Description : The California mussel is blue-black with strong radial ribs and irregular growth lines, often with surface eroded or worn. The interior of the shell is blue-gray, somewhat iridescent, darker at margins. The shell usually shows radiating ribs, especially around the outer edges. As they mature, they begin to secrete by a gland at the base of the foot a string-like substance . It is called a byssal thread. Byssal threads hold the mussels firmly to rocks and other mussels. Once they are matured, the mussels are unable to regenerate their byssal thread.

Size: Length 130 mm - 150 mm (largest collected off Baja California 250 mm)

Range: Alaska to southern Baja California.

Food: California mussels open their shells slightly to eat fine organic detritus (plankton) suspended in the seawater. Mussels of average size filter two to three liters of water per hour when feeding. Mussels have been observed to feed at temperatures as low as 7 degrees C and as high as 28 degrees C, and to survive salinities of 50-125 percent that of seawater.

Habitat: California mussels are abundant where there is surf along the outer coast. They form massive beds, sometimes several metres across, on surf exposed rocks and wharf pilings in the upper middle intertidal zone and offshore to 24 metres deep. Like barnacles, mussels like to gather in dense clusters in the middle intertidal zone. Mussel beds are often large enough to be a home to many other creatures, like snails and worms. California mussel beds accumulate a gritty mixture of sand and bits of shell which provide a home for a various assemblage of animals including a number of polychaete worms, snails, crabs and a blackish sea cucumber Cucumaria pseudocurata.

Reproduction: The breeding season extends throughout the year although peaks reported in California are in July and December. California mussels broadcast sperm or eggs, depending on their sex, into the sea where fertilization takes place. Sexes are separate.

Natural history and predators:

California mussels are good to eat. Coastal Indians knew this for ages and modern residents still collect them. In summer months mussels may become poisonous and they should be avoided because concentrations of toxins from red tide. In California, mussels are quarantined between May 1 and October 31.

They have blue-gray bodies that contrast against sea stars, their predators. Ochre sea stars are California mussels' main predator. Ochre sea stars affect the distribution of the mussels because the sea star generally occurs lower on the shore than the mussels and moves up to feed during high tide. Mussels are also eaten by shorebirds, crabs and snails (they drill holes in shells). When a predator eats the dominant species, then predation may prevent the dominant species from monopolizing a limited resource, thereby increasing diversity of species in that area.

California mussels are firmly attached to the rocks and each other by byssal threads. Scientists are trying to synthesize the material that forms the threads to use as a marine adhesive.

The pea crab (as one can see in the picture), Fabia subquadrata is found in 1 to 3% of California mussels along the central California coast and 18% of mussels along Vancouver Island. This is a parasite that lives within the shell, because they rob food from their host and sometimes damage one of the gills. Public health codes usually prohibit the marketing or serving of parasitized animals but since the pea crab is very tasty, organisms with this crab are sometimes sold.

Moreover, in a small portion of the population you can find imperfect pearls. These are of no value.

http://www.lanecc.edu/science/zonation/mussel.htm (accessed 31 January 2002)

Morris, R., P. Abbott and E. Haderlie. 1980. Intertidal Invertebrates of California. Stanford University Press, Stanford, California. 690 pages.