Development, surface exposure, and embryo behavior affect oxygen levels in eggs of the red-eyed treefrog, Agalychnis callidryas.
INTRODUCTION Egg envelopes, such as shells, capsules and egg cases, both protect developing embryos and impose constraints on their microenvironment, including diffusion barriers to oxygen. Development is an aerobic process, and embryonic oxygen demand increases as yolk is converted to metabolically active tissue. Thus hypoxia can slow development, kill eggs, and select for traits that improve the oxygen supply to embryos (Bradford and Seymour, 1988; Booth, 1995; Cohen and Strathmann, 1996; Seymour et al., 2000). Both parental traits and egg mass features that improve egg oxygenation are well documented [e.g. egg ventilation (Fernández et al., 2000), clutch size and shape (Strathmann and Strathmann, 1995), convection channels (Pinder and Friet, 1994)]. Embryo traits could also improve oxygen uptake ability. Here, we assess the contribution to respiration of embryo behavior and the external gills of embryos and hatchlings of red-eyed treefrogs. Hatching frees animals from the diffusion barrier of the egg and can be induced prematurely by hypoxia in many taxa (Petranka et al., 1982; Warkentin, 2007). In some species, hatching is both advanced by hypoxia and delayed by hyperoxia, suggesting that it occurs at a respiratory threshold (Latham and Just, 1989). While solving a physiological problem, hatching earlier may, however, create an ecological one. More mature hatchlings typically have greater sensory development and/or locomotor abilities (Fuiman, 2002), both of which can reduce mortality from predation (Sih and Moore, 1993; Warkentin, 1999a; Gomez-Mestre et al., 2008). If predators or other factors in the post-hatching environment impose selection for later hatching, then mechanisms to extend embryonic development will be favored (Warkentin, 2007). For embryos under respiratory constraint these include traits that improve oxygen supply or oxygen uptake capacity. Anurans have four respiratory surfaces: skin, lungs, external gills and internal gills. Their external gills are transient embryonic structures that are, in many species, little more than short externalized capillary loops of equivocal functional value (Burggren and Just, 1992). Moreover, some amphibian and fish embryos develop normally without convective oxygen transport, suggesting that diffusion is sufficient to supply their oxygen needs (Flores and Frieden, 1969; Pelster and Burggren, 1996; Territo and Burggren, 1998). In other anurans, both morphological elaboration of the external gills and environmentally regulated gill regression suggest that these structures serve an important function (del Pino and Escobar, 1981; Channing, 1993). The behavior of embryos and fetuses has been viewed primarily from a developmental perspective (Smotherman and Robinson, 1996). For instance, embryo movements play a role in neuromuscular and skeletal development (Robinson et al., 2000; Pitsillides, 2006). Embryo movements have seldom been examined for immediate utility, and functional roles for embryo behavior are documented in few contexts, mostly late in embryonic development [e.g. environmentally cued hatching (Warkentin and Caldwell, in press); care solicitation vocalizations (Brua, 2002)]. Behavior may, however, have immediate utility at earlier stages (Goldberg et al., 2008).