Meet the oldest living vertebrate: a shark you've probably never heard of.
When he traveled to Greenland for research, Peter G. Bushnell of the Indiana University South Bend would hear all of these grumblings. But he and his fellow scientists heard one slightly more intriguing rumor about the sharks over and over again: They lived to be incredibly old.
In a study published Thursday in Science, Bushnell and his colleagues report that the species actually has the longest known lifespan of any vertebrate on the planet. While their dating methods have a wide margin of error, they estimate that the sharks may live for four centuries or more. Even their most conservative lifespan estimates — about 272 years — puts the species well ahead of the current record of the bowhead whale (Balaena mysticetus), which has an estimated longevity of 211 years. The Greenland shark may be surpassed only by a 507-year-old clam.
"Great whites are up in the order of 80 or 90 years," Bushnell told The Post. "Which is impressive, but … well, hey, this isn't a contest."
The secret to the Greenland shark's longevity is pretty straightforward: These fish are very big and very cold.
"If you remember your basic high school chemistry, you know that temperature has a profound effect on chemical reactions," Bushnell explained. The hotter things get, the more quickly reactions happen. Greenland sharks live in waters that hover just above freezing. Their tissues are cold, and so are the chemical reactions occurring therein — including all the metabolic processes that turn food into fuel and run the body. Big animals are also known to have slower metabolisms than small creatures: A mouse has a much higher metabolic rate than an elephant. If your metabolic rate is slowed, everything in your body is slowed — including the process of aging.
Preliminary data on the metabolic rate of the shark shows that it's "way down there," Bushnell said.
George H. Burgess, an ichthyologist and fisheries biologist with the Florida Museum of Natural History at the University of Florida who wasn't involved in the new study, agreed that these qualities make the Greenland shark a likely candidate for superlative vertebrate age.
"I suppose if I had to pin my thoughts on the single species of shark that would live the longest, intuitively, the Greenland shark would be right up there at the top," Burgess said. But the maximum age suggested in the Science paper, he added, is higher than most scientists would expect. That doesn't mean it's necessarily incorrect, but more data is needed — especially since the researchers used a novel technique to date their shark specimens.
When you want to date a Greenland shark, the first thing you do is go looking for signs of nuclear bomb testing. No, really: Unlike bonier fish, Greenland sharks lack distinctive growth rings for researchers to count like those in a tree. So instead, they carbon date them by looking for the isotopes left behind when nuclear testing was in its heyday.
"That spike in radiocarbon isotopes in the atmosphere got incorporated into virtually all the food webs on the planet, and so into all of the tissues of living things," Bushnell said. In this case, the researchers looked for signs of these isotopes in the center of the lens of the eye of the sharks they studied. After being laid down early in fetal development, this tissue has little interaction with the outside world. In theory, it provides a snapshot of the isotopes in the atmosphere when the shark was first conceived.
Of the 28 female Greenland sharks studied, only the very smallest ones showed signs of having been gestated during the bomb pulse — which meant anything larger was at least 75 years old.
After that, Bushnell readily admits, the dating gets dicier. The researchers can use the same dating technique to trace changes in background levels of radioactive isotopes, but those changes happen slowly when you don't have a big "boom" to look for. When measuring time periods spanning just a few hundred years, the trend line becomes nearly flat.
By adding other factors into their mathematical model — the fact that a longer shark is bound to be older, for example, and that growth rates slow down over time — the researchers believe they've found reasonably accurate ages for all 28 of their sharks. They can say with 95 percent certainty that the oldest shark they found was between 272 and 512 years old, but they suspect it lived to be about 390.
The method, Burgess said, needs to be confirmed with further study just as much as the results themselves do.
"In the end, the proof is in the pudding, and we'll see how the technique works when it's used on other critters," he said.
If these age estimates are correct, then an average Greenland shark doesn't reach the size of sexual maturity until about 150 years of age. For Burgess, that's the real takeaway of the study.
"The business of whether it’s got another hundred years here or there is almost irrelevant," he said. "What matters is that this animal is around for a very long time."
Because of the cold waters of the deep ocean, many deep sea fishes and other animals have similarly long lifespans — and must reach a similarly ripe age before they can reproduce. The Greenland shark, despite having few local fans, isn't in any immediate danger of extinction; no one goes fishing for them specifically. But to Burgess, the results of the study, while exciting, are a reminder of how fragile life is in the deep ocean.
"The bottom line is, of course, that this makes the animals extremely vulnerable," Burgess said. "In an age where our technology is so good that we're able to move into the deepest parts of the sea, and in fact are forced to do so, because we've done such an efficient job of depleting fish in more shallow waters … the animals there live on a very precipitous edge, and there's not a lot of margin for error."