In the context of a complete revision of the Onchidiidae, it is shown here that Onchidium vaigiense Quoy & Gaimard, 1825 and Onchidium marmoratum Lesson, 1831 belong to a clade that is separate from all other onchidiid genera and so a new genus is described: Marmaronchis Dayrat & Goulding, new genus. Marmaronchis slugs are characterised by a unique combination of anatomical traits: intestinal loops of type I, rectal gland present, accessory penial gland present. Marmaronchis vaigiensis and M. marmoratus are cryptic externally and internally but are delineated as distinct species with both mitochondrial (COI, 16S, 12S) and nuclear (ITS2, 28S, H3) DNA sequences. Onchidium ambiguum Semper, 1880 and O. steenstrupii Semper, 1882 are proposed as new junior synonyms of M. vaigiensis. Marmaronchis slugs primarily live in the rocky intertidal and, unlike many onchidiids from Southeast Asia, they are not found inside mangroves. Both Marmaronchis species are geographically sympatric and can even be found at the same stations, but Marmaronchis vaigiensis is widely distributed, from the Nicobar Islands (Bay of Bengal) all the way to Vanuatu and the Philippines, while M. marmoratus is only known from New Ireland and Madang (Papua New Guinea). Several new geographical records are provided: Bali and Sulawesi (Indonesia) and Vanuatu for M. vaigiensis; Madang (Papua New Guinea) for M. marmoratus. The diversity of Marmaronchis slugs is compared to other onchidiid genera.

In the context of a complete revision of the Onchidiidae, it is shown here that Onchidium vaigienseQuoy & Gaimard, 1825 and Onchidium marmoratum Lesson, 1831 belong to a clade that is separate from all other onchidiid genera and so a new genus is described: Marmaronchis Dayrat & Goulding, new genus. Marmaronchisslugs are characterised by a unique combination of anatomical traits: intestinal loops of type I, rectal gland present, accessory penial gland present. Marmaronchis vaigiensis and M. marmoratus are cryptic externally and internally but are delineated as distinct species with both mitochondrial (COI, 16S, 12S) and nuclear (ITS2, 28S, H3) DNA sequences. Onchidium ambiguum Semper, 1880 and O. steenstrupii Semper, 1882 are proposed as new junior synonyms of M. vaigiensis. Marmaronchis slugs primarily live in the rocky intertidal and, unlike many onchidiids from Southeast Asia, they are not found inside mangroves. Both Marmaronchis species are geographically sympatric and can even be found at the same stations, but Marmaronchis vaigiensis is widely distributed, from the Nicobar Islands (Bay of Bengal) all the way to Vanuatu and the Philippines, while M. marmoratus is only known from New Ireland and Madang (Papua New Guinea). Several new geographical records are provided: Bali and Sulawesi (Indonesia) and Vanuatu for M. vaigiensis; Madang (Papua New Guinea) for M. marmoratus. The diversity of Marmaronchis slugs is compared to other onchidiid genera.

The family Ocypodidae is a group of intertidal brachyuran crabs found in tropical to temperate seas worldwide. While the family has historically included many subfamilies, most of these have now been given separate family status within the superfamily Ocypodoidea. The most recent classification recognises only two subfamilies, the ghost crabs: Ocypodinae Rafinesque, 1815; and the fiddler crabs: Ucinae Dana, 1851. The ghost crabs comprise 21 species in two genera, Ocypode Weber, 1795, and Hoplocypode Sakai & Türkay, 2013.