Phylogenetic relationships and genus-level taxonomy
The southeastern Australian helicarionid clade has previously been identified as a monophyletic radiation (Hyman et al., 2007; Hyman and Ponder, 2010) containing at least five genera (Helicarion, Brevisentis, Mysticarion, Parmavitrina and Peloparion). Here we expand this group to include Cucullarion and Ubiquitarion gen. nov. Synapomorphies of this clade include a spermatophore with spirally arranged branched spines, an epiphallic flagellum with internal cryptae, the absence of an epiphallic caecum and the presence of at most a very short vagina.
The reproductive characters that define this group occur singly or in combination in a number of other helicarionid taxa. For example, some species from northeastern Queensland have a spiraling, spinose spermatophore (e.g. Pravonitor kreffti, Hyman and Ponder, 2010) but do not share the other morphological characters. There are also several species with no epiphallic caecum (e.g. Levidens, Tarocystis, Sheaia, Hyman and Ponder, 2010). However, whether these similarities are symplesiomorphies of a larger helicarionid radiation or have indeed been acquired in convergence can only be resolved in the framework of a more complete phylogenetic analysis of the Australian Helicarionidae.
The members of the southeastern Australian clade represent a significant proportion of the ecological and morphological diversity of all Australian Helicarionidae. Members of this group are either leaf litter dwellers with fully developed shells (Brevisentis), small, arboreal or semi-arboreal semislugs (Peloparion, Ubiqutarion, Mysticarion), small to large litter-dwelling semislugs (Helicarion, Parmavitrina), or medium-sized semi-arboreal semislugs with a strongly reduced shell (Cucullarion).
Due to these significant differences in their general morphology and ecology, the south-eastern Australian helicarionids provide a suitable model to study evolutionary phenomena which have also occurred in the broader Australian Helicarionid radiation, such as limacisation.
Within this group, we found that all genera as now delimited formed well-supported (in terms of nodal support) and well-individualized (in terms of branch lengths) clades in the mitochondrial phylogenies. However, the phylogenetic relationships between these clades were poorly supported by bootstrapping, leading to substantially inconsistent topologies produced for different sequence datasets (COI, 16S, concatenated; Fig. 6). This lack of confidence in principal divisions in the phylogenetic tree poses a challenge to the inference of evolutionary trends among the southeastern helicarionid radiation.
However, it is still possible to draw some general inferences relating to character evolution in this group. Brevisentis, the only snail genus included in this study, was never the sister taxon to the rest of the clade. Consequently, under the premise that a fully developed shell represents the ancestral character state in this clade, shell reduction must have either been reversed in Brevisentis or has independently evolved multiple times within this clade. Some signs of slight shell reduction have been observed in the digestive and reproductive systems of Brevisentis jacksoniensis (Hyman, submitted), which may indicate a reversal of limacisation in Brevisentis. In any case, given the variability of body form in this group, shell reduction has evolved to a higher degree in some lineages (e.g., the rainforest semislugs belonging to Cucullarion and the Parmavitrina megastoma group) than in others (e.g., Mysticarion).