Anodonta californiensis

Considerable taxonomic confusion surrounds this species complex. Williams et al. (2017) retain Anodonta californiensis, A. kennerlyi, A. nuttalliana, and A. oregonensis based on their phylogenetic affinity to Eurasian Anodonta (Mock et al. 2004; Zanatta et al. 2007; Chong et al. 2008). O'Brien et al. (2019) supports the distinctiveness of A. nuttalliana and A. californiensis.

Anodonta dejecta was recognized by Turgeon et al. (1998), Graf and Cummings (2007), and Cummings and Graf (2010). This species is treated as a synonym of A. californiensis by Bequaert and Miller (1973) and the Arizona Game and Fish Department (2017) and is therefore placed in synonymy of A. californiensis by Williams et al. (2017).

Since the time of Call (1884) there has been much confusion regarding the taxonomic status of this and other floaters (Anodonta) of western North America. Isaac Lea (1838) described Anodonta wahlametensis, Anodonta nuttalliana, and Anodonta oregonensis from the same site ("Wahlamet [Willamette River], near its junction with the Columbia River [Oregon]") all in the same publication. Call (1884) considered Anodonta nuttalliana to include, as synonyms, Anodonta wahlametensis, Anodonta oregonensis, and Anodonta californiensis. Other authors (e.g., Burch, 1975, Clarke, 1981; Turgeon et al., 1998), however, have considered A. californiensis, A. nuttalliana, and A. oregonensis to be distinct. Some authors even continue to recognize Anodonta wahlamatensis as a distinct species (Frest and Johannes, 1995; Taylor, 1981; Henderson, 1929) while most place it in the synonymy of A. nuttaliana (Burch, 1975; Turgeon et al., 1998). Whether A. wahlamatensis should be removed from the synonymy of A. nuttalliana will depend on future anatomical and genetic work on western Anodonta. According to T. Frest, Anodonta nuttalliana has been revised to the following; Anodonta nuttalliana nuttalliana and Anodonta nuttalliana wahlametensis = Anodonta wahlametensis, and, Anodonta nuttalliana idahoensis and Anodonta nuttalliana californiensis = Anodonta californiensis (pers. comm. Amy Stock, WA-NHP, 1996). Mock et al. (2004; 2005) found a lack of resolution (very little nuclear diversity) in phylogenetic reconstructions of Anodonta (A. californiensis, A. oregonensis, A. wahlamatensis) populations in the Bonneville Basin, Utah, but there was a tendency for the Bonneville Basin Anodonta (tentatively A. californiensis) to cluster with A. oregonensis from the adjacent Lahontan Basin in Nevada. Zanatta et al. (2007) supported the monophyly of both Pyganodon and Utterbackia using mutation coding of allozyme data, but also resolved the Eurasian Anodonta cygnea to Pyganodon, Utterbackia, and North American Anodonta; indicating futher phylogenetic analysis of the Anodontinae is required including both North American and Eurasian species. In a phylogenetic analysis of western North American Anodonta using topotypic material as was available, Chong et al. (2008) found three deeply divided lineages: one clade including Anodonta oregonensis and Anodonta kennerlyi, one clade including Anodonta californiensis and Anodonta nuttalliana, and one clade including Anodonta beringiana.

This species was once thought to have been widespread in the Pacific Drainage from British Columbia into Mexico, but there is considerable taxonomic confusion as to the placement of the western North American Anodonta species. The current range however is patchy, and it has been lost from many parts of its former range due to many ongoing threats.

This species occurs in British Columbia, Canada, in the western United States from Washington to Wyoming, south to Arizona and west to California (Frest and Johannes 1995; Hovingh 2004; Mock et al. 2005; RARECAT 2025; NatureServe 2026). There is still some taxonomic uncertainty in this genus and so some records may be deemed that of congeners with additional taxonomic resolution (Williams et al. 2017; O'Brien et al. 2019).

This species is found in at least approximately 80 occurrences using a 10 km separation distance and records from 1994-2026 (Frest and Johannes 2000; Mock et al. 2004; Hovingh 2004; Nedeau et al. 2005; Cvancara 2005; WNHP 2008; Howard 2010; BCCDC 2013; Nevada Natural Heritage Program 2018; Arizona Heritage Data Management System 2021; RARECAT 2025; NatureServe 2026).

This species is threatened by sedimentation, nutrient, and chemical pollution from land conversion activities, including development, agriculture and industrial activities, dams and water diversions that change hydrological conditions and prevent habitat connectivity and recolonization, habitat alteration due to climate change, including increased water temperatures and drought, as well as predation by non-native fish and introduced crayfish, and a possible link to reduced populations of native fish that serve as larval hosts (WNHP 2008; BCCDC 2013; Nevada Natural Heritage Program 2018; Arizona Heritage Data Management System 2021).