Pop quiz: What’s the longest-lived vertebrate on Earth?
Many would guess the elephant, I suppose, because it’s a traditional symbol of longevity. But that would be a poor choice: Their life expectancy is only in the neighborhood of 65 years, short nowadays compared even to humans; the record-holder lived only to 86, well this side of the official human best, listed in the AnAge database at 122.)
Plenty would grasp for some kind of turtle or tortoise, a savvier selection for sure. The Galapagos tortoise is listed at 177 years by the folks at AnAge, and certain cousins are thought if not proved to have hung on even longer.
But the ocean is where the old-age action is, and until quite recently the bowhead whale was considered the most senior survivor among the vertebrates, at 211.
All that changed with the publication earlier this month of a curious paper in the journal Science, which credibly claimed to have established a lifespan of roughly 400 years for a Greenland shark, a large and resilient predator of the Arctic and North Atlantic.
The scientists who produced the paper did postmortem exams on 28 Greenland sharks of various ages. Establishing these was complicated because fish ages are usually determined by analysis of bone growth; sharks, of course, have frames of cartilage.
But through a shrewd application of carbon dating (details on that in a moment) they determined that the oldest individual was at minimum 272 years old at its demise in 2012 — so, considerably older than the record bowhead.
At maximum, they said, it could have checked out at 502, meaning it was born within a few years after Christopher Columbus ended his explorations of the New World.
And the safe midpoint of their calculation puts the shark’s age at 392 (with 95 percent confidence), which, if precisely correct, would mean this particular shark’s lifespan included both the Mayflower’s sailing for America and Barack Obama’s election to a second term.
Even older creatures
There are older living things on earth, to be sure: Bristlecone pines that have been around for five millenia. Some sponges in the AnAge database, which confines itself to animal species, live three times that long, or 15,000 years.
And some species of jellyfish are thought to be essentially immortal — but they cheat by reversing the aging process when threatened, turning themselves from full-grown adults back into polyps.
Much cited in the coverage of the Greenland shark paper (and mentioned in the paper itself) is another long-lived animal known to modern science, an ocean quahog nicknamed Ming by its keepers, which perished about 10 years ago at 507. Researchers knew it was dead because its heart stopped beating (did you know a clam could have a beating heart?), and they knew its age because they could count the growth bands in its shell.
The Greenland shark has long been known for its longevity, among other characteristics. It is among the world’s largest sharks — a rival size-wise to the great white — and the largest fish in Arctic waters. It’s also among the deepest diving, having been found as far down as 7,200 feet.
It has been harvested heavily by Icelanders despite its accumulation of toxic chemicals in its flesh. These can removed through repeated washing and processing, however, to produce an alleged national delicacy called kæstur hákarl. (Another superlative: Anthony Bourdain has described the dish as “the single worst, most disgusting and terrible tasting thing” in his dining experience.)
The huge shark eats other fish, naturally, but is also a prodigious scavenger whose gut has been found to hold hunks of horses, moose, polar bears and, in one case, the entire torso of a caribou. And it grows very slowly, according to the Science paper: Though it typically reaches a length over 20 feet and a weight over one ton, it adds but a centimeter per year to its length (your toenails probably grow faster).
Understanding this growth rate was a key objective of the research team, headed by Julius Nielsen of the University of Copenhagen, and the absence of calcified tissues to analyze led them to consider the now familiar tool of carbon dating, used in fields from forensics to archeology.
Eye tissues hold the key
Greenland sharks are born alive, typically in groups of 10 pups, and the tissue at the center of the eye lens is preserved by successive layers of lens material that accumulate, in what sounds rather like the formation of a pearl, throughout their lifetimes.
So the radioactive carbon content of the lens nucleus became a reliable key to establishing a birth date and also, in younger sharks, a growth rate, thanks to changes in carbon levels during the era of Cold War weapons testing. Excerpts from the paper, with statistics, footnotes and references omitted:
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. The period of rapid radiocarbon increase is a well-established time stamp for age validation of marine animals.
The two smallest animals had the highest radiocarbon levels, implying that they were indeed affected by the bomb pulse. However, given the variability of bomb pulse curves, no exact age can be assigned to these animals other than that they were born later than the early 1960s.
The third animal in the chronology, on the other hand, had a radiocarbon level slightly above those of the remaining sharks, placing its birth year close to the same time as the bomb pulse onset (i.e., early 1960s). We therefore assign shark no. 3 (total length 220 cm) an age of ~50 years in 2012 and consider the remaining 25 larger animals to be of prebomb origin.
The researchers also found chemical markers in the shark tissues associated with the era of industrial fossil-fuel consumption and a variety of marine pollution trends, which “taken together, seem to corroborate an estimated life span of at least 272 years for Greenland sharks attaining more than 500 cm in length.”
Longevity is no protection against all pressures, however, whether natural or industrial.
Because the 28 sharks used in the study were taken as accidental “bycatch” by fishing vessels seeking to harvest other species, the authors also called for “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.”
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The complete paper as published in Science, “Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus)” can be read here, but access isn’t free.