Phylogenetic assessment of the halophilic Australian gastropod Coxiella and South African Tomichia resolves taxonomic uncertainties, uncovers new species and supports a Gondwanan link

Abstract

Genetic and morphological data have suggested a Gondwanan connection between the three non-marine aquatic gastropod genera Coxiella Smith, 1894, Tomichia Benson, 1851 and Idiopyrgus Pilsbry, 1911. These genera have recently been included in the family Tomichiidae Wenz, 1938, however, further assessment of the validity of this family is warranted. Coxiella is an obligate halophile that occurs in Australian salt lakes while Tomichia occurs in saline and freshwater environments in southern Africa and Idiopyrgus is a freshwater taxon from South America. Despite their novel evolutionary and ecological characteristics, Coxiella, Tomichia and Idiopyrgus are poorly studied, and the lack of a contemporary taxonomic framework restricts our ability to assess the risk of declining habitat quality to these gastropods. We used data from mitochondrial (COI and 16S) and nuclear (28S and 18S) genes in 20 species from all three genera to undertake the most comprehensive phylogenetic test of the Tomichiidae to date. Bayesian and Maximum Likelihood phylogenetic analyses of a concatenated dataset (2974 bp) of all four genes strongly supported a monophyletic Tomichiidae. The COI analysis (n = 307) identified 14 reciprocally monophyletic lineages in Coxiella that comprised eight of the nine currently described species and at least six putative new species. Four distinct genetic clades of species with somewhat distinctive morphologies were found, each of which may constitute a distinct genera. In addition, four species of Tomichia were identified, including three described and one putatively new species. Current species descriptions of Coxiella do not account for the range of morphological variation observed within most described species, and although morphology is reasonably effective at delineating between clades, it is of limited use when trying to separate closely related Coxiella species. The improved understanding of the taxonomy and diversity of Tomichia and especially Coxiella will underpin future studies and conservation planning for these taxa.