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Tradeoff.
This brittlestar can regrow its 10-centimeter-long arms, but its muscles pay the price.

Credit: Hannah Wood; (inset) Sam Dupont

Echinoderms Wasted by Acid

By Erik Stokstad
ScienceNOW Daily News
7 May 2008

The chemistry of the problem is fairly simple. Carbon dioxide (CO₂) emitted into the atmosphere--either from natural sources or from the combustion of fossil fuels--dissolves in the oceans, where it converts to various forms of carbonate that ultimately release hydrogen ions and increase acidity. As industry and other sources have pumped out more CO₂, the oceans have become ever more acidic in recent years, especially the shallower waters (ScienceNOW, 17 February 2007).

Extra acid impedes certain marine creatures' synthesis of calcium carbonate, which they need to build shells and skeletons. Experiments have shown, for example, that the oceanic acidity predicted for the year 2100 quickly dissolves the shells of swimming snails called pteropods and hampers the skeletal growth of corals (ScienceNOW, 28 September 2005).

To gauge the vulnerability of another group of marine invertebrates--the echinoderms, which include sea stars and sea cucumbers--Wood and colleagues performed experiments on the brittlestar (Amphiura filiformis). Some of the creatures lived in tanks with normal seawater, whereas others had to deal with more acidic water. Wood measured the animals’ ability to regenerate their arms after an injury, which requires them to rebuild part of their skeleton.

All the animals regrew their missing limbs, but those in the more acidic water had to work harder. They increased their metabolism, as measured by their uptake of oxygen, by 40%. These brittlestars also increased the calcium content of all of their arms, compared with controls. The compensation could be a response to the acid, which dissolved 1% to 1.5% of their skeleton each day. Metaphorically, "they are swimming against the tide," Wood says. "They have to paddle harder just to stay where they are."

Rebuilding the missing limbs and their skeletons took a toll. Needing energy, the brittlestars in the more acidic water broke down their muscles. By the end of 40 days, their intact arms had 20% less muscle mass than did those from brittlestars in normal seawater, the team reports online 6 May in the Proceedings of the Royal Society B. This loss is a concern because brittlestars capture food with their arms, so the wasting could reduce their ability to survive. Other species could suffer from the loss of arm strength as well; the Norwegian lobster and other commercially important species feed on the arms of brittlestars, which would be less nutritious if their muscles are wasted. And brittlestars use their arms to churn the sediment, thereby improving the habitat for other animals and mixing nutrients and other chemicals between the water and sediment.

The loss of muscle highlights the different ways that increased acidity affects ocean organisms, says biological oceanographer Victoria Fabry of California State University, San Marcos. Physiologist Gretchen Hofmann of the University of California, Santa Barbara, says she expects that many processes other than calcification will be altered by increased acidity. "We need to look more deeply into an organism's physiology to really get the whole picture of how [ocean acidification] will impact future populations of marine organisms," she notes.

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