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Elephant Shark (Australian Ghostshark) - Callorhinchus milii
Topic Started: Jan 9 2014, 02:55 PM (4,523 Views)
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Elephant Shark (Australian Ghostshark) - Callorhinchus milii

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Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: Chimaeriformes
Family: Callorhinchidae
Genus: Callorhinchus
Species: Callorhinchus milii

The Australian ghostshark, 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 include elephant shark, makorepe, whitefish, plownose chimaera, or elephant fish.

It is found off southern Australia, including Tasmania, and south of East Cape and Kaipara Harbour in New Zealand, at depths of 0 - 200 m.

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Physical Description
Their length is up to 120 cm. Males of this species mature at about 65 cm. This fish has three cone pigments for colour vision (like humans); its dorsal fin has a very sharp spine. The spine has been reputed to be venomous, but no serious injuries have yet been reported. The ghost shark is easy to recognize due to the small club-like structure located on the snout. The mouth is located just behind the snout and the eyes which are often green are large and set high on the head. There is a single gill opening immediately in front of the pectoral fin origin on each side of the fish. The pectoral fins are large, providing a primary means of locomotion. There are two widely spaced dorsal fins. There is a spine located just anterior to the first dorsal fin; the first dorsal fin is much taller than the second dorsal fin and the anal fin is taller than the caudal fin. The caudal fin is broadest at the lower lobe origin and lacks a caudal filament. The upper lobe of the caudal fin is much longer than the lower lobe. The ghost shark has three pairs of hypermineralized tooth plates. The anterior tooth plates of the upper jaw are sharp and blade-like while the anterior tooth plates of the lower jaw are flat used for crushing hard prey items. The teeth of the ghost shark continue to grow and are not shed and replaced as they are in sharks. The gums of the upper jaw have enlarged papillae.

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The club-like projection on the snout of the ghost shark is used to search for prey. The end is covered in pores that sense movement and weak electrical fields. Ghost sharks feed primarily on shellfish and molluscs including the clam Maorimactra ordinaria.

From spring to autumn, adults migrate inshore to estuaries and bays and females lay their eggs on sandy or muddy substrates. The eggs are contained in large yellowish capsules. The egg partially opens enabling seawater to flow in to the egg capsules after a few months and juveniles emerge from the capsule after six to eight months as about 12 cm in length.

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In New Zealand, Australian ghostsharks are exploited commercially, particularly during spring and summer when they migrate into shallow coastal waters. In Australia, they are caught by southern shark gillnet fishery, particularly in Bass Strait and south-east Tasmania, though this fishery targets the gummy shark, Mustelus antarcticus, and will sometimes discard ghostsharks due to the considerably lower price they fetch at market. They are also a popular target of recreational fishers in Westernport Bay, Victoria and in the inshore waters of south-east Tasmania. Their white flesh fillets are very popular with ‘fish-and-chips’ restaurants in New Zealand, but less so in Australia.

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Recently, the Australian ghostshark was proposed as a model cartilaginous fish genome because of its relatively small genome size. Its genome is estimated to be 910 megabases long, 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 Australian ghostshark genome will serve as a useful reference genome for understanding the origin and evolution of vertebrate genomes including humans, which shared a common ancestor with the Australian ghostshark about 450 million years ago. Interestingly, studies so far have shown the sequence and the gene order ("synteny") are more similar between human and elephant shark genomes than between human and teleost fish genomes (pufferfish and zebrafish), though humans are more closely related to teleost fishes than to the Australian ghostshark. Recently, an Elephant Shark Genome Project has been launched to sequence the whole genome of the elephant shark.
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Elephant Shark Genome Decoded: New Insights Gained Into Bone Formation and Immunity

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Byrappa Venkatesh, PhD, holds an elephant shark, one of the world's oldest-living jawed vertebrates. Sequencing its genome offers new clues to why the skeleton of this fish is made of cartilage rather than bone and how the immune system evolved in higher organisms.

