Animals - Freshwater Ecology: Concepts and Environmental Applications - page 173

towed horizontally at a set depth and then retrieved. The meter indicates
the volume that was sampled. Alternatively, vertical tows can be taken.
Nets are available that close when a messenger is sent down the line that
is used to pull the net through the water. These nets can be used to sam-
ple vertically over a fixed range of depths.
Several techniques are available for removing water from a lake and
then filtering it. In all cases, the collection devices are clear to reduce the
chance that predator avoidance behaviors related to evading large, dark
objects do not decrease the amount of zooplankton collected. One method
is to take a Van Dorn sample and pour the water sample through a filter
to concentrate the zooplankton. Another method involves lowering a box
that encloses a known amount of water at a specific depth. The box has
an exit net where the water flows out after it is raised above the surface.
This device is called a Schindler trap.
Samples are preserved, generally with formalin, and returned to the lab-
oratory for counting and identification. Quick preservation is necessary to
prevent predatory species from consuming their prey before the sample is
transferred to alcohol for counting and possible longer-term preservation.
lectivity. The most obvious behavior is probably vertical migration in lakes,
with the population moving deep during the day to avoid sight-feeding
predators and moving to the surface at night to feed on phytoplankton.
The Decapoda includes the crayfishes, crabs, and shrimps. Most of the
species are marine, but hundreds of freshwater crayfish, shrimp, and crab
species have been described. The decapods live in lentic and lotic environ-
ments; some species have evolved to live in caves and groundwaters, and
others live in swamps and even wet meadows. They are the largest and
longest lived crustaceans in most freshwater systems. The bodies of shrimp
and crayfish are more or less cylindrical (Figs. 9.11F and 9.11G) with a
carapace that encloses at least a branchial chamber. Both freshwater shrimp
and crayfish can provide a food source for humans; therefore, aquaculture
of freshwater decapods is an important economic activity in some regions.
Freshwater crabs, crayfish, and shrimp can be serious pests in rice fields,
where they consume plants and disturb the water (Schmitt, 1965).
Crayfish are omnivorous and can be dominant consumers (Hobbs, 1991).
Freshwater shrimp are more often grazers or detritivores. The freshwater de-
capods can be important prey for fishes and other large animals. Some species
of crayfish have become nuisance invaders and may outcompete native species.
Isopoda (isopods) are known commonly as pill bugs or sow bugs in
terrestrial habitats. There are many marine and terrestrial species, but
freshwater forms can also be found. The isopods are strongly dorsal-
ventrally flattened (Fig. 9.11B) with length usually ranging from 5 to 20
mm. Many of the species in North America can be found in springs, spring
brooks, groundwaters, and streams. A few are found in the littoral zones
of lakes. Most isopods are thought to be detritivores and scavengers be-
cause they are often observed eating dead aquatic animals. The isopods
Caecidotea (formerly Asellus ) and Licerus have received some study