Data from: Ecological and evolutionary significance of a lack of capacity for extended developmental arrest in crocodilian eggs

Hypoxia within the oviducts maintains embryonic arrest in turtles at the pre-ovipositional stage, which expands the timeframe over which nesting can occur without compromising embryo survival. The arrest can be extended post-oviposition through incubation of eggs in hypoxia. We determined whether crocodilian embryos have this same capacity. We also tested whether increased oxygen availability during incubation alters hatching success. We incubated freshly-laid saltwater crocodile (Crocodylus porosus) eggs (N = 83) at 32°C in one of five treatments; control (normoxia; 21% O2), 3-day and 6-day hypoxia (1% O2), or 3-day and 6-day hyperoxia (42% O2). Incubation (~82 days) was then completed in normoxia. There was a significant effect of treatment on survival of embryos through to hatching (p < 0.001). The hypoxic treatments resulted in almost no hatching (6.7% and 0% survival for the 3- and 6- day treatments respectively), while the hyperoxic and control treatments resulted in normal to high hatching success (86.6%, 100% and 64.2% for the control, 3- and 6- day hyperoxic treatments respectively). Unlike turtles, hypoxic incubation of crocodile eggs failed to delay development. Our results provide the first experimental evidence that, unlike turtles, crocodiles do not exhibit embryonic arrest when incubated under hypoxic conditions immediately following oviposition. An absence of embryonic arrest is of ecological and evolutionary significance, as it implies that crocodilians lack an ability to avoid adverse environmental conditions through delayed nesting and that, unlike turtles, embryonic arrest may not be a potential explanation for the lack of viviparity in the order Crocodylia.

输卵管内的低氧（hypoxia）环境可维持龟类在产卵前阶段的胚胎阻滞（embryonic arrest），这使得龟类能够在不牺牲胚胎存活的前提下拓展筑巢的时间窗口。该胚胎阻滞现象可在产卵后通过将卵置于低氧环境中孵化得以延长。本研究旨在确认鳄类胚胎是否具备这一相同能力，并探究孵化过程中氧含量升高是否会影响孵化成功率（hatching success）。我们将83枚刚产下的湾鳄（Crocodylus porosus）卵置于32℃环境下，开展五种不同处理：对照组（常氧（normoxia），氧气浓度21% O₂）、3天低氧组、6天低氧组（氧气浓度均为1% O₂），以及3天高氧组、6天高氧组（氧气浓度均为42% O₂）。随后将所有卵转移至常氧环境中，完成剩余约82天的孵化周期。不同处理方式对胚胎孵化至破壳的存活率存在显著影响（p < 0.001）。低氧处理组的孵化效果极差，几乎无幼鳄破壳：3天低氧组与6天低氧组的存活率分别仅为6.7%和0%；而高氧（hyperoxia）组与对照组则展现出正常至较高的孵化成功率：对照组、3天高氧组及6天高氧组的孵化成功率分别为86.6%、100%与64.2%。与龟类不同，鳄类卵经低氧孵化后并不能延缓胚胎发育进程。本研究结果首次通过实验证实：与龟类不同，鳄类在产卵后立即置于低氧环境中孵化时，并不会出现胚胎阻滞现象。胚胎阻滞现象的缺失具有重要的生态学与进化学意义：这意味着鳄目动物无法通过延迟筑巢来规避不良环境条件；同时，与龟类不同，胚胎阻滞或许并非鳄目（Crocodylia）动物缺乏胎生（viviparity）现象的潜在解释机制。