Status and Threats
There is a global crisis of freshwater mussel decline and extinction. In the last 25 years especially, this crisis has developed on a monumental scale. North America, for example, has the greatest diversity of freshwater mussels by far, with over 300 of the 1000 recognized species worldwide. Of these, only 25% can be considered to have stable populations, and some 12% are believed extinct (Bogan, 1996). Of the three hundred U.S. species, 62 are listed as endangered and 8 as threatened under the federal Endangered Species Act- a total of 23%.
Nature Serve, (a non-profit conservation organization providing up-to- date knowledge on rare plants, animals, and communities), lists 68% of freshwater mussels as being at risk of extinction (Stein and Flack, 1997). "At risk" species are defined as those in one of four Nature Serve conservation status categories. These range from "presumed extinct"- i.e., not located despite intensive searching- to "imperiled" , defined by 6 to 20 occurrences or from 1000 to 3000 total individuals located. This percentage of "at risk" species is greater than that of any other animal or plant group tracked by the organization.
Most endangered mussel species are found in the American southeast in Alabama, Florida, Georgia, Tennessee, and Virginia where an extensive river system supports a wide variety of endemic species. In the New York metro area, Alasmidonta heterodon is listed as federally endangered, while seven species are listed as endangered, threatened, or special concern by New York State. Six other species found in northern or western New York are also listed, four of them federally. New Jersey also has a significant number of listed species. For a list of protective categories, go to: NY State Categories
For a list of all metro area taxa and their abundance, go to the Species Status page.
Predation and Parasitism
Predation is a limited threat in the New York area because relatively few animals regularly consume adult freshwater mussels. Muskrats are known to eat mussels, but are often species and size selective (Hanson et al., 1989; Neves and Odom, 1989; Watters, 1994d). Other animals documented to eat mussels include raccoons, mink, otters, some waterfowl, some turtles, and a few fish species such as the freshwater drum, Aplodinotus grunniens, some sturgeon, and certain catfish.
Various parasites (mites, leeches, flukes, distomids) are known to infect freshwater mussels causing death in rare instances. The most extensively studied of these are parasitic mites in the family Unionicolidae (see Vidrine, 1991; 1996).
Pollution has become an increasingly prevalent problem for all freshwater organisms across North America. Point source pollution includes that from industrial effluent pipes, wastewater release, and chemical spills. Non-point source pollution includes sediment accumulation, nutrient overloading, acid precipitation, and heavy metal increases.
Mussel response to these pollutants varies. Responses to toxicants can include a decrease in metabolism and respiration, tissue deterioration, reduction in growth rate, and death (Fuller, 1974; Goudreau et al., 1993). Sedimentation (see Channelization and Impoundment) from poor land use patterns can alter spatial distribution of stream sediments, depth and flow regimes, stream bank floral diversity, and aquatic vegetation composition (Karr, 1991). All are important factors in determining mussel diversity. Poor land use patterns (particularly from agriculture), resulting in loss of stream-side flora, can also contribute to increased nutrient loading as runoff from metropolitan areas, agricultural fields and pastures is heightened (Starrett, 1971; Carpenter et al., 1998; Fenn et al., 1998). Bauer (1988) found pollution (nitrate loading resulting in eutrophication) to be the leading cause of mortality among Margaritifera margaritifera in North Bavaria (Germany).
Chemical pollution is derived from a multitude of sources, but some that have been studied include chlorine from wastewater treatment plants (Goudreau et al., 1993), copper (Jacobson et al., 1993), and acid mine drainage. In the past, before wastewater pollution regulations were put into effect, human waste from urban areas was often pumped directly into streams in New England and New York. Mussels tend to be slow at colonizing polluted streams and many of the rare species are intolerant of even low levels of pollution.
Channelization and Impoundment
During channelization a river bed is dredged, often to allow passage of boat traffic. The streambed is scoured and sediments (as well as mussels) are physically removed in the process. Channelization is extremely detrimental to the survival of freshwater mussels. Mussels are physically removed from riverine habitats during this process. The resulting streambed is often coarser than the original, altering habitat for species that preferred the original habitat. Streambed alteration has been documented as a cause of the decline in Alasmidonta heterodon and A. varicosa in the northeast (Strayer and Ralley, 1993).
Impoundment involves the damming of a fluvial system, slowing or stopping its flow for varying time periods. Essentially converting a river to a lake causes many riverine species to perish (they cannot tolerate the sediment accumulation and deeper, colder water of reservoirs). Most of the less common species in the New York metro area are riverine species. Patches of fine sediment tend to be preferred habitat for riverine mussels such as Alasmidonta heterodon and A. varicosa (Strayer and Ralley,1993). Impoundment increases sediment loads upstream of the dam and erodes habitat (Coker et al., 1921; Parmalee and Hughes, 1993; Blalock and Sickel, 1996), while downstream reaches become dominated by cobble or boulders.
Dams also restrict fish distribution, and therefore mussel distribution as well (Walters, 1996). Deep release dams release cold water from the dam base at temperatures below that tolerable by many warm-water fish that serve as hosts for certain freshwater mussels. Surface-release dams may negatively affect survival of mussels that parasitize cold-water fish such as Margaritifera margaritifera and Alasmidonta varicosa.
Gene flow between mussel and fish populations also decreases following dam construction. When dams require repairs, their accompanying reservoirs are frequently drained exposing mussels to dessication and ultimate death.
A wide variety of introduced aquatic species have been documented in North America. Only a handful have been shown to cause significant declines in freshwater mussel populations (Mills et al., 1997; Strayer, 1999). The following discussion summarizes the status and impacts of two of the most common introduced species: Dreissena polymorpha, the zebra mussel, and the Asian clam, Corbicula fluminea.
