Clownfish lost at sea due to rising carbon dioxide levels

Ian Sample, science correspondent
The Guardian, Tuesday 3 February 2009

The tale of a clownfish that got lost at sea in the 2003 movie Finding Nemo may be a taste of things to come - as rising carbon dioxide levels could leave the fish unable to find their way around, say scientists.
Tests on clownfish larvae showed they became disoriented and were unable to find a suitable place to live if they were raised in seawater that had absorbed carbon dioxide from the atmosphere.
The effect is potentially devastating for a wide range of fish populations because many rely on odours in seawater to seek out the right habitats to live in, according to researchers investigating the impact of greenhouse gas emissions on marine life.
The world's oceans soak up vast quantities of carbon dioxide released by burning fossil fuels. By absorbing the gas, oceans become more acidic. Global ocean pH has dropped around 0.1 points since preindustrial times. But with increasing carbon emissions expected, this pH is predicted to fall a further 0.3 to 0.4 points by 2100.
Writing in the US journal Proceedings of the National Academy of Sciences, the scientists describe how clownfish larvae lose the ability to sense vital odours in more acidic waters, probably owing to the damage caused to their olfactory systems.
"They can't distinguish between their own parents and other fish, and they become attracted to substances they previously avoided. It means the larvae will have less opportunity to find the right habitat, which could be devastating for their populations," said Kjell Døving, a co-author from the University of Oslo.
Clownfish eggs are carried on the ocean currents. When they hatch, the larvae pick up scents leading them to reefs and anemones, where they make their homes.
In the study, scientists checked how well the larvae followed odours in normal seawater, which has a pH of 8.15, and compared it with their performance in slightly acidified seawater that mimics ocean conditions expected in 2100 and beyond.
At a pH of 7.8, the larvae stopped following odour trails released by reefs and anemones. Instead, they homed in on scents they normally avoided, including those released by plants and other organisms which thrive in the wrong kinds of habitat for the fish. The larvae also lost the ability to use smell to distinguish between their parents and other fish.
At pH of 7.6, the larvae were unable to follow any kind of odour in the water, and instead swam in random directions.