Phenotypic differences in terrestrial frog embryos: effect of water potential and phase

Abstract

SUMMARYThe terrestrial embryos of many amphibians obtain water in two ways; in a liquid phase from the substrate on which eggs are deposited, and in a vapour phase from the surrounding atmosphere. We tested whether the mode of water flux (liquid or vapour) affected the morphology and metabolic traits of the terrestrial Victorian smooth froglet (Geocrinia victoriana) embryos by incubating eggs both with a liquid water source and at a range of vapour water potentials. We found that embryos incubated with a liquid water source (ψ π =0 kPa) were better hydrated than embryos incubated with a vapour water source (ψ v =0 kPa), and grew to a larger size. Eggs incubated in atmospheres with lower ψ v values showed significant declines in mass and in the thickness of the jelly capsule, while embryos primarily showed reductions in dry mass, total length, tail length and fin height. The most significant deviations from control (ψ v =0 kPa) values were observed when the ψ v of the incubation media was less than the osmotic water potential (ψ π ) of the embryonic interstitial fluid (approximately -425 kPa). Despite the caveat that a ψ v of 0 kPa is probably difficult to achieve under our experimental conditions, the findings indicate the importance for eggs under natural conditions of contacting liquid water in the nesting substrate to allow swelling of the capsule.

Geographic variation in adult and embryonic desiccation tolerance in a terrestrial-breeding frog

“…Low soil water potentials reduced the wet mass of hatchlings and increased the occurrence of hatchling malformations, in line with earlier studies (Taigen et al 1984;Mitchell 2002a;Andrewartha et al 2008;Eads et al 2012). Embryos were preserved 6 -12 hours after hatching, and therefore differences in wet weight are unlikely to be the result of variation in body water content as hatchlings had ample time to hydrate.…”

“…In adult anurans, hydration state affects signalling behaviours and reproductive success (Mitchell 2001), locomotor performance (Gatten 1987;Hillman 1987), and predator avoidance and the ability to catch prey (Titon et al 2010). Similarly, water availability has major effects on offspring fitness, particularly in terrestrial-breeding species, as the outer capsule of their eggs is almost completely permeable to water (Bradford and Seymour 1988;Mitchell 2002a;Andrewartha et al 2008). Terrestrial embryos that develop on relatively dry soils are typically smaller (Mitchell 2002a;Andrewartha et al 2008;Eads et al 2012), develop more slowly (Bradford and Seymour 1985), have reduced survival (Martin and Cooper 1972;Bradford and Seymour 1988;Eads et al 2012) and are more often malformed (Mitchell 2002a;Eads et al 2012).…”

Patterns of Genetic Variation in Desiccation Tolerance in Embryos of the Terrestrial-Breeding Frog, Pseudophryne Guentheri

“…Consistent rainfall is necessary to keep terrestrial embryos moist, to initiate hatching, and to maintain sufficient standing water to allow the completion of metamorphosis (Bradford and Seymour 1988a; Thumm and Mahony 2005). In contrast, in low rainfall conditions, small, underdeveloped, or anomalous tadpoles are produced, which may be competitively disadvantaged, have decreased swimming performance, and be at a higher risk of predation (e.g., Andrewartha et al 2008; Arendt 2010). Although the population used in this study is not obviously declining, the forecast reduction in precipitation across southwestern Australia is a potential threat, as low‐rainfall years have a serious impact on recruitment by limiting fertilization success and reducing rates of embryonic and larval survival (N. J. Mitchell, unpubl.…”

“…Amphibians are likely to be highly susceptible to climate change due to their physiological and ecological requirements for available water (Carey and Alexander 2003; Lawler et al 2010), low dispersal abilities (Beebee 1995; Lawler et al 2010), vulnerability to fungal diseases (Blaustein et al 1994; Carey and Alexander 2003), and because their habitats are at high risk under future climate changes (Solomon et al 2007; Lawler et al 2010). Terrestrial‐breeding frogs in particular require wet conditions for the successful development of embryos, as the outer capsule of their eggs is almost completely permeable to water (Bradford and Seymour 1988b; Mitchell 2002; Andrewartha et al 2008). Frogs inhabiting southern Western Australia may be especially susceptible to a drying climate, as this region has already experienced a substantial decline in rainfall since the 1970s (Smith 2004; Hennessy et al 2008).…”

A comparative study of divergent embryonic and larval development in the Australian frog genus Geocrinia (Anura: Myobatrachidae)

“…Hatched tadpoles of G. victoriana showed reductions in dry mass, total length, tail fin length and fin height when reared out of water, compared with those reared in hydrated situations which grew larger Andrewartha et al (2008). The few embryos of the G. laevis group that hatched later than all others in the present study showed less foraging vigour, developed more slowly post-hatching, and metamorphosed last, supporting the possible fitness benefits of earlier (and larger) hatchlings as suggested by the above authors.…”