Bid to find shark repellent

Press, 13 December 1982, Page 40

Bid to find shark repellent

New York NZPA It was the biggest surprise of Stewart Springer’s life. Mr Springer, a veteran' shark researcher, -was in his laboratory dissecting a sand tiger shark that he had caught near Chandeleur Island, of Louisiana. To his consternation, when he reached into one of the animals oviducts, or birth canals, he was bitten. Each oviduct contained a living. 23cm shark pup. This discovery led him and others to undertake more than a decade of intensive studies of shark reproduction, whose findings, although still incomplete, have already amazed marine biologists' They have revealed, for example, widespread internal cannibalism in which one shark embryo eats scores of its potential brothers and sisters. • The research is part of a wide-ranging programme of studies in shark behaviour whose most practical goal is preventing, or at least minimising. shark attacks on people - and on underwater equipment, such as the United States Navy's sensitive submarine-monitoring systems. The research appears, for example, to be leading towards development of the

first truly effective shark repellent — one that might be included in every life jacket and wet suit. Mr Springer, who works at the University of Florida in Gainseville, has found that the sand tiger shark produces in a lifetime as many as 25,000 pea-sized eggs. Periodically 15 or 20 eggs pass from the ovary into each oviduct, where they are fertilised and packaged within an avocado-shaped egg case. Inside that case the shark embryos begin to develop. It is then, even though they are tiny, that their struggle for survival begins. For most of them it does not last long. The embryos begin eating one another until only one — the fiercest and fittest - remains. It does not starve, for soon a ‘ new egg case comes down the oviduct and is promptly eaten. After a year-long succes- ; sion of egg case deliveries the baby shark in each oviduct is one metre long — close to half the length of its 2.4 m long mother. Perry Gilbert of" Cornell University, a leading authority on sharks, points out that the baby at this stage is facing forward. To depart into the sea it must somehow perform a sharp U-turn inside its mothe‘

Research has demonstrated, or at least suggested, that embryos of several other shark species are cannibalistic or “ovaphagous” before birth. Among them are the great white, porbeagle, mackerel, mako and some (but not all) species of thresher shark. One clue to their prenatal behaviour is the stage at which they develop ' teeth. Samuel Gruber of the Rosenstiel School off Marine and Atmospheric Science at the University of Miami and L. J. V. Compagno of San Francisco State University have found fully functional teeth in immature embryos of the bigeye Thresher, which they suspect aids them in “cannibalising potential siblings." Such early dentition is otherwise rare in sharks. Marine biologists have been dumbfounded by the diversity of methods used by sharks to bring forth their young. Even closely related species, such as two members of the thresher family, depend on different techniques. Some sharks nourish their young internally through a’ primitive form of placenta. Others produce single eggs oi huge dimensions. Mr Springer is studying a shark of the genus Centrophorus that produces a single

egg the size of a baseball. Other species produce small eggs, but enough of them to provide food for the more successful embryos. One species of tiger shark may give birth to 80 pups at a time. Some sharks bear egg cases that drift in the sea until the babies hatch. Horn shark egg cases occur in a variety of exotic spirals, some with twisted appendages. The whale shark egg case is larger than a basketball. • In a review article on shark reproduction in “Oceanus," the Journal of the Woods Hole Oceanographic Institution.' Dr Gilbert hqs told how two families — the requiem sharks and hammerheads — use a placenta, in the form of a yolk sac, to nourish their young after thepups have exhausted the food supply in their egg& As in placental mammals, such yolk sacs transfer wastes from the offspring's bloodstream to that of the mother and carry nourishment in the opposite direction. ' Dr Gruber is also working to develop a powerful new shark repellent and. ways to lest its efi'ectivness. The Shark Chaser' developed under United States Navy auspices during World War Il proved of little value, but in 1970 s it was found that a

Red Sea fish, the Moses sole, exudes a substance that repels at least some shark species. Sharks have been seen to charge a Moses sole with open jaws, only to stop within centimetres of the fish. A strong dose of the active substance, called pardoxin, may cause the shark to behave’erratically or curl on the bottom of a test tank, belly up. The substance, a chain of 162 amino acids, is difficult and costly to synthesise. It quickly deteriorates, and is not as powerful as might be desired. Early in efforts to learn how pardoxin works. Eliahu Zlotkin.: a specialist in insect toxins at the Hebrew University. in Jerusalem, proposed that the key to its effect on sharks might be its surfactant properties. Surfactants, or surface-active agents, reduce . surface tension in liquids and help give detergents their cleansing properties. Some 17 commercial surfactants have been tested on captive sharks. Some, such as sodium lauryl sulate (used in toothpaste), have proved effective against sharks at one-quarter the concentration ' needed to obtain the same effect with pa rdrt vin