Fish

The idea of fishes sleeping is new, and sleep itself is a natural phenomenon we do not fully understand. Consequently, some scientists prefer to speak of fishes being in a state of suspended animation when they rest at night, rather than speaking of them as being asleep. It would be much simpler if these animals could close their eyes. At least that would give us some indication that they are asleep. There are, however, other symptoms that guide us, and these are very like the symptoms we know well in ourselves. Conger eels spend much of the day hidden in crevices among the rocks, where we can suppose they are resting if not sleeping. When congers are kept in an aquarium we can more easily watch what they are doing. The usual practice is to place a few drainpipes in a large conger aquarium. The eels spend much of their day in these, and if we watch carefully and over a period of an hour or more we notice that every now and then one of the congers will yawn copiously. Then it will turn over and wriggle its body for a moment or two as if making itself more comfortable. After this it will stay still for a long while, its only movements being the mouth slowly opening and closing as it takes in water, and the movement of the gill-chamber as water is expelled, the usual signs of a fish breathing. And these movements are slow as compared with those the fish makes when swimming. With the gray mullet, another sea fish, there are other indications of sleep. During the hours of daylight gray mullet swim in schools in which they are evenly spaced, but tightly massed, and each has its head pointing in the same direction. At night the schools break up and each individual fish goes to its own spot on the sea-bed. All the gray mullet are now well spaced out and they are facing in all directions. It is as though the sea-bed were a dormitory in which each fish had its own bed.

Should anything happen to disturb the sleeping mullet they all swim up to the surface and form into a school once again. This is the clearest possible sign that when they go to the sea-bed and separate they are all in a relaxed state, which is the prelude to sleep, whereas by day, with potential danger on all sides, they keep together for security.

GOING UP TO BED

Do fishes sleep ? This question was answered to some extent soon after the aquarium was added to the London Zoo, when the scientist in charge went into it one night and switched on the lights. He found that many of the fishes in the tanks were in an attitude of sleep. Since skin-diving and underwater television have become popular, we have learned more about sea fishes in their natural element. We know now that some of the fishes which have bodies flattened from side-to-side, actually lie on their sides at night, presumably fast asleep. That is, they really lie down to sleep. But from the many studies made in more recent years we
know that the position used may differ even in closely-related species. Some may lie down, others rest in the same upright position they use when swimming, and a few sleep in the vertical position with the head down and the tail up.

It may seem a somewhat trivial matter to bother one's head with, but it is more important than appears at first sight. For example, flatfishes that normally lie on the sandy bed of the sea rise slightly in the water so that they are floating a few inches off the bottom when sleeping. This it now seems, is why the best catches of flatfishes are made at night. The commercial trawl does not actually drag the bottom. Its lower end tends to travel up to a foot off the sea-bed. When used by day it catches some flatfishes, those that are alarmed at the trawl's approach and try to swim away from it, but misses those that remain buried in the sand. At night, with the flatfishes lying up from the sea-bed the net catches more of them.

The increased catch at night may also be due to some extent to the fact that the fishes are asleep. People who have studied fishes in their natural habitat, as contrasted with those who have watched them in aquaria, have found they could put a hand out towards a sleeping fish and actually pick it up. Catching flatfishes at night almost certainly has another advantage. One of the drawbacks to using nets is that small fishes as well as large are apt to be taken in them. Even if the small ones are later picked out and thrown back into the sea it does them no good, and we rely on young fishes to grow up and provide the seafood harvests in years to come. So anything that safeguards fishes too small for the market is worthwhile and it may be that the sole has an added advantage, although at the moment we know all too little to be sure on this point. What we do know is that some kinds of young soles will occasionally rise at night to the surface. With undulating movements of the body they swim almost vertically upwards. Arriving at the surface they curve the body into a saucer-shape, so that the fins along the sides of the body are just showing above the surface. Why they should in this way remain suspended instead of sinking is not clear. It has been suggested that the delicate fins and their supporting fin-rays make such intimate contact with the surface film that the body makes a sort of vacuum cup.

