Size does matter: integrative taxonomy and size evolution of threatened charopid land snails on Lord Howe Island (Gastropoda: Stylommatophora)

Abstract

Charopidae is a family of mostly minute land snails whose diversity is centered in the southern hemisphere. This family is represented on Australia’s Lord Howe Island in the Tasman Sea with 13 currently accepted species in seven genera, including Australia’s largest charopid species. We comprehensively revise the taxonomy of all endemic Lord Howe Island charopids using comparative morpho-anatomy and phylogenetic analyses of mitochondrial (COI, 16S) and nuclear (ITS2, ELAVI8) markers and demonstrate that these species represent two independent island radiations. We recognise these radiations as two distinct genera, Goweroconcha and Pseudocharopa, containing six and five species respectively. The two genera have distinct biogeographic affinities. Whereas the sister clade of Goweroconcha comprises several Australian genera, indicating its evolutionary origin likely being in Australia, the sister group of Pseudocharopa predominantly contains species endemic to New Zealand. Both genera have diversified on Lord Howe Island into flocks of species that differ most notably in shell size and, in the case of Pseudocharopa, to some extent, also in the degree of shell reduction. Hence, we demonstrate that while both charopid radiations include unusually large species, there is no evidence of the island effect, which tends to produce a narrower range of body sizes than observed in both groups.