Gas Permeability of the Epiphragm of a Terrestrial Snail, Otala lactea

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Abstract
Permeability of the epiphragm of Otala lactea (Helicidae) to O₂, CO₂, and water vapor was determined and used to assess the significance of the epiphragm as a barrier to evaporative water loss and respiratory gas exchange during dormancy. A distinct region of the epiphragm, known as the Kalkfleck, occupies 6% of the area but accounts for 98% of the diffusive conductance of the epiphragm for O₂ and CO₂. The high gas permeability of the Kalkfleck results from air channels that penetrate a loose aggregate of crystalline CaCO₃ granules. The rest of the epiphragm is relatively impermeable and includes a nonporous layer which is absent from the Kalkfleck. Rates of water loss and respiratory gas exchange of Otala were incorporated with epiphragm permeability measurements in diffusion equations to determine the relative contribution of epiphragms to resistance to diffusion of gases from Otala. These calculations indicate that epiphragms provide up to 20% of the total resistance to evaporative water loss during dormancy and that the Kalkfleck is necessary to permit respiratory gas exchange.