Electric gel gives sharks ability to sense heat
Sharks have a remarkable gel in their snouts that produces electricity in response to tiny temperature changes, giving them an added ability to spot prey, scientists have discovered.
The 'electric gel' lies in long channels, about 10 cm beneath the shark's skin, which connect pores on the surface with sensors called 'ampullae of Lorenzini'. The sensors are known to be highly sensitive to electrical fields generated in the water by the muscles of a wriggling or wounded fish - perhaps even the heartbeat of a human swimmer.
But Assistant Professor Brandon Brown, a physicist at the University of San Francisco, believes the ampullae are also finely tuned to temperature changes thanks to the thermo-electric gel. His work is published today in the journal Nature.
When a shark swims into an area where the water has become abruptly colder or warmer, the heat change is transformed into an electrical signal by the gel, which causes the sensors to fire, sending a message to the creature's brain.
Brown recovered gel samples from a black-tip reef shark and a white shark captured off the Californian coast, and tested them in a lab. He applied heat to one end of the sample holder and measured the change in the electrical field as the temperature shift moved through the gel.
He calculates that the gel delivers 300 microvolts of electricity for every 1ºC change in water temperature. Previous experiments with sharks have suggested that the sensors start firing in response to just 0.05 microvolts.
Put together, that suggests sharks could be sensitive to a temperature change of less than 1,0000th of a degree C.
Why would sharks need such a fantastically accurate thermometer? Brown suggests that this helps the predator to locate thermal fronts - areas where warm and cold water mix, often bringing up nutrients on which smaller fish feed.
The discovery could also prompt a rethink on how some creatures sense temperatures. In mammals, cold is sensed by pores called ion channels in certain nerve cells. If the temperature goes higher or lower, more or fewer potassium ions pass through these channels, producing corresponding variations in electrical current.
The transparent gel appears to be based on a family of proteins called glycoproteins, and if a synthetic substitute for it can be found, there are many exciting potential applications, Brown said.
Thermo-electrical materials - substances that translate heat change into electricity - already have numerous outlets, such as portable wine coolers. However, these gadgets are currently "very solid and very rigid," and a flexible gel offers big potential, he said.
"Presumably you could have a form-fitting gel-filled parka, it seems to me, providing battery-operated warmth or, alternatively, if you are in a really hot environment, like a firefighter, the gel could move heat out of your body," Brown added. "If you want to take heat out of something, thermo-electrics are a great way to go."