Comparing the reproductive biology of three “marsupial”, eu-viviparous gastropods (Cerithioidea, Thiaridae) from drainages of Australia's monsoonal north

Abstract

In molluscs, as often found also among other groups of animals, two distinct reproductive modes are realised, viz. oviparity and viviparity. The latter frequently occurs in various lineages of freshwater gastropods, among them the “marsupial” Thiaridae with a pantropical distribution including Australia. Here the family was found to comprise at least 11 species, of which five are endemic to the continent. Of those two are highly restricted to only the central northern most region, viz. Melasma onca and Sermyla venustula. In contrast, “Thiara” australis reveals a wider range in Australia’s monsoon tropics. Brood pouch contents of these three thiarids are analysed here in order to infer details of their reproductive modes in relation to the geographic origin of individual populations, seasonality and the biometry of their shells. We found pronounced species-specific differences in the reproductive modes of all three thiarids, with M. onca having significantly more progeny in the brood pouch compared to S. venustula and“Thiara” australis. At the same time, its progeny are smaller and hatchlings are less advanced than in the latter two species. In addition, we report drainage-based intraspecific differences in the first two species. Accordingly, the head-water region of the Roper River, which drains into the Gulf of Carpentaria, apparently provides better environmental conditions that allow both M. onca and S. venustula to produce more progeny with an increased shell height as compared to popoulations from other drainages within the two species’ ranges. We suggest, in accordance with earlier studies, to differentiate more than only one mode within eu-viviparous reproductive strategies in thiarids. Studying the influence of various factors potentially correlated with interspecific as well as intraspecific differences in their reproductive biology, such as female shell size and drainage specific habitat, we show that Australian thiarids are more flexible and adaptable than previously known (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)