Diversity and disparity ‘down under’: Systematics, biogeography and reproductive modes of the ‘marsupial’ freshwater Thiaridae (Caenogastropoda, Cerithioidea) in Australia

Abstract

We systematically revise here the Australian taxa of the Thiaridae, a group of freshwater Cerithioidea with pantropical distribution and “marsupial” (i.e. viviparous) reproductive modes. On this long isolated continent, the naming of several monotypic genera and a plethora of species have clouded both the phylogenetical and biogeographical relationships with other thiarids, in particular in Southeast Asia, thus hampering insight into the evolution of Australian taxa and their natural history. Based on own collections during five expeditions to various regions in Australia between 2002 and 2007, the study of relevant type material and the comparison with (mostly shell) material from major Australian museum collections, we describe and document here the morphology (of adults and juveniles) and radulae of all relevant thiarid taxa, discussing the taxonomical implications and nomenclatural consequences. Presenting comprehensive compilations of the occurrences for all Australian thiarid species, we document their geographical distribution (based on over 900 records) with references ranging from continent-wide to drainage-based patterns. We morphologically identify a total of eleven distinct species (also corroborated as distinct clades by molecular genetic data, to be reported elsewhere), of which six species are endemic to Australia, viz. “Thiara australis ”, Plotiopsis balonnensis, and “Stenomelania ” denisoniensis with wide distribution and Melasma onca, Sermyla venustula, and Ripalania queenslandica with more restricted ranges. In contrast, Thiara amarula and Stenomelania cf. aspirans as well as Melanoides tuberculata, Plotia scabra, and Sermyla riqueti are widely distributed also outside Australia, in particular in the Malay Archipelago and the Indo-West Pacific, respectively. The occurrences especially of the latter three species are discussed, concluding on quite distinct historical contingencies. Three thiarids species, viz. Stenomelania cf. aspirans, Sermyla riqueti and Plotia scabra, are recorded here for the first time for Australia. Based on a new taxonomic framework for the Thiaridae we point out some of the pertinent problems with naming and artificial delineation of species, revealing why typology and earlier practice of splitting was misleading in case of these truly “Darwinian snails”. We also report on finding two clearly distinct viviparous modes in Australian thiarids, discussing their distribution in certain fluvifaunal provinces and major drainage systems in concert with these reproductive features of life history tactics. While the live-bearing T. amarula, S. cf. aspirans, and R. queenslandica, that release veligers (ovo-viviparity), are found to have very restricted occurrences in rivers and streams in the Jardinian province of NE Queensland only, the five more widely distributed Australian endemics Plotiopsis balonnensis and “Stenomelania ” denisoniensis as well as “Thiara australis ”, Sermyla venustula and Melasma onca in the Leichhardtian province all brood and release shelled juveniles (eu-viviparity). Finally, we hypothesize on the evolutionary history and colonization of Australia by different lineages of these freshwater Cerithioidea. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)