The oceans absorb about half the carbon dioxide humankind releases into the atmosphere, and seawater is consequently acidifying.

That's a big problem for shellfish, corals, and certain other calcareous creatures, because lowered pH dissolves their shells and skeletons. Echinoderms — starfish and their relatives — have calcium-based skeletons, too, and so researchers have assumed they are likewise subject to slow dissolution.

Hannah L. Wood of the Plymouth Marine Laboratory in England and two co-workers decided to check. They took brittlestars (Amphiura filiformis), removed an arm or two, and then, to test how acidity affected regeneration of the lost arms, exposed the animals to seawater that was either normal (pH 8.0) or acidified (pH 7.7) — the standard worst-case prediction for the year 2100 — and pH 7.3).

To Wood's surprise, the brittlestars actually regenerated their arms faster in the acidified seawater than in the normal stuff, showing that they could lay down calcium effectively even under adverse conditions.

But there was a hidden cost. Both intact and regenerated arms had considerably less muscle mass in acidified seawater than they did in normal seawater. The low-pH animals consumed extra oxygen, so they were working hard, and Wood thinks they had to burn muscle to fuel the laborious regeneration. Weakened arms would undoubtedly affect feeding and reproduction.

Thus, even if it doesn't affect their calcification, low pH still costs echinoderms an arm and a leg.

The finding was detailed in the Proceedings of the Royal Society B.