Freshwater Fauna

the aggregate of animals that inhabit rivers, lakes, subterranean waters, and other freshwaters. Freshwater fauna includes various protozoans, turbellarians, gastro-trichs, rotifers, nematodes, nematomorphans, oligochaetes, leeches, gastropod and bivalve mollusks, crustaceans, ticks, mites, aquatic insects, and fishes and the aquatic spider (Ar-gyroneta aquatica). Many groups characteristic of marine fauna, such as echinoderms, cephalopod mollusks, and radiolarians, are absent among freshwater fauna or are represented by only a few species (sponges, coelenterates, polychaetes, bryozoans).

Freshwater animals include those that are primarily aquatic and those that are secondarily aquatic. All inhabitants of fresh-waters, that is, waters with a salinity no higher than 0.1 percent, are homeosmotic. This means that they are capable of maintaining an osmotic pressure of cellular, tissue, and cavity fluids that is higher than that of fresh water. Only a few freshwater animals can tolerate salination. Owing to sharp seasonal variations in freshwater conditions, many of the animals have developed adaptations for surviving unfavorable periods in a resting state. For example, the gemmules of sponges, the statoblasts of bryozoans, and the eggs of many crustaceans can survive prolonged desiccation and freezing. Some freshwater animals can live in warm waters with temperatures up to 40° to 50°C. The freshwater fauna of the tropics is the richest and most diverse.

Freshwater fauna probably originated from primarily aquatic marine animals that settled in freshwater lagoons and river mouths, mainly in tropical zones. Fossils of freshwater fauna dating as far back as the Devonian have been found. However, freshwater animals apparently existed in earlier periods of the earth’s history. Fossils of secondarily aquatic freshwater animals (pulmonates and insects) are known from the Jurassic.