Brittle Stars on Acid - A Really Bad Trip

While there is some debate over which creature is the coolest, it is generally understood* that marine molluscs and echinoderms both will face severe stresses coping with increased ocean acidification due to increased CO2 concentrations. The shelled molluscs use calcium carbonate to build their shells and echinoderms use it in creating their exoskeleton. Calcification is strongly related to ocean pH levels and there have been numerous studies looking at the impacts of pH on calcification rates and metabolism in marine inverts.

Much of the published research has been focused on the molluscs and hermatypic (reef building) corals, which has shown that acidification (with levels predicted for the year 2100) will have detrimental effects on calcification and on the metabolic rates of the animals under study.

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New research was published yesterday in the Proceedings of the Royal Society B on the effects of ocean acidification on the brittle star Amphiura filiformis and its arm regeneration. The researchers from Plymouth Marine Lab exposed A. filiformis to pH levels of 8.0 (control), 7.7 (IPCC predicted average ocean pH by 2100), 7.3 and 6.8 for a 40 day period. Regeneration effects were tested by detaching one leg from half the subjects and two legs from the rest in each pH group. The team found significant results for the length of regenerated arms, the calcium content and the metabolic rate caused by decreased pH levels.

Some of the results were surprising. Specifically, they found that the amount of calcium - which dissolves in acidic conditions - in regenerated arms was higher in acidified tests than in controls. They also found that non-regenerated arms maintained calcium levels in lower pH and even increased calcium content at a pH of 6.8. The researchers included a test of the calcium content of the separated arms as well. In acidified conditions the dead arms lost calcium as expected.

Brittle stars must increase calcification in the arms just to keep pace with the calcium disolution occuring due to pH. With regenerated arms having an even higher calcium content, this means A. filiformis must significantly increase the calcification under these conditions. The research found that the length of the regenerated arms was longer in acidified test conditions. All of this points to increased energy expenditure under acidified consitions in order to maintain calcium in extant arms and regenerate lost ones. This was confirmed by the oxygen uptake study, which found a significant increase in metabolic activity linked to incresed acidification.

Sounds pretty good then. A. filiformis can increase their metabolism and fuel an increaced calcification to cope with acidification. Except... the researchers also found that muscle wastage occurred and increased with greater acidification. It appears that the muscles are the fuel for the metabolism increase. The muscle wastage happened in regenerated and extant arms subjected to acidification at a rate of up to 20% muscle loss over 40 days. So they can regenerate faster and the calcium levels are higher, but the muscles are weaker and the net effect is a less capable arm. The trade offs to cope with acidification
are killing them.

And why do we care?

These brittle stars are an echinoderm model for what may happen as the ocean becomes more acidic. The fact that they will be increasing their metabolism just to maintain calcium levels in their skeleton means greater stress and muscle loss, which in turn may mean less effective use of the arms which they use to burrow and feed. Keep in mind this was a short term experiment. The wastage seen in 40 days, while not fatal, could explain the mortality seen in experiments involving smaller pH changes over longer periods of time.

Burrowing brittle stars, such as A. filiformis, can be siginifcant ecosystem engineers in soft bottom areas through bioturbation and are a major food source (through nipped off arms) for a number of commercially important fish and crustaceans. Other echinoderms responsd in a similar way to acidification. This is a phylum that is, in many ecosystems, a key linkage in ecosystem dynamics and trophic webs so understanding how they react to environmental changes will be important to predict ecosystem changes.

The study also highlights the fact that acidfication affects different organisms in vastly different ways. The effects need to be examined on an organism level instead of only looking at processes. There is still a lot of research needed here, but right now it doesn't look good for the brittle stars, at least A. filiformis.

Wood, H.L., Spicer, J.I., Widdicombe, S. (2008). Ocean acidification may increase calcification rates, but at a cost. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1--1. DOI: 10.1098/rspb.2008.0343