Sawfish (fish)
|
|
| Sawfish Conservation status: Endangered | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Missing image Sawfish-plate.jpg Sketching of a Sawfish | ||||||||||
| Scientific classification | ||||||||||
| ||||||||||
| Species | ||||||||||
|
Anoxypristis cuspidata |
Sawfish are related to sharks and rays. Their most striking appearance is a long, toothy snout. They possess a cartilaginous skeleton and no swim bladder. They are the sole family Pristidae of the order Pristiformes.
Their name comes from the Greek word "pristis", for 'saw'.
They are not to be confused with the sawshark, which is also in the order Chondrichthyes and bears a similar physical appearance.
| Contents |
Physical Characteristics
The most eye-catching feature of the sawfish is of course, their saw-like snout; called a rostrum. The rostrum is covered with motion and electro-sensitive pores that allow sawfishes to detect movement and even heartbeats of buried prey in the ocean floor. The rostrum acts like a metal detector as the sawfish hovers over the bottom, looking for hidden food. It has also been used as a digging tool to unearth buried crustaceans. When a suitable prey swims by, the normally lethargic sawfish will spring from the bottom and slash at it furiously with its saw. This generally stuns or injures the prey sufficiently for the sawfish to devour it without much resistance. Sawfishes have also been known to defend themselves with their rostrum, against predators (like sharks) and intruding divers. The "teeth" protruding from the rostrum are not real teeth, but modified scales. Due to the appearance of the fish, sawfishes are sometimes called "Sharks with Swords".
The body and head of a sawfish is flat as they spend most of their time lying on the sea floor. Like rays, the sawfish's mouth and nares are located on its flat underside. The mouth is lined with small, dome-shaped teeth for eating small fish and crustaceans; though sometimes the fish swallows them whole. Sawfishes breathe with two spiracles just behind the eyes that draw water to the gills. The skin is covered with tiny dermal denticles (skin-teeth) that gives the fish a rough texture. Sawfishes are usually light grey or brown; the Australian Smalltooth Sawfish (Pristis pectinata) appears olive green.
Like other elasmobranchs, sawfishes lack a swim bladder and use a large, oil-filled liver instead to keep them buoyant. Their skeleton is made of cartilage.
The eyes on a sawfish are undevelopped due to their muddy habitat. The rostrum is the main sensory device.
The intestines are shaped like a corkscrew, called a spiral-valve.
Sawfishes can range anywhere from 1.4 metres (Queensland Sawfish or Dwarf Sawfish - Pristis clavata) to over 7 metres (Pristis microdon).
Location and Habitat
Sawfishes are found in tropical and sub-tropical areas, and frequently swim far into rivers. They are also found in bays and estuaries.
There are 7 or 8 species of sawfish, depending on classification. 5 dwell in Australia. The other species can be found near Florida and the waters of southern United States.
Sawfishes live only in shallow, muddy water and can be found in both freshwater and saltwater. Most prefer river mouths and freshwater systems. All sawfishes have the ability to traverse between fresh and saltwater, and often do so.
Reproduction
Not much is known about the reproduction habits of the sawfish. Each individual lives around 25 to 30 years, and matures at 10 years.
Females give live birth to pups, whose semi-hardened rostrum is covered with a rubbery envelope. This prevents the pup from injuring its mother during birth. The rubbery envelope will eventually dissintegrate and fall off.
The sawfish is estimated to mate once every two years, with an average litter of around 8 pups.
Behavior
Sawfishes are nocturnal, usually sleeping during the day, hunting at night. Despite fearsome appearances, they are gentle fishes and will not attack humans unless provoked or surprised. The Smalltooth Sawfish is well known by fishermen as a prize game fish because of the fight it puts up once hooked. Capturing sawfishes is illegal in the United States and Australia.
Species
Australian sawfishes:
Pristis microdon - Freshwater Sawfish: Inhabits freshwater systems and have been found deep inland. Largest Australian freshwater fish.
Pristis zijron - Green Sawfish: Prefers muddy bays and estuaries. Most common sawfish.
Pristis clavata - Queensland Sawfish or Dwarf Sawfish: Also prefers muddy bays and estuaries. Lives along the northern coast of Australia. Relatively small compared to other species, only around 1.4m.
Pristis pectinata - Smalltooth Sawfish or Wide Sawfish: Live in muddy areas, appears green or bluish-grey. Also lives in the Caribbean and around the African coastline.
Anoxypristis cuspidata - Knifetooth Sawfish or Narrow Sawfish: Also lives in muddy areas, appears grey.
Atlantic Species:
Pristis perotteti - Largetooth Sawfish: Lives around the Caribbean and central american coastline. Seriously endangered in Central and South America, especially Lake Nicaragua.
Pristis pristis - The Common Sawfish: Once plentiful in the eastern Atlantic and Mediterranean, this species has become either critically endangered or is assumed extinct.
Conservation
All species of sawfishes are considered endangered, or critically endangered. Their main reason of death is bycatch by fishing nets, as their snouts often entangle them. The rostrum is also prized as a curiosity, and the fins eaten as delicacies, or as medicine. Their liver oil is also of importance to humans.
Loss of habitat is another threat to sawfish conservation.
Sawfishes are illegal to capture in the United States and in Australia. The sale of the Smalltooth Sawfish rostra is prohibited in the United States llegal under the Endangered Species Act (ESA); the others however, are not. This is due to the fact that most rostra on the American market is from the Smalltooth Sawfish. Very few laymen can differentiate the species which the rostra originated. It is generally advised not to purchase sawfish rostra.
Pristidae are difficult to conserve in aquariums because it appears they may require a blend of saltwater and freshwater to stay healthy; but the amount and duration of exposure are uncertain.