But some of the animals from the land, having once established themselves in fresh water, have become considerably modified to live in one particular and difficult part of the underwater world such as a torrent; others have achieved remarkable physiological adaptations, such as the ability to live without oxygen. It is among these specialists that we find the peculiar and characteristic freshwater types.
The main problem confronting an animal taking to the water is how to obtain its oxygen. Often the difficulty is not great, because many land animals live in damp places and have a moist surface. If they are quite small, this surface is all they require for respiratory purposes, and accordingly it does not matter greatly whether air or water is the medium beyond the layer of surface moisture. The problem is not quite as simple as this, but we need go no further for the present.
Other animals, whose land ancestors were probably less dependent on humid conditions, spend their lives in the water, but have developed a variety of methods whereby they can utilize atmospheric oxygen. The familiar water-beetles swim to the surface with the aid of their hind legs (which, with the transfer to water, have been modified into efficient paddles), and take in a bubble of air between their backs and their wing-cases. Some snails come to the surface to fill a lung. Mosquito larvae – the well-known wrigglers of the domestic water-butt – feed at the surface with breathing-tubes penetrating through to the air. The rat-tailed maggot has a remarkable telescopic appendage, so that it can walk on the bottom and keep in contact with the air at the same time. Other animals take in oxygen as a gas but nevertheless live perpetually submerged. Several groups – none of them very familiar to the layman – have a close-set pile of unwettable hairs. Withdrawal of oxygen for use by the body from the gas entrapped in this pile causes a partial vacuum, which is filled by oxygen dissolved in the water. Others tap the gas-filled tubes inside plants by means of some part of the body modified for the purpose.
Fresh water has presented invaders with a variety of problems, and no animal has solved all of them; none is found in all the habitats and most are confined within a rather restricted range.
The surface offers a rich hunting-ground, for many land animals fall into the water and are held there helpless by the force of surface-tension. Two groups of insects have mastered the art of living on the surface film, and are able to prey on these unfortunates. They are, however, confined to small pieces of water or sheltered bays, presumably because the effort of keeping station against the wind in the middle of a big sheet of water is too great. The pond-skaters have developed long legs and proceed somewhat after the manner of long-oared skiffs. The other group, the whirligig beetles, have shortened and flattened their legs till they resemble more the paddles of a canoe, by means of which they move over the surface with great rapidity. The way of life of these surface-dwellers calls to mind that of the wreckers who once gained a livelihood round our coasts, though they are not able to take measures to lure their victims to destruction.
Quiet shallow conditions, where mud settles and rooted plants – all incidentally of terrestrial origin – provide shelter, oxygen, and food, are probably the most easy to colonize; that is, they present the would-be inhabitant with least in the way of physical and chemical difficulties. But life is not easy in this habitat because there are so many different types of organism and competition between them is severe. The water-beetles and water-boatmen pursue their prey through the underwater jungle with the speed and grace of terrestrial felines; dragonfly nymphs lurk concealed like a crocodile in a waterhole; the tiny Hydra trails its tentacles in the water like a fisherman setting out his net. Against these marauders the caddis-larvae seek protection within a cumbersome house of stick or stone, and snails can withdraw into their more neatly made shell. But the snail’s shell, the caddis-larva’s house, the dragon-fly nymph’s protective coloration, and the beetle’s speed may alike prove unavailing when fish come nosing through the undergrowth in search of food. They are not the only enemies, and some birds and the water-shrew also hunt in this territory.
In deeper water, if it is too dark for plants to grow, the mud offers a substratum which is often rich, because the remains of animals and plants rain upon it from above. Here the chief inhabitants are mussels, worms, and midge-larvae, most of which are modified for burrowing and for feeding on minute particles. The diversity of form is not great, though the number of individuals may be colossal.
In yet deeper water conditions may be extremely difficult, because there is no oxygen for part of the year, but some animals, notably midge-larvae, have solved this physiological problem.
In the open water also the variety of form is not great, though numbers may be. Some small animals, such as the water-flea, resemble their counterparts in the sea, but the marine zoologist inspecting a freshwater catch is immediately struck by the lack of diversity. This is partly because many marine animals which lead a fixed or relatively sedentary life have a free-swimming young stage. This presumably serves the end of dispersing the species. In the circumscribed conditions of fresh water such a stage is of less advantage for this purpose, and might indeed prove a danger by being carried away to the sea before it was ready to settle. Freshwater animals which have come in direct from the sea have almost all lost the free-swimming stage, which their nearest marine relatives possess. Another striking feature of the animals of the open water is their small size. This is probably due to the absence of shelter and consequent vulnerability to predation. Some protection is obtained by transparency, a feature seen in both the sea and in fresh water, but the main defence is small size, rapid reproduction while conditions are favourable, and the formation of a resting stage as soon as they cease to be. Large animals could not reproduce fast enough to make good losses due to predation. It may be objected that some animals floating in the open sea attain a large size. In comparison with the sea most bodies of fresh water are very small and fishes feeding on the larger animals on the bottom in shallow water could easily make excursions to prey on any large organism that developed in the open water. Most of the sea is so far from land that this cannot happen, and predators and prey must develop in balance together.
One representative of that enterprising group the insects inhabits open water, and provides one of the most remarkable examples of adaptation that the animal kingdom has to show. It is the larva of a mosquito-like fly, and is known as the phantom larva, on account of the transparency already mentioned. The breathing-tubes, in other insects continuous from back to front, are vestigial except for paired air-sacs fore and aft. These are hydrostatic organs by means of which the animal can maintain itself at any desired depth. The earliest workers supposed that it functioned like a submarine, pumping fluid in or out of ballast tanks as required. Later observers, noting that the bladders never contained fluid, postulated that they worked like the swim-bladders of fishes, and secreted or absorbed oxygen. Finally, when a technique for analysing the minute quantities of gas in the bladders was evolved, it was discovered that the walls of the bladders expand or contract as the result of nervous stimulation, and the gases dissolved in the body-fluid diffuse in or out accordingly.
Rocky shores of lakes are startlingly barren places compared with those of the sea; there is no canopy of weeds nor incrustation of barnacles and other sessile animals. The two are not strictly comparable, because in fresh water there is no tide and the water may sink slowly to a low level and stay there during a long spell of fine calm weather. But even below this level there are not many living organisms. Probably this is due to a poor food-supply, for hard rocks and waters that are not rich in nutrient substances commonly go together. Perhaps this is an environmental niche that has not been completely filled; the fact that neither rocks nor wood are attacked by boring organisms in fresh water, as they are in the sea, suggests that this idea is not as revolutionary as it may seem at first. Further, the zebra mussel, the only freshwater bivalve that can attach itself to a hard substratum as the marine mussels do, has only recently entered fresh-water.
On a rock-face in a lake the only plants are green algae, and the only animal the freshwater limpet. This is actually descended from terrestrial stock, though superficially it resembles the marine limpet closely except in size. Smooth rock in a stream may have some covering of moss, which harbours many animals. If it is bare, it may be covered by great numbers of buffalo-gnat
