Juvenile Dungeness Crabs Are Smaller When Exposed to Ocean Acidification Conditions in the Laboratory

Ocean acidification, the consequence of atmospheric CO₂ dissolving in the ocean to form carbonic acid, is rapidly intensifying. A wealth of marine wildlife is known to be vulnerable to the expected drop in pH, including a number of crab species. The Dungeness crab (Cancer magister), the target of a commercially important fishery in the Pacific Northwest, is known to be sensitive to elevated CO₂ levels. In an effort to better characterize the degree to which C. magister is sensitive to ocean acidification, this study examined the size and shape of juveniles reared for over three hundred days in ambient (low) and future (high) CO₂ conditions across seven molt stages. Statistical analysis indicates that carapace widths and lengths were significantly smaller in crabs reared in high CO₂ relative to crabs reared in low CO₂. Effects on width were detectable at the third through the sixth instar, and effects on length were detectable at the fourth through the sixth instar. A proxy for carapace shape (width:length ratio) was significantly, albeit slightly, different for crabs in high CO₂ treatments from the second through the fifth instar. If dissolved CO­₂ levels continue to rise in the ocean, C. magister juveniles may be smaller at each stage. This could affect their role in US West Coast food webs and recruitment into fisheries, as smaller crabs are more susceptible to predation.