Greenland sharks have a unique eye structure in that the lens grows throughout an animal’s lifetime. The older an animal gets, the more layers are added to the lens. Scientists can’t count the layers as they would tree rings, but they can remove all the layers that have been added over the years until they reach the center, or the embryonic nucleus, of the lens.

This tissue is composed of proteins that were formed when the shark was a young pup. Scientists can analyze the chemical composition of the eye lens nucleus to estimate an animal’s age.

Radiocarbon dating of the 28 Greenland sharks’ lens nuclei revealed a maximum life span of at least 272 years, according to the study, published August 11 in the journal Science .