Thursday, July 15, 2010

July 15, 2010 : Elephant Shark

Elephant 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 meters to 500 meters. Their length is between 60 and 120 centimeters. Males of this species mature at about 65 centimeters. 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.

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 genome, 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 color vision (like H. sapiens).

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