SHARKS AND RELATED ANIMALS
Class Chondrichthyes (Sharks, Skates and Rays, Chimaeras)
also known as Elasmobranchia
These are an ancient group of fish which are cartilaginous. This means they have a skeleton made of cartilage, a material lighter and more flexible than bone. We have cartilage in our nose and ears. This group is more advanced than the cartilaginous Agnathans as they possess moveable jaws usually armed with well-developed teeth (i.e sharks). The mouth is often ventral (underneath the head) and they have rough sandpaper-like skin (made up of tiny scales).
These scales are placoid - dentine plates covered by enamel, similar to teeth, and often called denticles. The Chondrichthyes have also developed paired lateral fins for efficient swimming. Some other distinguishing characteristics include:
- Notochord is reduced but is present in an adult.
- Teeth rows are replaceable.
- They lack a swim bladder. Its function is partly carried out by an oily liver.
- Marine fishes, with only few species able to tolerate fresh water.
- Balance of water and salt in organism is maintained by unusually high concentration of urea in blood.
- All contemporary species have internal fertilisation. Male intromittent organs called claspers are developed on pelvic fins.
- They have 5 to 7 gill slits on each side of the body behind the head (Chimaeras have only 4).
The sharks and rays belong to the subclass Elasmobranchii and the chimaeras (also known as ratfishes and ghost sharks) to the subclass Holocephali. There are worldwide around 547 species of sharks and rays.
Superorder Selachii – Sharks
Selachians are characterized by a fusiform body and five to seven pairs of lateral gill slits. There are eight orders and about 350 species in the world. One hundred and sixty six species, approximately half of the world sharks, occur in Australian waters.
They have long, streamlined bodies, two dorsal fins, a large tail, small eyes and two ear holes. The skin is covered in minute placoid scales, with an outer enamel layer and an inner pulp layer. This gives protection and aids in locomotion by providing a firm base for muscle attachment. They are very well adapted for fast swimming and predatory feeding.
Sharks have specialised types of teeth, depending on their feeding habits:
• pavement teeth, found in rays and some sharks (eg. Port Jackson Shark Heterodontus portjacksoni). These are flat grinding plates used to crush molluscs and crab shells.
• awl teeth – long, tapering teeth with a very sharp point, used for grasping prey before swallowing it whole, eg. Mako (Isurus oxyrinchus) and Grey Nurse Sharks (Carcharias taurus)
• triangular teeth – these are flat and triangular with serrated edges, used for cutting and shearing large chunks of flesh, eg. Hammerheads (Sphyrna spp), White Sharks (O. Lamniformes, i.e. Carcharodon carcharias)
Teeth are set loosely in the jaws, and there may be many rows of teeth, but only the front row is used. The other rows grow to replace worn or damaged teeth in the front row. Sharks have changed little through time because they are so successful as they are.
Unlike bony fishes, sharks do not have a swim bladder that regulates buoyancy. Many sharks have large amounts of low density oil in the liver, which aids floatation. They swim by bending their bodies as they push forward with their tails. The pectoral fins are rigid, allowing very little movement, except to regulate direction and maintain stability. As water flows over the fins, lift is created which stops the shark from sinking.
All sharks are predators, using highly developed senses of smell, hearing and sight. They can detect minute vibrations and low frequency sound waves through sensory pits in the skin and Ampullae of Lorenzini in the snout, which they use to hunt prey. Most sharks eat fish, but some species eat plankton (the whale shark and the basking shark are filter feeders) or invertebrates such as crabs and worms. Most sharks hunt alone, but some hunt in groups. Some sharks (the catsharks, epaulettes and leopards) have modified teeth. They use grinding plates to crush their prey (similar to the sting rays).
All males possess claspers, paired sex organs at the base of the pelvic fins. The claspers are used to transfer sperm to the female where it fertilises developing eggs. Some sharks lay eggs, which are encased in a leathery shell and deposited on the seabed, but the offspring of most sharks develop inside the mother and are born live.
Most sharks swim continuously, forcing water through the mouth, over the gills, and through the gill slits. Oxygen is absorbed in the process. Those sharks that need to keep moving to ensure they obtain enough oxygen will die from suffocation if they stop (this accounts for the majority of the big predators caught in shark nets being brought up dead). Not all sharks need to swim however to obtain the oxygen they need. Some bottom dwelling species (dogfish, carpet sharks. Etc), pump water by rhythmically contracting the muscles that control the inlet and outlet valves of the gills. This is done whilst lying on the bottom of the ocean.
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