Phylogeny and systematics of the Pachychilidae of mainland Southeast Asia – novel insights from morphology and mitochondrial DNA (Mollusca, Caenogastropoda, Cerithioidea)

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Abstract

Pachychilid gastropods are a conspicuous element of the freshwater macro-invertebrate fauna of South-East Asia. In this region, three spatially separated groups of pachychilids can be differentiated mostly by means of their brooding strategy. The largest group comprises about 100 described species and displays an extended Sundaland distribution that ranges from north-east India in the west to southern China in the east and Borneo in the south-east. The most conspicuous synapomorphy shared by females is a subhaemocoelic brood pouch in which developing embryos are retained. Mitochondrial phylogenies based on partial sequences of the genes cytochrome oxidase C subunit I (COI) and 16S rRNA (16S) support the monophyly of this group of subhaemocoelic brooders. Amongst them, the two genera Brotia and Paracrostoma form a sister pair of clades both together forming the sister taxon of a third clade. This third clade itself is split into three subclades, which occupy separate ranges except for the overlap of two of them in Borneo. The subclades comprise species from (A) Thailand, Cambodia, and northern Borneo; (B) Java and south-western Borneo; and (C) Vietnam and southern China. In contrast with their spatial structuring, all species share a widely corresponding anatomy. Characteristic key features are a pallial oviduct with a seminal receptacle and an embryonic shell with a smooth, dome-like inflated apex. Differences in the histology of the pallial oviduct are not congruent with phylogenetic patterns as suggested by the mitochondrial trees. Given the lack of obvious distinguishing morphological characteristics, all species of the subclades A to C are assigned to a single genus, Sulcospira, with Adamietta as a junior synonym.