Tuesday, February 4, 2014

The Skink and the Crab - life in a Western Australian Salt Marsh


Photo credit R. Lloyd.
The striped slinks of the genus Ctenotus are the most diverse lizard clade of Australia with more than 100 known species. The Airlie Island Ctenotus is a dark olive-grey to light brown skink with a 64–69 mm body. Until recently it was known from the island and two mainland location.

The skink is threatened by buffel grass, an invasive plant that results in the loss of skink habitat. The weed has been successfully controlled on Airlie Island however, it is unknown whether it has been eradicated and if there is a future risk of habitat loss is on the mainland.

Human disturbance is also a threat to the skink with 3 ha or about 12% of the skink's range cleared of vegetation on Airlie Island  for the development of oil and gas processing facilities.

Many of the mainland skink populations are located close to the shoreline, and it is likely that rising sea levels and floods associated with climate change could impact the species.

Somaweera and Lloyd. (2014) conducted surveys for the skink and found populations along the northwestern coast, which appear to be highly restricted to specific habitats, primarily salt marsh communities adjacent to mangroves where lizards use crab holes for refugia. Genetic analysis of mitochondrial DNA showed very little variation between populations, suggesting a single widespread population or recent radiation.

Ctenotus angusticeps was previously recorded in most habitat types on Airlie Island, such as tussock grasslands and Acacia coriacea shrubland with coastal Spinifex  however then authors focused their survey efforts in salt marsh communities because of observations previously reported in the literature.  The landward fringe of mangroves are numerous and patchy along the northwestern coast. These are quite different from the habitats on Airlie Island and suggest the salt marsh communities are the ancestral habitat for this species, with the island population secondarily adapting to the habitat now present on the island. The authors observations confirm the Airlie Island striped skink is very habitat specific and they report it to be highly dependent on crab holes.

Wherever  the skink was found, crab holes of varying sizes were present and these were used by skinks to evading  attempts at capture. Initially, when adult lizards were observed they hid at the base of low vegetation, followed by retreating down a hidden hole among the samphire and marine couch grass. Juvenile
lizards usually relied on the protective cover of the vegetation to avoid capture, often taking refuge in the dense tussocks of marine couch grass. Both adults and juveniles were sometimes observed to retreat into samphire vegetation, sometimes up to 30 cm above the ground.

This species seems to be opportunistic in selecting retreat sites upon pursuit. To test whether lizards would use crab holes even in the absence of perceived threat the authors used a non-toxic fluorescent powder to track their movements. Two adult C. angusticeps caught early in the day were dipped in the ‘powder’ and
released. The next morning they tracked their movements from the previous day to separate crab holes where the lizards spent the night demonstrating the lizards use crab holes by choice.

The authors also report seeing lizards enter crab holes during rising tides and apparently remained in the burrows until the tide had receded - presumably surviving in air pockets in the burrows. The author's suggest this needs further investigation.

The  high frequency of crab hole usage by lizards in the salt marsh communities leads the authors to suggest crabs holes are an essential requirement for this species. Mangrove crabs could be considered  ecosystem engineers, a group of animals that physically create, maintain and modify their environment. Hence the spatial distribution and abundance of species that use these holes could vary with the distribution of the particular species of crabs.

Citation (on-line, no publisher given)
Somaweera MBR. R. Lloyd R. 2014. Status of the Airlie Island Ctenotus, Ctenotus angusticeps (Lacertilia: Scincidae), with notes on distribution, habitat and genetic variation. 

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