Name:  California Floater (photo: K.S. Cummings, Illinois Natural History Survey)
   (Anodonta californiensis)
Status: State Candidate (WA), Federal Species of Concern
Description: A freshwater bivalve with a thin, reddish-brown shell
Threats: Habitat loss, decline of host fish species, predation by non-native fish

Overview:  The California floater is a mussel that lives in the shallow areas of clean, clear lakes, ponds and large rivers.  They prefer lower elevations and a soft, silty substrate to burrow into.  Their common name is derived from the tendency of Adonata species to float to the surface of the water after death, which is a result of gas build-up behind their thin shells.  The life cycle of a California floater includes a parasitic larval stage (called a glochidium), during which it is dependent upon host fish, usually a member of the Gila genus (minnow), for food and dispersal.  Larval California floaters have two hook-like projections within their shells which they use to attach to the fins of certain species of native fish.  The fish hosts form cysts around the glochidia, but remain unharmed by these little hitch-hikers.  After it reaches a certain size, the glochidium releases itself from its host, undergoes metamorphosis and begins its adult life as a sedentary filter-feeder, straining bacteria, plankton and detritus from the surrounding currents with its gills.   Adults begin to reproduce after reaching 6 to 12 years of age, and can live as long as 100 years.  Although a female floater may release several million larvae during the course of one year, survivorship is extremely low due to the specific requirements of finding and attaching to an appropriate fish host.  The decline of native host fish species has been identified as a likely cause of decline in populations of this species.  Other factors that continue to heavily impact populations of California floaters include pollution, sedimentation due to excess logging and grazing, predation by introduced fish species, and dam-building.  Dams, in particular, have changed the physical, chemical, and biological environment of a large number of streams, both upstream and downstream of the structure, to the point that approximately 30% to 60% of the mussel fauna within those streams have been destroyed.

Freshwater mussels were an important food source for Native Americans, who also used them for building tools and for decorative purposes. Today, the mussel is still highly regarded commercially by the cultured pearl industry, which uses the shells for seed pearl production.  Many species of freshwater mussels have declined to the point of being listed as endangered, threatened or species of special concern.  It is of particular concern that so many populations of these bivalves are ailing because of their special status as indicators of aquatic environmental health.

Click on map to enlarge

Distribution:   California floaters used to be distributed from southern British Columbia south to northern Baja California, and east to Wisconsin and Arizona.  Today, however, their numbers have been depleted to the point that they are extinct throughout much of their former range, including Utah, the entire Sacramento River system, and most of Arizona.  As can be seen from this map, verified sightings in Washington are limited to a few sites in Curlew Lake (Ferry County).  In Oregon, California floaters can still be found in the lower Willamette and lower Columbia Rivers.   Idaho's California floaters are present in Elmore, Gooding, Jerome and Twin Falls Counties.


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