An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters Author Mehrotra, Rahul Reef Biology Research Group. Department of Marine Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand & Aow Thai Marine Ecology Center, Koh Mun Nai, Kram, Klaeng District, Rayong 21110, Thailand Author A. Caballer Gutierrez, Manuel American University of Paris, Department of Computer Science Math and Environmental Science, 6 rue du Colonel Combes, 75007 Paris, France & Museum national d'Histoire naturelle, 55 rue de Buffon, 75005 Paris, France Author M. Scott, Chad Conservation Diver. 7321 Timber Trail Road, Evergreen, Colorado, 80439, USA Author Arnold, Spencer Conservation Diver. 7321 Timber Trail Road, Evergreen, Colorado, 80439, USA Author Monchanin, Coline Aow Thai Marine Ecology Center, Koh Mun Nai, Kram, Klaeng District, Rayong 21110, Thailand & Research Center on Animal Cognition (CRCA), Center for Integrative Biology (CBI); CNRS, University Paul Sabatier, Toulouse III, France Author Viyakarn, Voranop https://orcid.org/0000-0002-2089-6356 Reef Biology Research Group. Department of Marine Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Author Chavanich, Suchana https://orcid.org/0000-0001-6266-7300 Reef Biology Research Group. Department of Marine Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand & Center of Excellence for Marine Biotechnology, Department of Marine Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand suchana.c@chula.ac.th text ZooKeys 2021 2021-06-09 1042 73 188 http://dx.doi.org/10.3897/zookeys.1042.64474 journal article http://dx.doi.org/10.3897/zookeys.1042.64474 1313-2970-1042-73 9CF986D86A474E179A67245C78FB8AFD 1BB0A10A35DD5541850FDAFFDB7119C2 Melibe sp. Figure 20E Material examined. Two specimens 15-22 mm , TT. Ecology. In soft sediment habitats beyond the coral reef, grazing upon the substrate. Depth 17-22 m. Distribution. Melibe engeli Risbec, 1937 is known across the Indo-Pacific including Mozambique ( Tibirica et al. 2017 ), the Red Sea ( Burghardt and Waegele 2014 ), the Philippines, Indonesia, Japan, New Caledonia, and Hawaii ( Gosliner et al. 2008 ). Melibe sp. 1 ( Gosliner et al. 2008 ) is known from Indonesia only. Recorded from Koh Tao as Melibe sp. 1 by Mehrotra and Scott (2016 : fig. 3D, E). Remarks. Externally similar to Melibe engeli and Melibe sp. 1 ( Gosliner et al. 2008 , 2018 ) in shape and some morphology, with similarities between the two also being noted by Yonow (2017) . Specimens recorded from Koh Tao range in colour from near colourless to strongly golden brown, although always at least slightly transparent. The body is covered in numerous small papillae, the oral hood is able to stretch to at least half of the length of the remaining body, which have four or five pairs of cerata. Cerata are mostly ovoid to cylindrical in shape, terminating in numerous large pointed white papillae. In smaller individuals, two papillae may dominate the apex of each ceras giving them a bifurcate appearance. Most intriguingly and substantially different from the morphology of M. engeli are the rhinophore sheaths which lack the 'sail-like' appendage but instead have a single long and thin almost 'whip-like' extension. While the overall body shape of M. engeli has been shown to go through numerous changes during development ( Burghardt and Waegele 2014 ), the specimens from Koh Tao do not entirely match M. engeli .