Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Mytilus (California mussel)

Mytilus californianus (Conrad 1837)

Phylum Mollusca, class Bivalvia, order Mytiloida, family Mytilidae

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Description

Mussel shell to about 130 mm long. Shell is a bluish-black color, often with eroded white valves and darker at margins. Anterior end of shell is sharply pointed. Prominent radial ribbing but also concentric growth lines present (Morris 1980).

Habitat and Geographic Range

Abundant, often on surf-exposed rocks and pier pilings. Found mainly in upper-middle intertidal zone on outer coast and can be found subtidal and offshore to 24 m (depth). Aleutian Islands, Alaska to southern Baja, California (Morris 1980).

Synonyms

None

Similar species

Bay mussels, Mytilus galloprovincialis and Mytilus trossulus, can co-occur with M. californianus but are most common in sheltered habitats because of their weaker byssal threads (Morris et al. 1980). They have smooth valves, lacking radiating ridges, with strong, elbow-curve at umbo and are less eroded than M. californianus.

Natural History

The California mussel forms extensive beds, which may be multi-layered (usually in the northern part of its range). Mussels attach to hard substrate by secreting byssal threads at the base of the foot (Morris et al. 1980). Byssal thread production appears to be possible only when water flow is < 50 cm/s. Although wave action in the intertidal results in flow rates much higher than this, mussel aggregations greatly reduce water flow within the beds and make possible the production of byssal threads (Carrington et al. 2008). Thick (³ 20 cm) beds of California mussels trap water, sediment, and detritus that provide food and shelter for an incredible diversity of plants and animals, including cryptic forms inhabiting spaces between mussels as well as biota attached to mussel shells (Paine 1966; MacGinitie & MacGinitie 1968; Suchanek 1979; Kanter 1980, Lohse 1993). For example, MacGinitie & MacGinitie (1968) counted 625 mussels and 4,096 other invertebrates in a single 25 cm² clump, and Kanter (1980) identified 610 species of animals and 141 species of algae from mussel beds at the Channel Islands. Kinnetics (1992) documented locational differences in the composition and abundance of mussel bed species. Northern sites had densely packed, multi-layered beds, but the more open southern sites had higher species diversity.

The California mussel spawns all year but spawning peaks in July and December in CA. Young mussels settle preferentially into existing beds at irregular intervals, grow at variable rates depending on environmental conditions and eventually reach ages of 8 years or more (see Morris et al. 1980, Ricketts et al. 1985). M. californianus is a filter feeder, and is quarantined from collection/consumption from late spring to early autumn because the toxin from a dinoflagellate accumulates in the tissue (Kozloff 1983). This toxin can cause paralysis and death.

While mussels can tolerate typical rigors of intertidal life quite successfully, desiccation likely limits the upper extent of mussel beds, storms tear out various-sized mussel patches and sea stars prey especially on lower zone mussels. Beds that are already patchy or thinned by human disturbance (e.g. via trampling or collection for bait) have increased susceptibility to wave damage. Mussels have also been found to be adversely affected by oil spills (Chan 1973; Foster et al. 1971). Recovery from disturbance varies from fairly rapid (if clearings are small and surrounded by mussels that can move in) to periods greater than 10 years (if clearings are large and recruitment is necessary for recolonization) (Vesco & Gillard 1980, Kinnetics 1992).

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