Freshwater pearl mussels (Unionoida) are among the world's most gravely threatened fauna. In eastern North America, the group's center of evolutionary diversification, 35% of the 297 native species are presumed extinct, with another 69% formally listed as endangered or threatened. Human-introduced pollution, especially from agriculture and industry, as well as other forms of habitat alteration (dredging, damming) have been blamed for much of the decline. Such factors can adversely impact not only the mussels themselves, but also the obligate fish hosts of their larvae, potentially resulting in population declines. Introduced species, especially the Asian clam (Corbicula) and two species of zebra mussels (Dreissena) have further impacted unionoid populations through competition for space and food resources. One-hundred ninety-five species of bivalves have been placed on the 2002 IUCN Red List; all but 10 of these are freshwater pearl mussels. Twenty-nine species of freshwater pearl mussels are protected under CITES.
Marine bivalves are much less affected by human activities. There are no known recent extinctions in this group, and none are currently listed as threatened or endangered. Many species are partially protected by local and national laws regulating the commercial and private harvesting of shellfish. The giant clams (Tridacna, Hippopus; Tridacnidae) are the single marine group regulated internationally, as a result of overcollecting; eight species are included on the 2002 IUCN Red List, and the entire family is protected under CITES.
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