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.

输卵管内的低氧环境可维持龟类在产卵前阶段的胚胎停滞状态，从而在不损害胚胎存活的前提下延长筑巢时间窗口。此类胚胎停滞可在产卵后通过将卵置于低氧环境中孵化得到延长。本研究旨在确认鳄目动物胚胎是否具备这一相同能力，同时探究孵化过程中氧气供应量升高是否会改变孵化成功率。我们将83枚新鲜产下的湾鳄（Crocodylus porosus）卵置于32℃环境下，分为5组处理：对照组（常氧，21% O₂）、3天低氧组（1% O₂）、6天低氧组（1% O₂）、3天高氧组（42% O₂）以及6天高氧组（42% O₂）。所有卵随后均在常氧环境下完成剩余孵化周期（约82天）。不同处理方式对胚胎孵化存活率具有显著影响（p < 0.001）。低氧处理组几乎无幼鳄孵化：3天低氧组与6天低氧组的存活率分别仅为6.7%与0%；而高氧组与对照组的孵化成功率则处于正常至较高水平：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，鳄卵经低氧孵化后并未出现发育延迟现象。本研究结果首次通过实验证实：与龟类不同，鳄目动物在产卵后立即置于低氧环境中孵化时，并不会出现胚胎停滞现象。胚胎停滞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利环境条件；同时，与龟类不同，胚胎停滞或许并非解释鳄目动物为何未演化出胎生现象的潜在机制。