Fig. 1

R-opsin expression in 7 dpf L. asellus larvae and eye photoreceptor cell morphology. a Larva with eyespot posterior to the prototroch (PT). b Expression of Las-r-opsin in the eye and the apical area (inlet). c Scheme of Las-r-opsin expression. d Las-r-opsin groups well supported within mollusk and other protostome r-opsins. Support values are given as Bayesian posterior probability/maximum likelihood bootstrap values (see “Methods” and Additional file 3 for uncollapsed tree and details of tree inference). e Specifically designed Las-r-opsin antibody reveals r-opsin protein in the eyes, as well as in apical and posterior cells (arrows). f Combination of Las-r-opsin in situ hybridization (cyan) and antibody staining (red) showing the spatial separation of the Las-r opsin protein in the apical area (right side) and the mRNA expression in the nucleus of an eye sensory cell. g, h Electron microscopic images of the photoreceptor cells (PRC) showing the flask-shaped morphology and the apical microvillar extensions. Apical surface on the right. (Scalebars 100 μm in a, b, e; 5 μm in f; 2 μm in g, h. PRC photoreceptor cell)