Shark, which would have reached sexual maturity at around 150 years, sets new record for longevity as biologists finally develop method to determine age
Species: Greenland shark (Somniosus microcephalus)
Habitat: deep in the North Atlantic and the cold surface waters of the Arctic
Fish that were alive during the Age of Enlightenment are still swimming strong. A Greenland shark has lived at least 272 years, making the species the longest-lived vertebrate in the world – smashing the previous record held by a 211-year-old bowhead whale. But it may have been as old as 500 years.
“We definitely expected the sharks to be old, but we didn’t expect that it would be the longest-living vertebrate animal,” says Julius Nielsen of the University of Copenhagen, Denmark.
Living deep in the North Atlantic and the frigid surface waters of the Arctic, Greenland sharks have a stable environment and grow just a few centimetres per year. Despite their slow growth, though, they reach more than 5 metres in length and are often the apex predator in their ecosystem.
Old blue eyes
It was once thought to be impossible to age Greenland sharks. Their skeletons, made of cartilage, lack the calcified growth rings of hard-boned vertebrates. And other fish are aged by measuring calcareous bodies that grow in their ears, but this doesn’t work for sharks.
Instead Nielsen and his colleagues focused on radiation in the sharks’ eyes. Nuclear bomb tests in the 1950s and ’60s blasted radioactive particles into the atmosphere.
Those particles entered food webs all over the world and show up in the form of radioactive forms of carbon in organisms that lived through that period. Because Greenland sharks’ eye lens tissue doesn’t change during its lifetime, it preserves the historic radiation.
After catching a 2.2-metre shark that showed radiation levels indicating it was born in the 1960s and was about 50 years old, the team calculated how fast the sharks grew.
150-year dry spell
The team estimated that one 5-metre animal was at least 272 years old – but could be more than 500 years old (392 +/- 120 years). Another was at least 260 years old, and could be more than 400 years old.
And the female sharks don’t seem to reach breeding age until they are about 150 years old. “They have to wait more than 100 years to get laid – I’m sure they’re not happy about that,” says Nielsen.
Despite the uncertainty in estimating birthdays deep in the past, it’s clear these sharks are centuries-old, says Aaron MacNeil at the Australian Institute of Marine Science.
“This is our best estimate of how old these things are, but I don’t think it’s the final word,” he says. “The key message here is that these things are living a very, very long time.”
Non-vertebrates can live longer, for example, some coral and sponges are thought to live thousands of years. Clams, too, can live for hundreds of years.
Pining for the fjords
Many aspects of the Greenland shark’s life remain unknown, says MacNeil.
We don’t know for sure where shark pups are born, but one hypothesis is that females give birth to live young in Arctic fjords, MacNeil says. It’s also unclear how much climate change will affect the species through changing its cold-water habitats, he adds.
Finding out more would help scientists determine whether the species has a population healthy enough for a fishing industry, says Nielsen.
While there seem to be many young Greenland sharks around, Nielsen believes many breeding-age sharks were harvested for oil around the time of the second world war, and because of the time it takes them to mature sexually, the population will be recovering for another 100 years. The long generation times could also make them vulnerable to habitat disturbances, he says.
Other mysterious deep-sea shark species could also have surprising life spans, says Neil Hammerschlag at the University of Miami in Florida. “Every single time you study a deep-sea shark, you make a new discovery,” he says. “This might be just the tip of the iceberg.”