Figure 7: Hypothesis for the functional significance of the blue stripes.

(a) The stripes’ reflectivity (blue, data set of Fig. 4b) and the linear absorption coefficient of sea water (black, data adapted from ref. 70). (b) Spectral irradiance at 0 (ASTM G173-03, 2012), 2 and 10-m depth below sea level before (light grey, grey and black) and after (light blue, blue and dark blue) light is reflected from a blue stripe. (c) Comparison of the two morphotypes of Patella pellucida. Limpets of the pellucida morph (top left) live visually exposed on the kelp fronds, while members of the laevis morph (bottom left) are usually found concealed in cavities of the kelp’s holdfast. Top left photograph, acquired with the help of Larry Friesen. (d) The toxic and unpalatable nudibranches Polycera elegans (left, on kelp) and Facelina auriculata (right) with similar blue features and overlapping habitats around the coast of Great Britain and Ireland. Nudibranch photographs courtesy of Josep Lluis Peralta and Jim Anderson.