Up to a quarter of the carbon dioxide from anthropogenic sources is absorbed by the world’s oceans, leading to ocean acidification. Recently, Philip Munday and his colleagues from James Cook University, Australia, have shown that clownfish (Amphiprion percula) and damselfish (Neopomacentrus azysron) larvae raised in approximate future conditions of acidified seawater lose the ability to detect homing and predatory cues. Consequently, these larvae have an increased mortality when compared with larvae raised in present day oceanic conditions. However, the physiological mechanisms behind this phenomenon remain shrouded in mystery. Göran Nilsson from the University of Oslo, Norway, in collaboration with Munday and their colleagues set out to determine whether behavioural changes in the coral reef fish larvae raised in acidified seawater were due to physiological changes in the brains of these fish. More specifically, the team looked at the effect that the inhibitory neurotransmitter γ-aminobutyric acid (GABA) has on the behaviour of these fish; their results were published in a recent issue of Nature Climate Change.
After raising the clownfish in either normal or seawater acidified at levels predicted for the end of this century, the researchers measured the preference of these two groups of clownfish larvae for seawater containing an olfactory cue, from either a predator or a non-predator. Just as in previous studies, the clownfish raised in normal seawater were strongly repelled by the predatory cue, while the clownfish raised in the acidified seawater were strongly attracted to the predatory cue. However, after the team treated the fish with gabazine, a GABA receptor inhibitor, both clownfish groups were repelled by the predatory cue, showing a reversal in the behaviour of fish raised in acidified seawater. These results indicate that the GABA receptor mediates neural function and therefore olfactory behaviour in clownfish.
Next, the team wanted to know whether gabazine would reverse abnormal behaviours seen in other coral reef fish exposed to acidified seawater. Just as humans are either left or right handed, damselfish larvae show a strong preference for turning left or right when faced with this choice in a maze. However, damselfish larvae that have been exposed to acidified seawater lose this preference. Could gabazine reverse this effect too?
The team caught damselfish larvae from the wild and exposed them to normal or acidified water for 4 days. They then tested the turning preference using a T-shaped maze and repeated the test after a 30 min treatment with gabazine. Again, treatment with gabazine reversed the behaviour: damselfish larvae that had been exposed to acidified water regained their turning preference.
Based on their findings, the researchers hypothesized that exposure to acidified seawater alters the acid–base balance of coral reef fish and this causes inhibitory neurons that contain GABA receptors to become excitatory, which may cause shifts in the fish’s olfactory and turning preferences. Therefore, inhibiting these now excitatory neurons with gabazine reverses the behaviour. Although many questions remain unanswered, these findings pave the way for future research on the impacts of ocean acidification on the physiology of fish and other aquatic organisms.