Chelodina expansa

Chelodina (Chelydera) expansa, commonly known as the broad-shelled river turtle or the broad-shelled snake-necked turtle,[2] is a pleurodire freshwater turtle and is the largest of the long-necked turtles.[3] The broad-shelled river turtle is one of the oldest maturing and longest living species of freshwater turtles in existence[4] and occurs in wide sympatry with Emydura macquarii and Chelodina longicollis. C. expansa is listed as ‘vulnerable’ in South Australia and ‘threatened’ in Victoria.[3]

Broad-shelled long-necked turtle
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Pleurodira
Family: Chelidae
Genus: Chelodina
Subgenus: Chelydera
Species:
C. expansa
Binomial name
Chelodina expansa
(Gray, 1857)
Synonyms [1]
  • Chelodina expansa Gray, 1857
  • Chelodina expansas Goode, 1967 (ex errore)
  • Chelodina oblonga expansa Blackmore, 1969
  • Macrochelodina expansa Wells & Wellington, 1985

Distribution and habitat

Broad-shelled river turtles are found throughout the Murray-Darling River system of South-Eastern Australia. A number of distinctive populations have been located across central and coastal Queensland areas.[2]

C. expansa is mostly found in turbid waters at depths of which are greater than three metres.[3] The turtle generally inhabits permanent streams [2] and may occur in rivers, oxbows, ponds in floodplains, backwater, and swamps [3] across its distributed region, however is mostly a river turtle.[2] The broad-shelled river turtle will tend to inhabit environments that are undisturbed and have moderate vegetation cover for nest construction. The turtle has shown a preference of aquatic habitats in structured environments, where submerged logs, root systems and dead trees occur. Factors such as shelter from predators, and food availability may influence the habitat preference of C. expansa. Seasonal changes including the water levels and flow may also influence the selected the habitat.[3]

Description

The broad-shelled river turtle has a broad, oval and flattened shell with a length of around 50 cm.[2] The carapace length is often longer in females than males.[3] The turtle has a rich brown to blackish-brown above, typically displaying fine dark flecks or reticulations. The turtle has a whitish or creamy colored underneath. The plastron is narrow and the shell does not display any noticeable expansion anteriorly. The shell is usually twice as long as wide and is broadest at the level of the bridge. The head is broad and highly depressed and the eyes are directly dorsolateral.[2] When extended, the neck may surpass the measurement of the carapace.[3]

Behaviour

The broad-shelled river turtle is mostly active from the Australian spring season (October) through to early autumn (April). The species tends to decrease in movement and feeding over the cooler winter months.[3] C. expansa is described to also show head bobbing and self-grooming actions. [5]

Chelodina expansa spends most of its time in water.[4] During periods when water levels are low, C. expansa will traverse over land to find other water sources. The turtle may also bury itself into mud and enter extended periods of aestivation until rain occurs and water levels increase.[4]

Reproduction

C. expansa usually nests during the Australian autumn[2] or in early winter when soils decrease in temperature.[6] It will also sometimes nest during spring. Although the female broad-shelled river turtle will travel up to one kilometre away from the bank to lay her eggs, it is more common for them to nest within 100 m of the waters edge. Along the Murray River nests are often constructed at the top of sandy ridges.[3] In Autumn nestings, the nest temperatures will initially decrease and then remain low during the winter period before slowly increasing during spring to reach a maximum temperature during summer.[7]

The female turtle constructs a nest by excavating a nesting chamber with her back legs to a depth of around 20 cm. She then deposits between 5 and 28 eggs before backfilling the nest with soil. The broad-shelled river turtle has shown a ‘body-slamming’ type behaviour when compacting nests. [8] This behaviour results in the formation of a hardened 'plug' which helps to seal the nest. These turtles will nest any time of the day or night with nesting being initiated by rain. Turtles tend to select sites that have higher sun coverage to construct their nests. The selection of nest sites is often a long way from the waters edge and this is seen as a behavioural strategy to minimise the risk of nest flooding. Hatchlings are however exposed to a number of challenges during their travels to the water.[8] and the incubation period is exceptionally long due to inherently slow embryonic development,[6] averaging between 324 and 360 days.[3] The development of the embryo also slows the process as it enters a two distinct diapause stages.[3][6] The primary diapause stage occurs inside the female, the secondary diapause is triggered through a drop in temperature and occurs after the egg is laid and once a small period of growth has occurred.[3] The young will generally hatch during Spring,[2] Asynchronous hatching is most likely maladaptive in the natural environment [6] Upon hatching from the egg the young remain entombed underground in the nesting chamber, waiting for heavy rain to trigger their release by digging their way out through the hardened soil.

