Photo Credit: NOAA OKEANOS Explorer Program, CC by 2.0)

Scientists have long suspected that the elusive Greenland sharks lived for a long time. It turns out they were right! A new study has revealed that the world’s second largest carnivorous fish may have a lifespan of as long as 400 years. To put it in perspective, that is about twice that of the previously known longest-living vertebrate, the Bowhead whale, which is known to live for up to two centuries.

The first clue that these deep sea dwellers live for a long time came in the 1930’s after a biologist in Greenland tagged 400 sharks and discovered that they grew only a centimeter a year. Given that Greenland sharks can grow to more than 20-feet in length, researchers realized that they must live for many years. However, nobody had investigated to see what that exact period was.

Photo Credit: Julius Nielsen/Science

John Steffensen, a marine biologist at the University of Copenhagen, and graduate student Julius Nielsen were curious to find out and decided to study the sharks more carefully. According to the researchers, the task is easy when the shark species have calcified vertebrate or fin spines. That’s because they contain stripes, which similar to growth rings in trees, can help determine the animal’s age. However, this technique was not available for Greenland sharks because they are “soft” sharks and have no bony structures.

An expert suggested the scientists look into the sharks’ eye lenses for answers. In some shark species, as the transparent tissue in the eye lenses becomes metabolically inactive over time, they are replaced by new layers. By peeling back the layers, researchers can find the one from the shark's infancy, and use it to analyze its age.

Photo Credit: Julius Nielsen/Science

The researchers managed to retrieve the eye lenses of 28 specimens for their study. Most of the sharks had been accidentally captured by trawling nets trying to ensnare other types of fish. However, instead of counting tissue layers, the researchers opted to use radiocarbon dating to estimate the shark’s ages. Utilized extensively by archaeologists, the method entails measuring the amounts of a particular carbon isotope absorbed by living tissue — in the case of the shark’s eyes, that meant examining the innermost, and hence the oldest, part of the lens.

The results of the study, published in the Science Magazine on August 12, revealed that the oldest shark in the group was probably about 392 years old when it died. However, given that radiocarbon dating is not foolproof, the scientists estimate that the real number is plus or minus 120 years. This means the shark could have been anywhere from 272 to 512 years old! Nielsen says, "Even the lowest part of the age range — at least 272 years — still makes Greenland sharks the longest-living vertebrate known to science."

Greenland sharks habitat (By Chris_huh (Own work) [GFDLor CC-BY-SA-3.0, via Wikimedia Commons

Researchers believe that the secret behind the longevity of these sluggish fish that are often called “sleeper sharks,” is the frigid waters of the Arctic and North Atlantic where they reside. Michael Oellermann, a cold-water physiologist at Loligo Systems in Viborg, Denmark, says the temperature helps slow down their growth and biochemical activity, extending their lifespan.

Shawn Xu, a geneticist at the University of Michigan, Ann Arbor, agrees. His research has shown that the cold environment helps activate anti-aging genes, rids the body of DNA-damaging molecules, and even helps fight off infections! Now if these ancient creatures could just figure out a way to avoid fishing trawlers, they could perhaps live even longer!

Resources: livscience.com, sciencemag.org, guardian.co.uk, BBC.co.uk