Jan. 8, 2014 — An international team of researchers has sequenced the genome of the elephant shark, a curious-looking fish with a snout that resembles the end of an elephant's trunk.
The elephant shark and its cousins the sharks, rays, skates and chimaeras are the world's oldest-living jawed vertebrates. But their skeletons are made of cartilage rather than bone, making this group of vertebrates an oddity on the evolutionary tree.
Now, by comparing the genome of the elephant shark with human and other vertebrate genomes, researchers at Washington University School of Medicine in St. Louis and elsewhere have discovered why the skeleton of sharks is cartilaginous. An analysis of the creature's genome, published Jan. 9 in the journal Nature, offers new insights into the genetic basis of bone formation and the molecular origins of adaptive immunity, which provides organisms with a more sophisticated immune response to pathogens.
Collectively, the findings have important implications for understanding bone diseases such as osteoporosis and for developing more effective therapies to treat these conditions. Findings related to the elephant shark's immune system provide new opportunities for studying adaptive immunity in humans and for formulating new strategies to fine-tune the immune response.
"We now have the genetic blueprint of a species that is considered a critical outlier for understanding the evolution and diversity of bony vertebrates, including humans," said senior author Wesley Warren, PhD, research associate professor of genetics at The Genome Institute at Washington University School of Medicine. "Although cartilaginous vertebrates and bony vertebrates diverged about 450 million years ago, with the elephant shark genome in hand, we can begin to identify key genetic adaptations in the evolutionary tree."
Among the cartilaginous fishes, the elephant shark was selected for sequencing because of its compact genome, which is one-third the size of the human genome. The fish lives in the waters off the southern coast of Australia and New Zealand, at depths of 200 to 500 meters, and uses its snout to dig for crustaceans at the bottom of the ocean floor.
By analyzing the elephant shark genome and comparing it with other genomes, the scientists discovered a family of genes that is absent in the elephant shark but present in all bony vertebrates, including the chicken, cow, mouse and human. When the researchers deleted a member of this gene family in zebrafish, they observed a reduction in bone formation, highlighting the gene family's significance in making bone.
In a surprise finding, the team found that the elephant shark appears to lack special types of immune cells that are essential to mounting a defense against viral and bacterial infections and for preventing autoimmune diseases such as diabetes and rheumatoid arthritis.
However, despite possessing a relatively rudimentary immune system, sharks exhibit robust immune responses and live long lives. The new discovery opens up the possibility of developing new strategies to shape the immune response in humans.
The researchers also determined that the elephant shark genome is the slowest-evolving among all vertebrates, including the coelacanth, a prehistoric fish popularly known as a "living fossil."
Furthermore, large chunks of elephant shark and human chromosomes were found to be highly similar, whereas the corresponding regions in fishes such as zebrafish and pufferfish were fragmented and scattered on different chromosomes. The markedly slow-evolving feature of the elephant shark genome further underscores its importance as a reference genome for studies aimed at better understanding the human genome.
"The slow-evolving genome of the elephant shark is probably the best proxy for the ancestor of all jawed vertebrates that became extinct a long time ago," said lead author Byrappa Venkatesh, PhD, research director at the Institute of Molecular and Cell Biology at the Agency for Science, Technology and Research (A*STAR), in Singapore. "It is a cornerstone for improving our understanding of the development and physiology of human and other vertebrates as illustrated by our analysis of the skeletal system and immune system genes."
Other collaborating institutions are: Yong Loo Lin School of Medicine at the National University of Singapore; Max-Planck Institute of Immunobiology and Epigenetics in Germany; University of Maryland in Baltimore; Hokkaido University Graduate School of Medicine in Japan; San Francisco State University; University of Toronto; Institut de Biologia Evolutiva and Institució Catalana de Recerca i Estudis Avançats, both in Barcelona; and the University of California at Santa Cruz.
The research was funded primarily by the National Human Genome Research Institute of the National Institutes of Health (NIH).


Journal Reference:
Byrappa Venkatesh, Alison P. Lee, Vydianathan Ravi, Ashish K. Maurya, Michelle M. Lian, Jeremy B. Swann, Yuko Ohta, Martin F. Flajnik, Yoichi Sutoh, Masanori Kasahara, Shawn Hoon, Vamshidhar Gangu, Scott W. Roy, Manuel Irimia, Vladimir Korzh, Igor Kondrychyn, Zhi Wei Lim, Boon-Hui Tay, Sumanty Tohari, Kiat Whye Kong, Shufen Ho, Belen Lorente-Galdos, Javier Quilez, Tomas Marques-Bonet, Brian J. Raney, Philip W. Ingham, Alice Tay, LaDeana W. Hillier, Patrick Minx, Thomas Boehm, Richard K. Wilson, Sydney Brenner, Wesley C. Warren. Elephant shark genome provides unique insights into gnathostome evolution. Nature, 2014; 505 (7482): 174 DOI: 10.1038/nature12826

The emergence of jawed vertebrates (gnathostomes) from jawless vertebrates was accompanied by major morphological and physiological innovations, such as hinged jaws, paired fins and immunoglobulin-based adaptive immunity. Gnathostomes subsequently diverged into two groups, the cartilaginous fishes and the bony vertebrates. Here we report the whole-genome analysis of a cartilaginous fish, the elephant shark (Callorhinchus milii). We find that the C. milii genome is the slowest evolving of all known vertebrates, including the ‘living fossil’ coelacanth, and features extensive synteny conservation with tetrapod genomes, making it a good model for comparative analyses of gnathostome genomes. Our functional studies suggest that the lack of genes encoding secreted calcium-binding phosphoproteins in cartilaginous fishes explains the absence of bone in their endoskeleton. Furthermore, the adaptive immune system of cartilaginous fishes is unusual: it lacks the canonical CD4 co-receptor and most transcription factors, cytokines and cytokine receptors related to the CD4 lineage, despite the presence of polymorphic major histocompatibility complex class II molecules. It thus presents a new model for understanding the origin of adaptive immunity.

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