Not all marine animals live in seas and oceans. There are plenty of others inhabiting the sweet waters of rivers, streams, lakes, and marshes.
Between the source and the sea, rivers hold five kinds of habitat. First comes the narrow headstream where algae and willow moss, cling to the boulders and shelter small flattened, crawling creatures like the smidges, mayflies, stone flies and caddis flies.
The downstream holds a broader stretch known as the trout back. Here the current pours over boulders and gouges out deep pools. Few plants can root. But small, aquatic insect larvae feed on morsels washed downstream. They themselves form food for trout. Dippers are tubby birds that swim under water with their wigs to seize young caddis flies.
Next comes the minnow reach. The river has at last slowed down enough to dump sand and shingle banks between the pools. Plants grow under water here. Apart from the larvae inhabiting the trout beck This is reach with dragonfly nymphs, freshwater shrimps and crayfishes. Minnows, trout, eels and other fishes find food and shelter in this gentler stretch. Some fishes like salmon comes up this far from downstream.
On its path toward the sea the river next enters its lowland reach. No other stretch contains so many kinds of animal. The slow pace of water flow and soft mud floor formed by the siltation and deposition of particles from the upstream goes to this level. Worms, insects, mollusks and crustaceans abound. Many help to feed fishes of the carp family - the barbell, bream, Rudd and roach. Such fishes in turn are snapped up by the predatory perch and pike.
In the few kinds of freshwater or saltwater animal can stand its brackish waters. But those that can are plentiful. A square of mud one pace across may house 10000 worms, or, 20,000 little snails that browse upon green, slimy algae. Fishes that swim here are most prone to fall prey of ducks, geese, wading birds and gulls those flock here in thousands.
In Australia, on the banks freshwater stream, there lives a rare species of egg-laying mammal. Called platypus, these are very good swimmers, even under water. Their bills are also like those of ducks and the webbed feet and a flat tail. While the beak grubs for food on river beds. These apart there are some inhabiting the typical tropical waters. The year round food supply and warmth help support a wealth of marine life - from insects and fishes to reptiles and amphibians. The strange variety of frogs include the goliath frog. Found in West Africa these are the largest variety of known frogs. Also strange is flat-bodied Surinam toad from South America. It never leaves the water. The female carries her eggs on her back in skin 'blisters'.
The upside down catfish from Africa guzzles food from the surface film on water. An elephant trunk fish has a long, trunk like lower lip that probes mud for the invertebrates. The Chinese algae eater has a vacuum cleaner mouth for browsing on under water leaves an stones. South Africa's hatchet fish has deep but knife-thin bodies. These thumb length creatures can take off and glide 5 meters(16 ft) through the air after insect prey or away from predators. The tiny looking piranha is one of the most ferocious freshwater fishes. These hard jawed saw teethed fishes with a sharp smelling sense attack their prey always in a large group. They start nibbling casually. The biting picks up as the nibbling continues until the prey is fully consumed leaving the skeleton.
The fresh water turtles with flattened shells streamlined for swimming inhabit all warm continents. Soft shelled turtles lack a hard protective shell, but make up for it by biting savagely.
Some fishes can give off electric impulses from nerves inside the body. Like South America's electric eel that produces an electric shock strong enough to stun a horse or scare predatory crocodiles.
Crocodiles and alligators are the real ferocious amphibians of this habitat. Crocodiles are the largest of all living reptiles. Equipped with hard jaws lined with grinding teeth these mighty monsters float log-like on the surface. They swim fast by lashing their long, deep, flattened tails. Some can drag cattle into water from quite afar.
The big monitor lizard include species almost as aquatic as a crocodile. Other reptiles include snakes that can swim well and prey on fish.
As water travels overland it gathers chemicals - for instance, mineral salts and the gases, oxygen and carbon dioxide. Water plants make food and grow with these. Living and dead water plants themselves form food for many animals that live in water. Other creatures prey on some of these. Body waste and dead animals in turn yield nourishment to fertilize the water plants. Thus sets off a fresh water food chain. Unlike the running waters the still water of a pool has its own special group of creatures. Yet their variety is no less amazing!
Around the soft, gently sloping edge of a pool with reeds, arrowheads, bulrushes and other tall plants. And it is because of these plants at the pool animal life here grows richest. The plants shelter and feed thousands of tiny crustaceans, insect larvae, mollusks and worms.
Between the water weeds the water spiders spin their underwater home filled with air bubbles that it brings down from the surface.
Newts, frogs and toads also make their habitat at the pool side rim. Frogs and tadpoles browse on soft vegetation. Newts larvae feed on worms and water fleas. Daphnia in turn eat minute green plants, mostly algae that drift about in water. Again the larvae and tadpoles may fall prey to big beetles that dive and hunt under water. Young dragonflies are another underwater threat.
Further out, in deeper water, lurk larger fishes. Most kinds eat water plants and worms or other small invertebrates. Trench and bream forage among underwater plants near the bottom. Not everywhere the same species of these still water creatures are found. It depends on the climatic zones.