Higher survival but smaller size of juvenile Dungeness crab (Metacarcinus magister) in high CO2

Dungeness crab (Metacarcinus magister) are the most valuable fishery on the U.S. West Coast and both larval and adult Dungeness crabs are important components of regional food webs. Previous experiments have shown decreased survival and a slower development rate for Dungeness crab zoea reared in water with high CO2, indicating a susceptibility to ocean acidification. In this study we reared late-stage megalopae and juvenile Dungeness crabs in both ambient and high CO2 conditions for over 300 days. Counter to expectations, crabs reared in high CO2 had a higher survival rate than those reared in ambient conditions and crabs in high CO2 transitioned more quickly in one of the stages (J5 to J6). However, crabs reared in high CO2 were generally smaller and had a higher resting metabolic rate than crabs in ambient CO2. We hypothesized that two separate mechanisms were in effect, with one process driving survival and a second process driving size and respiration rate. We further hypothesized that increased mortality in ambient CO2 could be caused by a CO2-sensitive microbial pathogen, but that size and respiration differences were caused by the direct effects of CO2 on the crabs themselves. Overall, the zoea stages seem more sensitive to CO2 than the megalopae and juvenile stages.