The effect of respiratory gases and incubation temperature on early stage embryonic development in sea turtles

Abstract

Sea turtle embryos at high-density nesting beaches experience relative high rates of early stage embryo death. One hypothesis to explain this high mortality rate is that there is an increased probability that newly constructed nests are located close to maturing clutches whose metabolising embryos cause low oxygen levels, high carbon dioxide levels, and high temperatures. Although these altered environmental conditions are well tolerated by mature embryos, early stage embryos, i.e. embryos in eggs that have only been incubating for less than a week, may not be as tolerant leading to an increase in their mortality. To test this hypothesis, we incubated newly laid sea turtle eggs over a range of temperatures in different combinations of oxygen and carbon dioxide concentrations and assessed embryo development and death rates. We found that gas mixtures of decreased oxygen and increased carbon dioxide, similar to those found in natural sea turtle nests containing mature embryos, slowed embryonic development but did not influence the mortality rate of early stage embryos. We found incubation temperature had no effect on early embryo mortality but growth rate at 27°C and 34°C was slower than at 30°C and 33°C. Our findings indicate that low oxygen and high carbon dioxide partial pressures are not the cause of the high early stage embryo mortality observed at high-density sea turtle nesting beaches, but there is evidence suggesting high incubation temperatures, particularly above 34°C are harmful. Any management strategies that can increase the spacing between nests or other strategies such as shading or irrigation that reduce sand temperature are likely to increase hatching success at high-density nesting beaches.

Transcriptomic analysis of preovipositional embryonic arrest in a nonsquamate reptile (Chelonia mydas)

“…With regard to turtles, hypoxia has been shown to upregulate HIF-1α in lung epithelial and dermal cells cultured from green sea turtle embryos 3-4 weeks after oviposition (Barlian & Riani, 2018). Mild hypoxia and hypercapnia after breaking of arrest was also shown to slow development in both green sea turtles and loggerhead turtles (Booth et al, 2020).…”

Increasing hypoxia progressively slows early embryonic development in an oviparous reptile, the green turtle, Chelonia mydas

“…During the 72 h treatment in hypoxia, white spots formed on 45% of eggs in the 3% oxygen treatment, suggesting that the embryos within these eggs were developing [ 1 , 30 ]. However, the latency between oviposition and appearance of the white spot was progressively extended by graded hypoxia, providing further evidence that embryonic development is slowed under hypoxic conditions [ 47 ]. The discovery in the current study that 3% oxygen is sufficient for turtle embryos to break arrest and resume development has important implications for research projects that use hypoxia as a method for safe transportation of turtle eggs [ 25 ].…”

A review of the effects of incubation conditions on hatchling phenotypes in non-squamate reptiles

“…Even a few hours of hypoxia can reduce hatching success (Ackerman 1981 ; Pike et al 2015 ), as can maintaining embryos in hypoxia-induced arrest for extended periods (Table 3 ; Rafferty et al 2013 ). Hypoxia, when combined with hypercapnia (see below), can decrease embryonic growth rates (Booth et al 2020 ). Low oxygen and high carbon dioxide concentrations occur regularly in nests (Booth et al 2020 ; Lutz and Dunbar-Cooper 1984 ) and thus, it is likely that gas concentrations regularly slow development rates.…”

“…Hypoxia, when combined with hypercapnia (see below), can decrease embryonic growth rates (Booth et al 2020 ). Low oxygen and high carbon dioxide concentrations occur regularly in nests (Booth et al 2020 ; Lutz and Dunbar-Cooper 1984 ) and thus, it is likely that gas concentrations regularly slow development rates. It is unlikely that developing embryos experience hyperoxia (i.e.…”

“…Generally, factors that limit oxygen entry also limit carbon dioxide removal, which in buried nests leads to reduced oxygen concentrations near the centre of egg clutches (Ralph et al 2005 ) and increased carbon dioxide levels (Ackerman et al 1997 ). Studies that control oxygen concentration while manipulating carbon dioxide concentrations are limited, although laboratory research on freshwater and sea turtles have shown that higher carbon dioxide levels (10–15%) result in female-biased sex ratios (Table 1 ), slower development rates, longer incubation durations and smaller hatchlings with larger residual yolks compared to low carbon dioxide levels (0–5%) (Booth et al 2020 ; Etchberger et al 2002 , 1992 ). In natural nests, embryonic carbon dioxide production (Booth 2000 ) and concentrations around the eggs increase throughout incubation (Lutz and Dunbar-Cooper 1984 ).…”