Animals - Freshwater Ecology: Concepts and Environmental Applications - page 160

several centimeters to 1 m long and about 3 mm wide. Their typical life
cycle includes adults that reproduce sexually and lay eggs. The eggs hatch,
and the larvae are ingested by a host animal (commonly an invertebrate,
although not always an aquatic host). Sometimes, this host is engulfed by
another predator, and the parasitic larva in-
fects the predator. The host must be in con-
tact with water for the mature adult to
emerge from the body cavity. At this stage
of development, the horsehair worm can be
almost as large as the host. Although repro-
ductive, the adult never ingests food and has
no functional digestive tract.

Sidebar 9.1.
Invasion by the Zebra Mussel

The zebra mussel was established in the Volga
drainage in Europe from its native Caspian Sea
drainages before 1800. It spread from the Volga
throughout most of the major river systems in
Europe following introduction to some far west-
ern European countries in the early 1800's
(Hutchinson, 1967). This invasion represents
the first major expansion of its range caused
by human activity. In 1986, a ship with water
from a European port, probably taking on cargo
at the St. Clair River, dumped ballast water and
released the zebra mussel into North America.
Since that time, the population has increased
its distribution to cover much of the Missis-
sippi drainage (Fig. 9.6). This explosive spread
has occurred partly because females can pro-
duce more than 1,000,000 eggs each reproduc-
tive cycle that give rise to easily transportable
veliger larvae (de Vaate, 1991; Sprung, 1993)
and partly because of the many ways the mus-
sel can be transported. Carlton (1993) reports
two natural transport methods (currents and
animals other than humans), and 20 human-
caused movements, including water traffic,
fisheries activities, and navigation. Ultimately,
this species probably will spread through much
of North America (Strayer, 1991). These mus-
sels attach tightly to any solid surface with
byssal threads, can reach tremendous densi-
ties (Fig. 9.7) and overgrow native species. This
invader has many potential effects on food
webs and ecosystems (Table 9.1).
The Hudson River provides a good case
study for examining the effects of the zebra
mussel on ecosystem properties (Strayer
et al., 1999). The zebra mussels were first ob-
served in the Hudson River in 1991 and by 1993,
densities were high enough that the mussels
filtered the entire water column every 1.2-3.6
d. The adult mussels are filter feeders and can
remove suspended particles including phyto-

Phylum Mollusca
The freshwater mollusks include two
classes, the Gastropoda (snails and limpets)
and the Bivalvia (clams and mussels). Mol-
lusks are widespread, conspicuous, and of-
ten abundant. They are soft-bodied and un-
segmented animals. Their body has a head,
a muscular foot, a visceral mass, and a man-
tle that often excretes a calcareous shell. They
form an important part of the biodiversity
and food webs of many aquatic ecosystems.
The gastropods constitute the most di-
verse class of the phylum Mollusca, with
about 75,000 species of marine and fresh-
water snails worldwide. Freshwater snails
are very diverse in North America, with
about 500 species present. The gastropods
have a univalve (one-piece) shell and a file-
like radula that is used to scrape surfaces
while feeding (Brown, 1991). Shell geome-
try can be simple and conical (Fig. 9.5F);
spiral and flat (planorbid; Fig. 9.5E), or spi-
ral and elevated (Fig. 9.5D). Reproduction
can vary among genera from parthenogenic
to sexual. Hermaphroditic snails occur. Re-
production occurs from once per year to
continuously, depending on the species and
the geographic region.
Snails feed on detritus, periphyton,
macrophytes, and occasionally carrion. They
prefer periphyton to macrophytes (Brön-
mark, 1985) and can be major consumers of
periphyton in aquatic systems. Despite the
protection of their shell, there are many im-
portant predators on snails. Some, such as
sunfish and crayfish, crush the shells. Other