Blacktip Reef Shark

Sharks are classified into eight orders, each according to common physical characteristics that group related species together. Although this seems simple, classifying sharks is not easy, because the actual characteristics used to define them are not always immediately obvious.

Some of the characteristics that are used to differentiate sharks include the number of gill slits, the number and type of fins, the presence of a membrane over the eye, the type of reproduction and even the structure of valves in the intestines. As a result, two species that look quite similar may be members of completely different orders, such as the mako shark and blue shark; while two others that look quite different can be members of the same order – dogfish and hammerheads, which both belong to the order Carchariniformes, are a good example of this.

Classifying sharks is so complicated that even experts cannot agree on precisely which species belong where. Some authorities put the frilled shark, of which there is only one known species, into a separate order, called the Chlamydoselachiformes. Meanwhile others assign a group known as the bramble sharks, whose two species most taxonomists place in the Squaliformes, to its own order called the Echinorhiniformes.

The eight most commonly recognised orders are:

Heterodontiformes: Bullhead Sharks, containing a single genus and only 10 species including the Horn Shark.

Orectolobiformes: Carpet Sharks. 34 species, including the Whale Shark, the Nurse Shark and the Zebra Shark.

Lamniformes: Mackerel Sharks.16 species, including well-known sharks such as the Great White and the Mako.

Carchariniformes: ground sharks. The most complex group with over 270 species, including Tigers, Hammerheads and Blacktip Reef Sharks.

Hexanchiformes: cow sharks. Four species, with either six or seven gills.

Squaliformes: Dogfish Sharks. About 115 species, including Dogfish, Catsharks and Lantern Sharks.

Squatiniformes: Angel Sharks. About 18 species, all with a distinctive flattened body shape.

Pristiophoriformes: Sawsharks. Eight species, all with distinctive saw-like snouts.