The longest-lived vertebrate
In the cold, deep waters of the Arctic and North Atlantic Oceans lives the longest-living vertebrate on Earth: the Greenland shark (Somniosus microcephalus). This species can reach lengths of over 7 meters and weigh more than 1,000 kilograms. Its longevity is extraordinary, with scientists estimating lifespans of at least 272 years and possibly more than 500 years. This extreme lifespan results from its very slow growth rate, which is less than one centimeter per year, and a sluggish metabolism adapted to the frigid deep-sea environment. They are slow swimmers, with a top speed under 2.9 kilometers per hour.
Determining the age of these animals was difficult for a long time. Traditional methods, like counting growth rings in vertebrae, are not possible because their cartilaginous skeletons are too soft to form distinct bands. The breakthrough came from an unexpected source: the Cold War.
An atomic age eye test
Scientists found a way to tell time using the shark's own eyes. The nucleus of the eye lens is formed before birth and its proteins are metabolically inert, meaning they remain unchanged throughout the shark's life. This tissue provides a pristine sample of the chemical environment from the time the shark was born.
In the mid-20th century, above-ground thermonuclear weapons testing released large amounts of carbon-14, a radioactive isotope, into the atmosphere. This "bomb pulse" created a distinct signature as the carbon-14 was absorbed into the marine food web. Any animal born during or after this period incorporated the elevated carbon-14 levels into its tissues, including the proteins in its eye lenses.
A 2016 study led by researcher Julius Nielsen analyzed the eye lenses of 28 female Greenland sharks. By measuring the levels of carbon-14, they could identify which sharks were born before or after the bomb pulse of the late 1950s and early 1960s. The smallest sharks showed the bomb pulse signature, confirming they were young. For sharks born before the tests, scientists used the carbon-14 data to calibrate a growth model. Their analysis estimated the largest shark in the study, at 502 centimeters, was approximately 392 years old, with a potential range of 272 to 512 years. The study also revealed that females do not reach sexual maturity until they are about 156 years old. This use of radiocarbon dating established the Greenland shark as the longest-lived vertebrate known.