MARINE NATURE NOTES

Otago Daily Times, Issue 21598, 19 March 1932, Page 2

MARINE NATURE NOTES

Written for the Otago Daily Times. By David H. Graham, F.Z.S. Biologist to the Marine Fisheries Investigation and Biological Station, Port Chalmers. FLOUNDERS. Flounders are represented in the Otago Harbour, coastal waters and tidal rivers by four species. The most common and well known sand flounder is the most favoured, and the one with which this article will mostly deal. The other species are the black flounder, greenback flounder and the yellow-belly flounder. Unauthorised names such as “ Tinplate ” and “ Three corner ” have been heard occasionally for the sand flounder. Flounders form an interesting subject as they are amongst the most remarkj able of the fishes in structure. Most other fishes exhibit a roundness of form, swim in a vertical position, and have perfect symmetry on both sides, even to the same spots or markings. In writing of flat fishes I am not including the well-known skate or stingray. It is true that they are flattened fishes and live on the bottom of the sea, where a flattened body is an advantage in securing their food. The skate and ray both have the back and the belly top and bottom. But there is a difference, in that the skate has become flattened downwards and the head is symmetrical, whereas the flounder and other similar fishes are flattened from side to side. The bottom side of the skate is the true lower surface, while the lower surface of the flounder is really the side and the upper surface is the other side. In other words what appears to be the back and belly of the flounder is in reality the right and loft side. The actual back and belly are the edges with the fins, which have become flattened out of all recognition to the normal condition. So when youlook at a flounder or other flat fish and see the white coloured or lower surface you are looking at the right side, and when you look at the coloured or upper surface of the flounder you arc viewing the left side.

The larval history and metamorphosis of these fishes arc remarkable, and have been well known to science for some considerable time, throwing a good deal of light on the evolution of fiat fishes. Why or when these flat fishes, as I shall continue to call them, developed this peculiar shape and began to remain at the bottom of the sea we cannot tell. Their ancestors in all probability swam like other fishes, such as cod. The moving of the eye to the other side gives a certain degree of malformation in the head of the fish—in fact, gives a curiously wry expression to the face of the fish.

The eggs of the flounder are pelagic or of the floating type. The young fish when first hatched out is perfectly symmetrical, with an eye on either side of its head like any other fish. It swims about the surface of the sea in a vertical or upright position. After several months, however, a change begins to take place, and the young fish begins to assume an attitude of swimming similar to that of its parent by turning over on its side. While swimming in this position the fish is really lopsided, and the eye on one side begins to move round towards the other eye. It slowly moves round to the upper edge of the head, and continues till it is close to its fellow. In this stage the fish may be anything from half an inch to one inch in length. It now sinks to the floor of the ocean, and moves about with both eyes on the upper sui’face as in the parent. This twisting of the eye to the other side leads to radical changes in the symmetry of the skull. The skull, as in other larval fish, is cartilaginous, and above each eye is a bar of cartilage, and. strange as it may seem, even before tbs eye begins to move, this is absorbed so that there will be no obstinotion to the moving of the eye. It is supposed by science that the moving of the eye from one side to the other is the outcome of habit, and so strongly is this inherited condition that preparation for the absorption of the cartilage above one eye begins as soon as the young fish is hatched.

Abnormal specimens of flat fishes have been found which further strengthen the theory of their once being round fishes. Specimens with an eye on either side are not rare in England; they usually have both sides coloured. Specimens have also been found here with a reversed condition; that is with the eyes and coloured skin on the under side. The usual colouring of flounders is grey with cloudy markings, the edges of the fins being darker. These fish, however, as I have previously explained, have the power of changing their colour to suit the ground on which they are living. Most visitors to this station have seen the experiments with them in six boxes with six different-coloured layers on the bottom. The flounders in each box have changed their spots to suit the surrounding condition where they are resting. Flounders kept here in tanks afford ideal conditions to observe their habits and movements. During the hours of daylight they remain in one position on the flooring of the tanks unless they are forcibly disturbed. The only movement is that of respiration, and little or no notice is taken by them of other animals in the tank; in fact, I have seen crayfish crawling over them while they still remained passive on the bottom. If they are kept in a tank with sand or gravel they will bury themselves so that considerable force is necessary to make them move away. But during the hours of darkness they are continually in motion, swimming round and round their tank. I have frequently entered the building and, on switching on the light, have found them swimming about, but within a few seconds they glide to the floor and again remain passive while the light is on.

The flounder’s manner of progression through the water is interesting. The long fins which fringe the edges of the body are made to undulate—that is, they move up and down from head to tail in a waving manner, and this gives them progression through the water. The speed these fish attain when disturbed is _ surprising. Though they attain a fair speed in a body of water, they are able to move with greater rapidity on the floor of the sea, when they shoot about with extraordinary rapidity. Probably the evenness of the sea floor gives the fins an added impetus to speed. Those who have done flounder spearing at night will recollect the zig zag speed at which they travel to escape the disturber.

- Most people consider that flounders are the best and sweetest of all fishes. Those caught inside the harbour are much better in quality than those caught outside the, harbour. They keep their condition better. The favourite depth outside for trawling for thein is from 14 to 18 fathoms, but they are caught in both shallower and deeper waters. Prom an examination of many of these fish, it appears that brittle starfishes form their favourite food. At certain seasons as many as six different species of shellfish arc devoured by them,. Four species of crabs and other crustaceans have been found in the stomachs, while four, different kinds of worms are also eaten. They have, therefore, quite a mixed diet. Last September we trawled in 18 fathoms and amongst the catch was a flounder about three inches in length without a tail. The formation where the tail should have been was perfect; there was a rounded edge forming a shape like an inverted letter U, the dorsal and ventral fins meeting at the base of the U.

The life history of the flounder has been worked out by a former worker, here (Mr Anderton), who found the eggs ripe in June and July. One flounder 13 inches in length was found to be carrying 2,342,984 eggs. The green back or grass flounder abounds at times in the harbour, and can be distinguished by the prominent snout. The yellow belly flounder is better known in estuaries along the coast line, such as Hooper’s and Papanui Inlets. The black flounder is found in tidal rivers and is sometimes called the river flounder.