Sawfish - sharks with swords

Sharks arose from a common ancestor to bony fishes over 400 million years ago (mya) during the Silurian period. Cartilaginous fishes went a different direction in solving issues of buoyancy, physiology, and locomotion. In the Late Jurassic period, approximately 163 mya, a major break occured within the shark line when certain sharks began to exploit a new niche, taking advantage of rich food sources on the sea floor. As these specialist sharks spent more time at the bottom, their bodies flattened, facilitating their feeding and resting on the substrate. This group became the rays, also known as batoids. The first true rays looked much like modern guitarfishes, with a shark-like body and pointed, flattened head.

True sawfishes, the family Pristidae, first trolled the murky margins of warm Eocene seas only 56 mya, evolving directly from an unidentified Paleocene guitarfish species. This family is characterized by the implantation of continuously growing rostral teeth into sockets on the edge of the saw. Roughly 40 species of modern sawfishes have been identified; only a handful survive today. Sawfishes appear very shark-like because they branched off from the rays very early, before more modified ray body types emerged such as stingrays or eagle rays.

When we look at a sawfish, we can tell about its lifestyle from its body form. Its shark-like tail propels it through the water with graceful undulations. The tail shape reveals that sawfishes are slow swimmers. A flattened body and head indicate a fish that lives on the bottom.

Behind each eye are openings called spiracles, often mistaken for ears. Spiracles are used by rays to inhale water when they rest on the bottom. The eyes are set on top of the head so they can see when partially buried. While sawfishes have decent eyesight, their habitat is so murky that vision is not very useful.

The remarkable snout, however, houses two sophisticated sensory systems, allowing sawfishes to feel the movements of distant animals, and even sense the heartbeats of buried prey. The toothy snout is usually between 1/4 and 1/5 of the sawfish's total length. The sharp awl-like teeth are not true teeth at all; they are really modified scales, which enlarged over time and became embedded deep inside the rostrum.

As with other rays, the mouth, nares (nostrils), & gill slits are located on the flat undersurface. The jaws are lined with a pavement of thousands of tiny dome-shaped teeth. Sawfishes eat only small fish and bottom-dwelling invertebrates, like crabs and shrimps, which can be crushed and swallowed whole through the mouth.

The skin is covered by a protective layer of enameled scales called dermal denticles (literally, 'little-skin-teeth'). Teeth in sharks and rays evolved from these rough tooth-like scales. Like most elasmobranchs, sawfishes are countershaded. The upper surface is dark to blend in with the substrate; the underside is white, to blend in with the surface when seen from below. Sawfishes range in color from light grey, to beige, to brown, and even olive green.

Like other elasmobranchs (sharks and rays), the sawfish has a cartilaginous skeleton and no swim bladder. Instead, elasmobranchs rely on a huge yellow, oil-filled liver to give them buoyancy. When sawfishes eat, the food passes from the stomach into a corkscrew-shaped intestine called the spiral-valve. Male sawfishes mate using a pair of rod-like organs near their pelvic fins called claspers.

Sawfishes are very lethargic animals, spending much of their day nestled in the muddy sea/river floor. At night, they scull slowly through the shallows, using their sensitive saw to find buried prey, which are then raked from the sediment to be consumed. It is useful to view the sawfishes' unique rostrum like a metal detector combined with a clam rake.

If small fishes, like mullet, swim past a hungry sawfish, this great ray will launch from the bottom, slashing its toothy weapon rapidly side to side. Gouged by the snout's awl-shaped teeth, injured fishes tumble to the sea floor, now immobilized and easy to catch.

Apart from its use in finding and disabling prey, the toothy rostrum is also a weapon of defense. When threatened, sawfishes will smack this jagged sword against attackers, whether they be sharks or fishermen. Generally, though, sawfishes are very gentle animals, prefering to lie quietly, undisturbed.

Very little is known about sawfish life history, but the late Dr. Thomas Thorson performed studies on a freshwater species, the Largetooth sawfish ( Pristis perotteti ) from Lake Nicaragua. According to his findings, this sawfish lives approximately 25-30 years, attaining maturity in about 10 years. Females give live birth and pup sawfishes are around 2.5 ft long at birth, reaching a maximum length of 23 ft.

A rubbery envelope surrounds the softened saw of fetal sawfishes to protect the delicate rostrum from injury. Just before birth, the rostrum hardens and the sheath then functions to protect the mother from her spiky offspring. This sheath dissolves shortly after birth and the baby sawfish begins to hunt small crustaceans and fish. It is thought that mating occurs every other year, with an average litter size of approximately 8 pups.

Sawfishes love muddy shallow water, and this is why many people are unaware of them. Few humans apart from tribal villagers and fishermen venture into sawfish domain. These elasmobranchs possess a remarkable physiological system allowing them to travel from the sea into freshwater at will. Some species seem to spend most of their lives in rivers and lakes. It is likely that sawfishes require a variety of habitats and salinities to complete their lifecycle; this factor complicates efforts to conserve space for these rays, and prevents captive breeding programs in aquariums...

There are probably 7 living species, although these animals are poorly studied, and estimates range from 4 to 10 species. Some prefer the warm shallow water of brackish bays and river mouths; other species are predominately freshwater, apparently able to complete their entire lifecycle in rivers and lakes. All sawfishes are classified under the family Pristidae, derived from an Ancient Greek term for sawfishes.

� Matthew McDavitt 1995 - 2004. Please do not reproduce text, photographs, or graphics without the written consent of the author. Quotations may be used for personal study and academic projects, but only with proper attribution to their source (website address and author name). Last update: APR 2004