03 February 2009

Acid oceans confuse baby Nemos

Increasing ocean acidity may interfere with baby 'Nemo's ability to smell their way to a reef and an anemone home. This was found in a recent study of larvae of the Clown anemonefish (Amphiprion percula). Another study had found that these fishes follow the scent of rainforest leaves to find their way to a home. The oceans are expected to become more acidic with rising CO2 emissions.

In Singapore, another kind of 'Nemo' called the False clown anemonefish (Amphiprion ocellaris) is often encountered on many of our reefs. And small ones are regularly seen.
False clown anemonefish (Amphiprion ocellaris)
A tiny little fish at Sentosa, very close to the reclamation site for the Sentosa Integrated Resort.
False clown anemonefish (Amphiprion ocellaris)
Small ones at Kusu Island.
Commensals in an anemone
A tiny fish which was even smaller than the anemoneshrimp at Pulau Hantu. Without their anemone hosts, these little fishes are unlikely to survive.

See also Losing Nemo on Not Exactly Rocket Science.

Acid oceans no laughing matter for clownfish
Rachel Nowak, New Scientist 2 Feb 09;
Just a few days after 150 marine scientists signed a declaration to draw attention to rising ocean acidity - dubbed "global warming's evil twin" – an international team has reported the first example of the potential for acid seas to directly affect animal behaviour.

When exposed to acidity levels in water similar to those expected by the end of the century, clownfish larvae fail to correctly discriminate between the smells they use to find a reef, according to the new study led by Philip Munday of James Cook University in Townsville, Queensland, Australia.

"Nearly all coral reef fish, and many other marine organisms, have a larval form that lives in the open water and needs to find its way to its reef habitat. If this is a wider issue, it could be serious," says Munday.

Like global warming, rising ocean acidity is due to carbon dioxide emissions. Around 40% of human CO2emitted in the past 200 years is now stored in the oceans, increasing acid levels. by a third, because CO2dissolves to produce carbonic acid.

Lab studies and modelling work suggest that, within decades, increased acidity will make it difficult for plankton, coral, mussels, oysters and other sea creatures to grow their calcium-based shells and skeletons. But until now no-one expected a direct impact on animal behaviour.
'Mom, I'm back!'

In the new study, the Munday team raised clownfish (Amphiprion percula) larvae in sea water with acidity at levels either current levels or equivalent to 1000 ppm of carbon dioxide -the upper level predicted by 2100 unless we can cut CO2 emissions.

In the lab, larvae that are at the age when they would usually be searching for a reef habitat are attracted to the scent of tropical tree leaves, and repelled by those of a type of tea tree plant that grows in swamps. The ability to discriminate between smells is thought to help the youngsters find a suitable reef habitat, as clownfish prefer to live on reefs surrounding vegetated islands. The larvae also avoid the scent of their parents, presumably to prevent inbreeding.

However, when larvae raised in acid water were given the choice of swimming in water carrying different scents, although they were still attracted by the leaves of a tropical tree called the golden penda, they were no longer repelled by tea tree leaves. Nor were they repelled by their parents' scent.

Quite how the acid water is altering scent discrimination remains a mystery, and there were no obvious anatomical changes in olfactory system, says Munday.
Rapid changes

"It is very concerning and unexpected. But one of the problems with ocean acidification is that it is happening quickly, and we don't know what to expect," says climatologist Janice Lough of the Australian Institute of Marine Sciences in Townsville.

A key question will be whether clownfish and other marine organisms can evolve to cope with increased acidity. "Their capacity to evolve may be limited because it is happening so fast, a hundred times faster than the less extreme increases associated with the end of previous ice ages," says Munday.

The Monaco Declaration (pdf format), which was agreed at a a meeting on the topic in Monaco last October, is intended to draw attention to problems of ocean acidification.

"We're trying to raise the alarm about this other impact of CO2 emissions that is independent of global warming," says Will Howard of the Antarctic Climate and Ecosystems Centre for Cooperative Research in Hobart, a signatory of the declaration. "Even if you believe this whole global warming thing is complete nonsense, you can't get away from the chemical impact of CO2 on the oceans."

To make matters worse, the geoengineering schemes suggested to date are designed to tackle global warming, but not ocean acidification. Some, such as dumping crop waste in the deep ocean could even exacerbate surface acidification, says Howard.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0809996106)

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