Osmotic and Ionic Regulation in the Aquatic Caecilian<i>Typhlonectes compressicauda</i>and the Terrestrial Caecilian<i>Ichthyophis kohtaoensis</i>

Abstract

The terrestrial caecilian Ichthyophis kohtaoensis and the aquatic caecilian Typhlonectes compressicauda were comparedfor osmotic and ionic regulatory capabilities. The aquatic form has a lower rate of osmotic water uptake (0.09 vs. 0.24 μL/10 g/h), suggesting a lower osmotic permeability. The terrestrial Ichthyophis reduces its osmotic flux to 0.17μL/10 g/h) after 3 d of acclimation to water. The aquatic Typhlonectes produces a more dilute urine and has a lower glomerular filtration rate (GFR) (0.139 mL/10 g/h) and fractional water reabsorption (9.5%) than the terrestrial Ichthyophis (0.267 mL/10 g/h, 56.9%). Ichthyophis increases urine flow by reducing fractional water reabsorption to 32.3% when placed in water; GFR does not increase significantly. Analyses of net and unidirectional ionic fluxes and transcutaneous potential differences showed that both species actively transport Na⁺ and Cl⁻ across their body surfaces. The aquatic Typhlonectes has an apparent Km of 0.15 rmM for Na⁺ and 0.34 mM for Cl⁻, while the terrestrial Ichthyophis has an apparent Km of 0.65 for Na⁺ and 0.32 for Cl⁻. The apparent Km values did not differ significantly. The Vmax (capacity) was 1.97μeq/10 g/h for Na⁺ and 1.16μeq/10 g/h for Cl⁻ in Typhlonectes; the corresponding values for Ichthyophis were 3.00 and 2.50. The values did not difer significantly for the two species.