Animals - Freshwater Ecology: Concepts and Environmental Applications - page 161

predators, such as leeches, flatworms, and some aquatic insect larvae, in-
vade the shells (Brown, 1991).
The freshwater bivalve mollusks are characterized by a shell with two
halves and enlarged gills with long ciliated filaments used for filter feeding
(McMahon, 1991). Most of the native North
American bivalves burrow in sediments. The
bivalves can be found in the benthos of
streams, lakes, and rivers. Several bivalve
species have recently invaded North Amer-
ica, including the Asiatic clam Corbicula flu-
minea (Fig. 9.5B), the zebra mussel Dreis-
sena polymorpha (Fig. 9.5C), and the quagga
mussel Dreissena bugensis . The zebra mus-
sel has caused significant economic and eco-
logical impact (Sidebar 9.1).
Unionid mussels are one group of bivalves
found in freshwaters. The diversity of the
unionid mussels (Fig. 9.5A) is high in North
America. Unionid mussels are unique in that
they have a specialized larval stage called
glochidia. The glochidial stage of the mussel
life cycle is important for dispersal. These
larvae attach to host fish species, mainly in
the gill region (but some attach to fins), and
encyst for 6-60 days. After encystement, the
larvae settle and develop into sedentary
forms with shells. Some adult female mus-
sels attract potential hosts with lures that
are muscular extensions of the mantle. These
lures may look like prey fish, and when po-
tential host fish attack them the glochidia
are released forcibly (Fig. 9.8). The unionid
mussels are also unique because of their long
life span. Individuals can live from 6-100
years; other bivalves live less than 7 years.
The long life span and infrequent reproduc-
tion make unionids vulnerable to human im-
pact. Of the 297 native species and sub-
species of North American mussels, 19 are
extinct, 62 are federally listed in the United
States as endangered or threatened, and 130
need further study to determine their con-
servation status. These species are linked to
many ecological processes through their role
as filter feeders (Vaughn and Taylor, 1999).
Conservation of mussels and other species is
discussed in Chapter 10. Unionids are also
the basis of a historically important pearl
and shell fishery, which continues at modest
levels.

plankton, small zooplankton, and detritus.
Amounts of all these suspended particles de-
creased concomitantly with mussel increases.
This consumption ultimately may impact fish
by lowering the amount of food available to an-
imals that feed on suspended particles and
serve as food for piscivorous fish. The increase
in zebra mussels has been accompanied by a
decrease in native bivalves, particularly native
unionid mussels. Two of the species of unionid
mussels probably will become locally extinct
in the Hudson River. Dissolved phosphorus and
water clarity have increased since the inva-
sion of the mussels, probably leading to in-
creases in macrophytes. The various ecosys-
tem effects can be viewed as positive or
negative. For example, an increase in water
clarity may be good, but the effects are obvi-
ously disastrous for some native species.
Zebra mussels have caused chronic prob-
lems in water intakes in both Europe and the
United States (Kovalak et al., 1993). Layers of at-
tachments up to 30 cm thick can clog pipes and
screens. Removal efforts are time-intensive and
costly, usually involving dewatering (shutting
down and drying) water systems and cleaning
with high-pressure water hoses (Kovalak et al.,
1993). Chemical controls such as treatment with
chlorine and other toxic chemicals are effec-
tive in confined areas (e.g., water pipes), but
toxic when released to the environment. The
economic impact was estimated to be $5 billion
in the Great Lakes region alone by the year 2000
(Ludyanskiy et al., 1993), and recent surveys put
actual costs at a minimum of $10 million per
year (O'Neill, 1997). Biological controls are be-
ing investigated, but no inexpensive method is
available that has been widely adopted. Bio-
logical controls may be somewhat effective;
crayfish (Perry et al., 1997), waterfowl (de Vaate,
1991), and some fish prey upon zebra mussels.
There is no good way currently to control zebra
mussels at the ecosystem level.