Damselfish in distress: on ocean acidification and suicidal reef fish

Munday L., Cheal A. L., Dixson D. L., Rummer J. L. & Fabricius K. E., 2014. Behavioural impairment in reef fishes caused by ocean acidification at CO2 seeps. Nature Climate Change 4:487-492.

Background

In Finding Nemo, the despondent protagonist Nemo wanders away from a school fieldtrip after his father Marlin mocks his impaired swimming ability, the result of a congenital lame fin. At least that is Pixar’s take on what happened. Science might say it was something in the water that emboldened the little talking clownfish to leave the safety of his reef to embark on a feature length adventure of a lifetime.

That something might have been carbon dioxide, the main culprit of ocean acidification, and prime suspect in anthropogenic climate change. When atmospheric carbon dioxide dissolves in water, it equilibrates to form carbonic acid. Acidification due to dissolved carbon dioxide is the same reason your dentist might have told you to avoid carbonated beverages. Sugar aside, just like the carbonation in a can of soda weakens your teeth, increases in acidity (decreases in pH) due to dissolved carbon dioxide can compromise coral structure, or otherwise alter the physiology of, for example, Nemo.

A study recently published by Munday et al. in Nature Climate Change suggests that reef fish exposed to higher levels of carbon dioxide exhibit reckless and vagrant behavior as compared to their more conservative counterparts in waters with lower concentrations of carbon dioxide. Past studies meant to simulate the effects of ocean acidification on fish behavior have shown that elevated exposure to the greenhouse gas desensitizes laboratory-reared fish to danger, such as predation. However, the question remains: how are native ecological communities affected by behavioral changes due to continuous exposure to elevated levels of carbon dioxide?

Methods

Munday et al. set out to address this question for the first time by studying behavioral differences in species of cardinalfish and damselfish in the context of the reef ecosystems they inhabit. The team took advantage of a unique natural “laboratory” afforded by reefs in Papua New Guinea situated on volcanic seeps, vents in the ocean floor that contribute elevated levels of carbon dioxide to the surrounding waters similar to levels projected as a result of rising anthropogenic carbon dioxide emissions.

Behavior of reef fish  situated near seeps could be compared against nearby “control” reefs with similar population structures, but with more typical carbon dioxide concentrations . Owing to the sedentary nature and limited home range of cardinalfish and damselfish, it is assumed that they have experienced the same environmental conditions since settling. This includes the prolonged exposure to elevated levels of carbon dioxide in the case of the seep fish.

Results

Consistent with previous results on laboratory-grown fish, Munday et al. found that juvenile reef fish from carbon dioxide seeps exhibited a desensitization to ecologically relevant olfactory cues. Seep fish displayed poorer decision making abilities in the face of simulated danger. When offered a choice between streams of water flavored with predator odors or no odor, seep fish chose the predator-flavored stream 90 percent of the time, while their control counterparts avoided it altogether.

Seep fish exhibited overall bolder behavior and an inability to distinguish between habitats, such as a lack of interest in shelter when it was offered. Seep fish spent an average of 12 percent sof their time in shelter as compared to 90 percent for control fish and emerged an average of six times faster from shelter than control fish as a result of disturbances.

Behavior of reef fish  situated near seeps could be compared against nearby “control” reefs with similar population structures, but with more typical carbon dioxide concentrations . Owing to the sedentary nature and limited home range of cardinalfish and damselfish, it is assumed that they have experienced the same environmental conditions since settling. This includes the prolonged exposure to elevated levels of carbon dioxide in the case of the seep fish.

Results

Consistent with previous results on laboratory-grown fish, Munday et al. found that juvenile reef fish from carbon dioxide seeps exhibited a desensitization to ecologically relevant olfactory cues. Seep fish displayed poorer decision making abilities in the face of simulated danger. When offered a choice between streams of water flavored with predator odors or no odor, seep fish chose the predator-flavored stream 90 percent of the time, while their control counterparts avoided it altogether.

Living dangerously. Preference of four species of juvenile reef fish from control and seep (CO2) reefs for water treated with “predatory” odors. The vertical axis is the percent of time a given species fish spent in the treated or control stream. Seep fish show a clear preference for predatory odors, while their counterparts in control reefs completely avoid predatory odors.
Seep fish exhibited overall bolder behavior and an inability to distinguish between habitats, such as a lack of interest in shelter when it was offered. Seep fish spent an average of 12 percent sof their time in shelter as compared to 90 percent for control fish and emerged an average of six times faster from shelter than control fish as a result of disturbances.

Leavin’ home too easy. Seep fish spend less time in shelter (top panel), and venture further from shelter (bottom) than their counterparts from control reefs.
Moreover, seep fish spent approximately equal amounts of time in streams of water flavored from their native habitats compared to control habitats, while control fish preferred their native waters even after adjusting for differences in carbon dioxide concentrations. In general, seep fish could be said to be more susceptible to being eaten, and hence less likely to survive? than their downtown counterparts.

What is perhaps most alarming is that the fish do not seem to be able to adapt to continuous carbon dioxide exposure.  Munday et al. considered differences in energetic demands resulting from elevated exposure to acidified waters to provide a possible physiological explanation for behavioral impairment as a result of carbon dioxide exposure. However, a lack of difference in metabolism in damselfish from seeps versus control reefs suggested that energetic demands could not account for behavioral differences. Instead, Munday et al. offer that acidification leads to changes in neurotransmitter function resulting in altered behavior.

Surprisingly, Munday et al. note that diversity and community structure do not differ between carbon dioxide seeps and control reefs for two of the sites. They explain this anomaly by the ability of seep reefs to recruit fish from the outside, as well as the smaller population of predators in the seep reef environments. If true, this theory raises alarming implications for an acidifying ocean, suggesting that as elevated dissolved carbon dioxide becomes the norm, populations might lose their ability to rebound. For a third site, Munday et al. observed significant differences in abundance of small fish species between seep and control reefs. They rationalize that these differences arise from substantial habitat-specific differences, rather than differences in dissolved carbon dioxide levels. Overall, the differences observed between the control and seep sites for the third location are less than their differences with the other two sites.

Conclusion

Although their findings have yet to be generalized across other ocean ecosystems, Munday et al. hint at the threat of biodiversity loss due to ocean acidification of the world’s oceans. While it may be impossible to know for certain the long-term effects of ocean acidification, if humanity fails to curb emissions of carbon dioxide to the atmosphere, many more Nemos may be lost, perhaps never again to be found.

Abrahim El Gamal, Oceanbites, 29 August 2014. Article.


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