Australian ghost shark

Taxobox | name = Elephant shark
status = LC | status_system = IUCN3.1
trend = stable

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image_caption = Elephant Shark, Melbourne Aquarium
regnum = Animalia
phylum = Chordata
classis = Chondrichthyes
subclassis = Holocephali
ordo = Chimaeriformes
familia = Callorhinchidae
genus = "Callorhinchus"
species = "C. milii"
binomial = "Callorhinchus milii"
binomial_authority = Bory de Saint-Vincent, 1823

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, reperepe, 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 Nile.

The elephant shark is an overall silver-gray color with an overlay of brown markings on the body and fins. it has a distinctive lobed, trunk-like snout in front of the mouth, which explains one of its common names. This lobe is probably a sensory device for detecting prey animals buried in the sea bottom. The two dorsal fins are high and triangular and the first has a strong folding spine in front of it. The upper lobe of the tail is elongate and shark-like, with a short lower lobe that is immediately preceded by the pointed anal fin, almost appearing to be part of the tail.

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 large eggs are contained in yellow-brown capsules measuring 25 by 10 centimeters that are laid on sand or mud in water less than 40 meters deep. The young hatch 6 to 8 months later, taking about 5 years to reach maturity. Elephant sharks are exploited commercially, particularly during spring and summer when they migrate into shallow coastal waters. About 1,000 tonnes of elephant shark are caught in New Zealand alone. 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 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.

References

* Database entry includes justification for why this species is of least concern
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*Tony Ayling & Geoffrey Cox, "Collins Guide to the Sea Fishes of New Zealand", (William Collins Publishers Ltd, Auckland, New Zealand 1982) ISBN 0-00-216987-8
* Nelson JS "Fishes of the world" (Wiley, New York 2006)
* P. R. Last and J. D. Stevens "Sharks and Rays of Australia" (Intl Specialized Book Service Inc June 1991) ISBN 978-0643051430
* Venkatesh B, Kirkness EF, Loh YH, Halpern AL, Lee AP, et al. (2007) Survey Sequencing and Comparative Analysis of the Elephant Shark (Callorhinchus milii) Genome. PLoS Biol 5(4): e101 doi:10.1371/journal.pbio.0050101
* Elephant shark genome sequencing project http://esharkgenome.imcb.a-star.edu.sg/ at Institute of Molecular and Cell Biology (Singapore)