129 — Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus)

Also C-14 dating of their eye lenses right?

— Shaena Montanari (@DrShaena) December 27, 2017

I first learned about Greenland sharks on what I believe was a NatGeo video podcast episode released in 2001. I watched this short video on my iPod Video and was astounded — was it possible that these sharks could have lifespans that rival almost any other animal on earth? And how could you even find that out?

This 2016 paper describes one possible strategy for just that: Nielsen et al sampled the eye lenses of 28 female Greenland sharks and applied radiocarbon dating to these crystalline structures. Unlike many types of animals whose calcified tissues can be “dated” by analyzing the growth zones of bone, Greenland sharks — like all sharks — have fully cartilaginous skeletons. Conventional chronology methods therefore cannot be used, and the eye lens method is one alternative.

$^{14}C$ radioisotope levels can be measured in relation to other stable nitrogen or carbon isotopes to determine the carbon source and calibrate the analysis. The researchers also looked for evidence of the “bomb pulse” — a perturbation to the levels of marine radiocarbon due to the atmospheric testing of thermonuclear bombs. This jolt to the levels of radiocarbon in the crystaline lenses of the Greenland shark help establish which sharks were born prior to and after the early 1960s — when the thermonuclear fallout effect reached the waters in which these sharks live.

Using other top predators of the region as a model, it is possible to establish expectations for the isotope signatures of the Greenland shark tissue.

Not only does this method indicate that the Greenland shark never stops growing — albeit at a very slow (<1cm/yr) rate — it also indicates that the Greenland shark lives many hundreds of years. Indeed, the only animals with longer lifespans are the ocean quahog — a variety of clam.

This makes the Greenland shark a particularly interesting model organism for studying the effects of man-made atmospheric and oceanic changes; it also makes them a target for conservation efforts: The long lifespans and slow metabolisms indicate a slow reproductive cycle, which means that the large number of Greenland shark in fishing bycatch may be putting a severe strain on the survival of the species.