Permeability of the oral epithelial layers in cnidarians

In order to characterize the permeability of the oral epithelial layers in cnidarians, we investigated the kinetics of transport of labelled ions (Ca-45, Na-22, Cl-36) and organic molecules (C-14-inulin-carboxyl, (14)Cala) through the oral tissue of two cnidarian species, Anemonia viridis (Forsskal, 1775) and Heliofungia actiniformis (Quoy and Gaimard, 1833) using the Ussing chamber method. In both species, unidirectional Ca, Na and Cl fluxes were the same in both directions (ectoderm towards endoderm and vice versa), the net Aux being equal to zero. The insensitivity a of these unidirectional transepithelial fluxes to metabolic inhibitor (1 mM sodium cyanide) and calcium channel inhibitor (100 mu M verapamil) and their linear dependence on calcium concentration suggest that these fluxes are simply driven by diffusion via a paracellular pathway. The epithelial layers were not permeable to inulin. Low-molecular weight amino acids such as alanine did not cross the epithelia but were absorbed by the ectoderm. The permeability coefficients indicate that the oral epithelial layers are leaky. It is suggested that the coelenteric cavity represents a compartment in which the ionic pool can be entirely renewed by simple diffusion. This process seems effcient enough to meet all calcium requirements in scleractinian corals.