Coelacanths, which have been around for 400 million years, were thought extinct until they were found alive in 1938 off South Africa. Scientists long believed coelacanths live about 20 years. But by applying a standard technique for dating commercial fish, French scientists calculated they actually live close to a century, according to a study in Thursday’s Current Biology.
Coelacanths are so endangered that scientists can only study specimens already caught and dead.

In the past, scientists calculated fish ages by counting big lines on a specific coelacanth scale. But the French scientists found they were missing smaller lines that could only be seen using polarized light — the technique used to figure out the age of commercial fish.

Study co-author Bruno Ernande, a marine evolutionary ecologist at France’s marine research institute, said polarized light revealed five smaller lines for every big one. The researchers concluded the smaller lines better correlated to a year of coelacanth age — and that indicated their oldest specimen was 84 years old.