Identifying conditions for ex-situ incubation of freshwater turtle (Chelodina oblonga) eggs to optimise hatching success

April L Sturm

The southwestern snake-necked turtle (Chelodina oblonga), endemic to south-western Australia is under threat from a range of human induced impacts, particularly in urban areas. Nesting females are particularly vulnerable to road mortality and predation, with eggs and juveniles also being consumed by both introduced and native predators. These threats have resulted in reduced recruitment of juveniles to urban populations and intervention is required to avoid localised extinctions. Ex-situ incubation of eggs has been identified as a key intervention to increase juvenile numbers, particularly in urban areas where natural recruitment rates are low. However, there is no research outlining ex-situ incubation requirements for eggs of this species. This study aimed to determine how incubation temperature regime and moisture levels influence hatching outcomes of C. oblonga eggs, as well as incubation periods, morphology, and post-hatching growth rates. Eggs of C. oblonga were collected both from natural nests and from recently deceased females and incubated in the laboratory under different temperature and moisture regimes. Four eggs from each cutch were randomly chosen for incubation and the remaining eggs were protected and left to incubate naturally in the field. Eggs were incubated under either a weekly fluctuating temperature regime (comparable to a natural nest temperature regime) or at a constant temperature of 28 °C (a frequently used temperature in reptile incubation that can decrease incubation time and alter normal physiological processes). Within each temperature regime, eggs were incubated on either a wet or dry substrate to determine whether moisture has any impacts on the incubation outcomes. Hatchlings were subsequently housed in an indoor aquarium and their growth rate measured daily (weight) and weekly (carapace length). Hatching success (i.e. eggs that hatched unassisted) was significantly greater under the fluctuating temperature regime at 73% and 60% for the wet and dry treatment, respectively, compared to 10% and 23% respectively, under a constant temperature regime. High numbers of late-stage embryonic deaths were observed in the constant temperature treatment. Post-mortem examinations found that these hatchlings were missing their caruncle (or ‘egg tooth’ which is required to hatch) and so, had become stuck inside their egg. Incubation period and time to white spot formation (a key early developmental stage) were also significantly longer for eggs in the fluctuating treatment than the constant temperature treatment, but the incubation period from white spot to hatching did not differ among the treatments. Weight at hatching, and neck length were all affected by the treatments with hatchlings from the fluctuating treatment being heavier with longer necks. Yolk sac size, limb size, growth rates and righting times were not significantly affected by the treatments. Eggs in the constant temperature treatment lost more weight at a greater rate than eggs under the fluctuating temperature regime. Moisture only affected egg weight loss, with eggs in the dry treatment losing more weight, more rapidly. Importantly, hatching success was higher in the fluctuating treatment in the laboratory than for eggs in nests left to incubate naturally at Bibra Lake. These findings demonstrate that ex-situ incubation of C. oblonga eggs can be done successfully under optimal conditions and is a viable option for any future programs attempting to increase juvenile recruitment in declining populations of C. oblonga.

Identifying conditions for ex-situ incubation of freshwater turtle (Chelodina oblonga) eggs to optimise hatching success

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