Australian Ghost Shark

The elephant shark, Callorhinchus milii, is a cartilaginous fish (Chondrichthyes) belonging to the subclass Holocephali (chimaera). Sharks, rays and skates are the other members of the cartilaginous fish group and are grouped under the subclass Elasmobranchii. Alternative names of the elephant shark include: Australian ghost shark, Makorepe, whitefish, plownose chimaeras or elephant fish. The latter is best avoided as it may be confused with elephantfishes (family Mormyridae) which are freshwater teleost fishes from tropical Africa and the Nile.

It is found off southern Australia, and south of East Cape and Kaipara Harbour in New Zealand, at depths of 200 metres to 500 metres. Their length is between 60 and 120 centimetres. Males of this species mature at about 65 centimetres. During spring, adults migrate inshore to estuaries and bays where mating takes place and the females lay their eggs on sandy or muddy substrates. The eggs are contained in large yellowish capsules. The gestation period is six to eight months. Elephant sharks are exploited commercially, particularly during spring and summer when they migrate into shallow coastal waters. The white flesh fillets of elephant shark are very popular with ‘fish-and-chips’ restaurants in New Zealand and Australia.

Recently, the elephant shark was proposed as a model cartilaginous fish genome because of its relatively small genome size. The genome of the elephant shark is estimated to be 910 Mb long (Mb = megabases = 1 million basepairs) which is the smallest among all the cartilaginous fishes and one-third the size of the human genome (3000 Mb). Because cartilaginous fishes are the oldest living group of jawed vertebrates, the elephant shark genome will serve as a useful reference genome for understanding the origin and evolution of vertebrate genomes including our own, which shared a common ancestor with elephant shark about 450 million years ago. Interestingly, studies so far have shown that the sequence and the gene order ("synteny") are more similar between human and elephant shark genomes than between human and teleost fish genomes (fugu and zebrafish) even though humans are more closely related to teleost fishes than to the elephant shark. Recently, an Elephant Shark Genome Project has been launched to sequence the whole genome of the elephant shark.

The elephant shark has three cone pigments for colour vision (like humans). The elephant shark has a dorsal fin with a very sharp spine. The spine has been reputed to be venomous but no serious injuries have yet been reported.