The Pacific Ocean is becoming so acidic that the exoskeletons of crab larvae are dissolving, according to a new study. Dungeness crabs are an economic staple of the West Coast, but scientists predict that ocean acidification will harm the estimated $220 million industry.
These partly dissolved crabs are striking examples of how man-made climate change is disrupting marine environments and a reminder that we are still discovering the full extent of its harmful effects. The study was published on January 22 in the journal Science of the Total Environment.
“Once this organism starts to reach the coastal waters, they are basically sucked in,” said first author Nina Bednaršek. “Those waters are very corrosive, and they cannot escape.”
When we burn fossil fuels, oceans absorb a third of the emitted carbon dioxide. Underwater, some of that gas dissolves and lowers the pH (the scale of how acidic or basic a liquid is) of the ocean in the process, making it more acidic. The lower pH bleaches coral reefs and makes oceans uninhabitable for marine life over prolonged periods. The last time the oceans were acidifying as quickly as they are now, it drove the extinction of 96 percent of all marine life.
Acidification is an even bigger problem on the Pacific Coast due to upwelling, in which the wind drives corrosive waters from the depths upward, impacting coastal fisheries.
The authors collected the data presented in the study in 2016 while aboard a National Oceanic and Atmospheric Administration (NOAA) research cruise. Scientists caught 50 larval-stage Dungeness crabs from10 different coastal regions and recorded information for each location, including its temperature, depth, and acidity. Back in the lab, Bednaršek and her team looked at the crabs under high-powered microscopes, a process that could take up to half a day for a single 1 centimeter-sized crab.
Their attention to detail paid off: Bednaršek noticed that crabs from more acidic sites tended to have partially dissolved exoskeletons, in the most extreme cases on both the outer and inner sides. Zooming in, she found that tiny bristles called setae had been uprooted in dissolved areas of the shells.
It’s unclear how losing the setae might affect the crabs, Bednaršek said. They seem to play a role in how the crab orients itself in its environment and defends against predators, and disrupting these functions could decrease a crab’s chance of survival, she added.
“There are some stations where the crabs are there and they look really compromised,” Bednaršek said. “I started immediately thinking, ‘Wow, if I see this kind of impact now, how is this going to look like in 10 years’ time?’”
Dungeness crabs are just the latest additions to the list of organisms harmed by ocean acidification. Bednaršek has studied pteropods—marine snails known as sea butterflies—and has found that acidification causes their shells to dissolve, too. One difference between these sea creatures, though, is that until this study, scientists did not think that Dungeness crab shells could dissolve, Bednaršek said.
Since a hard molecule called chitin comprises the exoskeleton of mature crabs, it was a “common paradigm” that the shells didn’t contain dissolvable crystalline structures, she said. But no one had looked at the composition of larval crabs’ exoskeletons, which turned out to have these structures that made them susceptible to dissolution.
As ocean acidification affects the availability and survival of Dungeness crabs, people can harvest crabs from less acidic regions of the coast or at different points in the season when acidification is not as pronounced, Bednaršek said. But humans will have to make big, sustained changes to keep from continuing down a harmful path: The study found that the past two decades of ocean acidification alone have led to a 10% increase in the dissolution of the crabs’ shells.
"It's kind of like a wake up call,” Bednaršek said. “We still need to start seriously thinking what to do in terms of not just adaptation, but mostly mitigation, like how can we start reducing these fast changes in the natural environment that are happening in front of our eyes.”