Greenland sharks — anything but voracious
Greenland sharks, also known as grey sharks or gurry sharks (Somniosus microcephalus) roam the North Atlantic and Arctic and are pretty good at eluding scientists. Accordingly, little is known about them. Nevertheless, an international team of scientists has succeeded in a recent study to investigate the slow swimmers in more detail. They found that Greenland sharks require very little food.
Greenland sharks, which belong to the sleeper shark family (Somniosidae), are the longest-living vertebrates — previous studies estimate that the animals can live at least 272 years, and possibly as long as 500 years. They probably do not reach sexual maturity until they are about 150 years old. They grow up to seven meters long and feed on pretty much anything that comes in front of their mouths: fish, marine mammals, crustaceans, octopods, squid, jellyfish, starfish, sea urchins and others. Scientists at the St Lawrence Shark Observatory believe that Greenland sharks feed primarily on carrion. Their habitat extends from the water surface to the deep sea.
In the current study, which appeared in the Journal of Experimental Biology, the researchers focused primarily on the quantity and composition of Greenland sharks’ diet. Although it is not easy to track the animals, they managed to equip 30 sharks with various sensors and satellite transmitters over a five-year period. When they analysed the data, they discovered something astonishing: an average-sized Greenland shark weighing 224 kilograms requires only between 61 and 193 grams of prey (fish and marine mammals) daily, which means that the animals’ metabolic rate is very low.
“As a lethargic polar species, these low field metabolic rate (FMR) estimates, and corresponding prey consumption estimates suggest Greenland sharks require very little energy to sustain themselves under natural conditions,” the authors said in their study. They suggest that the slow metabolic rate of Greenland sharks may be the key to understanding the exceptionally long lifespan of these animals.
Thanks to a close examination of the sharks’ slow lifestyle, the authors were able to characterise for the first time the energy budget and the role of endangered sharks as consumers in the wild. “This is essential given growing pressures from climate change and expanding commercial fisheries in the Arctic,” the researchers point out. With the new knowledge, scientists can predict how animals will probably cope with climate change and food becoming scarce.
Greenland sharks are designated “near threatened” by the IUCN, which keeps tabs on the status of species in the wild, but it is not known how many of the animals live in the North Atlantic and Arctic waters.
Julia Hager, PolarJournal
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