African coelacanths are very old. Fossil evidence dates their genesis to around 400 million years ago, and scientists thought they were extinct until 1938, when museum curator Marjorie Courtenay-Latimer noticed a live one in a fisher’s net.

Found off the southeastern coast of Africa, coelacanths also live a long time—scientists have suspected about 50 years. But proving that lifespan has been tough. (Coelacanths are endangered and accustomed to deep waters, so scientists can’t just stick their babies in a tank and start a timer.) Now a French research team examining their scales with polarized light has determined that they can likely live much, much longer. “We were taken aback,” says Bruno Ernande, a marine ecologist who led the study. The new estimated lifespan, he says, “was almost a century.”

His team from the French Institute for the Exploitation of the Sea, or IFREMER, found not only that individuals can live to nearly 100 but also that they have gestation periods of at least five years, and may not mature sexually until they’re at least 40. The results were published on Thursday in Current Biology. This slow-motion life highlights the importance of conservation efforts for this rare species, which is marked as “critically endangered” on the IUCN Red List. Only about 1,000 exist in the wild, and their long gestation and late maturity are bad news for their population’s resilience to run-ins with humans. “It’s even more endangered than we previously thought,” Ernande says.

“It will have enormous consequences,” agrees Daniel Pauly, an ichthyologist from the University of British Columbia, who was not involved in the study. Pauly is the creator of FishBase, a database of biological and ecological information about tens of thousands of species. If a fish takes decades to spawn, then killing it wipes out its potential to replenish the population. “A fish that needs 50 years to reach maturity, as opposed to 10 years, is five times more likely to be in trouble,” he says.

Coelacanths have thick scales that grow up to two inches long, and for decades ichthyologists have been debating how to read those scales for signs of age. In the 1970s, researchers noticed small calcified structures on them. They figured the rings were age markers, like tree rings. They disagreed, however, on how to count them: Some figured that each marking denoted one year; others believed that seasonal flips created two rings per year. At the time, the best guess placed their life expectancy at about 22 years. That conclusion, which meant that a 6-foot, 200-pound coelacanth is 17 years old, implied that they grow very quickly: “They would grow as fast as tuna, which is crazy,” Pauly says.

It’s crazy because these are animals with slow metabolisms, which should indicate slow growth. Coelacanths’ hemoglobin is adapted to that slow metabolism, which means they can’t take in enough oxygen to support a fast-growing fish. Some argue that their small gills are further evidence of oxygen limitations. They also live very passive lifestyles, resting most of the day in caves and lumbering slowly through the ocean’s twilight zone, down at 650 feet and below, when they do deign to move around. “Overwhelmingly, the biological features were pointing to a slow-living fish,” says Ernande.

Plus, scientists tracking the lives of individual coelacanths have known that 20 years is far too low. In the 1980s, researchers started sending submersibles and remote-operated vehicles down to a cave harboring 300 to 400 coelacanths. They returned to this spot for over 20 years. During each visit, they recognized individuals by their characteristic white markings. Only about three or four fish in this group would die, and an equal number of new ones would be born, each year. This observation provided striking evidence that coelacanths live long lives—even more than 100 years, that study argues.