Gel helps animals detect thermal fluctuations. (Shark sense).

Sharks possess uncanny skill at tracking down prey, but it's unclear how the animals sense their surroundings so acutely. New studies suggest that a clear jelly under a shark's skin keeps the animal informed about minute changes in seawater temperature that may serve as signposts to feeding grounds.

Brandon R. Brown, a physicist at the University of San Francisco, set out to characterize this mysterious gel. The salty brew of glycoproteins fills hundreds of electrosensory canals, called ampullae, that connect skin pores to subsurface nerve cells in sharks, skates, and rays.

After collecting gel from black-tip reef sharks and white sharks that had recently died at aquariums, Brown placed each sample in a tube and warmed one end. He then measured any voltage produced by the temperature difference along the gel's length. To his surprise, Brown found that a variation as small as 1[degrees]C would produce a voltage as large as 300 microvolts. From these data, reported in the Jan. 30 Nature, he concluded that a temperature change in seawater of less than a thousandth of a degree Celsius would induce a voltage in the gel filling the ampullae large enough for the shark to detect.

Brown wondered why a shark would require such exquisitely fine temperature detection. Sensitivity to one-thousandth of a degree could be a distraction to the animal unless it served a purpose, he says.

Scientists have known for years that sharks can home in on prey that congregate at thermal boundaries, where the ocean's temperature varies by a couple degrees over a kilometer or so. Brown conjectured that sharks use their supersensitive gel to detect these subtle boundaries.

"My guess is that sensing temperature is a pretty good strategy for finding food," agrees David W. Sims of the Marine Biological Association in Plymouth, England, who has studied sharks and their prey at thermal boundaries. Sharks may use these boundaries as "foraging corridors," he says.

Over the years, researchers have proposed that sharks use their ampullae to find their way and that the sensory canals play a role in detecting temperature. However, the questions of how and how well the canals might do so haven't been answered entirely, Sims notes.

Now, Brown's work indicates that "sharks seem to have the equipment to detect very small temperature changes," says Sims.