with blue-black, thick shells that are pointed at the anterior end, with strong radial ribs, and irregular growth lines. The shell exterior is often worn and eroded due to wave action, while the shell interior is blue-grey with an iridescent sheen. Studies of the shell microstructure show three calcified layers. The outer layer of the shell is calcite, with a middle layer of nacreous aragonite, and an inner prismatic calcite layer. Mussels secrete protein fibers called byssal threads from a gland in the foot. The byssal threads are used to attach the animals to hard surfaces and even to each other. Average adult size (130 to 150 mm) is attained in three years, though mussels up to 251 mm have been found in Baja California. Mussels in the lower intertidal zones usually grow larger and more rapidly than those found higher up in the intertidal zone, because mussels in deeper water are immersed for longer periods and have longer opportunities to filter feed. (Bayne, et al., 1976; Dehnel, 1956; Haderlie and Abbott, 1980)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    130 to 150 mm
    5.12 to 5.91 in


Mussels release their gametes into the water column, and fertilization is external. Within 24 hours of fertilization, the zygotes metamorphose into free-swimming trochophore larvae that develop into the veliger stage, again within 24 hours. The veliger is also known as the “straight-hinge” stage because of the appearance of the shell. Within two to three weeks, the veligers settle out of the water column, metamorphosing from planktonic larvae into a bottom-dwelling pediveligers. The young mussel now has a foot that it can use to move around on the ocean floor, searching for a suitable attachment surface. After attachment, young mussels can detach their byssal threads and crawl to a different location, then re-attach. Growth is fastest in mussel larvae in northern latitudes. In southern California, coast mussels reach about 86 mm in length within their first year of attachment, 120 mm at two years, and 150 mm by the end of three years. Highest growth rates of sexually immature mussels have been observed under conditions of high dinoflagellates abundance. Temperature plays a part in the growth rate to a certain degree. The mussels grow to an optimum size and then growth rate rapidly decreases. Mussels in warmer southern waters grow longer because the longer season is more conducive to rapid growth. Experimental studies have demonstrated that adult mussels in California and Alaska grow at similar rates. The largest difference in growth rate is determined by duration of tidal submergence. The mussels at lower intertidal regions are rarely exposed, and thus have more time to feed, growing faster. (Coe and Fox, 1944; Dehnel, 1956; Haderlie and Abbott, 1980)


As sessile, broadcast-spawning invertebrates, mussels do not attract, pursue, or defend mates, but simply spawn synchronously in response to chemical cues from nearby conspecifics. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Breese, et al., 1963; Haderlie and Abbott, 1980)

Some spawning occurs throughout the year in coast mussels. In California, the peak breeding times are during July and December. When spawning occurs in the wild, eggs and sperm are released into open water where fertilization takes place. In the lab, mussels can be induced to spawn through mechanical stimulation, by scraping the shells or by pulling the byssal threads. In laboratory experiments, spawning can be incited by bathing mussels in solutions of chemicals which mature the ovaries. Kraft mill effluent (KMI) is also effective in stimulating spawning. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Breese, et al., 1963; Haderlie and Abbott, 1980)

  • Breeding season
    Peak breeding times are during July and December, though spawning occurs year round.
  • Average gestation period
    24 hours
  • Average age at sexual or reproductive maturity (female)
    3 years
  • Average age at sexual or reproductive maturity (male)
    3 years

Mussels release their gametes into the water column, and fertilization is external. There is no parental involvement. (Breese, et al., 1963; Coe and Fox, 1944; Dehnel, 1956)

  • Parental Investment
  • no parental involvement


The coast mussel reaches adult size in three years. Beyond that point, no information was found on its ultimate lifespan. (Breese, et al., 1963)

  • Typical lifespan
    Status: wild
    3 (low) years


Communities of mussels can grow into very large groups, if predators are absent. A moderate-sized group may consist of about one million individual mussels. Newly-settled mussels often attach on top of older mussels in crowded mussel beds. This can prevent the mussels underneath from filter feeding and subsequently they die. Mussels lack antagonistic or territorial behaviors. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Breese, et al., 1963; Coe and Fox, 1944; Haderlie and Abbott, 1980)

Communication and Perception

When in their pediveliger stage, young coast mussels develop an eyespot. The eyespot is a photo-sensitive organ that allows the pediveliger to distinguish between light and dark, but this organ is lost after the young mussel attaches and grows larger. Intertidal mussels perceive submergence and exposure cycles, as noted by them initiating or ceasing their feeding behaviors. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Breese, et al., 1963)

Food Habits

Coast mussels are filter feeders. When the animals are submerged, their valves open slightly and the mussels use cilia to move water through the gills. Planktonic food suspended in the water is caught in mucus, sent to the palps for sorting, and then to the mouth. Mussels consume dinoflagellates, organic particles, small diatoms, zoospores, protozoa, unicellular algae, bacteria, and detritus. Phytoplankton is the main food source that provides energy for rapid growth. Average-sized mussels filter 2 to 3 liters of water per hour when feeding. Coast mussels share tidal rhythmicity with their rate of water propulsion. Mussels feed when they are submerged and feeding rates mirror the high tide cycle, even if mussels are taken from the underside of floats or if they are moved into a lab. This indicates that once feeding rhythm is set, it stays constant, regardless of outside influences. ("Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Coe and Fox, 1944; Dehnel, 1956; Haderlie and Abbott, 1980; Rao, 1953; Rao, 1954)


