Urodacus

Australia’s own scorpions
Published 23 September 2023

Popular lore represents the continent of Australia as some sort of death world, a land over-run with creatures (and a few plants) whose only goal in life is the extermination of unwitting trespassers within their demesne. This, of course, is largely rubbish; in most parts of Australia, it’s not the wildlife you have to worry about but the weather. Nevertheless, the Australian fauna is famed for its venomous snakes, its venomous spiders, its venomous fish. But one element that tends to get lost in the mix when discussing Australia’s purveyors of toxicity is its scorpions.

Inland scorpion Urodacus yaschenkoi, copyright Mark Newton.

Among the most distinctive genera of scorpions in Australia is Urodacus. This genus of about twenty known species (likely with more to be described) is unique to Australia; a supposed specimen from Saipan was regarded by Koch (1977) as mislabelled. Urodacus is placed in a family Urodacidae whose only other described member, the troglobitic Aops oncodactylus, is also found in Australia. The broader relationships of the family are uncertain though the urodacids are clearly part of the katoikogenic Scorpionoidea. In this clade of scorpions, embryos are not supported by a yolk prior to birth and instead receive nutrients through outgrowths of the ovariuterus onto which they attach by their chelicerae (Sharma et al. 2015), a sort of combination placenta/nipple. Urodacids may form a clade with the Madagascan genus Heteroscorpion, but the validity of this association remains disputed.

Urodacus planimanus (a pregnant female?), copyright Jean and Fred Hort.

Within Australia, Urodacus species can often be readily distinguished from other scorpions by their sheer size. Many Urodacus grow well over ten centimetres in length whereas other Australian scorpions tend to be much smaller. Other distinctive features include the presence of two pairs of lateral eyes on the carapace, a pentagonal sternum, and a single ventro-median keel on the first four segments of the tail (Koch 1977). Despite their size, Urodacus species are not commonly observed due to spending most of their time in burrows underground. In species associated with rocks, such as the unusually small U. manicatus, these burrows are relatively shallow and lead immediately to an expanded ‘living chamber’ under the rock. In species constructing burrows in open ground, burrows spiral downwards before reaching the living chamber; in U. yaschenkoi, the terminal chamber may be as much as a metre below ground (Koch 1978). The scorpions only emerge from their burrows to hunt when conditions are ideal, generally on dark moonless nights. The amount of time they can remain in their burrows without feeding can be remarkable. A colleague once told me that he had been keeping an individual (collected as part of a survey) in a terrarium in his garage but had not seen her for a while; when the entrance to her burrow collapsed, he assumed she had died. However, as is the way of things, he did not get around to cleaning away the terrarium for several months. As he dug the soil out of the tank, he was astounded to find the scorpion, still occupying her chamber and immediately responsive to being disturbed!

Black rock scorpion Urodacus manicatus, copyright Andrew Allen.

The primary reason scorpions tend to get overlooked when tallying Australia’s dangerous fauna is that, compared to species found elsewhere, Australian scorpions are not considered particularly dangerous. Only two human mortalities have been attributed to scorpions in Australian history, both involving infants and neither involving Urodacus (Nicholson et al. 2006). Stings from Urodacus may cause significant pain, but pain remains localised and fades within a few hours; no long-term effects seem to have been recorded. Whereas other non-burrowing scorpions may rely on their venom for hunting and defense, Urodacus place more reliance on the crushing strength of their pedipalps and the security of their burrows. Contrary to what you might have heard about the Australian wildlife, Urodacus species are but gentle giants.

References

Koch, L. E. 1977. The taxonomy, geographic distribution and evolutionary radiation of Australo-Papuan scorpions. Records of the Western Australian Museum 5 (2): 83–367.

Koch, L. E. 1978. A comparative study of the structure, function and adaptation to different habitats of burrows in the scorpion genus Urodacus (Scorpionida, Scorpionidae). Records of the Western Australian Museum 6 (2): 119–146.

Nicholson, G. M., A. Graudins, H. I. Wilson, M. Little & K. W. Broady. 2006. Arachnid toxinology in Australia: from clinical toxicology to potential applications. Toxicon 48: 872–898.

Sharma, P. P., R. Fernández, L. A. Esposito, E. González-Santillán & L. Monod. 2015. Phylogenomic resolution of scorpions reveals multilevel discordance with morphological phylogenetic signal. Proceedings of the Royal Society of London Series B—Biological Sciences 282: 20142953.

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