When you might live for up to 500 years, time moves a little different for you. A day might feel like a second, a week might feel like a day, and so on and so forth. So when time is this abstract thing, how do you space out meal time?
Such is the question for Greenland sharks (Somniosus microcephalus), who are the world’s longest-living vertebrate and one of the world’s longest-living animals. A sleeper shark that can reach lengths of 23 feet (7 meters), they are primarily found in the cold-water environments of the North Atlantic and Arctic Oceans. Little is known about these animals, but these slow-moving carnivores are often made up of several different types of aquatic animals (e.g. smaller sharks, seals, and fish). Eric Ste-Marie of the University of Windsor in Canada largely focuses his work just that — what Greenland sharks eat and how much they need to eat on a day-to-day basis in order to survive (scientifically known as their “metabolic rate”).
It turns out, not a lot! Eric Ste-Marie and colleagues reported the stunning field metabolic rate of Greenland sharks in the Journal of Experimental Biology, and found that these large predators only require only 61-193 grams of fish or marine mammal prey daily. The data comes from 30 Greenland sharks that Ste-Marie and his team tagged, sampled, and fitted with a biologger kit for five years. These kits recorded the shark’s movements, body temperature, and water temperature.
Using the data from the tags, the researchers were able to calculate the daily calorie requirements of the Greenland sharks; for example, a 500-pound (227 kg) individual would need between 2 – 6.5 ounces of fish or marine animal prey to live. “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 team writes in their report.
As Ste-Marie explained, the sluggish metabolic rate of these animals may be key in understanding how these animals have such extraordinarily long lifespans. This number is significantly lower than numbers from other sharks, as previous research has shown that a great white shark weighing 2,000 lbs (907 kg) would need to devour 66 lbs (about 30 kg) of fat every 11 days to thrive. For comparison, great white sharks are believed to live up to around 75 years according to some scientists.
By delving deeper in the slow lifestyle of Greenland sharks, Ste-Marie and his team have provided the first characterization of the energetics and consumer role of this vulnerable and understudied species in the wild. “This is essential given growing pressures from climate change and expanding commercial fisheries in the Arctic,” explain the authors. By knowing their nutritional needs, scientists can begin to predict how these animals may fare when food becomes scarce as a result of climate change, since the Arctic is warming three times the global annual average, driving many of the changes underway in the Arctic.