Scientists have discovered the Greenland Shark (Somniosus microcephalus) might be the most long-lived vertebrate in the world, being capable of reaching four hundred years old. The research paper was published in the journal Science last Thursday.
As implied by the name, the sharks are found in the cold waters of the Arctic and the North Atlantic seas. They live farther north than any other species of shark and is among the few truly sub-Arctic sharks.
The Greenland sharks are apex predators, feeding mostly on smaller fish, but rest of reindeer or polar bears have been found in their stomachs. They’re slow-moving predators with a top speed of 1.6 mph or 2.6 km/h and also slow in growth.
A previous estimate, made by tagging and measuring a Greenland shark in 1936 and then again in 1952 suggested a growth rate of less than an inch per year.
This makes sense since animals that live in the cold tend to have a much slower growth than those in warmer waters, ensuring their longer lifespans. This increase rate estimate suggested that the largest of the Greenland sharks could live up to 200 years.
The New Research
More recent research conducted on twenty-eight sharks accidentally caught in fishing nets give a newer estimate of their age: they could live up to 400 years.
The study used radiocarbon dating — something commonly used for fossilized remains and other archaeological finds — to estimate the ages of the various sharks used in the survey.
While this method’s accuracy takes a nasty hit when it comes to measuring newer organisms, the arms race between the United States and the Soviet Union, led to nuclear tests that noticeably raised the radiocarbon in the environment.
This means the researchers can at least know which sharks were born from the 1960s onward, and which ones were born before this.
Why use radiocarbon dating?
The sharks lack the appropriate bony structures that are used to measure the age in fish; they do not have otoliths (ear bones) to count their seasonal growth rings.
They also lack fin spines, and their cartilage skeleton has almost no calcified material, researchers can’t count the layers of these calcified parts to determine their ages. However, the amount of radiocarbon in the atmosphere is “recorded” in the tissue of an organism when it’s born.
The researchers compared the chemical signature found in the shark with a chart that records the variations in radiocarbon levels across the ages.
They utilized the core of the shark’s eye lenses, as it’s among the first bits of tissue that are created before the animal is born and remains metabolically stable, that’s where the most accurate chemical signature of the environment the fish was born into is.
An old and venerable animal
Two of the smallest sharks had a radiocarbon signature that made it clear they were born after the 1960s, making them 50 or less.
The third tiniest fish had a signature that placed her smack-dab in the middle of the “bomb pulse”, meaning she was born in the early ‘60s.
The remaining twenty-five sharks had signatures that indicate they were born before the bomb pulse. The researchers now had to determine exactly when.
As mentioned before, radiocarbon dating has a wide margin of error when it comes to more recent organisms; to narrow down their estimates, the researchers make the assumption that size determines age.
With this, the largest of their sharks (at 16 feet) has an estimated age of 392 years, with a margin error of 120 years.
While the margin of mistakes it’s still pretty big, the authors feel the study still demonstrates that the sharks are long-lived and that their considerations are sound.
Other scientists and experts have their skepticism about the results of the research; the use of radiocarbon dating makes it rather inaccurate, not to mention the possibility of other variables affecting the results.
“We know that in sharks and bony fish size is a very poor predictor of age. That’s why we spend millions of dollars every year to age fish — because we simply can’t use size […] Even though they have pretty wide error bars on those estimates, I think that even that is underestimating the error,” said a shark studying biologist from the Woods Hole Oceanographic Institution, Simon Thorrold.
Allen Andrews, another biologist from the Pacific Island Fisheries Center at the National Oceanic and Atmospheric Administration, notes that “with orange roughy [a large deep-sea fish] we’ve seen that maximum-size fish could be 20 years old or 100.”
However, Andrews doesn’t doubt the centurial ages of the sharks; he just comments that the data deserves to be scrutinized.
Lead author Julius Neilsen, from the University of Copenhagen, defended their reasoning for tying size with age since it was shown that the smaller sharks were also the youngest of the group (they were the only ones affected by the bomb pulse).
“I am 95% certain that the oldest of the sharks is between 272 and 512 years old. That’s a big range, but even the age estimate of at least 272 years makes it the oldest vertebrate animal in the world,” also said Neilsen.
Sources: CBS News