The oral epithelial layers of anthozoans have a polarized morphology: photosynthetic endosymbionts live within endodermal cells facing the coelenteric cavity and are separated from the external seawater by the ectodermal layer and the mesoglea. To study if this morphology plays a role in the supply of inorganic carbon for symbiont photosynthesis, we measured the change in pH and the rate of OH (H⁺) fluxes induced by each cell layer on a tentacle of the sea anemone Anemonia viridis. Light-induced pH increase of the medium bathing the endodermal layers led to the generation of a transepithelial pH gradient of ~0.8 pH units across the tentacle, whereas darkness induced acidification of this medium. The light-induced pH change was associated with an increase of total alkalinity. Only the endodermal layer was able to induce a net OH secretion (H⁺ absorption). The light-induced OH secretion by the endodermal cell layer was dependent on the presence of in the compartment facing the ectoderm and was sensitive to several inhibitors of ion transport. [¹⁴C] incorporation into photosynthates confirmed the ectodermal supply, the extent of which varied from 25 to >90%, according to availability. Our results suggest that the light-induced OH secretion by the endodermal cell layer followed the polarized transport of and its subsequent decarboxylation within the endodermal cell layer. This polarity may play a significant role both in inorganic carbon absorption and in the control of light-enhanced calcification in scleractinian corals.