If you eat crab, there’s a decent chance you’ve had king crab or snow crab — the type they often sell in grocery stores and restaurants as just big red arms — which are brought up from chilly waters in places like the Bering Sea off Alaska. The future for these species is uncertain; as with many tasty critters, they’re overfished and on the decline. But more worryingly, finds a study in PLOS One, the crabs are vulnerable to climate change.
Researchers from the National Marine Fisheries Service Kodiak Laboratory in Alaska studied two species of crab: the red king crab (the most sought-after kind of king crab) and a snow crab known as the Tanner crab. They raised young crabs for a little over half a year in tanks with sea water at one of three pH levels — 8.0, 7.8, and 7.5 — and then tracked the crabs’ growth.
The scientists were interested in the effects of ocean acidification on the crabs. As we pump more and more carbon dioxide into the atmosphere, the ocean absorbs some of that gas, which causes the water to become more acidic over time. Some areas of the world will be affected more than others, and northern regions are expected to become more acidic than tropical ones. The three different pH levels range from the current state of the water (8.0) to what will exist at the end of the century (7.8) to what could exist in the not-too-distant future (7.5).
Ocean acidification is a particular worry for organisms that rely on calcium carbonate to construct their skeleton or shell, like coral and crabs. If the pH of the water is too acidic, it will be too difficult, too costly for these organisms to turn calcium and carbonate into calcium carbonate to build their bodies.
At the 8.0 pH level, the crabs grew just fine. Some crabs died, but that’s normal. At a pH of 7.8, more crabs died; less than 40 percent of the red king crabs and about half of the Tanner crabs survived to the end of the study period. And at the 7.5 pH level, less than 40 percent of the Tanner crabs lived to the end, and all of the red king crabs died within 95 days. Growth rates were also affected by the acidic waters, with the crabs not growing as well when pH was low.
Given these results, the researchers predict that ocean acidification will cause a serious decline in these species by the end of the century, with red king crabs affected first. Even before then, though, small increases in acidity could affect crab growth. Because smaller crabs are more vulnerable to predators, they’re more likely to get eaten and not survive too long. Their smaller size could affect the predators that eat them (they won’t be as satiated by their crab meals), and that’s just the beginning of a host of changes that could sweep through the food web, affecting species that may not be directly impacted by water pH.
Crabs and other species may be able to adapt to more acidic waters. There are some cousins to the red king and Tanner crabs that live in deep places with lower pH, which shows that such adaptation is possible. But greater acidity at the surface could also open up these waters to the deep-sea species, resulting in a crab competition. What this means for the species we love to eat, well, only time will tell.
Image of red king crab courtesy of The Children’s Museum of Indianapolis, via wikimedia commons