Abstract
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 (approx. 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.
Highlights
Turtles and crocodilians exhibit a mixture of K- and r-selected species traits [1,2]
Eggs from the 6-day hypoxic treatment on average took 93 h longer than the control. Despite these observations being consistent with arrested embryonic development, there was a large variation in latency till opaque white spot formation for both of the hypoxic treatments, with approximately half of the eggs from each treatment forming spots prior to removal from hypoxia (7 of 15 for 3-day hypoxia and 7 of 14 for 6-day hypoxia; figure 1)
Once the time spent in hypoxia was accounted for (‘aerobic incubation time’; defined as total time excluding time spent in hypoxia) there was no significant between-group difference in latency to opaque white spot formation (H = 8.21, d.f. = 4, p = 0.08; figure 2b)
Summary
Turtles and crocodilians exhibit a mixture of K- and r-selected species traits [1,2]. They are all relatively long-lived species, with low adult mortality and take many years to reach maturity (Kselected traits) [3,4,5,6] They are all oviparous (egg-laying), produce a large number of small offspring and experience high rates of mortality during the life stages before maturity (r-selected traits) [7,8,9]. Crocodilians and turtles both lay eggs with relatively undeveloped embryos, having only reached the neurulation or gastrulation stages of development [10,11,12]. Because all crocodilians and turtles must deposit eggs into a nest, the reproductive success of the female and the phenotype of any resulting hatchlings are both heavily influenced by the abiotic and biotic conditions of the nest environment [14,15,16,17,18,19,20]
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