Increased atmospheric CO2 levels will have a major impact on marine life, in particular on corals and molluscs, through the effects of ocean acidification on calcification. A recent international collaboration by researchers from Australia, Canada and the USA suggests that non-calcifying species, such as fishes, may also be adversely affected, as increased acidity has subtle but important effects.
The researchers, led by Philip Munday of James Cook University in Australia, studied the consequences of increased levels of acidity on the behaviour of larval clownfish – the striking white and orange fish that featured in the film Finding Nemo. Clownfish live near the bottom of the sea, but their larvae (which are a dull brown) drift as plankton. As they mature, the young clownfish need to find the adult population and avoid predators. Both these behaviours rely on chemical cues, which may be susceptible to changes in pH.
Munday and his co-workers reared clownfish larvae at normal sea-water pH (8.15), and also at increasing levels of acidity, induced by higher levels of CO2. They then used a simple flume chamber, in which the fish can choose to position itself in one of two parallel water sources, to test the ability of larvae to respond to various chemical stimuli. Current CO2 levels are around 390 p.p.m. or 0.039%; in 1960 the figure was around 360 p.p.m. (an 8% increase in 40 years). When reared under 390 p.p.m. CO2, all ages of larvae were able to avoid predator cues; similar responses were seen in larvae reared under a slightly higher CO2 level (550 p.p.m.).
However, when larvae were reared at 700 or 850 p.p.m. CO2 (which some models predict we will see by the end of the 21st century), non-adaptive behaviour began to appear – some older larvae actually preferred the predator-cue water stream, in particular when reared at the highest CO2 level. To see whether this response was specific to the clownfish, the researchers repeated their experiment with larvae of the bright blue damselfish. The results were essentially the same – the longer larvae were reared at the highest dose of CO2, the greater the effect on chemosensory behaviour.
In a final test, the damselfish larvae were taken to the ocean and placed on reefs. Their behaviour in the wild mirrored their responses in the laboratory – larvae reared at higher CO2 levels showed riskier behaviours, venturing further from the reef than control larvae. These behaviours had lethal consequences: the larvae suffered significantly higher predation rates – up to 9-fold those of larvae reared at present-day CO2 levels.
Exactly how CO2 is affecting the behaviour of the fish is not clear. It may simply alter chemosensory responses, or it may also be altering the animals' general activity levels. Either way, these results suggest that ocean acidification will have multiple effects on this key part of the planet's ecology. If there were to be a remake of Finding Nemo in a few decades, it might not end so happily.
