Effects of hypoxia on egg capsule conductance

conditions (Barth, 1946; Gregg, 1962; Moore, 1940; Savage, 1935), which can result in slowed development and decreased survival (Mills and Barnhart, 1999). Oxygen partial pressures below 1.5 kPa have been reported within egg masses of several species (Bachmann et al., 1986; Pinder and Friet, 1994; Seymour et al., 1995; Seymour and Roberts, 1991). There are two major causes of hypoxia in amphibian eggs. First, many species utilize eutrophic wetlands and ponds as breeding sites (Collins and Wilbur, 1979; Stebbins and Cohen, 1995). Such habitats are likely to have a high biological oxygen demand (BOD) and low oxygen pressures, particularly at night. Second, respiratory gases must pass through substantial diffusion barriers to reach or leave the developing embryo. These barriers generally consist of a gelatinous egg capsule, the vitelline membrane and the capsular and/or perivitelline fluid (Salthe, 1963). However, in ambystomatids and most other salamanders, the embryo exits the vitelline membrane early in development and completes development in the capsular chamber (Salthe, 1963). Convection created by cilia on the surface of the embryo enhances oxygen transport in the capsular fluid (Burggren, 1985). Therefore, it can be assumed that most resistance to oxygen transport is in the egg capsule (Seymour, 1995). Oxygen must diffuse through the egg capsule because the gelatinous nature of the egg capsule prevents convection. The diffusive conductance of oxygen across the egg capsule can be described by the equation: