MARINE NATURE NOTES

Otago Daily Times, Issue 21538, 9 January 1932, Page 2

MARINE NATURE NOTES

By David H. Graham, F.Z.S.

Written for the Otago Daily Times.

Biologist to the Marine Fisheries Investigation and Biological Station, Port Chalmei’s. SKATE. Though not yet of commercial importance to New Zealand, the skate is an interesting animal in the aquarium, gliding about and flapping its large pectoral or side fing in a way that is suggestive of large bats. Its movement through the water is by a gentle waving or flapping of these fins, which constitutes practically the only means of locomotion. No one who has looked at a skate will need much description of the triangular snout, the broad fins, the long whip tail with email unpaired fins. The body is flattened from above downwards, unlike that of the flounder and other fiat fishes, which are flattened from side to side. The skate rests on its belly, while the flounder bests on its side. On the dorsal side the skin is studded with minute skin-teeth. Behind the lidiesa eyes are the spiracles or gill-slits, which communicate with the mouth cavity. The current of water that is taken in through these spiracles is larger and stronger than that drawn through the mouth, this probably being due to the fact of living on the floor of the sea with the mouth close to the sand. In this breathing habit they are different from their near relatives, dogfishes. The skin teeth or dermal denticles on the upper surface of the skate are very interesting, and serve the function of scales in other fishes. Each tooth or scale is based in bone, cored with ivory, and tipped with enamel. Most of the slime which is found on the skate comes from cells in the skin, known to science as goblet-cells. The skeleton is for the most part gristly and devoid of vertebrae, as in other fish. There are five or six spines round about the eyes, several on the back, and a number running from the body down the centre of the tail. The usual colour of the dorsal surface is brownish olive, with numerous small white spots scattered over the surface. Sometimes specimens have a number of almost black markings dotted about between the white dots. The under-surface is white, tinged with reddish colour, and small black dots round the mouth. The teeth are in 36 rows, with long, sharp points in the male; in the female they are rounded, with spinose ones behind. The under-surface presents a peculiar appearance, especially when the fieh is induced to move up against the glass sides of the tank. The resemblance to a human face is then seen, or, as some imaginative visitors say, it is like the face of the man in the moon. When seen in this attitude, the skate makes an irresistible appeal to visitors and causes much comment and laughter. These fish have spent so much time on the floor of the ocean that they have become extraordinarily flattened. Their shape, together with their similarity of colouring to the sea floor enables them to lie there almost unseen. In the observation tanks they spend most of the daytime on the floor, but occasionally they will rise to the surface and swim about or adhere to the sides, apparently with no effort or movement. Although they spend a great deal of their time on the ocean floor, they are occasionally seen swimming along the surface, probably chasing other inhabitants of the sea. During the hours of darkness those in captivity here spend some part of the night swimming round and round their tanks with their pointed beak out of the water, swimming the while at an angle of about 45 degrees. So powerful are the jaws of skates that they are able to crush the hard shells of shellfish with ease. Their food consists principally of shellfish (Zenatia acinacas and Zeathalia zealandica), worms (several species), and all forms of crabs, and an examination of some large specimens revealed the fact that they fed on lemon soles (Pelotretis flavilatus) up to seven inches in length, while medium-sized ones contained young red cod, also seven inches in length. Skates may bo caught on hand lines from rocks or by fishing from a boat or launch. Large numbers are caught by trawlers outside the Heads. The writer has seen them so plentiful in the trawl net as to make it almost impossible to haul it aboard. They are also caught in the harbour during seine netting. There is practically no sport in catching skates with a line; they may give a few jerks when hooked, but at once give up fighting, and are hauled aboard or out of the sea like a log, but they may possibly give an exhibition of splashing while leaving the water.

Skates grow to a very large size. Specimens up to seven feet long and five feet wide are caught outside the Otago Heads, but the size in which they are usually taken ranges from one foot to three feet across. The flesh is quite edible, but ia seldom seen on the market. People from Britain frequently ask where skates can be obtained, but owing to some unfounded prejudice, either due to their being slimy or to their being supposed to live on offal, no regular supply is in demand. The flesh is very nutritious and good for human consumption. In fact, research in Britain hag shown that skate flesh is next to herring in the amount of nutriment it contains. As a boiling fish it is equal to many other fish, but only those who have been used to eating it in other countries ask for it in New Zealand. When public prejudice has been overcome and our fisheries are more fully developed this fish will become more valuable. Among older fishermen the skate is a favourite dish, -and is usually called “ tawed ”or “ towed.” The edible portions are cut off, and in order to get rid of the slime which covers the skin they are fastened to a cord thrown into the sea and towed behind the launch. In London fish shops skate is usually shown in a crimped state, giving it a very appetising appearance.

Skates are caught in from five to one hundred fathoms, but are more numerous in shallow water, such as Blueskin Bay. The male skate is provided with copulatory modifications of the hind limbs, known as caspers, and fertilisation is internal. The ova are few and large, and the eggs are laid in egg capsules, each one containing a single embryo from which the young does not emerge until it has the form and habit of the parent. Nature usually provides fish with the power of laying large quantities of eggs, but in the ease of the skate only a few are deposited in the sea from one female. Nature has provided, however, that each one born is so well developed that it can take care of itself from birth. The embryo or egg is enclosed in a capsule of a horny nature, shaped like a pillow with four horns; or perhaps the best resemblance is to compare them to the oldfashioned carriers in which butchers used to hawk their meat. Each capsule measures about five inches by three inches, and is yellowish or green coloured, velvety to the touch when first laid, but becoming harder when exposed in the sea. Numerous fine hairs appear to adhere to them, and these again hold on to shells, weed, and debris in the sea. We have no knowledge in New Zealand just how long the young takes to develop, but the records in the Plymouth Aquarium in England show that it takes from four and a-half to five months to hatch out. The quadrangular egg cases are frequently washed up on benches, and form interesting natural history objects. There is a pair of long horns at one end, with another shorter and thicker pair at the other. One side of the capsule is raised and swollen, while the other side is sunken. The sides are quite thin, and turned over towards the sunken or hollow side.

The young skate develops in this capsule and when able to move keeps its tail in constant motion, this causing a current of

water to pass through the case from one one end and out the other, tliis giving it the necessary air for breathing. The water oozes through slits in the ends of the horns, and when the young fish is ready to emerge it escapes through the case at one end between the horns. The rate of development has been found to depend on the temperature of the surrounding water and varies from four to 10 months, according to the season. No one appears to know just how many egg capsules one skate lays in a season. One specimen in the Plymouth Aquarium laid 30 in the course of six weeks. During my time here I have examined many skates, and have not found more than two egg capsules approaching maturity and ready to be extruded. The ovaries of a skate on first appearance look more like those of a hen than a fish; in fact, the case of the skate is formed in a somewhat similar manner as the shell of a chick. Skates are found throughout the seas of both hemispheres. The exact relationship between sharks and skates is not clear, but we do consider that they are descendents from a common stock. In Iceland the skate is called skata; in Germany eshatten or merchatten. A very valuable oil is obtained from skate in America and the skin is manufactured into leather called “shagreen.” Two small skates, together with egg capsules, can be seen by visitors to the station.