You see fish of many shapes and sizes in the fishmonger's shop; they can be divided into two kinds—round fish and flat fish. Cod, Herring, Mackerel and Salmon are round fish. The flat fish are Plaice, Turbot, Brill, Halibut, Sole, Dab and Flounder.
Most people know the taste, as well as the look, of a Plaice; but few know much about its life in the ocean. Indeed, there are secrets in the life of this fish, and many other fish too, which still puzzle us.
Put a Salmon and a Plaice side by side, and it is plain that they live in very different ways. One is made to dart like an arrow, the other to lie flat. One is the shape of a torpedo, the other is flat like a raft. The shape and colour of the Plaice tell their own story of a life on the sandy, pebbly bed of the sea. And look at the eyes! Both are on the upper side of the head! What could be better for a fish that lies flat on the ocean floor?
The Plaice is the best known of these flat fish, so we will try to find how its life is spent in the deep sea.
Have you ever watched those little sailing-vessels which go a-shrimping? They carry a large net—a shrimp-trawl, it is called—which is drawn over the sandy home of the Shrimp. When the trawl is hauled up it may contain not only Shrimps, but the other dwellers in sandy places. Among these, sad to say, is often a mass of baby Plaice and other flat fish. Tiny little fellows they are, some hardly as large as a postage stamp. They are thrown aside, being of no use to the fisherman.
Now these babies are quite flat, darkish on the upper side, white on the other side, like the Plaice you see in the shop. They are not such new babies after all. Though such wee mites, it is more than six weeks since they left the egg; and, in that time, they have passed through wonderful changes, as you will see.
Plaice lay a great many eggs, which float about in the sea. Most are gobbled up by those sea-creatures—and they are many—who love fish-eggs for dinner. From each remaining egg a baby Plaice escapes. At first it floats upside down at the surface of the sea, and eats nothing at all. Then it rights itself, and begins to swallow the tiny creatures which swarm in sea-water.
Strange to tell, this baby Plaice is not a bit like its mother. It is not a flat fish now, but a "round" fish. It has one eye on each side of its head, and you would expect it to grow up like any other round fish.
For about a month this small, transparent youngster hardly alters. Then it grows deeper in the body, and begins to swim near the bottom of the sea. At last it lies on one side, and its life as a "round" fish is over.
A fish lying thus on its side would have one eye buried in the sand, and quite useless, would it not? But our young Plaice is changing its appearance very quickly. Its head is growing rather "lopsided." The eye next the sand is, little by little, brought round to the upper side, until it looks up instead of down. Its mouth gets a queer one-sided look, owing to the twisting of the bones in the head.
Many people think that the dark upper part of a flat fish is the back, and the white under part is the stomach. We have seen, however, that this is not so, for flat fish lie on one side.
For the rest of its life the Plaice will remain flat, with two eyes looking up, and a twisted head. But its colour alters. The side on which it lies is white; the upper side becomes brown and speckled, dotted over with red marks. This is a good disguise. Its enemies cannot distinguish the Plaice from the pebbles and sand around it. They might swim over it, and yet not see the thin, flat, brownish body pressed down on the bed of the sea.
Also, these flat fish have a wonderful way of changing colour. Put them on light sand, and they become lightish. Put them on dark sand and pebbles, and they soon match it by becoming brown and mottled. This is a most useful dodge where so many enemies abound, all swifter in the water than the slow-swimming flat fish.
If you look for flat fish in an aquarium, you will not easily see them. Now and again one will swim up, with a wavy motion of its body. On settling again, it shuffles and flaps about, works itself into the sand, hiding its edges well under, and then, hey presto! it is gone! If the flat fish are so hard to find in a tank, you may be sure it would be impossible to find them on the sea bed. They are poor swimmers, but perfect hiders.
As far as we can tell, they feed on other living creatures. The ocean floor is a huge dining table for them, where they find very mixed dinners. They eat small fish, sand-worms, shell-fish, Shrimps and young Crabs. The Plaice has strong, blunt teeth in its throat, and is well able to grind up the shells of Cockles and other mollusc's, swallowing the juicy contents.
Now we have seen that the Plaice is first a floating egg, and then a tiny transparent "round" fish. It sinks to the sea bed, lies on one side, and becomes a flat fish like its parents.
These little baby flat fish, not much larger than your thumb-nail, crowd in the shallow, sandy parts of the sea near the coast. There they often end their lives in the shrimp-trawl, as we have already noticed.
After leaving this "infants' school" the Plaice, and other small flat fish, go to deeper water. There they feed and grow fat. Our fishermen know where to find them. Indeed, these special fishing grounds are so well known that flat fish are scarcer than they used to be. Some kinds are much too dear ever to be seen on the poor man's table.
There is a special net for catching flat fish, called a trawl. This is a large net, dragged over the bed of the sea by ropes, or steel wire, attached to the sailing vessel or steam trawler. The net is kept open under water by means of beams or boards.
When the flat fish are disturbed, they rise a foot or two from the sea floor, and are then swept into the gaping mouth of the deadly trawl. Once in, there is no escape. There they remain, pressed together, until the net is hauled up and emptied.