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

Abstract

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.

Rediscovering the dancing semislug genus Cryptosemelus Collinge, 1902 (Eupulmonata, Ariophantidae) from Thailand with description of two new species

“…The purely shell-based taxonomy of the semislug groups provides insufficient evidence for their systematic classification because the highly reduced and rather featureless shells provide a dearth of informative characters. Convergence in shell characters has been demonstrated in Australian helicarionids ( Hyman and Köhler 2020 ). The lack of reproductive information has created a profound inaccuracy in their recognition and delimitation (e.g., Blanford and Godwin-Austen 1908 ; Solem 1966 ; Schilthuizen and Liew 2008 ; Hyman and Ponder 2010 ; Hyman and Köhler 2020 ).…”

“…Convergence in shell characters has been demonstrated in Australian helicarionids ( Hyman and Köhler 2020 ). The lack of reproductive information has created a profound inaccuracy in their recognition and delimitation (e.g., Blanford and Godwin-Austen 1908 ; Solem 1966 ; Schilthuizen and Liew 2008 ; Hyman and Ponder 2010 ; Hyman and Köhler 2020 ). At present, anatomy-based approaches or those integrating molecular analyses are likely to more successfully resolve these taxonomic problems.…”

“…Becoming a slug through the reduction of the shell has occurred multiple times among the stylommatophoran land snails; this has occurred particularly frequently within the limacoid snail families Ariophantidae Godwin-Austen, 1883, Helicarionidae Bourguignat, 1877, and Urocyclidae Simroth, 1889 ( Solem 1966 ; Hausdorf 1998 ; Hyman et al 2007 ; Hyman and Köhler 2020 ). This process, so-called ‘limacization’, has also produced intermediate forms between slugs and snails, known as ‘semislugs’, which are characterized by the presence of an external shell with a reduced number of whorls, and a loss of the ability to completely withdraw its body under the protective shell ( Collinge 1902 ; Blanford and Godwin-Austen 1908 ; Solem 1966 ; Barker 2001 ; Schileyko 2003 ; Schilthuizen and Liew 2008 ; Hyman and Köhler 2020 ). In addition, semislugs tend to show extensive development of the mantle lobes that can completely cover its shell, providing an increased surface area for gaseous exchange ( Tillier 1983 ; Barker 2001 ).…”

“…For the poorly known genus Cryptosemelus Collinge, 1902 such a systematic revision had been pending so far as there had been no additional records on this taxon since its first description. Collinge (1902) only gave a very brief description without providing any details on the genitalia, which bear important characters in semislug taxonomy ( Blanford and Godwin-Austen 1908 ; Solem 1966 ; Hyman and Ponder 2010 ; Hyman and Köhler 2020 ). This monotypic genus was described based on specimens collected during the ‘The University of Cambridge Expedition to the North-Eastern Malay States and Upper Perak’, known as the ‘Skeat Expedition, 1899–1900’ ( Gibson-Hill et al 1953 ), which visited a region now situated in southern Thailand and the northern part of the Peninsular Malaysia.…”

Systematic revision of the microcystid land snails endemic to Norfolk Island (Gastropoda: Stylommatophora) based on comparative morpho-anatomy and mitochondrial phylogenetics

“…In some land snail groups, the tendency to produce smooth, rather featureless shells lacking obvious distinguishing characters can also obscure the systematic relationships of terrestrial snails. This is particularly the case in the Helicarionidae, Euconulidae and Microcystidae, where shell features such as definite sculpture or columellar teeth are rare, yet the reduction or loss of these characters is frequently the result of convergence (Baker 1938;Hyman and Köhler 2020a). In such groups, testing historical, shell-based taxonomic concepts through the appraisal of anatomical and molecular evidence is paramount.…”