I7S 



linfrtehiiilcx — Our Living Resources 



Total number 



Fig. 1. Number of species and 

 subspecies of freshwater mussels 

 historically known to occur within 

 each state and the percentage now 

 classified as imperiled. 



Freshwater mussels from the 

 Tombigbee River at Memphis 

 Landing, Pickens County. 

 Alabama. Southern combshell 

 (Epiohlasma penita); female, top. 

 male, bottom. 



(Turgeon et al. 1988). Distribution data and con- 

 servation status were obtained from research pub- 

 lications, books, original data from biologists, 

 and a recent synopsis by Williams et al. ( 19931. 



The status categories were based on infor- 

 mation for each species throughout its geo- 

 graphic range. The conservation status cate- 

 gories for a mussel species were defined as fol- 

 lows: endangered — in danger of extinction 

 throughout all or a significant poilion of its 

 range: threatened — is likely to become endan- 

 gered throughout all or a significant portion of 

 its range: special concern — may become threat- 

 ened or endangered by relatively minor distur- 

 bances to its habitat; undetermined — historical 

 and current distribution and abundance have not 

 been evaluated recently: and currently stable — 

 distribution and abundance are seemingly sta- 

 ble, or may have declined in portions of range 

 but not in need of immediate conservation. 



Decline of Mussels 



The decline of freshwater mussels, which 

 began in the late 1800's. has resulted from var- 

 ious habitat disturbances, most significantly, 

 modification and destruction of aquatic habitats 

 by dams and pollution. Freshwater habitats suf- 



fer not only from direct alterations by humans 

 but indirectly from abuse of terrestrial habitats, 

 such as from siltation, especially evident if one 

 compares the levels of imperilment of aquatic 

 versus terrestrial species. Master ( 1990) recog- 

 nized 55% of North America's mussels as 

 extinct or imperiled, compared to only 7% of 

 the continent's bird and mammal species. 



Aquatic habitat loss comes in a variety of 

 forms such as from effects of dams, dredging, 

 and channelization, or from more subtle effects 

 of siltation and contaminants associated with 

 construction and agriculture. Dams, with their 

 altered flow regimes and attendant reservoirs. 

 have caused the extirpation of 309^-60% of the 

 native mussel species in selected U.S. rivers 

 (Williams et al. 1992: Layzer et al. 1993). 

 Siltation resulting from deforestation, poor agri- 

 cultural and land-use practices, and removal of 

 riparian vegetation can destabilize the stream bot- 

 tom and eliminate benthic organisms such as 

 mollusks (Ellis 1931). Many streams that look 

 healthy can be polluted by contaminants like 

 heavy metals, pesticides, and acid mine drainage. 

 The effects of pollution and habitat alteration on 

 mussels were reviewed by Fuller (1974). 



Competition from non-native mollusks also 

 has contributed to the loss of native mussel pop- 

 ulations. The Asian clam (Corbiciila flwnmea). 

 introduced to the U.S. west coast in the 1930"s, 

 has invaded nearly every watershed nationwide 

 (McMahon 1983). Local population explosions 

 of the Asian clam have adversely affected some, 

 but not all. native mussels (Belangeret al. 1990; 

 Leff et al. 1990). The recently introduced zebra 

 mussel {Dreissena polymorplui) appears poised 

 to decimate many of the remaining mussel pop- 

 ulations. Zebra mussels were discovered in the 

 United States at Lake St. Clair in 1988 and 

 spread rapidly throughout the Great Lakes. In 

 1991 they were found in the Illinois River, and 

 by late 1991 had spread to the Tennessee River 

 (Nalepa and Schloesser 1992). They are now 

 found throughout the Mississippi River and por- 

 tions of its major tributaries, even to southern 

 Louisiana. During the next 10-20 years, zebra 

 mussels will most likely spread throughout 

 most of the United States and southern Canada. 



The adverse modification and destruction of 

 aquatic habitats, along with the introduction of 

 nonindigenous species, have resulted in the 

 decline of freshwater mussels. The percentage 

 of imperiled mussel species for eastern states is 

 high (Fig. I). Of the 297 native mussel species 

 in the United States, 71.7% are considered 

 endangered, threatened, or of special concern 

 (Fig. 2), including 21 mussels that are endan- 

 gered and presumed extinct. Seventy species 

 (23.6%) are considered to have stable popula- 

 tions (Fig. 2), although many of these also have 

 declined in abundance and distribution. Many 



