A fish which can turn aside a hungry shark in mid-snap has become the focus of the search for naturally occurring shark repellents to protect swimmers and divers, scientists say.

The fish repellent is so potent that sharks exposed to it in experiments can go into convulsions or even roll over belly-up on the bottom of a tank.

Marine biologists told a convention of the American Association for the Advancement of Science recently that natural biological substances may replace chemical shark repellents discontinued a decade ago when they found to be ineffective.

The scientists, led by Dr. Bernard Zahuranec of the Office of Naval Research, said no replacement has been found so far.

But they said there was renewed hope for a repellent made from potent biolical toxins discovered in simple sea creatures over the last few years, particularly a toxin secreted by a species of fish called the Moses sole, a foot-long flatfish which inhabits the Red Sea and the western Indian Ocean.

Its name comes from a legend that the fish was flattened by being split down the middle when Moses divided the waters, said Dr. Eugenie Clark of the University of Maryland.

In a series of tests, individual Moses sole survived up to 28 hours in a tank with two captive reef white-tipped sharks, despite repeated attacks. In some cases, Clark said, the sharks would have the fish in their mouths before being driven away.

"At the end of the experiment, you take the fish out and if it doesn't have a scratch on it, even though it's been in the jaws," she said.

Scientists say the fish has a series of poison glands near its back and rear fins, which secrete a milky substance which interferes with the shark's gills. The gill is the lung-like organ through which fish breathe.

Dr. Eliahu Zlotkin, a biochemist at Hebrew University in Jerusalem, said it probably would be too expensive to make shark repellent from the fish toxin, Paradaxin, but he said a chemical substitute for it probably could be found.