The Helicarionidae is a group of land snails in which shell reduction, or limacisation, appears to have evolved repeatedly (Hyman et al., 2007). As a result this group harbours a wide range of morphological diversity, and includes snails with a full shell as well as semislugs, animals with a reduced, ear-shaped shell into which they cannot fully retract. The shells of all species, both snails and semislugs, are thin and glossy with reduced sculpture. Early species descriptions were primarily based on shells (e.g., Pfeiffer, 1849, 1850, 1852, 1853; Cox, 1864, 1868, 1871, 1873; Brazier, 1875a, 1875b, 1876b), as was the taxonomic framework put together in a series of papers by Iredale (1933, 1937a, 1937b, 1941, 1942), which included descriptions of 19 new, mostly monotypic genera. Subsequently, a number of small-scale anatomical investigations into single genera or species were published (Dartnall and Kershaw, 1978; Kershaw, 1979, 1981, 1982, 1988; Stanisic, 1993a, 1993b; Scott, 1995), but the underlying taxonomy essentially remained unchanged. More recently, a more comprehensive overview of Australian Helicarionidae was undertaken by Hyman and co-authors (Hyman, 2007; Hyman et al., 2007; Hyman and Ponder, 2010, 2016) utilizing both morphological data and mitochondrial DNA sequences. These studies resulted in the addition of several new species and genera as well as evidence to support the monophyly of some smaller subgroups among the Australian Helicarionidae. However, only around a quarter of Australia’s estimated 150 described species was included in these studies, with a high proportion from New South Wales (NSW), leaving the taxonomic structure of the entire group still unclear. Stanisic et al. (2010) documented the basic shell morphology and distributions of the NSW and Queensland (Qld) helicarionids and described 47 new species and 15 new genera based exclusively on their shell. This has raised the number of described species in Australia to 102. However, for more than half of these no anatomical data are known and their phylogenetic relationships have essentially remained unresolved.
Numerous studies in Australian land snail groups, such as the Camaenidae, have revealed the presence of both widespread conservatism in shell morphology, and in other cases, high levels of convergence (Stankowski, 2011; Criscione and Köhler, 2013a; Köhler and Criscione, 2015). Shell characters, such as shape, size and colour, appear to be closely linked to the adaptation to different habitat types, and therefore may be constrained by environment or lifestyle rather than by shared ancestry. Hence, these characters cannot reliably be used as a sole basis for a phylogenetic classification. There is evidence that shell reduction has evolved several times in the Helicarionidae (Hyman et al., 2007). As well as casting further doubt on the reliability of shell morphology in this group, this convergence in shell and body shape suggests that care must also be taken with the examination of anatomical characters, which are usually accepted as more reliable than shell morphology. During the process of limacisation, the size of the pallial cavity is reduced and some reproductive and digestive organs are moved into the foot. This can lead to convergence in anatomical characters; for example, many semislugs have developed an additional oesophageal crop, which is not present in snails (Tillier, 1984; Hyman and Ponder, 2010). The need to fit reproductive organs into a reduced space may also lead to convergence in genital morphology. For this reason, the use of molecular data, which already underpins much of modern taxonomy, is crucial to reliably resolve the systematic relationships among helicarionids.
The majority of Australian species live in rainforest habitats along the eastern coast. Although this group’s highest diversity is in the Wet Tropics region of northern Queensland, helicarionid species are also present in the subtropical and temperate rainforests of southeastern and mid-eastern Australia. A single clade made up of six genera dominates the helicarionid fauna of this region (Hyman et al., 2007; Hyman and Ponder, 2010). While the relationships among the Helicarionidae are in general poorly understood, this southeastern Australian clade, comprising the genera Helicarion Férussac, 1821, Mysticarion Iredale, 1941, Desidarion Iredale, 1941, Parmavitrina Iredale, 1937, Peloparion Iredale, 1937 and Brevisentis Hyman, 2007, groups together with strong support based on both morphology and on mitochondrial phylogeny (Hyman et al., 2007; Hyman and Ponder, 2010). The southernmost genus, Helicarion, is found from Tasmania to central NSW and will be the subject of a future paper. The remaining five currently recognized genera and one additional genus, Cucullarion Stanisic, 1998, including the fourteen species they contain, form the subject of this revision.
Anatomical data are available for eight species (Stanisic, 1998; Hyman, 2007; Hyman and Ponder, 2010); however, the remaining six species, some of which were recently described by Stanisic et al. (2010), are known only by their shell morphology. For several of these species a single specimen has been sequenced previously (COI and 16S; Hyman et al., 2007), for a phylogenetic study that was not concerned with resolving the species-level taxonomy. Herein, we comprehensively revise the taxonomy relationships of these six helicarionid genera from southeastern Australia based on comparative examination of the anatomy and morphology of all described species and some currently undescribed candidate taxa as well as analyses of partial sequences of the mitochondrial genes cytochrome c oxidase subunit I (COI) and 16S rRNA (16S) that sought to estimate the amounts of mitochondrial differentiation within and between species and to resolve their phylogenetic relationships.