Elysia flava Verrill, 1901 (Figure 6 (g)) Material examined Cala d’Aiguafreda, Begur (Spain), 41°57 ʹ 49”N, 3°13 ʹ 41”E, 30 July 2015, 2.3 m depth, 6 spcs, adults, L = 5–12 mm; Roqueo de los 14, La Herradura (Spain), 36°43 ʹ 13.2”N, 3°43 ʹ 43.5”W, 18 September 2016, 1.6 m depth, 3 spcs, adults, L = 7–10 mm. External morphology Body elongate, narrow, background colour green/yellow, dark green in parapodia, margin of parapodia wavy, white. Rhinophores short, apex white, with two white spots between eyes. Ecology Specimens found in July mating on unidentified algae. This species is strictly nocturnal, found during the day under rocks and at night on algae, especially green filamentous algae such as Cladophora. Distribution Caribbean Sea (GBIF.org 2021); Madeira (Cervera et al. 2004); Azores (Malaquias et al. 2009); Greece and Adriatic Sea (Ballesteros et al. 2012–2021); Spain: Canary Islands, Levantine coast, Catalonia (Cervera et al. 2004; this study), Andalucía (this study). Remarks This species is differentiated from sympatric Elysia by the yellowish colour of the body as well as the green dark colour in the parapodia (Thompson and Jaklin 1988). Superorder SIPHONARIMORPHA Family SIPHONARIIDAE Gray, 1827 Genus Williamia Monterosato, 1884

Published as part of Salvador, Xavier, Fernández-Vilert, Robert & Moles, Juan, 2022, Sea slug night fever: 39 new records of elusive heterobranchs in the western Mediterranean (Mollusca: Gastropoda), pp. 265-310 in Journal of Natural History 56 (5 - 8) on page 301, DOI: 10.1080/00222933.2022.2040630, http://zenodo.org/record/6758438