ROUND FISH INTO FLAT FISH

Press, Volume LXXV, Issue 22877, 25 November 1939, Page 19

ROUND FISH INTO FLAT FISH

(BPECIAXXT WHITTEN FOB THE FBZSS.)

[By DAVID H. GRAHAM, F.R.M.S.. F.Z.S.]

THE common sand flounder is known from one end, of New ( Zealand to the other, sometimes by different names in different parts. In Auckland he is known as the “dab,” but is no relation to the English dab; sometimes he is called

surface of the water. They are microscopic in size; 200 eggs side by side measure one inch. The baby fish is hatched from its egg less than a quarter of an inch in length, and swims with its body vertical and with an eye on each side. After three weeks or so, it is still swimming at the surface, and is less than half an inch in length. Shortly after this the baby flounder begins to broaden out and becomes wide in _ comparison with its length. It is at this stage that a niost* wonderful change begins; the gristle in front of the head becomes twisted; above each eye is a bar of cartilage. Now the left eye begins to move upwards or over; just a little and very gradually, until the eye has moved right over the edge of the head and both eyes lie close together on the right side and the bar of cartilage above the left eye has become absorbed and left a clear path for the eye to move. The mouth of a flounder has a peculiarly wry appearance from the twisting of the sktdl and bones. In this state the baby fish.is unhappy and at a loss to know which way to swim. It learned to swim vertically and keeps on trying to swim to suit its eyes and the result is a lop-sided movement; the baby fish becomes exhausted, and sinks to the bottom, and, finding it can move along the floor more easily than on the surface, stays there, and is now a flat fish about three months old and an inch in length.

“three corner,” from his triangular appearance: some call him “tin plate,” but why I am at a loss to know; the Maori called him “patiki,” and used to capture him at night on mud flats by means of long slender wooden spears. Commercial fishermen and retailers use the term

“flatfish” when speaking of soles, brill, turbot, and flounders; the term is used because of the flat body and also because the fish lie on their sides on the ocean floor.

Many people fall into the error of referring to the upper or dark side of a flounder as the back, and the under surface which is white as the belly. This is not correct,- as the flounder lies on its left side, while the dark-coloured side in the right side. If you take a flounder between your hands with the dark side against the palm of your right hand, the tail nearest your body, the right side is in your'right hand and the left side in your left hand; the fins on the top are the back fins, and the underneath fin is the ventral. It might surprise you to know that these fish at one time in the history of the earth always swam in the sea in a vertical position, just as you held the fish between the palms of your hands. In fact, they did it for so long that the newly-born flounders do still, and swim about in the sea with an eye on each side, and it is not possible to tell the difference between them and a red cod or other round fish.

Chameleon Habits These fish are so expert at changing their colours to suit the ground they are living on they might well be called the chameleons of the sea. The usual colour of a flounder is greenish grey. I once took 21 small flounders ranging from four to seven inches in length, placed seven water

The sand ftonndet lays anything from 1,000,000 to 3,000,000 eggs, which are buoyant and float at the

The Flounder Changes Colour and Shape '

tight boxes with sea water running into them side by side, apd put three flounders in each box. In box (1) there was clean, grey sand; <2) had a growth of brownish weed; (3) clay-coloured gravel; (4) blue and white broken cockle shells; (5) a black-tarred floor; (6) a large, white enamel dish, and (7), with a square and triangular linoleum pattern. The three fish placed in the sand changed in a few seconds, those placed in the brownish weed box took 10 minutes to change brown. Those put in the clay gravel took eight minutes; the fish took only five minutes to change to suit the colour of the cockle shells. The three placed in the black-tarred box needed 20 minutes, while those put into the enamel dish changed to a sickly white in a few seconds. Those placed in the box with the linoleum took days to change to suit the pattern, and it was interesting to see a flounder with a triangular pattern near the head and a square of another colour near the tail. After a few weeks of changing them from box to box, they became experts, and would change while a person watched them.

To do this “quick-change-artist” business, the fish' in some way use their eyes, unconsciously perhaps, to survey the bottom, and by some functional mechanism then, bring about expansion or contraction of the colour elements in the skin. • These latter contain pigments of : various colours. Mixed with them ■ * and underlying’ are numerous- j opaque white elements which do not i expand and contract like the others. * By varying the relative expansion, j and contraction of these bodies, the, fishes copy the colour pattern of th® „ bottom. In all my experiments, a ' blind fioandet was used, bat m n# instance did it change colour.