Lifeform of the week: Sawfish are armed to the teeth

Sawfish are equipped with a powerful weapon, but sadly the long, saw-like snout, or rostrum, can’t protect them from habitat loss and fishing nets.

Our planet’s waters house many strange-looking creatures, but few can match the distinct peculiarity of the sawfish. With its toothy protruding snout, or rostrum, the animal is an underwater standout. Unfortunately this memorable trait also renders sawfish vulnerable to certain fishing practices. Their numbers have dropped profoundly over the last century. Today they are listed as critically endangered.

 

Shark or Ray?

Looking sharky. Image Credit: Diliff

Depending on the angle from which you view it, your initial sighting of a sawfish may lead you to conclude that it is alternately a shark, a ray, or perhaps an exotic musical instrument that floated out from a sunken pirate ship. In actuality, sawfish are a kind of ray.* They are not as flat as the archetypal stingray, and their pronounced dorsal fins give them a rather shark-like profile, but don’t be fooled. The flattened underside on which their gills and mouth are located gives them away. Like other rays, sawfish also have openings called spiracles behind their eyes that are used to pull in water toward the gills.†

The skeleton of a sawfish (as well as those of their relatives) is made of flexible cartilage, rather than bone. Sawfish come in a range of sizes, with smaller species measuring 4 or 5 feet in length and larger ones reportedly as enormous as 23 feet long. Up to a third of a sawfish’s length is taken up by its signature saw.

Toothsome taxonomy

Pristis microdon (freshwater sawfish) specimen. I count 17 teeth per side. Image Credit: EvaK

Most species of sawfish belong to the genus Pristis. The structure of the toothy rostrum varies with species. In the largetooth sawfish, Pristis perotteti, the rostrum is tapered and has fewer teeth (about 15-20 per side). The smalltooth sawfish, Pristis pectinata, has a non-tapering snout with more teeth (22-32 per side). The teeth of the saw are not real teeth, but tooth-like projections known as denticles, and the saw is not connected to the mouth of the animal. If lost, the saw-teeth do not grow back.

Habitat

Sawfish are found in tropical and subtropical waters of both the Atlantic and Indo-Pacific oceans. They prefer shallow coastal areas, and spend much of their time near the bottom of their chosen body of water. Though they may favor one over the other, sawfish can move between saltwater and freshwater. They often travel through rivers and have been found in freshwater lakes such as Lake Nicaragua.

Gone Fishing

Scary saws. Image Credit: Alison Rawson

The rostrum is a versatile tool for obtaining food. Smaller sawfish employ it as a kind of shovel, digging through the muddy floor of a body of water to stir up tasty crustaceans and other bottom-dwelling fish. Larger sawfish use the saw as a weapon, swimming through schools of fish and slashing at their prey until they impale or otherwise immobilize something suitable for dinner.

Reproduction

Like all rays, sawfish reproduce via internal fertilization, and (like all rays except skates) they bear live young rather than laying eggs. When giving birth to an organism that so closely resembles a chainsaw, some precautions must be taken. Sawfish are generally born with their saw-teeth not fully erupted and the rostrum coated in a protective membrane. The denticles grow quickly after birth, but sawfish overall are slow to mature, taking up to 10 years to reach their adult size.

Can They Hurt You?

Image Credit: chidorian

Unprovoked attacks on humans are almost nonexistent, but you might want to avoid sneaking up on a sawfish, as they can be a little jumpy when cornered. They may not be dangerous, but they are most certainly armed. Armed to the teeth, you might say.

Nets and Other Threats

While they are hunted for their fins and their impressive toothed rostra, an even bigger problem for sawfish may be “by-catch” mortality. Those saws are easily entangled in nets cast for other critters. And, once caught, sawfish are difficult to remove from nets without killing or maiming them in the process. Their slow rate of growth and reproduction doesn’t help matters either. Populations decrease much faster than they can be recovered. And then there’s habitat loss. Animals that live in shallow waters really feel the pinch of coastal development.

As a result of the above threats, all species of sawfish are now listed as critically endangered by the International Union for Conservation of Nature (IUCN).‡ I wish I had better news for you, as sawfish are thoroughly amazing animals. Conservation groups are working on restoring populations and expanding international laws protecting these creatures, but it’s slow going.

There has been some progress. In 2007, the Convention of International Trade in Endangered Species (CITES) banned international trade of sawfish (except for limited trade for aquarium displays) and capturing them is already illegal in both the United States and Australia. It’s a start.

* Sharks and rays are both part of the same subclass, Elasmobranchii, so they are ultimately related.

† Spiracles allow rays to breath while lying on the floor of a body of water.

‡ That’s the highest risk level before “extinct in the wild.”

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Alex Reshanov