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A sawfish.
The rare fish's saws are lined with pores that detect electric fields, a new study suggests (file picture).

Photograph courtesy David Wachenfeld

Christine Dell'Amore

National Geographic News

Published June 15, 2011

They may not see dead people, but sawfish use a sixth sense based in their snouts to hunt and dismember prey, new research shows for the first time.

Previously scientists had suspected that sawfish—large ocean and freshwater fish found throughout the tropics—use their saws to probe sand or mud for prey.

Now, preliminary experiments suggest that the fish's long, tooth-lined saw are full of pores that can detect movements or electric fields of passing prey—acting as a sort of "distant touch," Barbara Wueringer, a sensory neurobiologist at the University of Queensland in Australia, said by email.

This skill is especially handy for nosing out dinner in murky or dark waters, Wueringer said.

The saw—a cartilaginous extension of the skull—also doubles as a weapon, the new research suggests. Lateral swipes can split smaller fish in half, she observed during experiments in the lab.

"We know so little about sawfish, even though these animals can grow really big"—up to 16 feet (5 meters). "To know that the saw acts like an antenna that can sense prey is amazing."

Making Sense of Sawfish

In addition to observing the animals, Wueringer dissected several sawfish that had been accidentally caught or died naturally.

She found that the sawfish's saws were full of tiny pores that signal the ability of an animal to detect the electric fields present in all living animals. Sharks and rays have these pores, as do some fish such as lungfish and even some mammals such as the echidna.

(See "Sharks Repelled by Metal That Creates Electric Field.")

By making "maps" of the skin of four species of rare sawfish, Wueringer pieced together where the pores are distributed on the saws. She then compared this data with pore placement on two species of shovelnose rays (shovelnose ray picture).

Determining where pores are most concentrated gives clues about animals' feeding behaviors, she said.

"For example, rays have their eyes on the upper side of their head, but the mouth is on the lower side. Pores that can detect electric fields around the mouth allow these animals to sense a fish when they are trying to ingest it— but cannot see it," she said.

In the sawfish, the pores were most concentrated on the upper sides of their saws, which should enable the predators to stalk fish in the space above their saws.

Sawfish Research May Help Rare Species

Overall, Wueringer hopes the research will help conservationists learn more about sawfish, especially the four species she studied, whose last stronghold is a remote bay of northern Australia.

In general, sawfish have dwindled dramatically in recent years, largely due to overfishing, both intentional and accidental—the sawfish's serrated snouts make the animals especially vulnerable to entanglement in fishing nets, according to the International Union for Conservation of Nature.

(See "Sawfish Is First Sea Fish on U.S. Endangered List.")

"In order to protect endangered species, we need to know as much as we can about them," she said.

"How a species catches its prey, and also which senses are involved in detecting the prey, is part of the basic understanding of a species."

Preliminary sawfish research presented at Fresh Science—a communication boot camp for early-career scientists—at the Melbourne Museum in early June.

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