This May Be Why Sharks Cannot Survive Well in Cold Water.

In the light of the setting sun, the water lapping against the rocky shore glistened like freshly spilt ink. Despite the fact that it is frigid to the touch, this cold environment is home to a wide range of wildlife including bowhead whales, polar bears, beluga whales, seals, walruses, and more – all threatened with extinction. It was here that Dr. Yuuki Y. Watanabe first experienced the spell cast by the Arctic Ocean.

“Previously I was studying Greenland sharks (Somniosus microcephalus), a shark in the Arctic, and found that they are unusually slow swimmers partly due to the effects of cold waters on muscle functions. They also have an unusually low metabolic rate, slow growth rate, and long life span (400 years according to a study),” says Watanabe. Known as the longest-lived vertebrate, they’re a species of sleeper shark that can reach 23 feet (7 meters) in length. Little is known about these slow-moving carnivores, which are mostly found in the cold-water environments of the North Atlantic and Arctic Oceans where they feed on several different types of aquatic animals (e.g. smaller sharks, seals, and fish). A recent study by Eric Ste-Marie, a scientist from the University of Windsor, has provided the first estimates of the resting and active metabolic rates of these sharks and found that these large predators only require only 61-193 grams of fish or marine mammal prey daily. “That is, Greenland sharks have an unusually slow lifestyle, partly due to cold waters,” explains Watanabe. “But I wondered how universal this pattern is among fish. Fishes in cold waters are not always slow. For example, Arctic char, a kind of salmon in the Arctic, is an active, fast swimmer in the same cold water as Greenland sharks. I started to think that there may be a fundamental difference between teleosts fish vs sharks & rays regarding the effects of cold temperature.”

As a scientist at the Research Center for Integrative Evolutionary Science, The Graduate University for Advanced Studies (SOKENDAI), Watanabe was led down a rabbit hole due to these massive sharks. The process of metabolism is the process of transforming foods into energy for the body to use for its functions. And though there exists numerous studies on fish metabolic rates, most have been about teleost (bony) fish and not sharks or their relatives. “Very few studies compiled the metabolic rate of sharks and rays. No studies, to our knowledge, compared the metabolic rates of teleost fish and sharks and rays in statistically solid manners,” says Watanabe. “That’s why we collected as many reports of metabolic rates for sharks & rays as possible from the literature. We then compared them to previously compiled datasets of teleost fish metabolic rates.”

In this new Nature paper, Watanabe’s team demonstrates that teleost fish have a higher metabolic rate in cold temperatures than sharks. And with this finding comes an intriguing question: could this be the reason we don’t see many cold-water elasmobranchs? “Teleost fish have a range of metabolic rate in cold waters, meaning that they show a variety of lifesyle ranging from sit-and-wait strategy to fast continuous swimming. By contrast, sharks and rays in cold waters almost always show low metabolic rate, meaning that they exclusively have slow lifestyle (as seen in Greenland sharks). [There are some] exceptions with unusual physiology and elevated body temperatures (e.g. salmon sharks).” According to the scientists, sharks and rays in cold waters have few options other than a slow lifestyle, which is why they are rare compared to teleost fishes in cold waters.

With oceans warming, what does this mean for sharks and rays? The study showed that sharks and rays are more sensitive to temperature than teleost fish, according to Watanabe: “Not only higher metabolic rates but also larger variations of teleost fish metabolic rates are, we think, important. […] Ocean warming may change the distribution and species composition of sharks and rays more dramatically than those of teleost fish.”

Watanabe believes the public needs to be aware that although sharks and rays are a type of fish, they are very different from teleosts. “The two groups diverged about 450 million years ago, which means that the difference between sharks & rays and teleosts fish is larger than the difference between human and crocodiles, for example. Therefore, what we know for teleost fish may be inapplicable to sharks and rays.” Although this theory is currently speculative, the team hopes this study shows that the energy metabolism of cold-water elasmobranchs needs to be studied at lower levels of biological organization (the molecular, organellar, cellular, and tissue levels) than the whole-organism level.

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