Distinguishing Between the Three Most Common Water Skink (Eulamprus) Species
Members of the genus Eulamprus, the Water Skinks, inhabit most of the east and south-east coast of Australia, including regions along the east end of the Victoria/New South Wales border, and around Adelaide. They often inhabit moist environments close to creeks and streams, but despite their name, they are regularly seen in locations away from water bodies, including around suburban residences, taking up residence under logs or under houses and so on. There are three commonly seen species, the Eastern Water Skink (Eulamprus quoyii), the Yellow-bellied Water Skink (E. heatwolei) and the Southern Water Skink (E. tympanum). There are, at the time of writing, two recognised subspecies of Southern Water Skink, (E. tympanum ssp. tympanum and E. tympanum ssp. marnieae), which I will not be addressing in this text. Furthermore, there are also two less frequently and more geographically restricted species, the Alpine Water Skink (E. kosciuskoi) and the Blue Mountains Water Skink (E. leuraensis), which are distinct enough to be identified easily.
Despite their frequent observation, many are having difficulty identifying which species of Water Skink they are looking at. There is quite a broad range of variety within each species, as well as overlap between species to add to the confusion. Here I will describe some of the diagnostic characters of the three most commonly seen, and frequently confused, species. At the end is also a dichotomous key I've written that may also be of assistance. The characters referred to is not a complete list of morphological features of each species, but those that help readily identify each one.
Eastern Water Skink (E. quoyii). https://inaturalist.ala.org.au/taxa/100709-Eulamprus-quoyii
The easiest and most reliable feature to identify Eastern Water Skinks is the clear, pale/yellow dorso-lateral stripe, which starts just above their eye-brow and extends down the length of their body, fading out by the time it reaches their hips. Unfortunately this feature can often be obscured by sunlight or camera flashes reflecting off the animals scales. Southern Water Skinks do not possess this stripe at all, but it can be observed in Yellow-bellied Water Skinks very faintly as a broken series of dots. Alpine and Blue Mountains Water Skinks also possess dorso-lateral lines, but are easily distinguished from Eastern Water Skinks by the overall dark, near black complexion of the Blue Mountains Water Skinks, and the Dark vertebral stripe of the Alpine Water Skink.
Yellow-bellied Water Skink (E. heatwolei). https://inaturalist.ala.org.au/taxa/38492-Eulamprus-heatwolei
In both Yellow-bellied and Southern Water Skinks, there is a mid-lateral division between their lighter lower lateral colour and the darker upper lateral colour extending down the length of their sides. This 'line' between the two intersects the tympanum (ear-hole). The pale colour hooks around the anterior and upper margin of the tympanum in Southern Water Skinks, whereas in Yellow-bellied Water Skinks it does not, and merely cuts straight across the ear-hole. Some specimens will have a pale coloured patch intersect the upper ear, providing the chance that E. heatwolei may be confused for a E. tympanum, usually there will be a dark division along the anterior margin of the ear to help make an identification.
Yellow-bellied Water Skinks also tend to have notably more dark banding over their eyes compared to Eastern or Southern Water Skinks.
Despite the name, the yellow colouring of the belly and of the legs is not unique to Yellow-bellied Water Skinks, and can be observed in the other species as well. I suspect it is likely breeding season colouration. The yellow can also be absent in Yellow-bellied Water Skinks to add to the confusion.
Southern Water Skinks (E. tympanum). https://inaturalist.ala.org.au/taxa/38483-Eulamprus-tympanum
In Southern Water Skinks, the snout appears to be proportionally shorter than the other species. They completely lack the aforementioned dorso-lateral stripe of the other species. The ventral colour hooks around the anterior margin of the tympanum, the upper posterior margin is dark.
Male vs. Female
I'm not aware of any morphological features that can be observed from a distance that distinguishes the sexes; however, the females can be identified when pregnant, as they will be noticeably wider than usual. The females give birth to live young rather than laying eggs like many other reptiles.
Key to Eulamprus:
1 Clear dorso-lateral stripe present
.......................................................................................................................................................................(Go to 2)
1* Dorso-lateral line absent or only marginally visible as a series of dashes or dots
.......................................................................................................................................................................(Go to 4)
2 Clear, dark vertebral line; yellow dorso-lateral stripe present, margined by dark colour on ventral side, presenting appearance of three dark lines running along the back
.......................................................................................................................................................................E. kosciuskoi
2* No vertebral line present
.......................................................................................................................................................................(Go to 3)
3 Very dark, near black, overall appearance, with yellow stripes/dots on lateral surface
.......................................................................................................................................................................E. leuraensis
3* Dark upper lateral region with light cream spots; lower lateral region has cream/pale coloured patching; cream coloured ventral surface; brass-brown to copper-brown dorsal surface with many to few dark spots; clear pale yellowish dorso-lateral stripe
.......................................................................................................................................................................E. quoyii
4 Pale ventral colour intersects, and hooks around the anterior margin of the tympanum (ear-hole), dark on the upper posterior margin (not to be mistaken with upper lateral patches that also may intersect ear-hole); no dorso-lateral line present; proportionally shorter snout
.......................................................................................................................................................................E. tympanum
4* Pale ventral colour intersects tympanum, does not hook around or border the margins of the upper half of the ear hole; dark eye-band usually present
.......................................................................................................................................................................E. heatwolei
Note: I would of liked to add annotated images for reference, but there doesn't appear to be an option to embed images into posts. I'm also happy to make corrections where necessary.
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