Coral Reef Fishes Show Decreased Antipredator Responses to Risk When Exposed to Increased CO2 Levels

Atmospheric levels of carbon dioxide are currently at 380ppm, and are expected to increase within the next few decades. Such increases in atmospheric concentrations result in increased levels of dissolved CO2 in the oceans. Elevated levels of dissolved CO2 increase ocean acidification and reduce carbonate-ion saturation. Scientists predict that these changes will have varied effects on marine species. A number of studies have demonstrated the variability in the responses of different aquatic species to increased levels of CO2. However, most studies have only compared species belonging to different orders. The study conducted by Ferrari et al. seeks to provide a comparison of physiological responses to elevated CO2 among closely related species. By doing so, they can test whether species sharing related life histories and ecology have similar tolerance levels to CO2. The ability to group species according to their CO2 tolerance could play a key role in predicting changes to marine communities in the future. Recent studies have demonstrated that CO2 exposure causes olfactory impairment in a number of reef fish. Given that reef fish rely upon their chemosensory ability to detect predators, any olfactory impairment is expected to increase mortality rates. Ferrari et al. tested the effects of CO2 exposure on the antipredator responses of four damselfish species; their research revealed that contrary to expectations, the four congeneric species varied drastically in CO2 tolerance. Additionally, using Pomacentrus chrysurus as a model species, their results suggest that reef fish larvae exposed to high levels of CO2 showed decreased antipredator responses to risk.—Cecilia Ledesma

Ferrari, M. C. O., Dixson, D.L., Munday, P. L., McCormick, M. I., Meekan, M. G., SIH, A. and Chivers, D. P. (2011), Intrageneric variation in antipredator responses of coral reef fishes affected by ocean acidification: implications for climate change projections on marine communities. Global Change Biology, 17: 2980–2986. doi: 10.1111/j.1365-2486.2011.02439.x

Maud C. O. Ferrari and scientists from various institutes collaborated on research submitted to Global Change Biology. The experiments for this study were conducted in November and December 2009 at the Lizard Island Research Station located on the Great Barrier Reef, Australia. Four damselfish species, P. chrysurus, P. moluccensis, P. amboinensis and P. nagasakiensis, were collected in light traps at the end of the planktonic larval stage. Once transferred to rearing aquariums, the fish were exposed to CO2-enriched air for 4 consecutive days. The tested levels of were approximately 700 and 850 ppm, levels of dissolved CO2 predicted under greenhouse gas emission scenarios. Immediately after the treatment period was over the fish were used in the experiment. Ferrari et al. measured the antipredator behavioral responses of the fish by quantifying their foraging, swimming activity, and microhabit use after detection of predator risk cues. These activities represent common antipredator responses in prey fishes. In order to compare behavior and mortality rates of fish exposed to CO2, P. chrysurus were placed on small patch reefs for about 11 hours and their behavior and survival were monitored.

The authors found that all four species showed varied antipredator responses when exposed to levels of CO2 higher than 390 ppm. While P. chrysurus, P. moluccensis, P. amboinensis and P. nagasakiensis all displayed significant losses of antipredator behavior, P. amboinensis remained less affected at CO2 levels of 700 ppm. The activities of CO2-treated P. chrysuru in the field were compared to those of control fish. The comparision indicates that P. chrysuru suffered much higher mortality rates, were more active, moved further and higher away from the reef, displayed higher feeding rates and were bolder than control fish. Thus, the CO2-exposed fish clearly lacked the adaptive antipredator responses to risk cues exhibited by control fish.

P. amboinensis appear as the more sensitive species; at CO2 levels of 700 ppm, it showed a 95% reduction in antipredator response. Meanwhile, at those same levels P. nagasakiensis only showed a 30% reduction in antipredator responses. Given similar history traits between the species, the authors cannot explain the variation in responses. They note that in order to better predict changes in marine communities under increasing CO2 levels future work should focus on the role of physiology in explaining these variations. Additionally, the reduced antipredator behavior observed in the lab translated into decreased survival rates under natural conditions; these fish showed as much as a fivefold to sevenfold incresae in mortality when placed in the reefs. These results indicate that interspecific variation in response to rising CO2 may result in changes to the composition of prey species; this would in turn effect the biodiversity of coral reef communities. However, the impact of increased levels of dissolved CO2 on marine ecosystems will depend on both the magnitude of variation between species and the ability of these species to adapt to changing environmental conditions.

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