A prehistoric fish that inhabits the waters off southern Australia and New Zealand evolves even slower than the coelacanth, a famous "living fossil" whose DNA has barely changed over hundreds of millions of years, scientists say.
The genome of the elephant shark "is evolving significantly slower than other vertebrates, including the coelacanth", they reported in the journal Nature on Wednesday.
Known by its Latin name as Callorhinchus milii, the elephant shark gets its monicker from a trunk-like snout with which it rummages for crustaceans on the ocean floor at depths of around 200 metres.
Despite its name, the creature is not a shark. Strictly speaking, it is a chimaera, a small group of fish that diverged from sharks, rays and skates, called elasmobranchs, around 420 million years ago.
Both groups are vertebrates whose skeleton is made of cartilage. They split from bony vertebrates around 450 million years ago.
Comparing the genomes of the elephant shark with that of humans and other vertebrates, the study found that the genetic code of C. milii is extremely compact, being less than a third that of humans.
Its genome, they found, has evolved even less than that of the coelacanth - a rare fish found off South Africa that is such a success in its habitat niche that it has hardly had to change over nearly 400 million years.
"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 Wesley Warren, an associate professor of genetics at the Washington University School of Medicine in St Louis, Missouri.
The genome could hold insights into how bones are formed, which could help the fight against the bone disease osteoporosis.
More secrets may lie in the elephant shark's immune system. Its defences seem rudimentary, lacking the kind of immune cells found in humans that combat viral and bacterial infection.
Despite this, its immune system is clearly robust and enables the fish to live a long life.
Elephant sharks, also called Australian ghost sharks, grow to around 120 centimetres in length.
The fish is occasionally netted as by-catch by commercial trawlers but is not considered endangered.
The Red List of the International Union for Conservation of Nature (IUCN) describes the numbers of elephant sharks as "abundant", helped by a ban on shark fishing in waters off Victoria.
It places the species in its "least concern" category of risk.
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