Sour times: Seawater acidification effects on growth, feeding behaviour and acid-base status of Asterias rubens and Carcinus maenas

The impact of seawater acidification on calcifying organisms varies at the species level. If the impact differs between predator and prey in strength and/or sign, trophic interactions may be altered. In the present study, we investigated the impact of 3 different seawater pCO₂ levels (650, 1250 and 3500 μatm) on the acid-base status or the growth of 2 predatory species, the common sea star Asterias rubens and the shore crab Carcinus maenas, and tested whether the quantity or size of prey consumed is affected. We exposed both the predators and their prey, the blue mussel Mytilus edulis, over a time span of 10 wk and subsequently performed feeding experiments. Intermediate acidification levels had no significant effect on growth or consumption in either predator species. The highest acidification level reduced feeding and growth rates in sea stars by 56%, while in crabs a 41% decrease in consumption rates of mussels could be demonstrated over the 10 wk experimental period but not in the subsequent shorter feeding assays. Because only a few crabs moulted in the experiment, acidification effects on crab growth could not be investigated. Active extracellular pH compensation by means of bicarbonate accumulation was observed in C. maenas, whereas the coelomic fluid pH in A. rubens remained uncompensated. Acidification did not provoke a measurable shift in prey size preferred by either predator. Mussels exposed to elevated pCO ₂ were preferred by previously untreated A. rubens but not by C. maenas. The observed effects on species interactions were weak even at the high acidification levels expected in the future in marginal marine habitats such as the Baltic Sea. Our results indicate that when stress effects are similar (and weak) on interacting species, biotic interactions may remain unaffected.