Sharks or their close relatives have been present in our oceans for more than 400 million years. They play an important role in the marine environment, similar to that of other top order predators such as birds of prey or big cats.
Sharks and their close relatives, the rays, belong to the class of animals called Chondrichthyes. They are characterised by having a skeleton which is made of cartilage, not bone, like the other fish groups.
There are about 165 shark species in Australian waters. These vary in size from whale sharks, which can reach 12m in length, to small dogfish less than 30 cm long.
The two largest sharks are the whale shark and the basking shark. The whale shark is regularly seen in tropical waters, and both sharks feed on small fish, plankton and jellyfish. In Western Australia, whale sharks regularly visit the coastline and form the basis of a significant tourism industry.
The sharks can be divided into two groups. The first group includes those sharks that live close to the bottom of the ocean. These species generally feed on other bottom-dwelling animals such as shellfish and crustaceans. They have a spiracle on the top of their heads to assist their breathing. This group includes catsharks, wobbegongs and leopard sharks.
The second group are the mid-water sharks that do not have spiracles. As a result, most of this group need to keep moving so that water can pass over their gills to assist their breathing. A few can breathe when resting by pumping water over their gills using their mouths. Open water sharks include the great white shark, tiger and reef sharks.
Sharks reproduce only slowly, which is one of the reasons they are vulnerable to exploitation. Some sharks produce, on average, only one offspring per year. A few sharks lay large eggs (some of which are called "mermaid's purses") from which well-developed young hatch, but about 70% of sharks, including most Great Barrier Reef species, give birth to live young called pups.
Many people have a fear of sharks that is disproportionate to the risk they pose. In reality, sharks have much more to fear from humans than we do from them. Some shark populations are at risk from exploitation as a fishery resource. At least 7,000 tonnes of shark are landed in Australia for sale each year and catches of some species have declined since the 1980s, indicating a potential decline in shark numbers.
CRC Reef research on sharks
There are no active projects specifically on sharks, although they form a significant element of the study of inshore fisheries by the Department of Primary Industries, Coastal fisheries resource monitoring in the Great Barrier Reef World Heritage Area (B4.5).
(Some material in this site was modified from Tropical Topics edited by Stella Martin from Queensland Parks and Wildlife Service)