By Dr. Becker
Some sharks swimming in the icy waters of the Arctic sea today were likely born 400 (and possibly more) years ago — a time when the Mayflower made its ocean journey from Plymouth to the New World.
The sharks — Greenland sharks, to be specific — were recently named the longest-lived vertebrates on Earth by University of Copenhagen researchers.1
The finding sheds more light on one of the most mysterious creatures in the ocean, as Greenland sharks have long been elusive. They like to live in cold, deep waters, so human sightings are rare, even with their impressively long lifespan.
There were some early clues that Greenland sharks may be record-holders when it comes to longevity. In 1936, a Danish research measured and tagged a Greenland shark and managed to recapture it (and re-measure it) in 1952.2
It had grown just 8 centimeters, which is important because Greenland sharks are some of the largest fish in the ocean, growing up to 7 meters (or nearly 23 feet) in length.
Assuming the sharks grow steadily, the finding proved the sharks to be incredibly slow-growing and also suggested they must be very long-lived to grow to such large sizes.
Radiocarbon Dating Reveals 400-Year-Old Shark
It can be difficult to determine the ages of some fish, but a commonly used method is to count the growth rings in ear bones called otoliths, similar to counting the rings in a tree. Layers of calcified tissue growing on shark back bones is used similarly to age certain species, but this method doesn’t work for Greenland sharks.
Julius Nielsen, a marine biologist from the University of Copenhagen who led the featured study, told BBC News, “ … [T]he Greenland shark is a very, very soft shark — it has no hard body parts where growth layers are deposited. So it was believed that the age could not be investigated."3
Proteins in the sharks’ eye lenses proved to be a novel way out of this dilemma, however, as some of the proteins are metabolically inert.
The Atlantic reported, “These structures are made of proteins that are added in layers throughout the shark’s life. Peel away the layers and you can eventually find molecules that were laid down at the animal’s birth.”4
The molecules can then be tested using radiocarbon dating. The lenses were tested for radioactive carbon-14, which peaked in the atmosphere when nuclear bombs were tested in the ‘50s and early ‘60s. When a spike in carbon-14 is detected in a sample, it therefore acts as a date stamp that can be used to figure out age.
Sadly, while Greenland sharks and humans don’t often interact, the sharks are sometimes caught as bycatch in fishing nets. Some of the sharks die as a result, which is how Nielsen and colleagues collected the lenses for the study, from 28 Greenland sharks in all.
One of the animals, the third-smallest of the bunch, was determined to be about 50 years old at the time of its death, which means it was born around the end of nuclear testing era.
The researchers were then able to create a mathematical model that factored in the shark’s age, growth speed, size at birth and geographical location, which was used to estimate the other sharks’ ages.
Nielsen told The Atlantic, “We can say with 95 percent certainty that they’re between 272 and 512 years old. But it’s most likely that it’s approximately 390.” The revelation means Greenland sharks flew past the former longest-lived vertebrate record-holder, a bowhead whale thought to be 211 years old.
Currently, the oldest animal known is Ming the clam, an invertebrate estimated to be 507 years old. If the Greenland shark’s upper age estimate turned out to be true, they could surpass even that!
Greenland Sharks Reach Maturity at 150
With such a length lifespan, the researchers estimated that Greenland sharks reach sexual maturity at 150 years old, when they’re about 4 meters (or 13 feet) long. Currently, it’s believed that most Greenland sharks roaming the oceans are sub-adults that won’t start producing new pups for another 100 years.
The animals are not considered endangered, but there’s still much that remains to be discovered. From the early 20th century to the 1960s, they were killed for their livers, which was used as machine oil, but this largely stopped after a synthetic alternative was discovered.5
During that time, more than 30,000 Greenland sharks may have been caught annually.6 Fishing nets remain one of their biggest modern-day threats.
The University of Copenhagen researchers hope the study will trigger more interest in these intriguing creatures to perhaps learn more about their mysterious existence. No one has ever seen a Greenland shark capture live prey, Nielsen said, yet a wide variety of food has been found in their stomachs.
Entire seals, moose hides, reindeer and polar bears have all been meals to Greenland sharks, as have fast-swimming fish like cod and halibut.
The latter is interesting, because Greenland sharks swim very slowly, around 0.8 mph (and maxing out at around 1.7 mph).7 Many experts believe the sharks scavenge some of their food, but Nielsen thinks they may also be ambush predators that sneak up on prey.8
It’s unknown how many Greenland sharks live in the wild — or even where they live. While they’re most commonly seen around Greenland and Iceland, this may only be due to the frigid water temperatures, even in shallow areas, which draws the sharks closer to the surface.
Some experts believe Greenland sharks may live just about anywhere with cold, deep water, and they’ve been spotted off the costs of Canada, Portugal, France, Scotland and Scandinavia.9
Also unknown is how the animals mate and where they give birth, or why the majority of them have parasitic crustaceans hanging from their eyes. It’s often said the sharks are blind, but newer research suggests otherwise. Perhaps the most burning question of all is how they’re able to live so long.
Their slow-moving nature may yield some clues, as they have a slow metabolism as well and can even slow down their life processes in very cold water (sort of like animals do during hibernation).10 “Maybe they have anti-ageing mechanisms, too,” Nielsen told The Atlantic, “but I don’t know about that. I’m just a Greenland shark nerd.”11