Analysis of cutaneous and internal gill gas exchange morphology in early larval amphibians, Pseudophryne bibronii and Crinia georgiana
This study uses stereological techniques to examine body, internal gill and cardiovascular morphology of two larval amphibians, Pseudophryne bibronii and Crinia georgiana, to evaluate the roles of diffusive and convective gas exchange. Gosner stage 27 specimens were prepared for light microscopy and six parallel sections of equal distance taken through the body as well as a further six through the heart and internal gills. Body, internal gill and heart volume as well as body and internal gill surface areas were determined. The harmonic mean distance across the internal gills was also measured and used to estimate oxygen diffusive conductance, DO2. The species were of similar body size and surface area, but the heart and internal gills were larger in P. bibronii, which may represent precursors for greater growth of the species beyond stage 27. The much larger surface area of the skin compared to the internal gills in both species suggests it is the main site for gas exchange, with the gills supplementing oxygen uptake. The sparse cutaneous capillary network suggests diffusion is the main oxygen transport mechanism across the skin and directly into deeper tissues. A numerical model that simplifies larval shape, and has an internal (axial vessels) and external oxygen source, confirms that diffusion is able to maintain tissue oxygen with limited convective input.