An Ancient Arctic Deep Sea Dweller
Darcie Anderson
Location: Scotland
Darcie Anderson is in her third year studying Marine Science with Oceanography and Robotics at the Scottish Association for Marine Science. In her free time she advocates for the ocean as a part of the World Ocean Day Youth Advisory Council, the Plastic Oceans UK Youth Network, and in her work with AYUDH Europe.
Greenland sharks, Somniosus microcephalus, are the largest fish species in the Arctic Ocean. This understudied, mysterious, deep-dwelling species may have a major impact on the arctic ecosystem, and is therefore increasingly researched, often using new techniques.
Despite living throughout the Arctic in regions inhabited by humans for thousands of years, there is little traditional ecological knowledge, known as qaujimajatuqangit in Inuktitut (the Inuk language), about them.They live in the deep sea and their flesh is toxic to humans, so they are rarely eaten. As the Greenland Ice Shark is understudied, the little qaujimajatuqangit that does exist about this species has proved useful in verifying scientific findings.
Despite their flesh being toxic for human consumption in its raw state, it can be made edible using specialised fermentation and drying processes. It can then be fed to sled dogs by indigenous people in Greenland and eaten as a delicacy in Iceland. From the early 20th century until the 1960s, there was a large-scale Greenland shark fishery with up to 150,000 sharks caught annually, as their liver oil was used similarly to whale blubber for manufacturing and industrial products. The industrial usage of Greenland shark liver oil has presumably been responsible for large scale population decline of the species. Sharks currently consumed by humans and sled dogs may be bycaught, however a small directed fishery of Greenland sharks remains.
Greenland sharks are found throughout the Arctic Ocean but have been known to live as far south as the boreal Atlantic, off the Georgian coast, provided they are in waters around 1ºC. They have been known to live throughout the water column, found at over 2200m deep in some instances, however, they have an age-based depth distribution, with middle-aged sharks being found throughout the water column, whereas young and old sharks tend to live in deeper waters.
Their populations and distributions are less known than would be expected of a species with such a long history of commercial exploitation and importance to a vital ecosystem. In 2018, the first local abundance estimates of Greenland sharks were published by a Canadian team of scientists, who used Baited Remote Underwater Video cameras (BRUVs) to study Greenland sharks in the eastern Canadian Arctic. Typically, longlines are used to survey shark abundance, however Greenland sharks are more likely to die on these fishing lines than other species as they become more stressed and have an unusual habit of wrapping themselves up in the line, hence why BRUVs, which pose no risk to Greenland sharks, were used instead. From 30 BRUVs, 142 sharks were sighted, predominantly in deep, warmer waters, above 0ºC, and a potential nursery location was identified. These important findings are useful to build evidence for marine protected areas and to better utilise marine spatial planning to conserve Greenland sharks.
Figure 1- Images of Greenland Sharks taken from BRUVs. Devine et al., 2018
As these are a slow growing and large species, reaching over 5m in length and increasing in length by less than 1cm per year, scientists suspected that Greenland sharks were a long-lived species.
In 2016, a team from various universities, research institutes and aquariums in Denmark, Greenland, Norway, the United States of America and England published findings that the Greenland shark is the oldest known living vertebrate in the world, possibly reaching up to 512 years old.
Similar to counting tree rings (dendrochronology) the growth rings on otoliths, small bones near a fish’s brain, are used to determine a fish's age. Radiocarbon dating from bones is another well accepted method for ageing fish. However, as sharks belong to a group of cartilaginous fishes, the Chondrichthyes, they do not have otoliths, or any other bones, making aging sharks far more complex than ageing bony fishes.
When searching for tissue to radiocarbon date, the scientists landed on the eye lens nuclei. Eye lenses are made from crystalline proteins and are split into the embryonic nucleus, foetal nucleus, adult nucleus and outer cortex, named for when these proteins were formed. As such, proteins formed during development are retained in the eye lens throughout an animals life, which can be used to age the shark.
Radiocarbon dating compares levels of different carbon isotopes, that is versions of carbon with different numbers of neutrons. Of the 15 isotopes of carbon, 12C and 13C are stable, and of the 13 unstable, radioactive isotopes of carbon, 14C decays the slowest, with a half-life of 5730 years, and is therefore most useful to determine the age of tissue. As it has a known rate of decay and known atmospheric concentration, we can compare the levels of stable 12C and 13C to unstable 14C to determine when a tissue was formed.
