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By the Teeth of Their Skin

24 March 2003 (All day)
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Tail teeth. Toothlike scales, about half a millimeter long, cover the tail of a dogfish.

The toothlike scales that coat sharks, rays, and skates help protect them from predators and parasites, cut down on scrapes, and possibly allow them to them zip through water faster. Now researchers have discovered that sharks' skin can even help them rip prey apart--a surprising use of the scales that hadn't been seen before.

Whereas adult great white sharks and other predators can rely on sharp teeth and powerful jaws to help dismantle large prey, juveniles may not pack enough bite to pull apart victims too big to swallow whole. While observing young sharks in the lab, researchers at the Marine Biological Association in Plymouth, United Kingdom, serendipitously discovered that the fish might use their toothy tushes instead.

Fish behaviorist David Sims and ecologist Emily Southall videotaped feeding juvenile spotted dogfish (Scyliorhinus canicula). The sharks curved their tails forward until their body armor hooked oversized pieces of squid held in their jaws and then rapidly flicked their heads back to tear off bite-sized chunks. "This is a really novel feeding behavior and a previously undescribed use of scales," says ichthyologist Philip Motta of the University of South Florida in Tampa, who was not involved with the research. "It's almost similar to tool use in other animals, but in this case the shark is using a body part evolved for a different function for feeding purposes."

Using their sandpaper skin as a food processor could help baby sharks tackle large prey such as crustaceans, the researchers suggest in a report published online 20 March in the Proceedings of the Royal Society. Adult dogfish were also seen "scale rasping," but only rarely. Southall and Sims suggest that adults, with their larger mouths, simply don't need to use their skin to eat.

S. canicula belongs to the most species-rich shark family, so the researchers suggest that scale rasping might be common among sharks. "That the behavior is present in hatchlings only a few days old is an interesting finding, because it suggests this is a programmed behavior and one that is probably conserved through evolution," Sims says. Motta notes that the tiny teeth used for scale rasping were longer than elsewhere on the sharks' bodies; looking for similarly long spines in other species should help researchers identify other scale raspers, he says.

Related sites
David Sims's page
The lesser spotted dogfish

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