Northern snake-necked turtle (Chelodina oblonga)

About Chelodina oblonga

In Australia C. rugosa (previously C.oblonga) is widely distributed throughout the northern wet-dry tropics (Kennett et al 1993) and occurs from the Kimberley region of Western Australia to Cape York Peninsula, Queensland (Cogger 2014). Chelodina rugosa has been recorded from both the Gilbert and Flinders catchments of the Gulf of Carpentaria (University of Canberra 2014; DSITIA 2014).  Chelodina rugosa inhabits swamps, billabongs, waterholes and slow-flowing rivers throughout its range (Cogger 2014). C. rugosa could be considered a floodplain specialist as the species displays a preference for ephemeral waters with the highest densities occurring on seasonally inundated, shallow (<2 m), vegetated floodplains (Kennett et al 1993). During the subsequent dry season populations retreat to more permanent waters, however if those refuges continue to dry reproductive activity is curtailed and turtles seek refuge in aestivation sites (Kennett et al 1993). C. rugosa aestivates underground after burying itself in the muddy bottom of receding water bodies (Kennett et al 1993).

Most freshwater turtles leave the water to lay their eggs so turtles inhabiting the tropics often have only a very short period which to find the relatively dry ground suitable for nesting (Georges et al. 1993). Chelodina rugosa however, has developed an unusual nesting strategy whereby it can lay its eggs underwater (Kennett et al., 1993a; Georges et al. 1993). Eggs are deposited in nests dug into the edges of waterholes that it inhabits (Kennett et al., 1993a). This species’ ability to lay its eggs underwater represents an important advantage, giving the embryos a developmental ‘head start’ over other turtle species in the area, helping to ensure that hatchling emergence is timed to coincide with benign flow conditions (Cann 1998). Hatchling growth in C. rugosa is characterized by two phases. There is an initial phase of relatively slow growth under the partial influence of initial egg size and incubation duration, followed by a second phase of relatively rapid growth under the partial influence of water temperature and mass at hatching (Fordham, unpubl. Data).

Chelodina rugosa is highly fecund, laying between 6 and 21 eggs per clutch (Kennett 1999) and potentially up to 3–4 clutches of eggs per season (Fordham, unpubl. data). Eggs are hard shelled and ellipsoid in shape (Kennett et al 1993). Eggs can survive inundation because they can undergo embryonic diapause until the waters recede and the inundation-induced hypoxia ends (Kennett et al 1993a). Eggs that have not begun to develop can survive immersion for up to 12 weeks under experimental conditions but die if immersed after embryonic development has commenced (Kennett et al 1993a; Kennett et al 1993b). Immersion of nest containing partially developed eggs represents a significant risk to C. rugosa (Georges pers. comm.). Late-term embryos may prolong their stay in their egg until favourable conditions return by entering a form of aestivation but the cost of maintaining a fully developed embryo may be high and cannot be maintained indefinitely (Kennett 1999). Post hatching survival is negatively correlated with duration of egg inundation and water temperature. Evidence suggests that inundation of C. rugosa eggs for 6 weeks, incubation of embryos at 28 °C and raising hatchlings in 28 °C water will yield the best overall outcomes (Fordham et al 2007). The combination of a narrow window in time suitable for nesting and extreme unpredictability in when the window is open would make it very difficult for C. rugosa to persist in the floodplains if its nesting requirements were similar to those of other freshwater turtles (Kennett et al 1993). The ability of C. rugosa to nest in flooded ground during the wet season allows it to exploit a longer nesting period, and potentially produce more clutches per year, than would be the case if dry nesting sites were required and females were unable to ‘store’ eggs in the ground (Kennett et al 1993).

The life history strategies that this turtle species has developed have made them tolerant of variable climatic and flow conditions however alterations to hydrology do have the potential to represent risks to their long-term persistence. For example, an unseasonal rise in water level as a result of flow supplementation may inundate northern snake-necked turtle nests and cause egg mortality during periods where nesting sites would normally be drying (Cann 1998). For this reason, C. rugosa has been selected as an ecological asset for the Gulf plan area, reflecting it’s strong dependence on various aspects of the flow regime and vulnerability to infrastructure development that has the potential to disrupt these natural flows.