This species has received the most attention and caused the greatest negative impact in the New York area.
The zebra mussel was accidentally introduced into Lake Erie in December, 1987 (Leach, 1993) and again later in Lake St. Clair in June, 1988 (Hebert et al., 1989), most likely as veliger larvae in the ballast water of ships arriving from Europe. Since that time, the species has spread quickly into New England, south to Louisiana, and west to Oklahoma and Minnesota. A second dreissenid mussel was accidentally introduced into the Great Lakes in the same manner, the quagga mussel, Dreissena bugensis (Spidle et al., 1994).
The zebra mussel was introduced into New York in the Hudson River near Catskill in May 1991 (Strayer and Powell, 1992). It has since spread south to West Haverstraw, while others have moved eastward from the Great Lakes into the Mohawk River (Mills et al., 1993). Now widespread, they have also spread from the Hudson River basin into Vermont (Lake Champlain) and Connecticut (East Twin Lakes).
Whittier et al. (1995) provided regional assessments of potential for the spread of zebra mussels in northeastern lakes based on knowledge of their alkalinity and calcium requirements. All areas are of high risk in the New York Metro area except the eastern half of Long Island (Suffolk County), parts of the Susquehanna and Delaware River watersheds (eastern Broome, Delaware, western Greene, Sullivan, and Ulster Counties), all of the eastern half of Connecticut, and all of the southern half of New Jersey.
Zebra mussels have already had serious and costly economic impacts on North American industry, causing billions of dollars in damage, subsequent repair, and removal. Utility plants have experienced clogging of intake pipes, increased corrosion of piping, and pump and holding tank fouling. Serious impacts on recreation and fishing have also occurred.
Zebra mussels have a significant impact on unionoids. Like marine blue mussels, zebra mussels attach to solid objects by means of a tuft of adhesive hairs called a byssus. These may include rocks, bottles, cans, boat hulls, flow pipes, and native mussels. Densities of over 10,000 individuals have been reported on a single mussel in use as a substrate.
Both the zebra and quagga mussel have extremely high reproductive potential requiring internal fertilization, no fish host, and produce multitudes of veliger larvae (see Hebert et al., 1991; Mills et al., 1996). Fouling by zebra mussels in western Lake Erie caused mortality and reduced fitness among unionoids, though to different degrees depending on the species (Haag et al., 1993). Infested specimens of Amblema plicata were found to have higher ammonia excretion rates, lower ratios of respiration to nitrogen excretion, lower water clearance rates, and more depleted energy stores than non-infected specimens from the same area (Baker and Hombach, 2000).
Zebra mussels can extirpate native unionids from lakes and rivers by extensively fouling shells and outcompeting them for food. Evidence from the Hudson River suggests zebra mussels can reduce food concentrations to levels too low to support unionid mussel reproduction and survival (Strayer, 1999).
These tiny animals can alter the entire freshwater ecosystem, increasing water transparency, decreasing suspended organic matter, decreasing primary phytoplankton production, decreasing zooplankton, and physically altering the macrobenthic community (Karatayev et al., 1997).
Specimens are spread through transfer between water bodies of bait buckets, bilge water, ballast water, ducks or waterfowl, fouled crayfish or other organisms, live specimens, or by transfer of boats or trailers with attached specimens or vegetation.
The most complete and up-to-date information on zebra mussel impacts and control can be found in Boelman et al. (1996) obtainable as a CD-ROM from the U.S. Army Corps of Engineers.
First introduced into the Columbia River near Knappton, Washington, in 1938 (Counts, 1986), possibly as a food item, Corbicula fluminea, the Asian Clam, now occurs in nearly 40 states. This species probably reached New York State sometime before 1997.
In the metro area, the Asian clam occurs in the lower Connecticut River (Morgan et al., 1991), and in Twin Lakes in northwestern Connecticut. Its New York occurrence is limited to rivers and small lakes on Long Island, including Massapequa Lake (Foehrenbach and Raeihle, 1984; Norman Soule, pers comm.), and its New Jersey localities include the Raritan River in Middlesex and Somerset (Trama, 1982) and the DelawareRiver near Newbold Island, Wright Point, and Trenton (Fuller and Powell, 1973; Counts 1991). Pennsylvania localities include the Ohio and Delaware Rivers; the Beaver River in Beaver County; the Monongahela River at Lock and Dam Number 8; and the Schuykill River at the Limerick Power Station and Fail- mount Dam (Fuller and Powell, 1973; Counts, 1991).
In addition to fouling power plants and irrigation pipes, this species can alter benthic substrates and compete with native mussels for food, though to a lesser degree than the zebra mussel, and may consume larval mussels or glochidia (Leffet al., 1990).
Means of dispersal include bait buckets, water currents, passive introductions with water plants, and as food or pet trade items. Prior reports of transport by water fowl are unjustified as mortality would likely occur during digestion (Thompson and Sparks, 1977).
Low temperature appears to be a limiting factor in this clam's dispersal, with 35-37ºF being the lower limit (Graney et al., 1980). However, despite high winter mortalities, Corbicula fluminea may be adapting to colder climates without the benefit of thermal refuges such as power plant effluent (e.g. the Connecticut population downstream of Haddam Nuclear Power Plant), recent populations discovered in Colorado in the Colorado (Crane et al., 1996), Platte (Kreiser and Mitton, 1995), and Arkansas River basins (Cordeiro and MacWilliams, 1999). For information on freshwater mussel studies, go to: Surveys & Study