Sawfish uses snout to find and stun prey

By Jenna Hanson 5 August 2014
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Sawfish use electrical signals in their long saw-like snout to detect, stun and then stab them.

FRESHWATER SAWFISH (Pristis spp.) are a critically endangered species found in the freshwater estuaries and rivers of northern and western Australia, but little is known about these unusual creatures.

Although it was previously thought the sawfish uses its long snout to rake through the sand for prey, this new study shows the freshwater sawfish has thousands of electroreceptors on its saw, which it uses to detect the weak electrical signal coming from live prey. It then attacks the prey using swift swiping motions.

“I was surprised to see how skilled sawfish are with their saw,” said lead author Dr Barbara Wueringer from the University of Queensland.

Sawfish predators

Scientists like Barbara have long been intrigued as to what the sawfish – considered sluggish, bottom-dwelling animal – uses its prominent serrated snout for. The best explanation so far has been the sand-raking one.

Sawfish are close relatives of the giant and eastern shovelnose rays (Glaucostegus typus and Aptychotrema rostrata), which look like a sawfish but without a saw, but just why they developed a saw-snout is an enigma.

“When you look at a sawfish in an aquarium, you will instantly ask yourself why an animal would have such a unique adaptation and what it may be used for,” says Barbara. “We know that sawfish are closely related to shovelnose rays and that both groups have evolved from a shovelnose ray-like ancestor, [so] by comparing aspects of [their] predatory behaviours, we could find out why the saw evolved.”

Video of sawfish hunting

In order to see these behaviours, Barbara and her team filmed recently captured freshwater sawfish while they were feeding on chunks of dead fish and again when they were presented with weak electric fields to simulate live hidden prey.

Their film showed that sawfish don’t use their saw to rake through the sand in search of food, but quite the opposite – the sawfish held their saws at an angle of 15 degrees away from the bottom when they were searching for food.

Moreover, when the sawfish were presented with the weak electric fields – a signal to simulate signals emitted by prey – they reacted by swiftly turning towards them and manipulating them with swift lateral swipes, designed to impale the prey on their saw.

“Both the morphological and behavioural results of our study contradict [the standard information], at least in freshwater sawfish,” says Barbara. “Now we know that sawfish are not sluggish bottom dwellers as previously believed, but agile hunters that hunt in the three-dimensional space of the water.”

Saving the sawfish

Not only does this study contradict the standard information in textbooks, but it may also be incredibly important for saving these elusive and endangered creatures.

“Worldwide, sawfish have become very rare,” says Barbara. “But in order to protect these animals we need to know more about their behaviours, how they hunt and which habitats are important for their survival.”

The freshwater sawfish has numerous threats in the wild, including habitat modification, trophy fishing, indigenous harvesting and the shark fin trade. However, the largest factor that has caused the most rapid decline in sawfish numbers is net fishing, as their saws easily become entangles in nets.

“In Australia and Asia the local sawfish populations receive protection, but even then some researchers capture live sawfish without a saw,” says Barbara. “What this means is that someone had caught the animal previously and removed the saw either to keep it as a trophy or to make it easier to get the animal out of a net.”

This new study shows that removing the saw from these creatures drastically reduces their chance of survival.

Aquarist Martin Garwood from Sydney Aquarium believes that this research has provided invaluable insights into the behaviour of these species and important evidence for new investigations into the methods used to fish in the areas sawfish favour.

“This research implies that, while freshwater sawfish are very talented predators, their talents leave them very vulnerable to net fisheries,” says Martin. “Their saw is adept at finding prey but is at a high risk of entanglement, which unfortunately means they’re attracted to fisheries’ nets full of fish which they easily get caught in – [which] highlights the need to finds ways of reducing by-catch.”