Paris (AFP)

The coelacanth, a species of "prehistoric" fish threatened with extinction, is much more vulnerable than imagined, according to a study of marine biologists released Thursday.

They established that Latimeria chalimnae, sometimes referred to as a "living fossil" and established in Indian Ocean waters around the Comoros, lives five times longer than previously thought.

It also reproduces at a very late age, over 50 years old, a peculiarity that puts it at risk of being caught before it has been able to perpetuate the species.

"We knew its population was in danger, but with these new parameters, we show that the coelacanth is particularly in danger", told AFP Kélig Mahé, marine biologist from Ifremer, and main author of the study published in Current Biology.

There are perhaps 1,000 to 2,000 individuals left off East Africa of this legendary species, which is on the Red List of the International Union for the Conservation of Nature, he said.

400 million years old, this placid and imposing animal, with a blue coat and spotted with white, bears the premises - a lung and an ancestor of legs - of the passage, a little later, of fish to the first terrestrial vertebrates. .

It was believed to have been extinct for at least 70 million years, until fishermen brought back specimens in 1938.

- Underwater caves -

Rare studies gave him a life expectancy of about twenty years, for an adult height reaching two meters.

"We compared it to bluefin tuna, which is a fish made for super fast growth, and we found that weird," reports Kélig Mahé, alerted by his colleague Marc Herbin, from the Museum of Natural History in Paris, who owns the world's largest collection of coelacanths.

Because a lot of things did not fit with the little that we know about the animal.

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"We know that it has a very slow metabolism (unlike bluefin tuna) and that in general such a species has a slow growth", underlines the researcher.

"He lives alone in underwater caves, between 100 and 1,000 m deep, and comes out at night to feed."

Another clue, the supposed gestation time of the embryo in the female's body was two years, an unusually long duration for an existence of barely 20 years.

The initial scans counted the growth streaks of scales, visible under a microscope, which give age of a fish as do trees with growth rings on the trunk.

- Five-year gestation -

Kélig Mahé's team resumed the experience, with more substantial resources than the previous ones.

Specialized in estimating the age of "commercial" fish species, it determines between 30 and 40,000 per year.

Under normal light, the researchers found the streaks of the first analyzes.

But by using polarized light, which distributes the electric field in a given direction, "we discovered lots of micro-streaks, about five for each seen in normal light."

It is these micro-streaks that determine the age of the fish.

An analysis confirmed with another laboratory providing a 3D image of the scales.

As a result, the oldest specimen studied reaches 85 years of age in this species with a life expectancy that is probably 100 years old.

Her gestation period was also revised upwards, from two to five years, by studying two embryos kept at the Museum.

A record in the animal world, where the elephant was in the lead with more than two years for gestation.

But this relative slowness puts it in great danger for its reproduction, because the researchers determined that the coelacanth reached sexual maturity around the age of 50.

"It's very rare, even if there is a similar case with deep-sea sharks," says Mahé.

As the coelacanth "must reach this age in order to reproduce, it should not be a problem until then" to ensure its offspring.

© 2021 AFP