Greenland sharks are some of the slowest-swimming sharks, which attain a maximum swimming speed about half the maximum swimming speed of a typical seal. Therefore, biologists have wondered how the sharks are able to prey on the seals. Greenland sharks apparently search out seals and ambush them while they sleep. Greenland sharks have also been found with remains of polar bear, horses, moose, and reindeer (in one case an entire reindeer body) in their stomachs. The Greenland shark is also known to be a scavenger, but to what extent carrion (almost certainly the origin of the reindeer) figures into the slow-moving fish’s stomach contents is unknown. It is known that the species is attracted by the smell of rotting meat in the water.
It’s been reported from mark-release-recapture studies that the sharks grow only about 1 cm per year (due, no doubt, to their cold habitat), so their large size suggested to some scientists that they might be very old. This suggestion was supported by a new paper in Science by Julius Nielsen et al. (reference below, access probably not free). Using a unique method of dating these sharks, they found that they could be up to 500 years old, attaining sexual maturity only after 150 years. That makes them the longest-lived vertebrate on record, far longer-lived than the previous recordholders, Aldabra tortoises and bowhead whales—a bit more than 200 years each. (See the bottom for the longest lived animals that we know about.)
The way that Nielsen et al. aged the sharks (they sampled 28 females between 2011 and 2013) was to use radiocarbon dating on the proteins in the eye nucleus, whose center forms when the shark is still a fetus. (Note: the sharks weren’t killed for this study: they were “by-catch”, accidentally caught in fishing nets.)
It turns out that nuclear bomb testing in the 1950s and 1960s created a spike in radioactive carbon (carbon 14) that was absorbed into the marine environment, and then into animal proteins, so you can see a spike in the amount of radioactive carbon occurring in specimens caught beginning around 1960 (figure from the paper):
The authors found that the highest amounts of radioactive carbon were found in the eye nuclei of smallest sharks, which were presumably born about 50 years ago. They also did radiometric dating of the eye nuclei of the other sharks, which were born before the pulse and could be dated using conventional methods. Because once the eye nuclei are formed in utero the proteins (and the carbon they contain, derived from the environment at that time) do not change further, forming in effect, biological artifacts that can be dated just like ancient wooden artifacts.
The analysis was a bit more complicated than this, but you can read the paper and its references for details. The upshot is that there’s opportunity for error—not only in the radiocarbon dating itself, but in their use of Bayesian statistics, which requires prior assumptions about age and growth rate.
Given this, the authors are still confident that their estimates are pretty accurate within the error limits shown. The figures that everyone wants to know are in bold (my emphasis):
The model estimated asymptotical total length to be 546 ± 42 cm (mean ± SD), a size matching the largest records for Greenland sharks, and the age estimates (reported as midpoint and extent of the 95.4% probability range) of the two largest Greenland sharks to be 335 ± 75 years (no. 27, 493 cm) and 392 ± 120 years (no. 28, 502 cm). Moreover, because females are reported to reach sexual maturity at lengths >400 cm , the corresponding age would be at least 156 ± 22 years (no. 19, 392 cm) (table S2). Amodel was 109.6%, demonstrating that samples are in good internal agreement, implying that the age estimates are reliable.
The error limits put the upper age limit of the biggest shark as 512 years and the lower limit at 272 years, with the point estimate at 392 years. That means the shark was estimated to have been born in 1624, and could have been born as early as 1504 (that’s 60 years before Shakespeare was baptized). The Guardian says this about the point estimate of the oldest female:
But not everyone is convinced that Greenland sharks can live for four centuries. “I am convinced by the idea of there being long lifespans for these kinds of sharks, [but] I take the absolute numbers with a pinch of salt,” said Clive Trueman, associate professor in marine ecology at the University of Southampton.
Trueman agrees that it is possible to get a record of the early life of a vertebrate from eye lens proteins. However, the fact that the proteins in the centre of the eye lenses, and hence the carbon-14 within them, came from nutrients taken in by the shark’s mother adds a number of uncertainties to the calculations, he says.
Campana says while the approach taken by the researchers is sound, he remains unconvinced that Greenland sharks live for almost 400 years. But, he adds, “future research should be able to nail the age down with greater certainty.”
2.) Is this the longest lived animal? No, not by a long shot. Sponges and corals, which are animals, can live millennia, with some Antarctic sponges estimated at 10,000 years old. However, for animals we’re more familiar with, the record longevity known with reasonable certainty is held by a clam. As I mentioned in 2013, a specimen of the ocean quahog Arctica islandica—a clam nicknamed “Ming”—was snatched from the sea floor off Iceland and dated at 507 years old using growth rings. Pity that the heartless scientists killed it, for who knows how long it might have lived? Like the shark above, this is a cold-water organism. Cold environments can put physiological limits on growth rates by slowing down metabolism, and that might have something to do with extreme longevity. Who knows?
To close, here’s an email exchange I had with Matthew about this paper:
Matthew: And why don’t most vertebrates live for a long time anyway, Mr Professor?
Me: Antagonistic pleiotropy? How the hell do I know?
Matthew: He he. The more you know, the more you realise we know nothing about anything.
Me: Nothing about anything? Not how many hydrogen atoms in a normal water molecule? Not when we split off from the ancestors of chimps? Not how old the universe or the Earth are?
Matthew: You know what I mean. Don’t be a curmudgeon