Diet

The broad-shelled river turtle is highly specialized and entirely carnivorous.[3] C. expansa primarily preys upon frogs,[9] crustaceans, aquatic insects, and small fish.[3]

Predators

The introduced red fox (Vulpes vulpes) and ravens (Corvus spp.) are the major threat to the nests of the broad-shelled river turtle. Predation of nests by feral foxes along the Murray River is in excess of 93% which means there is little recruitment of young turtles into an ageing population. As turtles are long lived animals, and adults are still commonly seen in the wild, this can lead people to falsely believe that the population is still healthy. Other nest predators include monitor lizards, ibis and feral pigs. Predation on nests is the main challenge for C. expansa, however it is thought that the turtle becomes aware of predators, mostly from olfactory and visual signs. Once the turtles have hatched and dug their way from the nest chamber they face a variety of predators while they travel overland to water; these can include birds and lizards in addition to feral foxes and cats. Many young turtles also die from dehydration if they fail to reach the water in time, and some are killed whilst trying to cross roads. Once they reach the water they are subject to more predators such as fish. In the disjunct Queensland populations, the main predator of hatchlings is the spotted barramundi.[3]

Threats

The broad-shelled river turtle is infected with parasitic adult worms (D. pearsoni and D. megapharynx). The worms appear to be host specific and live in the intestines of C. expansa.[10]

For photographs and distribution maps of Chelodina Expansa please see: https://commons.wikimedia.org/wiki/Category:Chelodina_expansa.

References
  1. Fritz Uwe; Peter Havaš (2007). "Checklist of Chelonians of the World" (PDF). Vertebrate Zoology. 57 (2): 335–336. ISSN 1864-5755. Archived from the original (PDF) on 2010-12-17. Retrieved 29 May 2012.
  2. Cogger, Harold (2014). Reptiles and amphibians of Australia. CSIRO Publishing.
  3. Ercolano, Ellyn (2008). "Aquatic and terrestrial habitat use of the Australian freshwater turtle, Chelodina expansa". Independent Study Project (Isp) Collection.
  4. Loudon, F.K.; Spencer, R.J. (2012). Applying theories of life history and ageing to predict the adaptive response of Murray River turtles to climate change and habitat. Wildlife and Climate Change: Towards robust conservation strategies for Australian fauna.
  5. Legler, J.M. (1978). "Observations on behavior and ecology in an Australian turtle, Chelodina expansa (Testudines: Chelidae)". Canadian Journal of Zoology. 56 (11): 2449–2453. doi:10.1139/z78-330.
  6. Booth, D (2002). "The breaking of diapause in embryonic broad-shell river turtles (Chelodina expansa)". Journal of Herpetology. 36 (2): 304–307. doi:10.2307/1566007. JSTOR 1566007.
  7. Booth, D.T. (1998). "Nest temperature and respiratory gases during natural incubation in the broad-shelled river turtle, Chelodina expansa (Testudinata: Chelidae)". Australian Journal of Zoology. 46 (2): 183–191. doi:10.1071/zo98003.
  8. Booth, D.T. (2010). "The natural history of nesting in two Australian freshwater turtles". Australian Zoologist. 35 (2): 198–203. doi:10.7882/AZ.2010.008.
  9. Chessman, B.C. (1983). "Observations on the Diet of the Broad-Shelled Turtle, Chelodina Expansa Gray (Testudines: Chelidae)". Wildlife Research. 10: 169–172. doi:10.1071/WR9830169.
  10. Jue Sue, L; Platt, T.R. (1999). "Description and life-cycle of three new species of Dingularis ng (Digena: Plagiorchiida), parasites of Australian freshwater turtles". Systematic Parasitology. 43 (3): 175–207. doi:10.1023/a:1006163819279.


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