How the sawfish wields its saw… like a swordsman

If you ever saw a sawfish, you might wonder if someone had taped a chainsaw to the body of a shark. The seven species of sawfish are some of the wackier results of evolution. They all wield a distinctive saw or ‘rostrum’, lined with two rows of sharp, outward-pointing ‘teeth’. But what’s the saw for?

Barbara Wueringer has an answer: the saws are both trackers and weapons. They’re studded with small pores that allow the sawfish to sense the minute electrical fields produced by living things. Even in murky water, their prey cannot hide. Once the sawfish has found its target, it uses the ‘saw’ like a swordsman. It slashes at its victim with fast sideways swipes, either stunning it or impaling it upon the teeth. Sometimes, the slashes are powerful enough to cut a fish in half. Even less dramatic blows can knock a fish to the sea floor, and the sawfish pins it in place with its saw.

Many people have speculated about how sawfish use their weapons. Maybe they rake through the sand in search of buried prey. Maybe they cut chunks out of whales. Maybe they slash their way through schools of fish. But the number of hypotheses outweighs the amount of evidence. Very few people have actually seen sawfish using their saws.

Wueringer wanted to cut through these competing ideas, for she has been fascinated by sawfish for years. “I think when you are interested in sharks and rays, there are some species that everyone is fascinated with,” she says. “Sawfish are definitely one of them.”

She fed young freshwater sawfish (Pristis microdon) with pieces of mullet and tuna, and filmed them as they ate. She found that they never rake through the sand. Instead, they’re active predators that use their saws to rapidly slash at floating fish or pin prey to the ground. They would even attack electrodes that simulated the weak electric fields of living fish.

Sawfish first appeared between 59 and 62 million years ago. Their ancestor probably looked quite similar to their closest modern relatives – the shovelnose rays, or guitarfishes. It would have been halfway between the streamlined shapes of sharks and the flattened forms of rays. From this basic body, the sawfish evolved their distinctive headgear by lengthening a piece of cartilage in their skulls. It also modified some denticles – tooth-like structures on the skins of sharks and rays – into the ‘teeth’ of the saw.

Wueringer saw clues to the sawfish’s origins when she filmed the feeding behaviour of giant and eastern shovelnose rays. Both species also pin prey to the floor as sawfish do with their saws. And when they attack free-floating electrodes, they spiral around and bump into them with their heads. In the wild, these attacks may help to stun a target. Eventually, the sawfish evolved a weapon that would do the job more effectively.

Amazingly enough, the sawfish’s snout is not unique. A second group of fish – sawsharks – have very similar bladed heads, and both groups evolved independently. Sawfish evolved from rays, while sawsharks evolved from sharks. They’re one of the most spectacular examples of convergent evolution – the process where different species turn up for life’s party accidentally wearing the same clothes.

There are differences, however. The teeth on a sawshark’s rostrum regularly fall out and grow back. But those on a sawfish grow continuously throughout their lives, like the teeth of rodents. Indeed, Wueringer saw that some sawfish would randomly scrape their teeth on the ocean floor, perhaps to sharpen them.

Now, she wants to understand how sawsharks use their saws. “When you read books about sharks, most of the time you will find a statement saying that sawsharks use their saw to find and manipulate prey, but I am sure we will find some surprises with that,” she says.

Meanwhile, the sawfish is in trouble. It is critically endangered, partly because fishermen hack off its fins to supply the growing demand for sharks’ fins. But even if people aren’t seeking out these species, their nets can entangle the spiky saws all the same.

Wueringer hopes that her research will help to reduce these accidental kills. “There are researchers developing different devices to minimize bycatch of sharks and rays, including little magnets that you attach to the fishing gear,” she says. This simple measure could produce electric fields strong enough to repulse sawfish and other sharks or rays. And thanks to Wueringer’s work, we now know “which fields the animals react to, and which field strengths they are attracted to”.

Reference: Wueringer, Squire, Kajiura, Hart & Collin. 2012. The function of the sawfish’s saw. Current Biology, citation tbc.

Video by Barbara Wueringer; photo by Diliff

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