What is the longest-living animal with a backbone?

Somewhere in the cold, dark North Atlantic there may be a shark that was already swimming before the American Revolution. It is slow, half-blind, and in no hurry about anything. Including dying.

The short answer

The Greenland shark (Somniosus microcephalus) is the longest-lived vertebrate known to science. In August 2016 a team led by Julius Nielsen of the University of Copenhagen published a study in the journal Science estimating that the oldest shark in their sample, a large female, was about 392 years old, give or take 120 years. That knocked the previous vertebrate record holder, a bowhead whale of around 211 years, off its perch.

The real story

Working out a shark’s age is surprisingly hard. Most fish can be aged by counting growth rings in hard structures, a bit like tree rings. Sharks have cartilage instead of bone, so there is nothing tidy to count. The Copenhagen team needed a cleverer clock.

They found one in the eye. The core of a shark’s eye lens is made of proteins that form before birth and are never replaced. Lock in the carbon at birth and you have a time capsule. The researchers radiocarbon-dated the lens nuclei of 28 Greenland sharks, and used a neat trick from the Cold War: nuclear bomb tests in the 1950s sent a spike of radiocarbon through the oceans. Only a few of the smaller sharks carried that bomb signature, meaning they were born after the tests began. Everything bigger was older, and the team combined the radiocarbon data with growth estimates to work out how much older.

The answer was startling. The sharks grow roughly 1 centimetre a year, so a big one has been growing for a very long time. The study suggested Greenland sharks live at least 272 years, and the females may not be ready to breed until they are around 156 years old. Imagine waiting a century and a half for your first date.

Not every scientist is convinced by the headline figure. The 120-year margin of error is enormous, and some outside researchers have called the nearly 400-year estimate optimistic. But even the cautious end of the range leaves the Greenland shark comfortably ahead of every other animal with a backbone that has been reliably measured.

Why people get it wrong

Giant tortoises are the famous old-timers of the animal world. They are big, wrinkly, slow and photogenic, and stories of individual tortoises reaching great ages get shared widely. They fit our picture of what ancient should look like.

Greenland sharks, on the other hand, live in deep, icy water where almost nobody sees them. You cannot keep one in a zoo enclosure and throw it a birthday party. Until scientists found a way to read the clock hidden in their eyes, there was simply no reliable way to know how old they were, so they never entered the conversation. The record was hiding in plain sight, or rather in plain sight of no one.

“Mate, that shark was ordering its first meal when Newton was still in short pants, and it still hasn’t hit puberty. Respect your elders.”