PERMEABILITY OF THE ORAL EPITHELIAL LAYERS IN CNIDARIANS

In order to characterize the permeability of the oral epithelial layer s in cnidarians, we investigated the kinetics of transport of labelled ions (Ca-45, Na-22, Cl-36) and organic molecules (C-14-inulin-carboxy l, (14)Cala) through the oral tissue of two cnidarian species, Anemoni a viridis (Forsskal, 1775) and Heliofungia actiniformis (Quoy and Gaim ard, 1833) using the Ussing chamber method. In both species, unidirect ional 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 met abolic inhibitor (1 mM sodium cyanide) and calcium channel inhibitor ( 100 mu M verapamil) and their linear dependence on calcium concentrati on suggest that these fluxes are simply driven by diffusion via a para cellular pathway. The epithelial layers were not permeable to inulin. Low-molecular weight amino acids such as alanine did not cross the epi thelia but were absorbed by the ectoderm. The permeability coefficient s indicate that the oral epithelial layers are leaky. It is suggested that the coelenteric cavity represents a compartment in which the ioni c pool can be entirely renewed by simple diffusion. This process seems effcient enough to meet all calcium requirements in scleractinian cor als.