For the study of biogeography, particularly for testing vicariance scenarios, it certainly helps if the a priori likelihood of frequent dispersal in a group is low. Based on this premise, some groups of freshwater molluscs should present ideal candidates for biogeographic testing across wide geographic and geological time scales.
There are two major groups of freshwater gastropods, pulmonates (having a lung) and prosobranchs (with a gill). The prosobranchs are not a natural group in the sense of being monophyletic, though. There are some evolutionary differences between pulmonates and prosobranchs which are relevant for biogeography, e.g. freshwater pulmonates are known to be less speciose than their freshwater ‘prosobranch’ counterparts, which also have been calculated to be 1.5 to 10 times more endemic. In general, pulmonates have the largest range sizes of all freshwater gastropods. Nonetheless, despite widely distributed taxa, there are many restricted and even narrowly endemic groups both among pulmonates and prosobranch gastropods.
Pachychilidae: This family of prosobranch freshwater gastropods (Caenogastropoda: Cerithioidea) is distributed with about 10 genera and more than 200 species throughout the tropics. Diversity is highest in Southeast Asia, where about 60 % of all species are found. All Asian taxa are (ovo-)viviparous and most species have rather small distribution ranges. In Wallacea, pachychilids are represented by a single endemic genus on Sulawesi (Tylomelania Sarasin & Sarasin, 1897) with about 60 species. Its sister group is Pseudopotamis on the Australian Torres Strait Islands, suggesting an origin from the East.
Distribution of pachychilids in Southeast Asia, two typical species are shown
Species of Tylomelania from the ancient lakes of Sulawesi are often very colourful (© C. Lukhaup)
Viviparidae: These prosobranch freshwater gastropods commonly known as pond snails have a worldwide distribution in temperate to tropical areas with the notable exception of South America. More than 75 % of all described genera (30+) are only found in Southeast and southern East Asia. In Wallacea, two genera (one endemic) are found on Sulawesi, while the Philippines are home to another four genera, two of which are endemic. The lack of a phylogenetic framework for the entire group has so far prevented any serious biogeographic analysis of viviparids. In order to overcome this obstacle, a molecular phylogeny of pond snails is currently in progress with a focus on Asian taxa and particularly the origin of the Wallacean species.
Thomas von Rintelen
Planorbidae: Planorbidae is the taxonomically and evolutionarily most diverse family of freshwater pulmonate gastropods (Pulmonata: Hygrophila). Within Planorbidae shell shapes range from limpet-like, planispiral to high-spired forms. High-spired planorbid gastropods occur in Australia, Papua New Guinea, West Irian, Sulawesi and many other parts of Indonesia. The Australian high-spired planorbids are seen as Gondwana relicts. The Wallacea is particularly rich in planorbid snails of varying levels of endemism. Some taxa, particularly on Sulawesi, developed unique anatomical and shell features and even life-history strategies. Planorbidae are ideal candidates to address a variety of biogeographic hypotheses. They are currently used in a comparative phylogeographic framework for clarifying biogeographical relationships among Wallacea and Australia (picture: Miratesta celebensis from Lake Poso, © T. Hauffe).