The sacoglossan Thuridilla hopei (Verany, 1853) shows highly diverse chromatic patterns. Based on the morphological examination of specimens from different Mediterranean localities, 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
