Feeling sluggish: The extreme semislugs of Australia (Stylommatophora,Helicarionidae).

The extreme semislugs of the family Helicarionidae, defined as having shells of fewer than three whorls, comprise at least five independent lineages in Australia. Five genera of extreme semislugs from Queensland are treated in detail herein. We comprehensively revise the systematic classification of these genera using comparative morphology and mitochondrial phylogenetics. Parmacochlea and Thularion together comprise a single radiation, but both genera are retained based on reciprocal monophyly and the presence of distinct morphological differences, including a more developed shell, a significantly larger body size, the presence of a distinct vagina, and the absence of a penial verge in Thularion. We synonymize Parmacochlea ferrea with P. fischeri; reinstate P. smithi; give revised species descriptions for P. fischeri, P. smithi, and Thularion semoni; and describe three new species, P. balios, P. furca, and T. stanisici. The formerly monotypic genus Antiquarion is herein expanded to contain eight species. We give a revised species description for the type, A. collinsae, and introduce seven new species, A. boondah, A. chooreechillum, A. evelynensis, A. limbus, A. occultus, A. ravenshoe, and A. swausi. In addition, we describe one new genus and species, Limpidarion kukuyulangi, and give a revised species description of the monotypic genus Fenestrarion and its type species, F. brianna. We demonstrate that these four semislug groups are independent lineages, but share some morphological characters that are likely to be adaptations to shell reduction, including a unilobed ovotestis, a shortened or absent vagina, and often (but not always) a reduced flagellum and epiphallic cecum. [ABSTRACT FROM AUTHOR]