Juvenile sea stars exposed to acidification decrease feeding and growth with no acclimation potential

Ocean acidification has the potential to affect growth and calcification of benthic marine invertebrates, particularly during their early life history. We exposed field-collected juveniles of Asterias rubens from Kiel Fjord (western Baltic Sea) to 3 seawater CO₂ partial pressure (pCO₂) levels (ranging from around 650 to 3500 μatm) in a long-term (39 wk) and a short-term (6 wk) experiment. In both experiments, survival and calcification were not affected by elevated pCO₂. However, feeding rates decreased strongly with increasing pCO₂, while aerobic metabolism and NH ₄⁺ excretion were not significantly affected by CO ₂ exposure. Consequently, high pCO₂ reduced the scope for growth in A. rubens. Growth rates decreased substantially with increasing pCO₂ and were reduced even at pCO₂ levels occurring in the habitat today (e.g. during upwelling events). Sea stars were not able to acclimate to higher pCO₂, and growth performance did not recover during the long-term experiment. Therefore, the top-down control exerted by this keystone species may be diminished during periods of high environmental pCO₂ that already occur occasionally and will be even higher in the future. However, some individuals were able to grow at high rates even at high pCO₂, indicating potential for rapid adaption. The selection of adapted specimens of A. rubens in this seasonally acidified habitat may lead to higher CO₂ tolerance in adult sea stars of this population compared to the juvenile stage. Future studies need to address the synergistic effects of multiple stressors such as acidification, warming and reduced salinity, which will simultaneously impact the performance of sea stars in this habitat.