This distinctive species of freshwater turtle was only described by scientists in 1994. “This turtle was of great interest to the International turtle community as turtles of similar size were known of a century before” according to Dr Peter Pritchard, a world-renown turtle scientist. As one of Australia’s largest freshwater turtles, it is intriguing how this species evaded scientists for so long.
Name: Mary River turtle
Scientific name: Elusor macrurus
Conservation status: Critically endangered
Geographic range
It is restricted to the Mary River, Queensland, mostly inhabits the main river trunk and larger creeks.
Description
As an adult, it has a low streamlined shell (unique amongst Australian turtles), a moderately short neck, webbed fore and hind limbs. The upper side of the shell (carapace) is dark olive to almost black, and underneath the shell (plastron) is a bone colour. The head and legs are grey in colour. The crown of the head is dark and distinctly smooth. The rear legs may have a pinkish tinge. The tail of adult males is massive being about 200 mm in length and about 260mm circumference. The structure of the bones within the tail (caudal vertebrae) that form a hook which is unique amongst modern turtles worldwide.
Size
Hatchlings are approx. 33mm (straight line length of carapace, SCL). The largest adult recorded was 436mm (SCL) and weighed 8.08kg.
Sexual dimorphism
Adult males are significantly larger than females.
Diet
They feed on algae, macrophytes, aquatic insects, bivalves and gastropods.
Reproduction
Age at maturity is unknown but indications suggest about age 20.Nesting occurs after spring rains and continues into summer. Females produce a single clutch per year with incubation taking from 50 – 79 days. An average clutch size is 15 eggs. The sex of hatchlings is not determined by temperature during incubation.
Conservation genetics
Microsatellite diversity of the wild population is low (average Hs = 0.554). Mitochondrial DNA diversity is extremely limited with only two haplotypes found in the wild.
Habitat and Ecology
The Mary River is a dynamic river system. Most of the flow occurs during austral summer and early autumn when river heights can vary significantly, while during the winter months, the flow is relatively stable.
The Mary River turtle primarily inhabits riffle zones and pools in the river. It requires foraging areas (mostly riffle zones), suitable basking sites (emergent rocks and large logs), sites for nesting (open areas on the riverbank), and nocturnal resting areas which provide safety from predators.
The Mary River turtle has two methods (bimodal) of respiration. When it comes to the surface it uses its lungs. Within its tail is a deep cavity lined with gill-like structures that are used for extracting oxygen from the water. This allows the turtle to stay submerged for longer periods and has led to it being known as a ‘bum-breather”. A hatchling has been recorded remaining submerged for 2.5 days in optimal temperature and oxygen conditions.
Pet trade
From 1960 to 1974, up to 12,000 eggs were collected from nesting bank each season. Once hatched, the tiny hatchlings were sold as ‘penny turtles’ through the pet trade. No-one knew these belonged to an unknown species.
Threats to survival
Habitat modification: Their habitat has been modified by over a century and a half of human activity. The Gympie gold rush began in 1867. Unnaturally large quantities of sediment choked the river from the gold rush period until 1904. Channels had to be excavated within the sediment to permit the river to flow. Several mercury-contaminated sites remain around Gympie. Sand and gravel extraction from instream and the floodplain increased fourfold during the 1970s and 1980s to meet the needs of the building industry. Extensive areas of the riparian zone have been altered by agricultural and urban land use. This has led to a depletion of large woody material throughout the main trunk of the river. All these activities in combination with major floods (1895, 1955, 1968, 1974, 1989, 1992, 1999, 2011 and two in 2013) have caused a significant increase in the sediment load in the river. The construction of a tidal barrage downstream of Tiaro in 1982 drowned out numerous riffle zones and converted 35km of flowing water into still water. It is likely that the instream conditions consequent of the construction of the barrage has favoured fork-tailed catfish (Arius graeffei), a known predator of Australian freshwater turtles.
Since the 1970s, a dam on the Mary River has been considered a viable option to augment the water supply for the Greater Brisbane region. The Queensland government has identified an unallocated strategic water reserve of 150,000 ML in the Mary Basin. This has the potential to reduce flows in the main channel and further degrade the habitat of the Mary River turtle.
Introduced plants and animals: Numerous plants and animals have been introduced to the Mary River that have had a major impact on wildlife. Rafts of invasive floating aquatic plants such as salvinia (Salvinia molesta) and water hyacinth (Eichhornia crassipes; during 2007, rafts of 18km in aggregate formed) modify water quality through reduction of light, temperature and dissolved oxygen. The introduction of native and non-native predatory fish species such as sooty grunter (Hephaestus fuliginosus), saratoga (Scleropages leichardti), and golden perch (Macquaria ambigua) are a threat to aquatic wildlife. Predation of nests by the introduced European red fox (Vulpes vulpes) appears to have increased since the 1960s. Wild domestic dogs and monitor lizards (Varanus panoptes, V. varius) predate turtle nests. Trampling of nesting banks by livestock during the nesting season is likely to impact on the developing embryos.
Recreational activities: The extent of mortality from recreational activities such as the deliberate killing of turtles caught on fishing lines or the number killed by motor boat strike is unknown.
Climate change: Researchers found that elevated temperature (32°C) during incubation resulted in lower hatching success, smaller hatchlings, reduced post-hatch growth rate, and they were less fit, i.e. took longer to right themselves on land, had lower mean stroke force and spent less time swimming.
Did you know that the Mary River turtle is the 29th most endangered reptile in the world according to the Zoological Society of London’s EDGE program (Evolutionary Distinct and Globally Endangered)??
Note: References used in compiling this information are included are included in the Resource Library.
