Saw VII: The Revenge of the Sawfish

by Katie Pratt

This is a sawfish. I’m pretty sure it’s named for its large, chainsaw like, nose (I know, it’s not really its nose, but I’ll get to that in a second), but I could be wrong. And until recently, what the sawfish does with its saw has been a bit of a mystery.

Sawfish are a species of ray, and live in both salty and fresh water in the sub-tropics. What I incorrectly referred to as its nose is actually called a rostrum, and is an extension of the fish’s cranium. It is made of cartilage, save for the boney teeth that stud the periphery. But unfortunately this rostrum, due to its propensity to get tangled in fishing nets and lines, has resulted in a dramatic decline in the number of these fish, and they are now considered critically endangered.

For this reason, studying sawfish in the wild is extremely difficult. Different theories about the purpose of the saw-like rostrum have been postulated, including foraging through the muddy estuary floors that make up their primary habitat, or sawing flesh from large marine carrion (such as dead whales).

In a paper published this week in Current Biology, a group of Australian researchers report a rare opportunity to study newly-captured wild sawfish.

There are many examples of elongated rostra in jawed fish, including those of the prey-whacking billfish and the fish-sensing paddlefish. Scientists had thought, however, that these two features of the rostrum were completely separate. That is, no fish uses its rostrum for both attacking and detecting prey. But as Wueringer et al found, sawfish do.

Marine predators often detect their prey using electrical signals called dipoles. The researchers therefore used dipole decoys to study how the wild sawfish behaved towards their prey. Using the closely related shovelnose ray as a control, sawfish were shown mock prey either in the water column or close to the floor of the tank. When they detected prey, the sawfish didn’t just try and clamp its jaws around it as the shovelnose ray did. Instead, they swatted at the fish with their barbed rostrum: (Check video below of saw fish doing cool stuff)

But why? Aside from looking pretty cool, this behavior actually serves to stun the fish. This then lets the sawfish eat its dinner in the best possible orientation: Head first. Fish scales and fins are designed to allow them to move through water quickly, and are therefore oriented from front to back, so swallowing a fish tail first results in a nasty situation where these scales and appendages drag the wrong way.

So there you have it, folks. There is a right way to swallow a fish whole. I had no idea…

Katie Pratt is a graduate student in Molecular Biology at Brown University. She has a passion for science communication, and in an attempt to bring hardcore biology and medicine to everyone, she blogs jargon-free at www.katiephd.com. Follow her escapades in the lab and online on Twitter.

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