If you want to eat, go where the food is. That's one reason sharks tend to linger along boundaries between colder and warmer water. Now a team of physicists have discovered how sharks perceive minute differences in water temperature, and it turns out to be quite unusual.
Sharks detect prey by sensing the electric fields of passing fish. They pick up these signals using a network of organs in their snouts called ampullae of Lorenzini. Gel-filled canals connect the ampullae to pores in the shark's skin, and the gel transmits the electric field properties to the sensory nerves connected to the ampullae. Researchers also knew that these ampullae notified the shark of temperature changes, but they couldn't figure out how. Clearly, sharks didn't employ any method of temperature sensing that had been described before.
In mammals, proteins called ion channels embedded in the cell membrane translate temperature information into electrical signals that activate nerve cells. Based on the characteristics of these proteins and sharks' sensitivity to even tiny temperature changes, "people tried to make calculations," says physicist Frank Moss of the University of Missouri, St. Louis, but "the numbers just didn't work out." No ion channels--at least not any that had been studied to date--could produce such acute temperature sensitivity.
Physicist Brandon Brown of the University of San Francisco and his colleagues devised a way to test the gel's response to a temperature change. They placed some gel inside a quartz container and briefly heated one end of the sample so that it was slightly warmer than the other end. By measuring the voltage at either end of the gel, the researchers found that the temperature gradient produced a corresponding voltage gradient. When the temperature difference fades, so does the voltage difference.
In a live shark, Brown says, suddenly swimming into slightly colder water would produce a gradient between the ampullae, under the shark's skin, and the pores, on the skin's surface. The gel translates that temperature difference into an electrical stimulus that it transmits to the shark's nervous system. Brown says the gel responds to temperature changes as small as one-thousandth of a degree Celsius.
"It's outstanding," says Moss, adding that the work "beautifully explains why these ampullae are so temperature sensitive."