MARINE NATURE NOTES

Otago Daily Times, Issue 21242, 24 January 1931, Page 3

MARINE NATURE NOTES

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

Biologist to the Marine Fisheries’ Investigation and Biological Station, Port Chalmers.

“SEA BISCUIT” OK “CAKE URCHIN” AND “PERCH.”

A specimen of the above has been forwarded to me by Mr E. R. Watt from Warrington, who states that it is now difficult to find this animal, which was once plentiful at that place at low tide. Mr Watt falls into the permissible error • —in which many others have done the same—of calling it a shell, thinking it was one of the shellfish family. A shell it certainly is—that is, the outer covering of an animal, but it does not belong to the shellfish family. It is two subkingdoms lower in the scale of life to mollusca, being in the same class as “ sea urchins.” To naturalists and those children who frequent the seashore, it is the well-known “ sea biscuit,” “ sand cake,” or “ cake urchin.” In fact, it is so flat and round, and when devoid of bristles is not unlike the biscuits which our mothers cut out of dough with a circular tin. lu America it is known as the “ sand dollar.” It derives the name “ sand cake ” on account of its living in sandy situations.

This specimen is about three inches across, but it is known to grow to four indies in diameter. It is so flat, bLng very slightly raised in the centre, that no one but a person with a knowledge of science would associate it at first sight with “ sea eggs ” or “ sea urchins,” to which family it belongs, but, when dried in the sun and when the bristles are removed, the same lines or joints of the plates can be seen as in the spherical “ sea egg.” It belongs to the class Echinodcrmata, “ cchino ” meaning spines and “ derma ” skin —in other words, spiny skin. The family name is Clypeastroid; “ clypeus ” is Latin for shield, and “ aster ” a star. “ Oid ” means like. All together, they mean “ a shield like a star,” the animal, when denuded of spines, having five rays like a star. The scientific name is Arachnoidos placenta. “ Arach ” is Greek for spider, and “ oides ” means like —“like a spider.” “Placenta” is Latin for flat cake-

The mouth is on the under surface and furnished with a masticatory or chewing apparatus. The teeth are not so complicated as in the sea egg, being simply flat pieces arranged around the mouth without any muscular bands by which the teeth move as in the “ seaegg.” It is rather a curious fact, considering the radiate structure pf these spiny animals, that the teeth, instead of moving up and down like those animals with backbones, or from right to left like crabs and crayfish, move concentrically—that is, move in a circle towards the centre, grinding against each other the food before it can pass to the alimentary canal. These cake urchins live principally on small forms of life found in sandy or mud flats. They are common in large shallow harbours. I can remember on several occasions finding them in thousands in the Ohiwa Harbour in the Bay of Plenty, lying in shallow pools, as close to each other as it was possible for them to be. On the upper surface in the centre will be seen what looks like a circular piece of perforated stone. It is through this perforated stone that water is admitted to the animal to fill the tube feet. Around the outside of the stone arc five genital pores, through which the eggs or sperms of the animals are extruded. If you examine one of these dried cake urchins, you will find it is made up of ten pairs of raised plates, with five grooves running from the centre to the edge, continuing on the under surface to the mouth. Down these grooves arc innumerable holes through which tube feet are pushed and controlled by an excellent water system. It is by these tube feet that the animal is able to move about or convey food to its mouth. Around the mouth are a number of hollow outgrowths which function as gills. When alive the animal is of a reddish brown colour and covered with spines of a silky golden nature. Each spine is cancellated—that is, chambered, each vertical chamber consisting of slender fibres and plates which appear under, the microscope like fine net work. One has just cause to wonder how these hard, shelly looking animals can grow. To the naked eye there is no visible place to add to the structure, but Nature has made a wonderful provision for the growth of these spiny creatures. The 20 plates mentioned above dovetail into each other with surprising regularity, yet each plate is capable of being added to to allow the animal to grow. The fish are found throughout New Zealand, and the same species is recorded from Australia, Phillipines, Torres Straits, East India Islands, Solomon Islands, and Bay of Bengal. They are frequently found by fishermen in their trawl nets, off Tyrone and Blueskin Bay. Fossil species have been found at Castlecliff, Wanganui, in the tertiary geological formation. Mrs "k. Campbell, of Balclutha, has sent three fish caught in the Molyneux, and asks if they are “ sea perch.” These arc not “ sea perch ” but “ river perch,” scientifically called perca fluviatilis. “ Perea ” is Latin for perch, and “fluviatilis” is Latin for river, hence the name river perch. These fish were introduced into New Zealand from Tasmania about 50 years ago, and are to be found in many parts of the South Island. Personally, I have not observed them in any of the rivers of the Auckland province or Bay of Plenty districts. The river perch is a handsome fish, distinguished by dark brown bars on the sides. The side fins are nearly yellow, and the under ones bright scarlet. The general colour is olive green, but old specimens may be dark brown. It is a common fish in ,the British Isles, except the North of Scotland, but not found in swift-running streams. It is also an inhabitant of Europe, except the Iberian Peninsula, and extends through Transcaspia and Siberia. River perch rove about in shoals in search of shell fish, worms, insect larvae, and little fishes on which they feed. Mrs Campbell had the interesting experience of watching nn adult perch slowly swimming up the river with a number of young ones following it, the adult every now and again looking around to see that they were following. The perch is fished for a great deal in England. Iznak Waiton and Hood both mention the difficulties they had in tempting them to bite. Hood indicates that long hours of vigil were spent in trying to catch them: — Not a nibble has ruffled my cork—• It is vain in this river to search, then; I may wait till it’s night Without any bite, And at roost time have never a perch then. Perch arc capable of living a long time out of water, and, if occasionally moistened, will survive many hours; in fact, in some parts of Germany and Wales, they are caught and kept alive by being moistened. They are hawked from house to house, and those not disposed of arc taken back to ponds and released, to he caught and sold at some future occasion. Mrs Campbell states that perch are easily caught with a minnow as bait, and are in their best condition for eating in April and May. The little blue crab sent by Master Jack Stewart is our common shore crab, petrolisthes elongates, commonly known as the “ porcelain ” crab. It can be found under stones and debris at low tide jn large numbers. The females spawn in late summer or early autumn.