A FISH'S SLEEPING BAG

Parrot-fishes live in tropical waters, on both sides of the Atlantic, among coral reefs where they feed on the succulent seaweeds growing on the corals and rocks. The teeth in both upper and lower jaws are joined to form cutting edges, so that they look more like a parrot's beak. The parrot-fish bites off pieces of seaweed with these teeth but masticates them with special teeth in the throat. A few years ago it was discovered not only that parrot-fishes sleep but that certain species use a kind of sleeping bag. As night falls, some species of parrot-fish dress themselves in an envelope of slime (or mucus), given off from glands in their skin. There is an opening in the front of this envelope, guarded by a flap, which allows water to enter, and there is a hole at the back to allow the water to escape. So a parrot-fish is able to draw water in, to pass across its gills, even while enclosed like a car in its plastic cover for the night. In other words it can still breathe although completely wrapped up. In the morning, the parrot-fish breaks out of its 'nightgown' and goes on with its normal activities.

Why does a parrot-fish do this ? It may be a method of preventing silting up of its gills during the time it is sleeping on the sandy bottom, or it may be a protection against enemies, by keeping the fish hidden. Whatever may be the purpose of this mucus envelope the use of it is somewhat laborious. A fish may take as much as half-an-hour to secrete it, and it will take as long to break out of it in the morning.

UNDER THE ICE

We sometimes read that certain fishes living in temperate latitudes, where rivers and lakes may become iced over in winter, hibernate. The current view among scientists is that fishes do not hibernate in the strict sense,, so we speak about it as winter rigidity. Nevertheless, it is not just a case of fishes becoming frozen stiff when the temperature falls to a low level. They do, at least, do something to mitigate the effects of the cold. So a state of winter rigidity is something more than a passive state. One of the best examples is seen in the carp, a freshwater fish found across Europe and Asia, and also introduced into the United States. During the summer months the adult carp keeps to moderately deep water, preferably where there is a muddy bottom, feeding on vegetable matter as well as insect larvae, worms and freshwater shrimps. In winter carp move into even deeper water and there they spend the season of cold weather in groups of up to a hundred, forming tight circles with all their heads at the centre. The tench is another member of the carp family which is found in the rivers of Europe and northern Asia, and it also has been introduced into the United States. Its winter rigidity is accompanied by an even more pronounced and deliberate change in behaviour. Tench bury themselves in the mud. When dug up and thrown out onto the bank they could be dead for all the movement they show-until somebody taps one with a stick, when it promptly shows signs of life.

DEEP-SEA HIBERNATION?

The basking shark grows to a length of up to 45 feet and a weight of 4 to 5 tons, but it feeds on microscopic plankton. This it obtains by swimming slowly through the water with its capacious mouth wide open. Water enters its mouth and flows out through the gill-slits in the side of the throat, and it is there that the plankton is filtered out and swallowed. On each side of the gills are numerous filaments, up to 4 ins. long, known as gill-rakers. There may be as many as twelve hundred rakers on each side of a gill, and together these form a fine meshwork through which water can flow, leaving the plankton behind.

Basking sharks are found in temperate seas of both hemispheres and are particularly numerous off the coasts of Europe in summer. In winter they disappear. Moreover, nothing is known of their breeding habits, and young basking sharks are very seldom seen. It has been presumed that they go down into very deep water in winter, and that they breed there. It has also been supposed that much of this time is spent lying inactive on the sea-bed, in a sort of hibernation.

It is only within the last few years that even the slenderest clues have been obtained as to what happens to them. These clues came when "a small male basking shark accidentally swam into a submerged dry-dock in San Francisco Bay. Another small basking shark was washed up on the coast of Holland in winter. Both had lost their gill-rakers. The theory is that at the end of the summer basking sharks shed their gill-rakers as they go down to deeper waters, and there they fast until the warmer weather brings them up to the surface again, where they grow a fresh set of rakers. Much of this is guesswork, but in support of it some calculations have been made. It has been calculated that a 22-foot basking shark would need to take each hour enough food to produce 663 calories, merely to give it the energy to swim about to collect food. More than this would be needed for the energy needed in addition for growth, so we can say that during the summer the shark needs to take in food hourly to a calorific value of 700 calories or more In November there is a sharp drop in the amount of plankton in the waters these sharks frequent. Then, they would be able to obtain food giving little more than 150 calories an hour. So, it is argued, the basking shark solves its problem by throwing away its gill-rakers, worn by a season's use, refrains from feeding and sinks to the bottom to rest while hibernating. Possibly, like hibernants on land, its bodily processes slow up tremendously. So we have a picture of these huge fishes lying inert deep in the sea for the winter, after having either laid their eggs or produced living young. Which they do is not known, and may not be known, until some chance clue puts us on the track of this further aspect of its biology.