How can you tell how old a shark is?? That question has bugged scientists for many years now and it certainly isn’t something that is easily done – it’s not as if they come with a birth certificate. In some species you can count the rings in their vertebrae, as you would the rings of a tree trunk, but this doesn’t tend to work for most species, especially those in the deep. Now an international team of scientists are using eye lenses to age Greenland sharks and, in doing so, may have just stumbled across the oldest living vertebrate on earth today.
Having sampled a range of shark sizes, it was estimated that the largest shark in this study, a 502cm female, could be 392 years old (or as old as 512)! That would make it one of the oldest recorded vertebrates to date, the second oldest being the bowhead whale, previously estimated to reach 211 years old. Furthermore, they estimate that female Greenland sharks don’t reach sexual maturity and can’t start reproducing until about 156 years of age. That’s one old first time mother right there!
Greenland sharks the largest shark species belonging to the “dogfish” group or Squaliformes and can reach a staggering length of 7.3m (just longer than a shipping container or seven, one-metre long rulers). They live in the North Atlantic and while they are typically considered a deep-sea shark, they can be found anywhere from the surface down to 2000m. Don’t let their appearance of a drunken, small-eyed, log fool you though, these large predatory fish are capable of taking on fish, whales, sleeping seals and even moose (or mooses, or is it meese…..what ever the plural for a moose is, this individual was found choking on one ).
We knew that Greenland sharks probably grew very slowly and that meant that they could reach very old ages but until recently, there was no solid proof. This current research has used a method called radiocarbon dating to reveal just how old these mysterious sharks might get. Eye lenses grow continuously throughout the lifetime of a fish. By sampling the very core of their eye lens, researchers were able to sample biological material that was made, and has remained unchanged, since the birth of that shark. Don’t worry, there aren’t a hoard of one-eyed greenland sharks out there, scientists utilised dead individuals accidently caught onboard fishing boats.
Without getting too technical, radiocarbon dating uses the radioactive decay of 14C isotopes to estimate the age of a material (same method they use for dating bones and archaeological trinkets). Because the material at the core of the eye lens has remained unchanged since birth, it is possible to measure the level of 14C present at death and then use this to determine the shark’s age.
The exact ages of the sharks here should be taken with a pinch of salt however. Although not discussed in the paper, old-deep-sea reserves of 14C and maternal transfer of older 14C could lead to overestimations of ages. To think an animal could live for maybe 500 years without being eaten or contracting a disease is quite remarkable. It would also be good to validate this technique against other aging techniques within other species, such as vertebral ring counts.
Regardless of the precise ages, this study provides a promising novel technique that could determine the age of hard to study deep-water shark species. Knowing that these sharks reach such old ages and can’t start reproducing until quite late on in life, has implications on the management and conservation of this species. An animal that grows slowly and reproduces late is likely to be highly sensitive to fishing pressures and may be susceptible to extinction. IF these sharks could grow up to 500 years old, imagine the stories it would have.
Paper: Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus) – Nielsen et al 2016. Science. 353. 702 – 704: DOI: 10.1126/science.aaf1703
More information about the project: http://saveourseas.com/update/greenland-shark-expedition-in-disko-bay-greenland-may-2014/
More information on Greenland sharks: http://www.geerg.ca/greenland-shark.html
Written by Christopher Bird