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Ocean acidification may push many fish to the brink

By / June 30, 2009

Orange clownfish require a keen sense of smell to find homes where they can safely grow from larvae to adults. Ocean acidification has been shown to undercut that sense of smell.

Melanie Stetson Freeman / The Christian Science Monitor / File

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So, the US House has passed an energy bill aimed at helping the US kick its dependence on fossil fuels and help deal with global warming.

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White sea bass and orange clownfish will be happy to hear that.

Recent research on ocean acidification, often dubbed global warming's "evil twin" (as if human-triggered global warming itself was a "good twin"), is beginning to show that acidification has the potential to disrupt the life cycles of fin fish, in addition to its effects on coral and shell-building marine organisms of all shapes and sizes that marine scientists have long fretted over.

Two recent studies highlight the concerns.

One, published last week in the journal Science, shows that acidification can lead to excessive growth in a key component in the organ a fish uses to sense movement and orientation -- analogous to the human inner ear.

The component is called an otolith. It is made from aragonite, a form of calcium carbonate particularly vulnerable to ocean acidification. Otoliths rest atop tiny hairs in liquid-filled sacs. With a change of direction or speed, the otolith bends the hairs in a new direction and with differing intensity, signaling the change to the fish.

Researchers wanted to know what would happen to otolith development in more-acidic waters. So a team of US and Japanese scientists, led by David Checkley at the Scripps Institution of Oceanography in La Jolla, Calif., reared two batches of eggs from white sea bass in tanks of sea water.

The water in one tank had seawater with today's pH. Two other tanks held water at successively lower pHs (more acidic). In one of these latter two, the scientists lowered the pH to levels projected for 2100.

Ordinarily, as the pH falls, marine creatures that build aragonite shells have a harder time doing it -- the shells are thinner or non-existent. Surprisingly, otoliths in the larvae swimming in more-acidic waters, grew up to 17 percent larger than the otoliths in larvae in the least acidic tank.

The implication is that the larvae somehow try to regulate their body chemistry in ways that overcompensate for the low pH of the water they inhabit, leading to larger, rather than smaller, otoliths.

How this affects larvae behavior is unclear at the moment. Little work has been done in this area, notes Simon Thorrold, a marine biologist at the Woods Hole Oceanographic Institution in Woods Hole, Mass.

However, "fish have evolved to have otoliths of a certain size, a certain density, and a certain shape," Dr. Checkley said during a phone chat.  "Now there's a mismatch between the mass of the otolith and the hair cells on which it sits."

The deck already is stacked against larvae in a stable population of fish. A female may release upward of a million eggs over her lifetime, Checkley explains. Of those, only two are likely to survive to reproduce. Any physical abnormality is likely to be harmful to fish at a point in their development when they already are most vulnerable to predators or starvation.

It will take additional work to be more specific about the effect larger otoliths have on fishes ability to avoid predators, find food, or stay upright in turbulent water. But researchers have done a fair bit of work on the importance of a good sense of smell to fish larvae. And they've found that more-acidic waters can scramble a larva's sense of smell in dangerous ways.

In research reported in February, a team of scientists from Australia, Russia, and Norway demonstrated that orange clownfish larvae reared in seawater with pH levels as low (more acidic) as those projected for 2100 were attracted to smells they ordinarily avoided. They also avoided smells that ordinarily would attract them. A good sense of smell is vital to larvae as they hunt for reefs and suitable places within a reef to hunker down while they grow into full-fledged fish.

Larvae in seawater similar to today's pH levels had no trouble telling the difference in smells that would help them find a home and heading that way. The results appeared in the Proceedings of the National Academy of Sciences.

"A loss of larval olifactory capacity in marine organisms through acidification could have significant consequences for marine biodiversity," the team concludes.

The one-syllable translation: If fish lose their sense of smell as they now know it, they are toast.

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