Greenland sharks are not seen often, because they live in the depths of the Arctic Ocean, but scientists know they can weigh over a ton and live rather long lives. How long? Recent research determined one was 335 years old, and another was 392 years old! Of course, there’s a margin for error, but if true, that means there may be Greenland sharks alive now that were around when the Pilgrims landed at Plymouth Rock. But wait, how do they figure a shark’s age?
Fortunately, they had access to a good number of Greenland sharks; unfortunately, that’s because those sharks had been accidentally caught in fishing nets and scientists’ long lines between 2010 and 2013. All 28 female sharks used in the study had been fatally injured by the time they landed onboard—some by other sharks, and some by fishing equipment—and so all were euthanized. After the sharks’ deaths, researchers measured each shark and took tissue samples from the lenses of its eyes.
The scientists used radiocarbon dating on the samples to see if they could age the sharks. Once again, they had good data thanks to a bad situation—in this case, nuclear warfare. Scientists have known since the 1950s that nuclear bomb tests leave permanent molecular marks on sea creatures. Consequently, the appearance of bomb-related changes in an animal’s tissue can be seen as a sort of time stamp. But because these changes persist, even animals born after any given bomb can be marked by it if the animals they eat were alive during the test.
So it's a bit more complicated than cutting them open and counting the rings. Read more about Greenland sharks at mental_floss.