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8 BIOLOGICAL REPORT 90(5) Biology and Ecology of Mollusks in Streams Richard J. Neves by U.S. Fish and Wildlife Service Virginia Cooperative Fish and Wildlife Research Unit Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 The indigenous freshwater mollusk fauna of the United States consists of about 765 species, in- cluding 500 species of snails, 40 species of finger- nail clams, and 225 species of mussels. Two major subgroups of snails, pulmonates (lunged) and proso- branchs (giBed), have adapted to most freshwater environments. Pulmonate snails lack an operculum, respire via a pulmonary sac, are hermaphroditic (monoecious), and form light shells for a fairly ac- tive existence. In contrast, pros branch snails have an operculum, respire through 'lls, have separate sexes, and form heavier shells f r a more sedentary lifestyle. The 15 families of s ails in freshwater rivers in the southeastern nited States are dominated by the Pleurocerid . These snails are small to medium in size and ha e distinctive solid, dextral shells, which are used for species identifica- tion. Pleurocerids are most abundant on cobbles and boulders in rocky shoals. Freshwater fingernail clams (Pisidiidae) and the exotic asiatic clam (Corbicula fluminea) are small bivalves (<35 mm) that occupy both lotic and lentic habitats. Pisidiids are hermaphroditic, reproduce once or twice each year, and brood their young in marsupial sacs. The asiatic clam is also hermaphro- ditic, spawning twice a year and brooding embryos in its gills. It lives 2-3 years and achieves a max- imum size of 35 mm in most rivers. Since 1924, this species has spread from the Pacific Northwest and now occurs in roughly 40 States. Because of its high reproductive potential, fast growth rate, short generation time, and high population densities, this. exotic clam may compete with-and even- tually displace-some of the indigenous bivalve fauna. Ofthe 225 freshwater mussel species (Unionidae) in the United States, 34 are listed by the Federal Government as endangered; most of these occur in the Southeast, and many more species have been proposed for listing. The mussel reproductive cycle is similar for all species. During the spawning period, males release sperm into the water column. The sperm are then taken in by females during siphon- ing. Eggs are fertilized and incubated in the gills until larvae (glochidia) are mature. Glochidia are released into the water and must attach to the fins or gills of appropriate host fishes to encyst and metamorphose to the free-living juvenile stage. Juveniles are about 200 ~m and require roughly 5 years to become sexually mature. Mussels are the longest-lived freshwater mollusks; individuals of some species have been aged at more than 50 years. The extreme longevity and the annual release of prodigious numbers (10,000-100,000) of glochidia per female provide the necessary reproductive potential to contact appropriate fish hosts and con- tinue recruitment. Habitat requirements for most unionids are fair- ly specific: clean, flowing water and a stable sub- stratum of mixed particle sizes. In the Southeast, declines in the mussel fauna have been attributed principally to river impoundments, siltation, and water pollution. Numerous hydroelectric and flood control reservoirs have eliminated riverine condi- tions in most drainages, altering substrate and changing fish species composition. Poor land-use and mining practices have silted many streams and rivers, eliminating or reducing mussel populations in many river reaches. Point and nonpoint pollution have resulted in acute and chronic mortalities from herbicides, toxic spills, heavy metal discharges, sewage plant effluents, coal waste deposition, and many other anthropogenic activities. Over the last decade, an improvement in water quality in many rivers has occurred, which may allow the natural recolonization and recovery of some species. How- ever, most endangered species are not likely to recover without the implementation of specific activities identified in recovery plans. Additional research and improvements in environmental quality