Fig. 3

Refractive indices of tunics and seawater (a), and the difference between the two, in the visual light range (400–700 nm) (b). Refractive indices of the tunic of Thetys vagina (previously frozen specimen) and Rhopalaea sp. (fresh specimen) were calculated from ellipsometric parameters Ψ and Δ (Additional files 2 and 3). The curve for seawater is based on the equation of Quan and Fry [22] that is valid from 400 to 700 nm. The vertical dotted line indicates the wavelength of D-line that was used for the measurement with the Abbe refractometer