News in Science

Sharks use snot to hunt prey

Sharks use a gel-like substance on their heads to pick up electrical signals from the water, possibly to follow a bloody trail, according to a new study.

As the process, known as electroreception, can override the animals' other senses, such as taste and smell, the discovery may help to explain sharks preference for bloody victims.

The researchers say the results might explain why sharks chase bleeding prey, even when other 'easy target' prey is around and why the gushing blood obscures shark vision and smell.

"The gel contains various proteins and salts, so it's similar to mucus, only with a [jelly-like] consistency. Basically, it's shark snot," says lead author Dr R Douglas Fields, chief of the Nervous System Development and Plasticity Section of the US National Institutes of Health.

There are several reports of swimmers towing wounded mates to shore, with the shark still going after the injured person instead of the rescuer, says Fields.

"Bloody salts produce a strong electrical field that sharks can detect [with the gel]," says Fields, whose findings are published in the journal Neuroscience Letters.

However, the findings negate an earlier study, published in the journal Nature, which says shark gel serves as a semiconductor, generating electricity in response to temperature changes.

Author Professor Brandon Brown, of University of San Francisco, agrees to issue a correction.

"Brown's paper, which came out in 2003, inspired my recent study, since I had my suspicions," Fields says.

First, find your snot

For his own work, Fields and colleagues extracted the gel from skate pores.

The pores, which sharks and rays also have, are part of an organ system known as the ampullae of Lorenzini.

Skin membrane cells sense electricity, causing positively charged calcium ions to rush in. The charge moves through the gel before reaching nerves that send the electrical signals to the fish's brain.

As Brown did, Fields and his team inserted two silver wires into the gel and heated or cooled one end of the wire's holder.

Sure enough, this generated electricity. But Fields found out that the electricity was simply caused by an electrochemical reaction between the silver and the gel. When non-silver devices were heated or cooled, the gel did not generate electricity.

"So the gel is nothing but a conductor that allows electrical signals to move from the membrane to the brain," Fields says.

Hunting with snot

But the slimy substance plays a big role in hunting. It allows the fish to detect very faint electrical fields, which prey emit when they swim or bleed.

"Imagine that a shark is swimming between two points of a 1.5 volt battery, with one battery end dipped into Long Island Sound and the other located in the waters off of Jacksonville, Florida," Fields says.

"Despite the incredible distance, the shark could easily determine if the battery was switched on and off. That's how sensitive its electroreception is."

Harold Zakon, a professor of neurobiology at the University of Texas, supports the new findings.

"Dr Fields had every reason to be sceptical of Brown's results," Zakon says. "The generation of voltages at metal-electrolyte interfaces is a bugaboo for electrophysiologists who take great care to ensure that such voltages do not obscure or mislead."

Now that the sharks' electricity detection process is better understood, Fields and his colleagues hope the information may one day lead to better shark repellent devices that he says could lure sharks away from swimmers.