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A "fossil" fish can live for an impressively long time - perhaps for up to a century, according to a new study.

The coelacanth was thought to have a life span of around 20 years, but new estimates suggest it is a centenarian of the ocean, alongside sharks.

French researchers studied marks on the scales of museum specimens - much like tree rings tell the age of trees.

They believe the fish reproduces only in late middle age and can be pregnant for as long as five years.

Slow-growing fish that produce few young are particularly vulnerable to extinction pressures, such as climate change and overfishing.

Knowing the coelacanth's life history might help to enforce even stronger protection and conservation measures, said Dr Bruno Ernande of the University of Montpellier, France.

"One very important framework for conservation measures is to be able to assess the demography of the species," he told ���˿��� News. "With this new information we will be better able to assess it."

The coelacanth was long thought to have gone extinct until famously turning up in a fishing net off South Africa in 1938.

Two populations were subsequently discovered living off the eastern coast of Africa and another off the coast of Sulawesi, Indonesia.

The African population is classed as critically endangered, with possibly only a few hundred individuals left.

'Peculiar life history'

Dr Kélig Mahé of the North Sea Fisheries Research Unit in Boulogne-sur-mer, France, said the coelacanth appears to have one of, if not the slowest, life histories among marine fish, close to that of deep-sea sharks and roughies (deep-water fish such as the orange roughy).

"Our results thus suggest that it may be even more threatened than expected due to its peculiar life history," he explained.

"Consequently, these new pieces of information on coelacanths' biology and life history are essential to the conservation and management of this species."

In future studies, the scientists plan to perform further analysis on coelacanth scales to find out whether growth rate is related to temperature. The answer will provide some insight into the effects of climate change on this vulnerable species.

The ancestors of the coelacanth evolved 420 million years ago, surviving the shifting of continents and the asteroid strike that wiped out the dinosaurs.

Dwelling in caves on the ocean floor, individuals can grow to 1.8m (6ft), tipping the scales at more than 90kg (200 pounds).

The research is published in the journal Current Biology.