Introduction
There is a global crisis of freshwater mussel decline and extinction.
In the last 25 years especially, this crisis has developed on a monu-
mental 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 or NJ State Categories
For a list of all metro area taxa and their abundance, go to: Status
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 Threats
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
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.
Introduced Species
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.
Zebra Mussel
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.
Asian clam
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
35-37°F 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
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New York Metropolitan Region and New Jersey Freshwater Mussel Identification Handbook