Nov. 6, 2007 -- Sharks, rays and skates use a gel-like substance on their heads to pick up electrical current signals from their water environments, and possibly to follow a bloody trail, according to a new study.
Since the process, known as electroreception, can override the animals' other senses, such as taste and smell, the discovery may help to explain why sharks pursue bloody victims, even when other "easy target" prey is around and the gushing blood obscures the shark's vision and smell.
"The gel contains various proteins and salts, so it's similar to mucus, only with a jello-like consistency. Basically, it's shark snot," said lead author R. Douglas Fields.
There are several reports of swimmers towing wounded buddies to shore, with the shark still going after the injured person instead of the rescuer, said Fields, who is chief of the Nervous System Development and Plasticity Section of the National Institutes of Health.
"Bloody salts produce a strong electrical field that sharks can detect" with the gel, he explained.
The findings, which have been accepted for publication in the journal Neuroscience Letters, negate a prior study that claimed shark gel serves as a semiconductor, meaning that it generates electricity in response to temperature changes. The author of that paper, B.R. Brown, agreed to issue a correction.
"Brown's paper, which came out in 2003, inspired my recent study, since I had my suspicions," Fields said.
For his own work, Fields and colleagues Kyle Fields and Melanie Fields 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, no electricity was generated by the gel.
"So the gel is nothing but a conductor that allows electrical signals to move from the membrane to the brain," Fields said.
The slimy substance plays a big role in hunting, however. 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 said. "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 skeptical of Brown's results," Zakon told Discovery News. "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 said could "decoy sharks away from swimmers."
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