The Greenland shark, Somniosus microcephalus, is the longest-living vertebrate on the planet with a lifespan that’s thought to possibly extend beyond 500 years. Even at the most conservative estimate of 272 years, it beats any other species on Earth – but the way we came to discover their extreme longevity begins with the atomic bomb.
Aging Greenland sharks is difficult because they lack the calcified tissues researchers sample when studying things, like the ear stones of fishes, which can reveal age a bit like growth rings in a tree. To overcome this obstacle, researchers in a 2016 study used radiocarbon dating techniques to investigate the eye lens of Greenland sharks.
The eye lenses of 28 female Greenland sharks were studied to test for their radiocarbon levels and stable isotopes, which are dictated by the diet of the shark’s mother rather than the sampled animal. This is because of the unique way in which eye lenses retain some structural details that were present on the first day of the shark’s life.
“In vertebrates, the eye lens nucleus is composed of metabolically inert crystalline proteins, which in the center (i.e., the embryonic nucleus) is formed during prenatal development,” explained the authors. “This tissue retains proteins synthetized at approximately age 0: a unique feature of the eye lens that has been exploited for other difficult-to-age vertebrates.”
Where an atomic blast comes into this is a phenomenon known as the “bomb pulse” that’s been identified as an indicator of marine animals’ ages because of the way it influences the radiocarbon levels, as enormous amounts of the radioactive isotopes C-12 and C-14 were released into the environment.
“Since the mid-1950s, bomb–produced radiocarbon from atmospheric tests of thermonuclear weapons has been assimilated in the marine environment, creating a distinct 'bomb pulse' in carbon-based chronologies,” explained the authors. “The period of rapid radiocarbon increase is a well-established time stamp for age validation of marine animals.”
The artificially high levels of radiocarbon were incorporated into the tissues of all living things born after the bomb pulse, acting as a biomarker for age as the tissues of animals born beforehand look very different in radiocarbon dating terms. For our Greenland sharks, that meant the highest amounts of radioactive isotopes were found among the eye lenses of the youngest sharks.
For the remaining large sharks, modeling revealed the average lifespan to be at least 272 years, though the biggest ones were estimated to be nearer to 335 and 392 years old. The team also estimated their age at sexual maturity to be 156 years old, making for a seriously long adolescence.
Part of the reason for these deep-sea giants’ incredible longevity is the extremes of cold they live in. The Arctic environment calls for a seriously slow metabolism, something that’s associated with longevity in a range of animal groups.
While an impressive record, as the researchers warned, Greenland sharks’ incredibly long lifespans and the time it takes for them to reach sexual maturity also make them vulnerable, as every animal of reproductive age represents a massive investment in terms of years spent on Earth.
“Our estimates strongly suggest a precautionary approach to the conservation of the Greenland shark, because they are common bycatch in arctic and subarctic groundfish fisheries and have been subjected to several recent commercial exploitation initiatives,” they concluded.
400 years is pretty impressive, but the ocean quahog still has the Greenland shark beat.