Biodiversity Journal, 2014, 5 (2): 117–130 Phenotypic diversity of Thuridilla hopei (Vérany, 1853) (Gas- tropoda Heterobranchia Sacoglossa). A DNA-barcoding ap- proach Giulia Furfaro 1 , Maria Vittoria Modica 2 , Marco Oliverio 2 , Juan Lucas Cervera 3 & Paolo Mariottini 1* 1 Dipartimento di Scienze, Università degli Studi di “Roma Tre”, Viale Marconi 446, 00146 Rome, Italy; e-mail: giuliafurfaro@hotmail.it; paolo.mariottini@uniroma3.it 2 Dipartimento di Biologia e Biotecnologie “Charles Darwin”, Università di Roma “La Sapienza”, Viale dell’Università 32, 00185 Rome, Italy; e-mail: marco.oliverio@uniroma1.it; mariavittoria.modica@uniroma1.it 3 Departamento de Biología, Facultad de Ciencias del Mar y Ambientales, Campus de Excelencia Internacional del Mar (CEI•MAR,) Universidad de Cádiz. Polígono Río San Pedro, s/n, Ap.40. 11510 Puerto Real (Cádiz), Spain; e-mail: lucas.cervera@uca.es * Corresponding author ABSTRACT The sacoglossan Thuridilla hopei (Vérany, 1853) shows highly diverse chromatic patterns. Based on the morphological examination of specimens from different Mediterranean locali- ties, we have observed that in spite of this great variability in colours of T. hopei, two major chromatic morphotypes are related to bathymetry. Specimens from deeper water exhibit blue darker and more uniform patterns than individuals from shallower water, which show a more variable, dashed and spotted arrangement of light blue, yellow, orange, white and black pigmentation. A molecular genetic analysis using the mitochondrial COI and 16S rDNA markers has confirmed that all these extremely different chromatic morphotypes belong to a single specific entity, i.e. T. hopei, a sacoglossan with a wide distribution, from Macaronesia in the Atlantic, to the easternmost Mediterranean Sea. KEY WORDS Sacoglossa; Thuridilla hopei; colour morphotypes; Mediterranean Sea; Atlantic Ocean. Received 28.01.2014; accepted 21.03.2014; printed 30.06.2014 Proceedings of the Seventh Malacological Pontine Meeting, September 9 th -10 th , 2013 - San Felice Circeo, Italy INTRODUCTION The plakobranchid sacoglossan genus Thuri- dilla Bergh, 1872 is represented in the northeastern Atlantic and in the Mediterranean Sea by two species, T. hopei (Vérany, 1853) (Carmona et al., 2011; Malaquias et al., 2012) and the amphiatlantic T. mazda Ortea et Spinosa, 2000, recently recorded from the Azores (Malaquias et al., 2012). The brightly coloured T. hopei lives from the lower intertidal down to about 35 m depth, and generally is found crawling on hard substrate. It feeds suctorially on photophilous algae, in particular on Derbesia tenuissima (de Notaris) Crouan et Crouanand Cladophora vagabunda (Linnaeus) van den Hoek (Marín & Ros, 2004; Händeler & Wägele, 2007; Händeler, 2011) and it retains functional algal chloro- plasts from its food for few days (Marín & Ros, 1989, 2004). The aposematic chromatic pattern of this slug is related to the presence in its tissues of toxic compounds like the diterpenoids thuridillin A, B and C and nor-thuridillin (Gavagnin et al., 1993; De Rinaldis, 2012). During the last decade, the high levels of in- traspecific chromatic variation of T. hopei has been well documented in literature (Trainito, 2005; Händeler, 2011; Carmona et al., 2011 and refer- ences therein), as well as depicted in the Sea Slug MONOGRAPH