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aFW, freshwater; S, saline; B, benthic or attached; P, planktonic. Source: Horne and Goldman (1994).

aFW, freshwater; S, saline; B, benthic or attached; P, planktonic. Source: Horne and Goldman (1994).

TABLE 15.7 Characteristics of Some Common Insect Larvae

Common Name and Order

Habitat

Food Habits

Examples

Nymphs

Stoneflies (Plecoptera) Rapids

Mayflies (Ephemeroptera) All waters

Damselflies (Odonata) Dragonflies (Odonata) Water bugs (Hemiptera)

Larvae

Caddisfly (Trichoptera) Water beetles (Coleoptera) Order Diptera (and family) Mosquitoes Blackflies

True midges (chironomids) Horseflies

Slow and stagnant Slow and stagnant All waters

All waters Slow or stagnant

Pools at surface Rocks in rapids All waters Beds in pools

Mainly carnivorous Mainly herbivorous Carnivorous Carnivorous Carnivorous

Carnivorous Carnivorous

Herbivorous Herbivorous Herbivorous Carnivorous

Nemoura

Baetis, Ephemerella

Libellula

Notonecta (backswimmer) Gerris (water strider)

Dytiscus (diving beetle)

Culex

Simulium

Chironomus

Source: Needham and Needham (1962); Horne and Goldman (1994).

is not reached until about 5% of saturation. This is attributed to the presence of hemoglobin in the blood of the midge, which improves the efficiency of oxygen transfer.

Other invertebrates include various worms and mollusks (Table 15.8). Included are nematodes, oligochaetes, leeches, bivalves (clams), and snails. Most are detritivores. Leeches are predators that attach to fish. Some nematodes are internal fish parasites. Bivalves are becoming uncommon in American rivers. Snails feed on macrophytes and, along with insect larvae, can control their abundance. Tubifex can become so abundant in highly polluted streams that they form red mats on the bottom.

The zebra mussel, Dreissena polymorpha, is an exotic species imported to North America from Europe. It probably hitched a ride in the ballast water of a ship that released it near Detroit, Michigan, in 1985 or 1986. Since then it has been spreading rapidly, carried by currents and shipping. Zebra mussels cause great ecological and economic harm

Figure 15.12 Insect larvae and nymphs: (a) Ephemeroptera (mayfly); (b) Diptera, Chironomus; (c) Trichoptera (caddisfly); (d, e) Trichoptera case. (From Standard Methods, 10th ed. # 1955 American Public Health Assoc., Inc.)

Chironomid

Chironomid

1 cm

Figure 15.13 Zoobenthos of the profundal zone. (From Home and Goldman, 1994. © McGraw-Hill, Inc. Used with permission.)

1 cm

Figure 15.13 Zoobenthos of the profundal zone. (From Home and Goldman, 1994. © McGraw-Hill, Inc. Used with permission.)

by occupying growing surfaces formerly used by other organisms and by growing on intake structures of power plants and drinking water plants, obstructing their flow.

Fish and Fisheries Fish occupy and often dominate all the trophic levels from detriti-vore to herbivore to top carnivore (Table 15.9). They serve as prey to other fish and to aquatic birds or other animals. Since streams receive most of their nutrient input from terrestrial sources, harvesting of fish by birds, bears, or humans represents a return of some of those nutrients. Fish are the most motile of aquatic species; thus, they can live and breed in different places, and move in response to changes in food supply or other conditions.

Freshwater fish are hypertonic: Their bodily fluids are more saline than the water they swim in. Thus, they tend to absorb water by osmosis and lose salts by passive diffusion. To counter this, they discharge large volumes of dilute urine, and their cells absorb salt by active transport. They do not drink.

TABLE 15.8 Common Invertebrates Other Than Insects

Taxonomic Group

Feeding Method

Examples

Turbellaria (flatworms)

Carnivores

Dugesia

Nematodes (roundworms)

Carnivores, herbivores, parasites

Dolichodorus

Annelida

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