Ocean acidification and responses to predators: can sensory redundancy reduce the apparent impacts of elevated CO(2) on fish?

Carbon dioxide (CO(2)) levels in the atmosphere and surface ocean are rising at an unprecedented rate due to sustained and accelerating anthropogenic CO(2) emissions. Previous studies have documented that exposure to elevated CO(2) causes impaired antipredator behavior by coral reef fish in response to chemical cues associated with predation. However, whether ocean acidification will impair visual recognition of common predators is currently unknown. This study examined whether sensory compensation in the presence of multiple sensory cues could reduce the impacts of ocean acidification on antipredator responses. When exposed to seawater enriched with levels of CO(2) predicted for the end of this century (880 μatm CO(2)), prey fish completely lost their response to conspecific alarm cues. While the visual response to a predator was also affected by high CO(2,) it was not entirely lost. Fish exposed to elevated CO(2,) spent less time in shelter than current-day controls and did not exhibit antipredator signaling behavior (bobbing) when multiple predator cues were present. They did, however, reduce feeding rate and activity levels to the same level as controls. The results suggest that the response of fish to visual cues may partially compensate for the lack of response to chemical cues. Fish subjected to elevated CO(2) levels(,) and exposed to chemical and visual predation cues simultaneously, responded with the same intensity as controls exposed to visual cues alone. However, these responses were still less than control fish simultaneously exposed to chemical and visual predation cues. Consequently, visual cues improve antipredator behavior of CO(2) exposed fish, but do not fully compensate for the loss of response to chemical cues. The reduced ability to correctly respond to a predator will have ramifications for survival in encounters with predators in the field, which could have repercussions for population replenishment in acidified oceans.