Elevated pCO₂ reinforces preference among intertidal algae in both a specialist and generalist herbivore

Ocean acidification (OA) can induce changes in marine organisms and species interactions. We examined OA effects on intertidal macroalgal growth, palatability, and consumption by a specialist crab (Pugettia producta) and a generalist snail (Tegula funebralis) herbivore. Moderate increases in pCO₂ increased algal growth in most species, but effects of pCO₂ on C:N and phenolic content varied by species. Elevated pCO₂ had no effect on algal acceptability to herbivores, but did affect their preference ranks. Under elevated pCO₂, electivity for a preferred kelp (Egregia menziesii) and preference rankings among algal species strengthened for both P. producta and T. funebralis, attributable to resilience of E. menziesii in elevated pCO₂ and to changes in palatability among less-preferred species. Preferred algae may therefore grow more under moderate pCO₂ increases in the future, but their appeal to herbivores may be strengthened by associated shifts in nutritional quality and defensive compounds in other species.