There are many different animals that prey on coast mussels. The larval forms are fed upon by zooplanktivorous fish, including the shiner surfperch Cymatogaster aggregata. Small mussels are eaten by crabs, shore birds, and by predatory gastropods that bore holes through the mussel's shell and feed on its soft tissues. Larger mussels are preyed upon by sea stars (particularly Pisaster ochraceus), the sea otter, and humans. The lower limit of mussel populations in the intertidal zone is primarily determined by the presence of the predatory sea star Pisaster ochraceus. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Coe and Fox, 1944; Dehnel, 1956; Haderlie and Abbott, 1980)

Ecosystem Roles

A mature coast mussel bed grows to such proportions that it becomes the structural basis of a community to many other marine creatures as well. Over 300 species have been identified from a mussel bed community. Gastropods, oysters, annelid worms, and even octopuses make their homes in and around the mussel bed, exploiting the interstitial spaces between the mussels as refuges from predators. Mussels also compete for attachment and growing space with barnacles in areas exposed to heavy wave action, with mussels emerging as dominant competitors, crushing and smothering acorn barnacles (Balanus glandula), gooseneck barnacles (Pollicipes polymerus), and macroalgae. Mussels may be parasitized by a pea crab or sea spider; these symbionts consume bits of the mussels' gill and gonad tissue. (Haderlie and Abbott, 1980; Lubchenco and Menge, 1978; Ricketts, et al., 1992)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

Native American coastal tribes ate coast mussels and people still eat them today. Coast mussels have also been used as bait by recreational fishermen since the early 1900’s. According to California State Tax records and the Department of Aquaculture Harvest Survey Database, the amount of mussels harvested in the wild is significantly smaller than what is harvested from aquaculture. In 1986 the value of cultured mussels was $267,693 with the value of wild caught mussels at only $10,282. Since 1986, the differences between harvest rates have stayed consistent, with aquaculture far outproducing wild-harvested mussels. Between 1986 and 2008, the total economic value of the California mussel fishery was $14,499,845. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Haderlie and Abbott, 1980; Rao, 1953; Rao, 1954)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

Mytilus californianus must not be harvested during the summer months, because of the high concentration of dinoflagellates in the water. Dinoflagellates are planktonic microorganisms that may be ingested in great quantities by the mussels. Some dinoflagellate species contain a toxin which can become concentrated in mussel tissue and may cause paralysis and death if humans consume contaminated mussels. ("Culture of Mussels", 2001; "Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries", 2008; Haderlie and Abbott, 1980)

  • Negative Impacts
  • injures humans
    • carries human disease

Conservation Status

Coast mussels are not considered endangered; they are in fact, quite prolific, and among the most common invertebrates present on the rocky shores of California. While not under specific conservation protection measures, they are protected where they occur within MPAs (Marine Protected Areas) along the northeastern Pacific Coast. (Haderlie and Abbott, 1980)

Other Comments

Mussels are extremely tasty in a garlic wine sauce!


Elizabeth A. Neal (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Angela Miner (editor), Animal Diversity Web Staff.


Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


A substance that provides both nutrients and energy to a living thing.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

native range

the area in which the animal is naturally found, the region in which it is endemic.


chemicals released into air or water that are detected by and responded to by other animals of the same species


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate

year-round breeding

breeding takes place throughout the year


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


California Department of Fish and Game. Culture of Mussels. unknown. Sacramento, CA: California Department of Fish and Game. 2001. Accessed December 07, 2013 at‎.

California Department of Fish and Game. Status of the Fisheries Report 2008: Culture of Mussels (Mytilus spp.) and Mussel Fisheries. 20. Sacramento, CA: California Department of Fish and Game. 2008. Accessed October 28, 2013 at

Bayne, B., C. Bayne, T. Carefoot, R. Thomson. 1976. The physiological ecology of Mytilus californianus Conrad. Oecologia, 30(1): 211-228.

Breese, W., R. Millemann, R. Dimick. 1963. Stimulation of Spawning in the Mussels, Myltilus edulis, Linnaeus and Mytilus californianus Conrad, by Draft Mill Effluent. The Biological Bulletin, 125/2: 197-205.

Coe, W., D. Fox. 1944. Biology of the California Sea-Mussel (Mytilus californianus). III. Environmental Conditions and Rate of Growth. The Biological Bulletin, 87/1: 59-72.

Dahlhoff, E., B. Menge. 1996. Influence of Phytoplankton Concentration and Wave Exposure on the Ecophysiology of Mytilus californianus. Marine Ecology Progress Series, 144: 97-107.

Dehnel, P. 1956. Growth Rate in Latitudinally and Vertically Seperated Populations of Mytilus californianus. The Biological Bulletin, 110/1: 43-53.

Gilbert, M. 1973. Growth Rate, Longevity and Maximum Size of Macoma balthica (L.). The Biological Bulletin, 145: 119-126.

Haderlie, E., D. Abbott. 1980. The Clams and Allies. Pp. 355-411 in R Morris, D Abbott, E Haderlie, eds. Intertidal Invertebrates of California. Stanford, California: Stanford University Press.

Lubchenco, J., B. Menge. 1978. Community Development and Persistence in a Low Rocky Intertidal Zone. Ecological Monographs, 48/1: 67-94.

Rao, K. 1953. Rate of Water Propulsion in Mytilus californianus as a Function of Latitude. The Biological Bulletin, 104: 171-181.

Rao, K. 1954. Tidal Rhythmicity of Rate of Water Propulsion in Mytilus and its Modifiability by Transplantation. The Biological Bulletin, 106/3: 353-359.

Ricketts, E., J. Calvin, J. Hedgpeth. 1992. Between Pacific Tides (5th ed). Palo Alto, CA: Stanford University Press.