Ocean acidification exacerbates the effects of paralytic shellfish toxins on the fitness of the edible mussel Mytilus chilensis.

High latitudes are considered particularly vulnerable to ocean acidification, since they are naturally low in carbonate ions. The edible mussel Mytilus chilensis is a common calcifier inhabiting marine ecosystems of the southern Chile, where culturing of this species is concentrated and where algal blooms produced by the toxic dinoflagellate A. catenella are becoming more frequent. Juvenile Mytilus chilensis were exposed to experimental conditions simulating two environmental phenomena: pCO2 increase and the presence of paralytic shellfish toxins (PST) produced by the dinoflagellate Alexandrium catenella. Individuals were exposed to two levels of pCO2 : 380 μatm (control condition) and 1000 μatm (future conditions) over a period of 39 days (acclimation), followed by another period of 40 days exposure to a combination of pCO2 and PST. Both factors significantly affected most of the physiological variables measured (feeding, metabolism and scope for growth). However, these effects greatly varied over time, which can be explained by the high individual variability described for mussels exposed to different environmental conditions. Absorption efficiency was not affected by the independent effect of the toxic diet; however, the diet and pCO2 interaction affected it significantly. The inhibition of the physiological processes related with energy acquisition by diets containing PST, may negatively impact mussel fitness, which could have important consequences for both wild and cultured mussel populations, and thus, for socioeconomic development in southern Chile.