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 * Elysia cf. tomentosa Jensen, 1997 Figure 6B Material examined. One specimen 37 mm , CB. Ecology. In soft sediment habitats outside the coral reef. Observed feeding on a pinnate form of Caulerpa racemosa ( Forsskal ) J. Agardh, 1873 which is found chiefly in soft sediment habitats beyond the coral reef. A more lenticular/globular form of the algae can be abundant in some shallow, degraded reef habitats; however, specimens from Koh Tao not been observed associated with this variety. Depth 12-20 m. Distribution. At present Elysia tomentosa is considered widespread across the Indo-Pacific (but see remarks below) including Iran ( Oladi et al. 2018 ), Madagascar, Malaysia ( Gosliner et al. 2008 ), Australia ( Jensen 1997 ), Reunion , India, Singapore, the Philippines, Indonesia, Japan, New Caledonia, and Hawaii ( Jensen 2015 ). Likely introduced to the Mediterranean alongside the highly invasive Caulerpa spp. ( Zenetos et al. 2010 ). Here representing a first record for Thai waters. Remarks. Recent molecular investigations have found that specimens recognised as Elysia tomentosa likely correspond to a complex of at least six species ( Krug et al. 2013 ). Discussions about the identity of specimens identifiable by black marginal lines along the parapodia and the similar species Elysia expansa ( O'Donoghue , 1924) have yet to be resolved ( Rudman 2009a ; Krug et al. 2013 ). Additionally, Oladi et al. (2018) recently documented a species in this complex from Iran that matches a sequence of E. cf. tomentosa from the Andaman coast of Thailand, by Cornelius Swennen (GenBank accession number KC573755.1); however, no record of this species from the waters of Thailand has been found in the literature to date. Greater sampling efforts from more locations, with an assessment of internal characters, are needed to clarify this complex, including specimens from Koh Tao.