How a shark’s eye became a clock
Most tissues continually break down and rebuild, which makes their age difficult to separate from the animal’s. The center of a shark’s eye lens is different: its proteins form while the shark is developing and are then largely left in place. That makes the lens a small archive of the carbon available before birth.
Scientists can measure carbon-14, a radioactive form of carbon, in that tissue. Carbon-14 levels in the atmosphere rose sharply during mid-20th-century nuclear weapons testing, and some of that signal eventually moved through the ocean food web. If it appears in a lens, it can help establish whether the shark was born before or after that rise. For older sharks, the measurement helps set a broad age estimate rather than identify an exact year.
Why the estimate has a wide margin
The lens does not contain annual rings like a tree. Researchers have to interpret its carbon signal against changing conditions in the ocean and the shark’s food supply. The result is therefore an estimate with substantial uncertainty—not a precise birthday certificate.
That uncertainty matters because a difference of decades is still a small slice of a life that may span centuries. Combined with the sharks’ slow growth and late maturity, the evidence points to an animal whose population cannot quickly replace individuals lost to fishing or other pressures. A shark caught today may have been alive before many of the technologies and institutions that now shape the North Atlantic even existed.
