Abstract
A new species, Onchidium melakense Dayrat & Goulding, sp. nov., is described, bringing the total to four known species in the genus Onchidium Buchannan, 1800. Onchidium melakense is a rare species with only nine individuals found at three mangrove sites in the Andaman Islands and the Strait of Malacca (western Peninsular Malaysia and eastern Sumatra). The new species is delineated based on mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) DNA sequences as well as comparative anatomy. Each Onchidium species is characterized by a distinct color and can easily be identified in the field, even in the Strait of Malacca where there are three sympatric Onchidium species. An identification key is provided. In addition, Onchidium stuxbergi (Westerlund, 1883) is recorded for the first time from eastern Sumatra, and Onchidium pallidipes Tapparone-Canefri, 1889, of which the type material is described and illustrated here, is regarded as a new junior synonym of O. stuxbergi.
Highlights
IntroductionOnchidiids are true slugs (lacking an internal shell) which breathe air with a lung and die if they are immersed in water for a few hours
Onchidiids are true slugs which breathe air with a lung and die if they are immersed in water for a few hours
We follow an integrative approach to taxonomy based on: 1) a re-examination of all types available and a comprehensive review of the nomenclature to address the application of all existing species- and genus-group names; 2) extensive field work to observe species in their habitat and to collect fresh material; and, 3) species delineation using DNA sequences to complement comparative anatomy
Summary
Onchidiids are true slugs (lacking an internal shell) which breathe air with a lung and die if they are immersed in water for a few hours. Onchidiids are found worldwide, but the highest species diversity is in South-East Asia, especially in mangroves where onchidiids are among the most abundant animals. Onchidiid slugs are most closely related to veronicellids, which are true slugs but which, unlike onchidiids, are fully terrestrial, and to Stylommatophora, the land snails and slugs (Dayrat et al 2011). The taxonomy of the Onchidiidae has been in a state of chaos for many years (Dayrat 2009). In the past few years, the Dayrat laboratory has been revising the taxonomy of the entire family one clade at a time (Dayrat et al 2016, 2017, 2018, 2019a, b; Dayrat and Goulding 2017; Goulding et al 2018a, b, c).
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