Here's an interesting bit of snail biogeography. This is Rhachistia histrio (Pfeiffer) from Forty Mile Scrub in NE Queensland. This species is found over a wide stretch of coastal Queensland. It also occurs in Vanuatu and New Caledonia. And it might have been introduced to both places from Africa.In his 1959 study of land and freshwater molluscs of Vanuatu, American malacologist Alan Solem suggested that R. histrio had been brought to Vanuatu from Madagascar or eastern Africa. From there, it had spread to New Caledonia and Queensland with the transport of indentured labour.
It's an intriguing tale but the evidence for translocation is flimsy. Solem listed the localities of R. histrio in Queensland as Port Denison (= Bowen region), Mt Dryander, Port Curtis, Bundaberg and Maryborough. He wrote:
All are near the active centers for the blackbirding trade between the New Hebrides and Queensland during the late 1800s. Thus there was ample opportunity for accidental importation either to or from Queensland.
He synonymised R. histrio with Bulimus bidwilli, a species described in 1868 by Cox, who believed it to be native. Although B. bidwilli had been collected in natural areas (not the usual pattern for introduced species), Solem dismissed this. He argued that at least one species of Indian Rhachistia introduced into Africa was now found out in the bush. Why shouldn't this one be capable of the same thing?For a start, it's by no means clear that R. histrio from Africa is the same species as R. histrio from the Pacific. That's the key to the whole story.
And what's the relationship between the populations in Australia, New Caledonia and Vanuatu? Have they been shifted around by human intervention? Or is R. histrio just a widespread species? Or is it really two or three different species with similar shell patterns? (It happens!)
Now there's a great hypothesis just waiting to be tested by a keen molecular biology student. So what are you hanging around here for?
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Solem, A. (1959). Systematics and zoogeography of the land and freshwater Mollusca of the New Hebrides. Fieldiana: Zoology 43: 1–359.
