Fig. 3

Studied species and their ONT eyeshine. a-d Habitus and optic disc (OD) shape of Tripterygion delaisi (a), Tripterygion melanurus (b), Parablennius zvonimiri (c), and Amphiprion ocellaris (d). Fish images a, b, and c were taken in the field, d in the laboratory. Scale bars equal 1 cm. OD images were taken through a spectrophotometer with attached endoscope under dorsal illumination of the fish’s head. The black dot is the detection area of the spectrophotometer and was positioned within the OD during measurements. The bluish tint is due to the cyan filter used for illumination. e Absolute radiance measurements (n = 5, mean ± SD) of a diffuse white reflectance standard (DWS), T. delaisi’s optic disc with ONT eyeshine (ONTE), and the adjacent retinal PET eyeshine, all under the same conditions (see Additional file 6). The DWS served as a proxy for illumination intensity. f Mean transmission efficiency of the study species’ ONT eyeshine and retinal background relative to the DWS (T. melanurus: n = 2, all others: n = 5). Transmission is poor for short-wavelength light in all species, but consistently rises with increasing wavelength. T. delaisi features the highest proportional transmission, reaching about 8% at the red end of the visible spectrum