Carbon dioxide enrichment alters predator avoidance and sex determination but only sex is mediated by GABA _A receptors

We hypothesized that near-future elevated CO ₂ would affect the antipredatory behavior of two freshwater organisms; a pulmonate gastropod (Physella columbiana) and a cladoceran crustacean (Daphnia magna). Studies have found that pCO ₂ and increased acidification due to CO ₂ impedes fright responses to predators by activating GABA _A receptors. After administration of predator-derived kairomones and conspecific alarm cues, we also briefly exposed some of the animals to gabazine which is a GABA _A receptor antagonist to restore a fright response. We found that added carbon dioxide negatively affected the antipredatory behavior of both species but gabazine did not reverse this effect. To further examine the effect of CO ₂ and gabazine, we also tested the effect of stressful crowding, cold, and acidic conditions on the production of male daphnid offspring. An increase in ratio of male to female offspring is a common and expected response to stress by daphnids. We found that stress increased the production of males and gabazine reversed this at a pH of 5.5 but not at pH 6.2 or 6.5. Our study suggest that while the main negative effects of anthropogenic CO ₂ enrichment can be robust, the myriad indirect effects of CO ₂ make predictions about future predator prey systems less clear.