Sea slug night fever: 39 new records of elusive heterobranchs in the western Mediterranean (Mollusca: Gastropoda) Author Salvador, Xavier 0000-0003-3039-7566 Author Fernández-Vilert, Robert 0000-0002-2812-4414 Author Moles, Juan 0000-0003-4511-4055 text Journal of Natural History 2022 2022-05-18 56 5 - 8 265 310 http://dx.doi.org/10.1080/00222933.2022.2040630 journal article 10.1080/00222933.2022.2040630 38f6df24-838f-409a-90df-b56087e6a9bc 1464-5262 6758438 Palio nothus ( Johnston, 1838 ) ( Figure 3 (a)) Material examined Cala Maset caves, Sant Feliu de Guíxols ( Spain ), 41°47 ʹ 10”N , 3°2 ʹ 44”E , 16 January 2018 , 1.7 m depth , 5 spcs, adults and egg masses, L = 8–14 mm ; 7 May 2018 , 1.4 m depth , 1 spc ., adult and egg masses, L = 15 mm ; L = 8–14 mm ; 11 March 2020 , 2 m depth, 7 spcs, juveniles. External morphology Body short, thick, background colour black with numerous white papillae. Mantle margin white. Rhinophores with lamellae, beige or white. Branchial leaves dark brown. Ecology Specimens were found mating and laying the egg masses on the bryozoans Amathia lendigera (Linnaeus, 1758) at the entrance of a cave, on overhangs. Egg masses were white with a soft brown colour and laid in a ‘C’ shape over the bryozoans. Juvenile specimens, only found in 2020, were also crawling on A. lendigera . Slugs are more active at night and may go unnoticed by divers and other observers during the day. Distribution North Sea ( England and Norway ; Johnston 1838 ; Evertsen and Bakken 2013 ); Mediterranean Spanish coast: Catalonia ( Ballesteros et al. 2016 ; this study). Remarks This species apparently has an Amphi-Atlantic and Boreo-Arctic distribution, but its identity may need confirmation since is easily confused with Palio dubia (M. Sars, 1829) ; therefore, several records may have been misidentified ( Thompson and Brown 1984 ). Genus Polycera Cuvier, 1816