Dissecting an island radiation: systematic revision of endemic land snails on Lord Howe Island (Gastropoda: Stylommatophora: Microcystidae)

Abstract

The Lord Howe Island Microcystidae currently comprise two endemic genera containing nine species. We revise their taxonomy comprehensively using comparative morphology and phylogenetic analyses of the mitochondrial genes COI and 16S and demonstrate that the Lord Howe Island microcystids probably represent a single radiation. Based on our findings, we recognize Melloconcha, Tribocystis and Annacharis as junior synonyms of Innesoconcha and provide a revised generic diagnosis of Innesoconcha and redescriptions for all species. In addition, we reinstate Innesoconcha segna from synonymy and describe Innesoconcha doppelganger sp. nov., bringing the total number of accepted species in the genus to 11. Most species are well differentiated, in terms of both morphological differences and basal branch lengths in the mitochondrial trees. However, members of the Innesoconcha catletti species complex reveal lower levels of genetic and morphological differentiation and are likely to represent examples of more recent, perhaps even ongoing, speciation which might be driven by reinforcement. We recommend conservation assessments of all species, particularly Innesoconcha aberrans and Innesoconcha grata, which are rare species that have declined in abundance over the past 50 years, and note that Innesoconcha rosacea and Innesoconcha delecta appear to have become geographically more restricted. Innesoconcha miranda and I. segna are probably extinct.

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

“…The only exception is Dendrolamellaria mathewsi , which is a helicarionid, as outlined further below. Since all the microcystids on Norfolk Island have a reasonably simple reproductive system, lacking characters such as penial darts or stimulators, penial caecae or an epiphallic corona, these are placed in the Microcystinae, implying a close relationship with taxa from Tonga and Samoa, and the Cook and Austral Islands (Baker 1941 b ), Lord Howe Island (Hyman et al 2023) and potentially the Philippines, Indonesia and Madagascar (MolluscaBase, see http://www.molluscabase.org), although the taxonomic placement of species from the latter areas is based solely on shell morphology. Iredale (1913) also noted a close similarity between Fanulena from Norfolk Island and Diastole expositum (Mousson, 1893) from the Kermadec Islands, again based solely on shell morphology.…”

“…We prefer to treat Microcystidae as an independent family due to the considerable morphological differences that separate this from Euconulidae (including ovoviviparity, the insertion of the bursa copulatrix on the male side and the partial separation of the prostate; Baker 1938) and a lack of synapomorphies that unite the two groups (Hyman et al 2023). Accordingly, we elevate the three tribes to subfamilies.…”

“…This proposal was followed by some (e.g. Stanisic et al 2010;Hyman and Köhler 2020b;Hyman et al 2023) but not others (e.g. Hyman et al 2007;Bouchet et al 2017).…”

“…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 2020 a ). In such groups, testing historical, shell-based taxonomic concepts through the appraisal of anatomical and molecular evidence is paramount.…”

“…Apart from a few cosmopolitan species, microcystids are absent from mainland Australia, New Zealand and New Caledonia (Solem 1966; Powell 1979). Yet endemic species are present on Lord Howe Island (Hyman et al 2023), Norfolk Island (Iredale 1945) and the Kermadec Islands (Iredale 1913), for example. Three subfamilies are currently recognised within the Microcystidae: Microcystinae, recorded from the Cook and Austral Islands, Samoa, Tonga, the Philippines, Indonesia, Timor, China, Christmas Island and Madagascar; Liardetiinae, found throughout the Pacific and in South-East Asia; and Philonesiinae, found only in Hawaii (Baker 1941 b ).…”