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Boom and bust don’t hit much harder than in the Bering Sea. After reaching historically high numbers, the population of snow crabs there cratered by 90 percent following a heat wave in 2018 and 2019. Some 10 billion disappeared. Water temperatures had risen 3 degrees Celsius, but that probably didn’t kill the crabs by overheating them, as you might assume.
“It looks like starvation was likely a key player in the collapse,” says fishery biologist Cody Szuwalski of the National Oceanic and Atmospheric Administration’s Alaska Fishery Science Center, lead author of a recent paper describing the collapse. “There were record numbers of crab, something we've never seen before. And it was also hotter than we've ever seen before. That boosted their metabolism, which meant they needed more food. And that's what points at starvation.”
Metabolic change is a less-talked-about, yet brutal and widespread, consequence of global warming. As ocean temperatures rise, so do the metabolisms of animals from fish to crustaceans to zooplankton. They need more food, and it isn’t always available, which is what appears to have contributed to the snow crabs’ population collapse.
“You heat up a crab or anything else, everything gets faster, up to a certain point when it can't handle it anymore,” says Woods Hole Oceanographic Institution evolutionary biologist Carolyn Tepolt, who studies metabolism but wasn’t involved in the snow crab work. “It isn't just those really high extreme temperatures. It's what happens when you increase that need for energy, essentially, beyond the point that the environment can provide it.”
“Metabolism” refers to the chemical processes that keep a living thing … a living thing. “I usually just define it as kind of the sum of all the chemistry inside the body that provides us energy,” says Earth scientist Curtis Deutsch of Princeton University, who studies metabolism in marine organisms but wasn’t involved in the new study. “For most organisms on Earth—all ectotherms, so pretty much anything except for mammals, which is the vast majority of everything on land and in the ocean—that rate of metabolism accelerates in a kind of exponential way with temperature.”
On average, an organism’s rate of energy consumption goes up by 6 percent for every degree Celsius of warming. “If you're metabolizing—converting energy from food into activity and cellular repair, and all the things that a living thing uses energy for—if that rate has gone up by 6 percent, it means that you need to feed 6 percent faster,” Deutsch says. “It also means that you need to breathe 6 percent faster.”
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That’s doubly problematic because of what happens chemically and biologically as the ocean warms. Hotter water is less dense, so it tends to form a layer at the surface, while cooler water sinks to the depths. This is known as stratification. Think about swimming in a lake in the summer—take a dive and the warm water gets real cold real quick.
This warm water at the surface forms a sort of cap that prevents nutrients from mixing upwards. That deprives the microscopic plants known as phytoplankton of the nourishment they need to properly proliferate. That means less phytoplankton to feed the tiny creatures known as zooplankton, and then less zooplankton to feed larger animals like fish. Even the creatures down on the seafloor, like snow crabs, rely on life growing at the surface, which becomes a crucial source of energy once it dies and sinks. Stratification interrupts that dynamic, reducing the flow of organic material into the depths.
At the same time, less gas dissolves in warmer water than in colder water. Unfortunately for the inhabitants of a warming ocean, that means less oxygen is available. But for every degree of warming, an oceanic creature needs on average 6 percent more oxygen as its metabolism speeds up. “So it's sort of a double whammy,” says Deutsch. “You need more and you get less.”
Throw ocean acidification into the mix, and now there are even more problems. As humanity pumps carbon dioxide into the atmosphere, more of that gas dissolves in the ocean, which increases the acidity of the water. This is the acidification that’s threatening corals, since it makes it harder for them to build their calcium carbonate skeletons. Creatures with exoskeletons, like crustaceans, are also struggling with acidification, as they have to spend more energy to construct their armor. That, in turn, affects their metabolism. “They have to pay some energetic or metabolic cost for it,” says Deutsch.
What organisms eat might change as well. In lab experiments, marine ecologist Wave Moretto has exposed brown box crabs to different temperatures and offered them clams and mussels. The former prey required twice the force for crabs to crack open with their claws than the latter. “What we found was that despite the crabs in the warmer temperature being able to generate stronger pinch scores, they were selecting the mussels preferentially that had the lower breaking force, so the easier prey item to eat,” says Moretto, who did the research while at the Scripps Institution of Oceanography but is now at Oregon State University. “Then we saw the crabs in the cold treatment had a really strong preference for the clams, which have a higher breaking force.”
The preference change might have had something to do with the crabs’ nutrient needs changing as their metabolism speeds ramped up. If they’re generating higher pinch forces at higher temperatures, that might also expand the list of the kinds of prey they can tackle. But that may not hold true for other crab species. Other groups of animals, like zooplankton and fishes, might undergo their own subtle dietary transformations as temperatures rise.
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Certain species might actually benefit from rising temperatures. “Ultimately, there are going to be winners and losers in climate change—this shake-up of ecosystems in the ocean,” says Szuwalski, the author of the snow crab paper. “So the snow crab was a big loser this time through. But in the Bering Sea, we also had a few other species that seem to benefit from the marine heat wave. Sablefish, they're more in the Bering Sea than we've seen before.” (Sablefish are a deep-water species native to Alaska and the Pacific Northwest.)
Ocean heat is transforming ecosystems; some species are moving north, for instance, as the Arctic rapidly warms. That may introduce new predators for native species to contend with—or alternatively provide more prey for native species to dine upon. The issue of metabolism change adds an extra wrinkle. It shows that a species can be harmed even if it’s not killed outright. These are known as “sublethal effects”: If an animal’s metabolism goes up and it can’t find enough food, it may not starve to death, but its growth might be stunted. “If you have a limited amount of energy to go around, your energy preferentially goes to maintenance,” doing just what it takes to survive, says Tepolt. “Then anything extra can go to extras, essentially—to you doing a little bit better than surviving, maybe growing more or growing faster.”
That may be the difference between being able to reproduce or not. Especially for females, who have to develop eggs, reproduction is extremely expensive in terms of energy. It’s one of the first things a body sacrifices when there’s an energy shortfall. “Life cycle and development rate, as a function of temperature, does matter in terms of whether they can reach some critical life stage or not, and whether they can maintain the population,” says Rubao Ji, a senior scientist also at the Woods Hole Oceanographic Institution. “You're more vulnerable, but in the meantime, there are more needy predators.”
Put another way: Higher temperatures mean hungrier mouths to feed. If a fish can’t eat enough to grow big and strong, it might be less likely to escape a bigger predator, and less likely to reproduce. If an invasive species moves into its habitat, that native fish’s population might get squeezed both by supercharged predation and decreased reproduction.
All of this can add up to a mass die-off, driven by the changes in how energy moves through ecosystems. What happened to the snow crabs is but a hint of the wild population swings to come.
