Environmental Engineering Reference
In-Depth Information
topic, aquatic invertebrates are classiied into three major phyla: Rotifera, Mollusca, and
Arthropoda.
Phylum Rotifera is a group of microscopic (50-500 μm in length), free-living organisms
made up of over 2000 described species. Although this group contains a diverse array
of species, including some that are capable of withstanding extreme conditions, 95% are
found in freshwater ecosystems and 5% are found in marine ecosystems. Rotifers often
make up an important component of the plankton in both freshwater and marine eco-
systems. Normally, 40-500 organisms are found per liter of freshwater lakes or ponds.
Rotifers are an important source of food for fish; therefore, they play a significant role in
determining the structure of the aquatic community and in mediating the flow of energy
throughout the freshwater ecosystem (Pechenik 1996).
Phylum Mollusca is the second largest animal phylum in regard to numbers of described
species. There are approximately 50,000-120,000 described species of living mollusks as
well as over 70,000 described fossil species. The majority of these species belong to the
classes Cephalopoda, Gastropoda, and Bivalvia. About 75%-80% of all living mollusks
belong to the class Gastropoda. Gastropods, which include snails, slugs, conchs, sea slugs,
and their relatives, are found in freshwater, marine, and terrestrial ecosystems and are
considered carnivorous, herbivorous, and detritivorous in the food chain. Class Bivalvia,
which includes clams, oysters, and mussels, consists of 7000 species, 10%-15% of which
are found in freshwater ecosystems. Both gastropods and bivalves are primarily benthic
organisms, although many mollusks spend their larval stages as plankton to aid in disper-
sal (Pechenik 1996). Mollusks are very diverse, not only in size and superficial structure
but also in behavior and habitat. A unique characteristic of the organisms of this phylum
is their shell, which is secreted by a thin membrane called the mantle. Although many
cephalopods, such as octopi and squid, do not have shells, they are classified as mollusks
due to the presence of the mantle and mantle cavity.
Organisms in the phylum Arthropoda have an exoskeleton (external skeleton), segmented
body, and jointed appendages (Triplehorn and Johnson 2005). Although new species are
continually being described, it is estimated that there are over 5 million species of arthro-
pods worldwide. Common aquatic arthropods that humans are most familiar with include
lobsters, crabs, and shrimps. However, aquatic insects constitute the majority of described
species. Although there are 13 different arthropod classes, the majority of aquatic arthro-
pods belong to the classes Insecta (mayflies, dragonflies, mosquitoes, etc.), Arachnida
(water mites, spiders, etc.), Malacostraca (shrimp, crayfish, and crab), Branchiopoda (fairy
shrimp, clam shrimp, water fleas, etc.), and Maxillopoda (barnacles, copepods, etc.). Two
major groups of aquatic arthropods, which are the focus of this chapter, include class
Insecta and subphylum Crustacean (classes Malacostraca and Maxillopoda), which are
abundant and important members of the aquatic and terrestrial ecosystems.
The class Insecta is the most diverse class of animals (over 750,000 described species),
which has been found in every ecosystem on the planet. Because of their importance in
both agriculture and human health, they are often a major target of pesticide applications.
For example, mosquitoes, which inhabit practically all parts of the earth except the polar
regions, can carry disease-causing organisms, such as the malarial parasite, Yellow Fever
virus, Dengue virus, or filarial worm. As a result, persistent pesticide compounds were
widely used to control potential mosquito vectors. Aquatic insects are made up of benthic,
nekton, neuston, and plankton species, which vary in their feeding strategies. In addition,
aquatic insects may have varying degrees of pollution tolerance, creating unique ecologi-
cal niches for many species. Because insects are both numerous and diverse in aquatic
ecosystems, they have received a lot of attention for their use in monitoring water quality.
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