14C naturally occurs in the atmosphere, and has naturally remained rather stable for several thousand years, however, between 1945 and 1963, testing of nuclear bombs, primarily in pacific islands by European nations, significantly increased the radioactive 14C in the atmosphere worldwide. As this carbon entered the oceans, it travelled through food chains and formed tissues in many organisms. Therefore, we see a drastic increase in animals 14C tissue content formed during the 1950’s-60’s, known as the bomb pulse, which can be very helpful as a marker to determine the age of organisms.
The team of scientists used radiocarbon dating on the eye-lens nuclei of 28 female Greenland sharks, using the increase in 14C from the bomb testing as a benchmark to calibrate age against. Using the bomb pulse recorded in three of the sampled sharks, all born after the 1960’s, the scientists dated the other sharks with their relative stable carbon to unstable carbon ratios to these three sharks to determine the age of all sharks sampled, and this revealed that the Greenland shark is the oldest living vertebrate on earth.
The sharks were found to reach sexual maturity at around 156 years old, living up to at least 272 years old, and possibly 512 years old, potentially outliving the ocean quahog, an arctic clam species living to 507 years old.
Due to their late age of sexual maturity and presumed low birth rate, Greenland sharks are very vulnerable to exploitation, as their ability to recover from population decline by having pups is a slow process. Despite half a century of large scale, directed exploitation, there appears to be a small, yet widespread and relatively strong population of Greenland sharks throughout the Arctic ocean, and even down into the Atlantic and Pacific basins.
Greenland sharks are an unusual, understudied species. They are the garbage cans of the Arctic, eating nearly anything they can get their teeth around. Despite being almost blind from parasitic copepods on their eyes, and being slow swimmers, they are apex predators. Essentially every new thing we learn about them is strange. They are the longest living vertebrate in the world, but as we continue to damage the Arctic, through changing the climate, exploiting oil and catch them in our fishing nets, accidentally or not, we are risking their survival. What a loss it would be to have a world without these mysterious, strange sharks, both from an environmental perspective, where they regulate ecosystems and sink carbon to regulate our climate, and for the joy of discovery we would lose if they were gone.
The scientists studying these sharks are not just learning about them for curiosity's sake, but to protect them. Knowing how old they live, and that they reach sexual maturity at such a late stage emphasises how important it is to minimise fisheries and bycatch, to prevent sharks being taken from the population before they can even reproduce. It emphasises how important long term management is, and how we should stop pollution from entering the oceans. Despite many of these sharks being older than the United States of America, in the last few decades we have polluted so many dioxins, PCBs and other chemicals into our oceans that these centuries old animals have incredibly high concentrations of pollutants in their tissues, which may impact their development and reproduction for centuries to come. Humans are changing the earth at an incredibly fast rate, and these ancient ocean giants remind us how long these impacts may last, and how much is at stake from our destruction.
References:
Devine BM, Wheeland LJ, Fisher JAD. First estimates of Greenland shark (Somniosus microcephalus) local abundances in Arctic waters. Scientific Reports [Internet]. 2018 Dec 1 [cited 2021 Mar 19];8(1):1–10. Available from: www.nature.com/scientificreports
Nielsen J, Hedeholm RB, Heinemeier J, Bushnell PG, Christiansen JS, Olsen J, et al. Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus). Science [Internet]. 2016 Aug 12 [cited 2021 Mar 19];353(6300):702–4. Available from: https://science.sciencemag.org/content/353/6300/70
Nielsen J, Hedeholm RB, Simon M, Steffensen JF. Distribution and feeding ecology of the Greenland shark (Somniosus microcephalus) in Greenland waters. Polar Biology [Internet]. 2014 Jan 1 [cited 2021 Mar 21];37(1):37–46. Available from: https://link.springer.com/article/10.1007/s00300-013-1408-3
Strid A, Nn H, Rundsdó Ttir J, Päpke O, Rundur Svavarsson J, Ke Bergman A˚. Dioxins and PCBs in Greenland shark (Somniosus microcephalus) from the NorthEast Atlantic. Available from: www.elsevier.com/locate/marpolbul