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Demystifying a Sea Monster

Credit: David Wachenfeld, Triggerfish Images

A freshwater sawfish rests on the bottom. Credit: David Wachenfeld, Triggerfish Images

By Barbara Wueringer

The use of the sawfish’s saw has been widely speculated upon, but a recent study has finally revealed its dual purpose.

Catching prey might be one of the most difficult tasks that animals face every day. This task can be even more difficult for aquatic predators, whose prey can escape into three-dimensional space.

Sharks and rays have developed a variety of hunting strategies, but one of their close relatives, the sawfish, has an adaptation that might simplify this daunting task – its saw. Our research has found that the saw is both a sensor to detect the sawfish’s prey and also a weapon that is used to dismember prey. The findings have changed our understanding of the hunting skills of sawfish.

But let’s take a step back and find out what a sawfish exactly is. Sawfish are rays. As such, they belong to the group of cartilaginous fishes and are closely related to sharks. They are some of the few rays that possess a shark-like body.

Sawfish also possess a long rostrum or snout – the “saw” – which bears teeth on its lateral edges. These teeth have evolved from the skin teeth, or denticles, of sharks and rays. If you have ever touched a shark or ray you will know that their skin feels like sandpaper.

Sawfish are often confused with swordfish. Swordfish are bony fish, which means that their skeleton is made of bone. They live in the open oceans and their close relatives are marlins and sailfish.

Sawfish, on the other hand, are closely related to shovelnose rays, which look like a sawfish without the saw. These two groups have both evolved from an ancestor that looked like a shovelnose ray.

For our study we compared sawfish and shovelnose rays, which allowed us to shed light on evolutionary reasons of the saw.

There is another group that evolved from the same ancestor – the sclero­rhynchids, which literally means “stone noses”. Sclerorhynchids only grew to about 1 metre in length, and their saw was sturdier than that of sawfish. It may have also been heavier: rostral teeth were regularly replaced and whole rows of unused dormant teeth were present under the skin. These teeth moved into position when the active tooth fell out, which is comparable to the oral tooth replacement mechanism of sharks.

Sawfish rostral teeth, on the other hand, grow continuously like the teeth of rodents, providing maximal functionality at minimal weight.

Maybe sclerorhynchids used their saw to defend themselves against marine dinosaurs, but both groups fell victim to the mass extinction event at the end of the Cretaceous, around 65 million years ago.

How sawfish use their saw has been widely speculated upon. Sawfish were thought to use their saw to cut pieces of flesh out of whales, and to kill dugongs. There were also reports that sawfish rake through the sand with the saw to uncover hidden prey, and that they slash at schooling fish.

For our study we worked with 19 recently captured freshwater sawfish (Pristis microdon). The animals were juveniles under 2 metres in length.

Until they reach adulthood, freshwater sawfish inhabit the brackish and freshwater regions of rivers in Northern Australia. They reach adulthood at about 3 metres in length (10 years of age) and then move into marine waters.

We fed our sawfish dead mullet, and filmed them. The videos were later analysed, and the behaviours that sawfish displayed during feeding were categorised.

We found that sawfish use two different strategies depending on where the first contact with the prey item is made.

Sawfish can move throughout the three-dimensional space of the water, and if the first contact occurs there they will make rapid lateral swipes at their prey. These can impale the prey on the rostral teeth, and sometimes the impact can be strong enough to split the fish in half. Sawfish then move the prey to the ground.

Prey encountered on the ground or brought to the ground are sometimes manipulated with further lateral swipes of the saw, in contact with the ground.

Prey are also sometimes pinned down with the underside of the saw. Pinned sawfish continue to move, enabling them to be turned around for head-first ingestion. If that does not occur, the fish is further manipulated and turned around by the jaws.

Turning a fish around to ingest it head-first is important because wild sawfish feed on catfish, which possess poisonous spines that might pierce the sawfish’s throat when ingested with the tail first.

The sawfish’s saw has a second function – as an antenna that enables prey detection even in murky waters or at night. Weak electric fields surround every living being. They are created by different ion concentrations inside the body, and sharks and rays have found a way to detect them.

Contrary to common belief, sharks and rays do not detect the heartbeat of their victims but the uniform DC background field that surrounds every living organism. All species tested to date, including sawfish, react to these fields at a distance of about 40 cm from prey, which equals a natural field strength of about 10 nV/cm.

But how do sawfish detect these weak electric fields? They possess tiny electro­receptors called the ampullae of Lorenzini, which are distributed in the skin of their head and saw. From the outside, these receptors are visible as skin pores. Each pore belongs to one receptor that can independently detect an external electric field.

When a sawfish comes across the electric field of hidden prey, the electro­receptors closer to the prey will detect a stronger field than those further away, thus providing the sawfish with a directional clue about the prey’s location.

In our experiments we presented weak electric dipole fields to shovelnose rays and sawfish. The species used were freshwater sawfish (Pristis microdon), eastern shovelnose rays (Aptychotrema rostrata) and giant shovelnose rays (Glaucostegus typus).

When reacting to electric fields on the ground, each of these species always bit the centre of the field. As this behaviour is also known from other species of sharks and rays, we assume that it evolved before the sawfish’s saw.

Moreover, as freshwater sawfish never tried to rake through electric fields on the ground, we assume that they do not use their saw to search for prey hidden in the ground.

Most interestingly, only the sawfish produced rapid lateral swipes of the saw towards electric fields suspended in the water. While shovelnose rays did react towards electric fields in the water, it remains unclear if they would have the behavioural repertoire to manipulate prey suspended in the water. However, from their reactions we conclude that shovelnose rays will investigate an obstruction in the water, and if the obstruction is related to a weak electric field, the field will be the centre of investigation.

We hope that the knowledge of how sawfish detect their prey can be used in the development of mechanisms that prevent the animals from becoming caught in commercial fishing gear.

Moreover, the knowledge that sawfish are not sluggish bottom-dwellers, as previously believed, but agile hunters that use the three-dimensional space of the water may influence conservation measures. For example, this may influence which fishing practices are allowed in sawfish habitat.

All species of sawfish are endangered globally, and all four species of Australian sawfish are protected under the Environmental Protection and Conservation of Biodiversity Act and under various state legislations. They are now considered no-take species, and if caught they have to be released alive.

However, in recent years more and more images have emerged of recaptured sawfish that had their saws removed previously. This practice may be more common in areas like Florida and Australia, where sawfish are protected and where recreational and commercial fishermen are common in sawfish habitats.

We assume that saws are removed from live animals by trophy hunters or to render the animal defenceless before removing them from fishing gear like nets. Not only is this practice illegal, but it also strips the animal of its most important tool, thus minimising its chance of survival. Moreover, it is important to note that the trade of sawfish or their body parts is now banned by the Convention for the International Trade of Endangered Species, which is best known for the trade ban of ivory.

The results of our study may also influence how captive sawfish are fed. If sawfish become so rare that they have to be bred in captivity in order to release some individuals back into the wild, it will become important to be able to provide these animals with stimuli that resemble their natural prey.

Barbara Wueringer is now an Adjunct Senior Research Fellow at James Cook University’s School of Marine and Tropical Biology. The study was conducted at both the University of Queensland and the University of Western Australia.