At four to five meters in length, the Greenland shark (Squaliformes, Somniosus microcephalus) is the largest fish native to the Arctic waters. Getting that big must take a while, and scientists have long known that these sharks grow less than one cm per year. So these sharks probably live a very long time, but little was known about their longevity and maturation.

In an investigation recently published in Science, a team of researchers used radiocarbon dating to put together a timeline of the Greenland shark's lifespan.

Because Greenland sharks lack bones—they’re cartilaginous fish—conventional methods of tracking growth, like carbon dating of bones, won't work. Instead, the team used a modified radiocarbon dating technique that has worked before on other boneless animals: tracking the chronology of the eye lens. The eye lens nucleus is composed of inert proteins. The central portion of the lens is formed during prenatal development, and during growth, the tissue retains the original proteins, which were largely made before birth.

As a result, carbon-dating these proteins can help determine how long ago the shark was born. For this work, researchers performed radiocarbon dating on the eyes of 28 female sharks that were collected in Greenland during scientific surveys that took place between 2010 and 2013. According to the radiocarbon dating, these sharks live at least 272 years.

In the past, atmospheric testing of thermonuclear weapons caused bomb-produced radiocarbon. This radiocarbon subsequently assimilated into the marine environment. The occurrence of these events created a “bomb pulse” that is visible in radiocarbon dating data. In the field, it’s well-established that the presence of the radiocarbon bomb pulse is a time marker for the early 1960s.

The two smallest sharks in the survey (220 cm or less) are presumably the youngest, and they exhibited the highest radiocarbon levels (>99 pMC). The scientists think this indicates these two sharks were affected by the bomb pulse. Unfortunately, due to variability of bomb pulse curves that model its effect on radiocarbon dating, assigning an age to these animals is impossible, though it's likely that they were born later than the early 1960s.

Another shark was found to have a slightly elevated radiocarbon level compared to the other sharks, possibly because it was born close to the onset of the bomb pulse. Using this information, the scientists determined the shark’s age to be ~50 years in 2012. The remaining 25 larger sharks were almost certainly born prior to the detonation of nuclear bombs.

The largest of these pre-bomb sharks, at just over five meters long, is estimated to be 392 ± 120 years old.

In addition to determining longevity, the scientists wanted to determine the age at which Greenland sharks begin to reproduce. Through analysis of sharks that did not exhibit the “bomb pulse” radiocarbon indicator, the team determined that the reproductive age of the sharks was at least 156 ± 22 years, based on other results that indicated females only start reproducing once they reach four meters in length.

This investigation reveals that the Greenland shark is among the longest-lived vertebrate species, with a life expectancy exceeded by only one other ocean dweller (a species of whale). Since it takes them more than a century to reach reproductive age, conservation efforts are important to help keep this population from dwindling.

Science, 2016. DOI: 10.1126/science.aaf1703 (About DOIs).