California floater mussel (Anodonta californiensis)

Top, interior, and side views of several California floater shells collected by Lea 1852 and marked with museum collection number 86393
California floater mussel museum specimens (Smithsonian National Museum of Natural History Collections )
Closeup of group of three California floater mussels shallow freshwater habitat
California floater mussels in shallow freshwater (William Leonard - Copyright)
Category: Molluscs
State status: Candidate
Vulnerability to climate change (More details)

Moderate

If you see this species, please share your observation using the WDFW wildlife reporting form. Providing detailed information such as a photo and exact coordinates will improve the confidence and value of this observation to WDFW species conservation and management.

In Washington, the California floater mussel is known to occur in the Columbia River system and in a few other lakes and rivers in eastern Washington. The population size of California floater is low in the state, and the trend is declining. This mussel is declining or extinct over much of its range due to habitat alteration and pollution.

Freshwater mussels have been greatly affected by dams and annual water drawdowns, as well as degraded water quality resulting from development and agriculture. Many historical sites no longer support mussels, and many local populations no longer successfully reproduce.

Description and Range

Physical description

California floater is a bivalved freshwater mollusk. They have an oblong or ovoid shape, and they are up to 5 inches long. Their thin shell varies in color, including greenish-yellow, yellow-brown, light brown, or black. The posterior dorsal margin of the shell may have an upraised structure, forming a "wing." For more details (including photos), see the Xerces Society’s guide to Freshwater Mussels of the Pacific Northwest.

Taxonomic notes: Recent genetic research suggests that the California and winged floaters belong to a single clade, and that this clade exhibits basin-specific substructuring and may contain at least six distinct groups. A clade is a group of organisms that share a common ancestry. However, before new species or genus level designations are made, the taxonomy for the entire Unionidae family needs to be resolved.

Ecology and life history

Freshwater mussels are found in shallow habitats in permanent bodies of water, including creeks, rivers, and ponds generally at low elevations. California floaters occur in natural lakes, reservoirs, and downstream low-gradient reaches of rivers in pool habitats. Because their thin shells are prone to damage, floaters favor habitats of sand and silt substrates in lower gradient streams than those favored by western pearlshells and western ridged mussels; sandbars near the mouths of tributary streams or below riffles are important habitats. 

Watch this short video for a special glimpse into the watery world of the California floater mussel.

Mussels tend to concentrate in areas of streams with consistent flows and stable substrate conditions. They are often absent or sparse in high-gradient, rocky rivers, but are frequently encountered in low-gradient creeks and rivers, perhaps because they provide a variety of habitat conditions, reliable flow, good water quality, and diverse fish communities. The mussels are filter feeders that consume phytoplankton and zooplankton suspended in the water.

Freshwater mussels have a complex life cycle. They have separate sexes. California floater mussels grow quickly, reach sexual maturity in four to five years, and probably have a maximum life span of about 15 years. During breeding, males release sperm into the water and females filter it from the water for fertilization to occur. Embryos develop into larvae called glochidia, which are released into the water and must encounter and attach to a fin or gill filaments of host fish. Glochidia form a cyst around themselves and remain on a host for several weeks. They subsequently release from the host fish and sink to the bottom, burrow in the sediment and remain buried until they mature.

Like other freshwater mussels, California floaters rely on host fishes to reproduce and disperse. Because freshwater mussels are not able to move far on their own, their association with fish allows them to colonize new areas or repopulate areas from which they have been extirpated. During their lives, mussels may move less than a few yards from the spot where they first landed after dropping from their host fish. Host fish of the California floater may include Chiselmouth (Acrocheilus alutaccus) and northern pikeminnow.

