Abstract
Pioneering studies into the effects of elevated CO2 on the behaviour of reef fishes often tested high-CO2 reared fish using control water in the test arena. While subsequent studies using rearing treatment water (control or high CO2) in the test arena have confirmed the effects of high CO2 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 CO2) and experimental test water (control or high CO2) 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 CO2 experiments. Specifically, we tested if: (1) using control or high CO2 water in a two channel flume influenced the response of larval clownfish to predator odour; and (2) using control or high CO2 water in the test arena influenced the escape response of larval damselfish to a startle stimulus. Finally, (3) because the effects of high CO2 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 CO2 water. Larval clownfish reared from hatching in control water (496 µatm) strongly avoided predator cue whereas larval clownfish reared from hatching in high CO2 (1,022 µatm) were attracted to the predator cue, as has been reported in previous studies. There was no effect on fish responses of using either control or high CO2 water in the flume. Larval damselfish reared for four days in high CO2 (1,051 µatm) exhibited a slower response to a startle stimulus and slower escape speed compared with fish reared in control conditions (464 µatm). There was no effect of test water on escape responses. Treatment of high-CO2 reared clownfish with 4 mg l−1 gabazine in high CO2 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 CO2 levels.
Highlights
Rising concentrations of atmospheric carbon dioxide (CO2) have caused an increased uptake of CO2 by the ocean, leading to a decline in seawater pH and changes in the relative concentration of carbonate and bicarbonate ions, a process called ocean acidification (Sabine et al, 2004; Doney, 2010)
Latency to respond significantly increased in fish reared in elevated CO2 (F1,6 = 7.76,p = 0.032), with fish displaying latencies that were 30% longer compared with fish reared in control water (Fig.2A)
Escape speed and maximum speed responded with both kinematic variables declining significantly in fish reared in high CO2 compared to fish reared in control water (Fig. 2C; F1,5 = 6.81,p = 0.048 and Fig. 2D; F1,6 = 9.016,p = 0.0239)
Summary
Rising concentrations of atmospheric carbon dioxide (CO2) have caused an increased uptake of CO2 by the ocean, leading to a decline in seawater pH and changes in the relative concentration of carbonate and bicarbonate ions, a process called ocean acidification (Sabine et al, 2004; Doney, 2010). High CO2 affects the kinematics of predator–prey interactions, with juvenile prey exposed to elevated CO2 allowing predators to get closer before responding (Allan et al, 2013) and exhibiting reduced escape speeds and distances compared with fish reared at current-day CO2 levels (Allan et al, 2013; Allan et al, 2014) These changes in behaviour alter the outcome of predator–prey interactions, leading to significantly increased rates of mortality of small juveniles in mesocosm experiments (Ferrari et al, 2011b) and in fish transplanted to natural coral reef habitat (Munday et al, 2010; Munday et al, 2012; Chivers et al, 2014)
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