Toward a systematic revision of the Southeast Asian freshwater gastropod Brotia H. Adams, 1866 (Cerithioidea: Pachychilidae): An account of species from around the South China Sea

Abstract

The systematics of freshwater gastropods attributed to Brotia H. Adams, 1866 based on conchological similarities has long remained confusing. Previously, only scattered information was available on the taxonomy and virtually nothing on the phylogenetic relationships of these viviparous snails that are widely distributed in Southeast Asia. To resolve the systematic confusion, the relevant types were re-examined together with additional material from various museum collections, supplemented by personal field collections. We here provide a detailed characterization of Brotia sensu lato and, as a basis for a taxonomic revision, comparative conchological (including embryonic shell characters), anatomical (including histology of the reproductive organs) and biogeographical data for the most crucial constituent species including their synonymies. On the basis of conchological differences, radular features, and especially reproductive anatomy and biology, we suggest to differentiate within Brotia sensu lato three major species groups representing evolutionary lineages. In addition, the proposed systematization is supported by geographical patterns. Accordingly, Brotia sensu stricto, with the type species B. pagodula (Gould, 1847) and the widely distributed B. costula (Rafinesque, 1833), occurs on the Southeast Asian mainland from the Ganges region to Sumatra. Brotia s.str, is characterized, in part, by an initially open embryonic shell with protruding soft tissue and, consequently, delayed calcification of the apical part of the shell causing a typical wrinkled texture of the embryonic shell. In contrast, species occurring on Java, i.e.B, testudinaria (von dem Busch, 1842), on Borneo, e.g. B. pageli (Thiele, 1908), and in South China, such as B. hainanensis (Brot, 1872), are distinguished by smooth, dome-shaped protoconchs and lack the peculiar mode of ontogeny established for B. pagodula and B. costula. While these constituent members of the Brotia s.str. group retain embryonic stages in a subhaemocoelic brood pouch, two other groups with a distinct reproductive anatomy in each can be distinguished: One group of pachychilid species endemic to the island of Sulawesi exhibit an uterine brood pouch. In contrast, a pallial brooding strategy is found in the third group of taxa synonymized with B. asperata (Lamarck, 1822). Both taxa that are endemic to Sulawesi and the Philippines, respectively, are also characterized by distinct gonoduct morphologies and form separate clades. The systematic and biogeographic implications of the new systematization for Brotia are discussed, using a recently developed synopsis of Southeast Asian palaeogeography.