A new study on the lifespan of the Greenland shark has established that this fish may be the longest-lived vertebrate on Earth. While marine biologists have long suspected that this species of shark had a long lifespan — one individual, caught twice, with each catch more than a decade apart, had shown growth of less than a centimeter per year — researchers had no definitive way of dating individual specimens, as dating fish involves counting the layers in their bones. Sharks, on the other hand, have cartilage skeletons that don’t exhibit this layering, making dating them difficult.

Julius Nielsen, a doctoral student in marine biology at the University of Copenhagen, was struck by the sheer size of an accidentally-caught Greenland shark that he saw while he was on a research expedition — the creature was 5 meters (16.5 feet) long, and at a growth rate of less than one centimeter (0.4 inches) per year, that implied that the fish was extremely old.

Nielsen hit on the idea of using the shark’s eye lenses to date the individual fish, a method currently used for figuring out the age of long-lived bowhead whales: the center of the creature’s eye lens develops before it is born, with successive layers growing onto the lens as the shark develops. The center of the lens can then be radiocarbon dated, using a specific carbon isotope that was released into the environment by nuclear testing in the 1950s and 1960s, absorbed by the sharks’ eyes before they are born. But out of the lenses they had from 28 individual sharks, only 3 of them contained the isotopes — meaning the remaining 25 sharks were born before nuclear testing was begun, and were over a half-century old.

“We realized immediately, Jesus Christ, all of our sharks are extremely old — they’re much older than we expected.”

For the older sharks, Nielsen used regular radiocarbon dating, that yielded some astonishing results: the two largest specimens, at 4.83 meters (15.8 feet) and 5.02 meters (16.5 feet), had average ages of 335 and 392 years old, respectively! The study also estimates that individuals of the species don’t reach sexual maturity until they are 150 years old — a very long time to wait for that special someone, indeed.