Effects of elevated CO ₂ on predator avoidance behaviour by reef fishes is not altered by experimental test water

Pioneering studies into the effects of elevated CO ₂ on the behaviour of reef fishes often tested high-CO ₂ reared fish using control water in the test arena. While subsequent studies using rearing treatment water (control or high CO ₂ ) in the test arena have confirmed the effects of high CO ₂ on a range of reef fish behaviours, a further investigation into the use of different test water in the experimental arena is warranted. Here, we used a fully factorial design to test the effect of rearing treatment water (control or high CO ₂ ) and experimental test water (control or high CO ₂ ) on antipredator responses of larval reef fishes. We tested antipredator behaviour in larval clownfish Amphiprion percula and ambon damselfish Pomacentrus amboinensis , two species that have been used in previous high CO ₂ experiments. Specifically we tested if: 1) using control or high CO ₂ water in a two channel flume influenced the response of larval clownfish to predator odour, and 2) using control or high CO ₂ water in the test arena influenced the escape response of larval damselfish to a startle stimulus. Finally, 3) because the effects of high CO ₂ on fish behaviour appear to be caused by altered function of the GABA-A neurotransmitter we tested if antipredator behaviours were restored in clownfish treated with a GABA antagonist (gabazine) in high CO ₂ water. Larval clownfish reared from hatching in control water (496 μatm) strongly avoided predator cue whereas larval clownfish reared from hatching in high CO ₂ (1022 μatm) were attracted to the predator cue, as has been reported in previous studies. There was no effect of testing fish using control or high CO ₂ water in the flume. Larval damselfish reared for 4 days in high CO ₂ (1051 μatm) exhibited a slower response to a startle stimulus, slower escape speed and a shorter escape distance compared with fish reared in control conditions (464 μatm). There was no effect of test water on escape responses. Treatment of high-CO ₂ reared clownfish with 4 mg l ⁻¹ gabazine in high CO ₂ seawater restored the normal response to predator odour, as has been previously reported with fish tested in control water. Our results show that using control water in the experimental trials did not influence the results of previous studies on antipredator behaviour of reef fishes and also supports the results of novel experiments conducted in natural reef habitat at ambient CO ₂ levels.