Sawfish don’t saw

Spiked snouts slash, impale and whack prey

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Sawfish use their fearsome-looking snout to sense and slash at prey, new experiments with wild-caught fishes have found. istolethetv/Flickr

Sawfishes use their spiked snouts as a combination sword, antenna and serving spoon — but not much at all as a saw, scientists have found.

Figuring out how the fishes use their whopper snouts has been tricky, says Stephen Kajiura of Florida Atlantic University in Boca Raton. Most of these rare and endangered relatives of sharks and rays live in murky waters or have been trained to captive-feeding procedures in big aquarium displays.

But Barbara Wueringer of the University of Western Australia in Crawley got a rare chance for a more natural look at a freshwater sawfish, Pristis microdon, in tanks in Australia just after they were collected from the wild. “Now we actually have some empirical evidence” for what the saw actually does, Kajiura says.

What didn’t show up was any kind of sawing motion. Instead the fish were more likely to slash their saws in swordlike swipes that impale prey dangled in open water, the researchers report in the March 6 Current Biology. The saw lets its owner fling or whack prey to the bottom, pin it and then, like a cook arranging food on a plate, manipulate the catch so it can be swallowed headfirst.

“If you try and swallow a fish, swallow it the right way,” Kajiura says. Scales, fins and spines are streamlined for motion in water and thus angle backward. Trying to choke down a meal the other way gives prey a chance to flex and scratch and make life miserable for the predator.

The ability to whack prey from a distance gives sawfishes an advantage over predators that have to get their mouths over something before nailing it, notes fish ecologist Michael J. Miller of the University of Tokyo’s Atmosphere and Ocean Research Institute in Chiba, Japan.

The saw can sense as well as snag, picking up the displacement of moving water and the weak electric fields that give away the position of prey. Wueringer set up metal rods creating electric fields in the water and found that setting off an electric signal with no prey fish attached still prompted the sawfish to try to bite.

This experiment clarifies that just sensing an electric field is enough to trigger a sawfish attack, says neuroscientist Carl D. Hopkins of Cornell University, who has studied the electric world of fishes. “It really is an amazing story.”

The saw of Pristis microdon whips back and forth to cut down prey in open water and then helps the fish position its dinner at the correct angle for a good gulp.

Found in “Sawfish don’t saw” online at: http://www.sciencenews.org/view/generic/id/338962/title/Sawfish_don%E2%80%99t_saw

Video credit: B.E. Wueringer et al/Current Biology 2012

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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