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Center for Biological Diversity:
Ocean Acidification

New Scientist, January 15, 2012

Carbon dioxide encourages risky behaviour in clownfish

By Wendy Zukerman

Carbon dioxide in the ocean acts like alcohol on fish, leaving them less able to judge risks and prone to losing their senses. The intoxication adds to the threats that global warming and ocean acidification pose to marine ecosystems.

Around 2.3 billion tonnes of human-caused CO2 emissions dissolve into the world's oceans every year, turning the water more acidic.

Philip Munday and colleagues at James Cook University in Townsville, Queensland, Australia, have previously found that if you put reef fish into water with more CO2 than normal in it – similar to the levels expected in oceans by the end of the century – they become bolder and attracted to odours they would normally avoid, including those of predators and unfavourable habitats.

Munday and his colleague Göran Nilsson at the University of Oslo, Norway, have now discovered that CO2 leads to riskier behaviour by interfering with a neurotransmitter receptor called GABA-A.

The pair reared clownfish (Amphiprion percula) larvae in seawater with normal (450 microatmospheres) and elevated (900 microatmospheres) CO2 levels. When they reached adulthood, the fish were given a choice between a water stream containing the odour of common predators such as the rock cod (Cephalopholis cyanostigma) or a stream lacking predatory odours. Those reared in high levels of CO2 swam towards rock cod's scent around 90 per cent of the time, whereas those that had enjoyed normal levels of CO2 avoided the predator's scent more than 90 per cent of the time.

Sobriety restored

Treating the clownfish bred under CO2-rich conditions with gabazine, a chemical that blocks the GABA-A receptor, helped them to regain their senses, though: fish treated this way swam towards the predatory smell only 12 per cent of the time.

"The fact that we could use a specific blocker for the GABA-A receptors to reverse the behavioural alterations proves that this receptor is involved in the CO2 effects," says Nilsson.

In a second test, using juvenile damselfish (Neopomacentrus azysron) from Lizard Island in the Great Barrier Reef, the team found that high levels of CO2 destroyed their natural tendency to turn left or right in certain situations – a crucial factor in shoaling. A bath with gabazine restored this "handedness".

Exciting effect

The GABA-A receptor sits on dendrites, the wiry projections of a neuron that detect chemical signals from other neurons. When the neurotransmitter GABA binds to its receptor, the receptor opens and a flood of negatively charged chloride and bicarbonate ions rush into the cell and prevent it from firing. This means that GABA has an inhibitory effect on the neuron.

When CO2 accumulates in the fish, it alters the distribution of ions. Now, when the receptor opens, chloride and bicarbonate ions escape out of the cell, exciting the neuron instead. "This would have strong effects on the function of neural circuits in the brain," says Nilsson, and may make ultimately make the fish behave in a way that increases its likelihood of being eaten.

The influence of CO2 "is likely to be far broader than just applying to the two species that were considered in this study", says Anthony Fowler, a fish ecologist at the South Australian Research and Development Institute in Adelaide, who was not involved in the study. Most animals, both vertebrates and invertebrates, have GABA-A receptors, he says.

Nilsson and Munday suspect that water-breathing animals, such as fish and crustaceans, are especially at risk. These creatures generally have lower blood CO2 levels than air-breathers, so it is more difficult for them to cope with the acid boost.

This article originally appeared here.

Photo © Paul S. Hamilton