Geographic range

Historically, California floater mussels were widespread west of the Continental Divide from British Columbia to Baja but are extirpated from many areas by dams. It is problematic to determine the distribution of this species because of its morphological similarity and confusion of taxonomy with the winged floater; this range description may prove to apply to several distinct species. Frest and Johannes (1995) reported the range has been reduced and extant populations were found in the following areas: the Middle Snake River in Idaho; the Fall and Pit Rivers in Shasta County, California; the Okanogan River in Chelan County, Washington; and Roosevelt and Curlew Lakes in Ferry County, Washington. This species is extirpated from much of its historic range, including the Willamette and lower Columbia Rivers and the Central Valley in California.

For a map of range-wide distribution and conservation status of this species, check out NatureServe Explorer.

Climate vulnerability

Sensitivity to climate change

Low-
Moderate

There is limited information regarding the sensitivity of California floaters to climate change. This species, which has already experienced significant declines over the past few decades, is generally found in shallow pools of freshwater streams and reservoirs with good water quality and a sufficient abundance of small fish who serve as hosts for mussels during their transition from the larval to juvenile stage. Therefore, their main sensitivity is likely to stem from climate-induced changes in water quality and host fish abundance. For instance, increased intensity of winter storms could lead to higher flow in rivers and increased nutrient runoff, both of which would degrade and reduce available mussel habitat. Additionally, increases in water temperature could lead to altered abundance of host fish for larval stage mussels, thus leading to declines in abundance. This species may also be sensitive to summer droughts, which could lead to shallower water levels in the pools that serve as mussel habitat, and potential air exposure and mortality, particularly since mussels have limited mobility and thus limited ability to respond to changes in habitat.

Confidence: Low

Exposure to climate change

Moderate-
High

  • Increased water temperatures
  • Altered flow regimes
  • Drought
Confidence: Low

Regulations

Rules and seasons

For current rules about harvesting shellfish in Washington, be sure to check the statewide Shellfish/Seaweed Species Rules.

Conservation

This species is identified as a Species of Greatest Conservation Need (SGCN) under the State Wildlife Action Plan (SWAP). SGCN-classified species include both those with and without legal protection status under the Federal or State Endangered Species programs, as well as game species with low populations. The WDFW SWAP is part of a nationwide effort by all 50 states and five U.S. territories to develop conservation action plans for fish, wildlife and their natural habitats—identifying opportunities for species' recovery before they are imperiled and more limited.
This species is identified as a Priority Species under WDFW's Priority Habitat and Species Program. Priority species require protective measures for their survival due to their population status, sensitivity to habitat alteration, and/or recreational, commercial, or tribal importance. The PHS program is the agency's main means of sharing fish and wildlife information with local governments, landowners, and others who use it to protect priority habitats for land use planning.

Conservation Threats and Actions Needed

  • Agriculture and aquaculture side effects
    • Threat: Water level fluctuations; pollution.
    • Action Needed: Protect water quality.
  • Resource information collection needs
    • Threat: Taxonomic uncertainty may mean one or more taxa are in greater decline.
    • Action Needed: Taxonomic clarification.

See the Climate vulnerability section for information about the threats posed by climate change to this species.

Resources

References

Frest, T. J. and E. J. Johannes. 1995. Interior Columbia Basin mollusk species of special concern. Final report to the Interior Columbia Basin Ecosystem Management Project, Walla Walla, WA. Contract #43-0E00-4-9112. 274 pp. plus appendices.

Jepsen, S., C. LaBar,and J. Zarnoch. 2012. Profile: Western pearlshell (Margaritifera falcata). The Xerces Society. 24 pp.

Jepsen, S., C. LaBar,and J. Zarnoch. 2011. Profile: California floater (Anodonta californiensis) / Winged floater (Anodonta nuttalliana). The Xerces Society. 31 pp.

Jepsen, S., C. LaBar, and J. Zarnoch. 2011. Profile: Western ridged mussel (Gonidea angulata) The Xerces Society. 19 pp.

Nedeau, E. J., A. K. Smith, J. Stione, and S. Jepsen. 2009. Freshwater Mussels of the Pacific Northwest. 2nd edition. The Xerces Society. 51pp.

WDFW publications

PHS Program

Other resources