Fish Prefer the Smell of Predators in Acidified Conditions

Ocean acidification as a result of increased CO2 emissions by human activity has been shown to have a variety of secondary effects on marine organisms. Dixson et al. (2010) studied the ability of orange clownfish, Amphiprion percula, to detect the presence of predators by smell under acidic conditions. Newly hatched and settlement stage larvae prefer waters that contain no trace of other fish—either predatory or non-predatory. The authors simulated the more acidic conditions predicted for the year 2100 to determine whether fish raised in more acidic conditions would be able to detect predators. Although newly hatched larvae in acidic conditions navigated away from predator signals, settlement stage larvae preferred water containing the chemical signature of predators over waters without any chemical signal and waters containing the chemical signature of non-predators. The results suggest that clownfish may exhibit risky settlement choices as oceans continue to acidify.—Emily Putnam

Dixson, D.L., Munday, P.L., Jones, G.P., 2010. Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology Letters 13, 68–75.­

          Dixson and colleagues at James Cook University compared the ability of settlement-stage larvae and newly hatched Amphiprion percula to detect predators. Two species of predators were chosen­, Cephalopholis cyanostigma and Pseudochromis fuscus. Two species of non-predators, Acanthurus pyroferus and Siganus corallinus, were chosen as further controls on the ability of the clownfish to detect predator signals. The clownfish were raised in water either at pH levels consistent with today’s oceans or at pH levels predicted for the year 2100. The fish were then placed individually into chambers with two streams of water. These tests focused on six combinations: normal water vs. normal water, predator 1 vs. normal water, predator 2 versus normal water, non-predator 1 vs. normal water, non-predator 2 vs. normal water, predator 1 vs. non-predator 1, predator 2 vs. non-predator 2. The clownfish were monitored for stream choice every five seconds for two minutes. To account for side preferences unrelated to chemical signals, the fish were allowed to rest for one minute while the streams were switched, then monitoring was repeated.
          Although newly hatched and settlement-ready clownfish raised in today’s waters preferred normal water and water with non-predatory chemical cues over those of predators, Amphiprion percula larvae raised in acidified conditions preferred water containing the chemical signatures of predatory fish and could not make a distinction between the chemical signatures of predatory vs. non-predatory fish. Newly hatched clownfish from acidified conditions followed the preferences of fish from today’s waters. The difference between newly hatched and settlement-age fish reflects the vulnerability of these two ages. Newly hatched fish by today’s standards would avoid the chemical signals of all fish and swim to the open ocean, whereas settlement-ready fish need to settle on reefs where they are more likely to be near other fish species. This could account for the discrepancy in sensing abilities between newly hatched and settlement-age orange clownfish individuals.
The larvae’s ability to detect predators can be directly related to the chance of survival. Fish that are unable to detect chemical cues will be more likely to settle near predators and will increase their chance of becoming food. This risky behavior could contribute to a large decrease in the population sizes and could even lead to extinction. One important thing to note is that the fish in the acidification samples were treated using acidity levels predicted for 2100. It is possible that a gradual increase of ocean acidity could instigate the adaptation of senses to be able to distinguish predators even under acidified conditions. However, the ability to adapt and the exact pH levels at which these fish are affected has yet to be determined. Dixson et al. have shown that increases in ocean acidity can affect the ability of fish to determine the presence of a predator and to distinguish between predators and non-predators.

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