The Fish that Lives to 500

Age always seems to be an imporant question. Determining the age of a fossil is a critical step in obtaining valuable information about the history of an extinct animal and is typically done by way of radiocarbon dating. This process measures the rate of carbon-14 decay in a sample and compares it to a constant in an effort to get an idea of how old something was when it perished. There are different methods used to age animals that are recently deceased that involve examining their bones and teeth, but in the case of fish, their ear stones or otoliths provide us with this information. Calcareous layers are added to the otoliths each year as the fish grows and by taking a cross section of the ear stone and counting the rings, a fish can be aged much in the way a tree can by counting the growth rings. But this is not the case with sharks.are different methods used to age animals that are recently deceased that involve examining their bones and teeth, but in the case of fish, their ear stones or otoliths provide us with this information. Calcareous layers are added to the otoliths each year as the fish grows and by taking a cross section of the ear stone and counting the rings, a fish can be aged much in the way a tree can by counting the growth rings. But this is not the case with sharks.

Since sharks’ skeletons are composed of cartilage instead of bone, there are not many options for hard body parts where growth layers could be deposited. This has led researchers to instead study the dense proteins in the lenses of their eyes, which are laid down in vitro and can be dated using the carbon-14 method. A recent study on Greenland sharks using this technique revealed a remarkable discovery; they might be the longest living vertebrates on the planet.

Greenland sharks are huge North Atlantic cousins of the Great white that can exceed lengths of six metres and weigh more than 900 kilograms. According to Julius Nielsen, the lead author on the Greenland shark research out of the University of Copenhagen: “The Greenland shark's eye lens is composed of a specialised material - and it contains proteins that are metabolically inert, which means after the proteins have been synthesised in the body, they are not renewed any more. So we can isolate the tissue that formed when the shark was a pup, and do radiocarbon dating." It was already well established that Greenland sharks had a long lifespan, but even this team was shocked by what their data revealed. They examined 28 Greenland sharks, all females that had been caught as bycatch in fishing nets, and determined that the oldest shark was likely close to 400 years old. Nielsen pointed out that the range of possible ages stretches from 272 to 512 years. Until now, the record holder for the longest-lived vertebrate was the bowhead whale at 211 years, though the oldest known animal is still an Icelandic clam that lived for 507 years.

Greenland sharks are thought to reach sexual maturity once they are around four metres long, and this research suggests that it would take them until the age of about 150 to even reach this size. Their extremely slow growth (~1cm annually) and reproductive rates make these sharks particularly susceptible to overfishing, and because of their extreme longevity, it is actually possible that their population is still recovering from being heavily fished prior to WWII. "When you evaluate the size distribution all over the North Atlantic, it is quite rare that you see sexually mature females, and quite rare that you find newborn pups or juveniles," Mr. Nielsen explained. "It seems most are sub-adults. That makes sense: if you have had this very high fishing pressure, all the old animals - they are not there anymore. And there are not that many to give birth to new ones. There is, though, still a very large amount of 'teenagers', but it will take another 100 years for them to become sexually active."

This fascinating new information confirms just how important it is to understand the natural history of our marine life so we can better manage their populations and promote effective conservation initiatives. 

*OG first published a natural history essay of Greenland shark – “Mysterious Predators of the Frozen North” by Doug Perrine in edition No 10, 2009

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by Alex Rose with picture by Doug Perrine ( OG 38)

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