Close up of shell interior shown in image 17 Patella pellucida. Length 10.2mm. Early adult, Growth Stage 4 frond dweller (GS4f), on Laminaria. March 2014. Littoral. West Anglesey, Wales. .
Same specimen as image 15 Patella pellucida. Length 10.2mm. Early adult, Growth Stage 4 frond dweller (GS4f), on Laminaria. March 2014. Littoral. West Anglesey, Wales. , but on pale background so translucent juvenile growth is paler than when on black, while more opaque skirt of adult growth is brown.
There is a large patch of black over the head (1). The colour of the rays is created by the light interference of zig-zag lamellae of calcite selectively reflecting blue/green in water. Colour saturation is intensified, and contrast with the visible white body within the shell is increased, by black, non-crystalline, colloidal particles (2) underlying parts of the rays, and a fine black spray over most of the interior surface of the shell (3). If the shell is rotated, different lines appear and disappear as the light’s angle of incidence on them changes (Li et al., 2015, “supplementary movie1” at www.nature.com/articles/ncomms7322#supplementary-information ). Flashing on and off of the rays as the Laminaria waves about and changes the angle of light incidence, may distract from the animal’s outline, or it may camouflage by resembling neighbouring iridescent weeds such as Chondrus crispus flic.kr/p/ZmLnQA . Li et al. (2015) suggested that P. pellucida may have Batesian mimicry of nudibranchs with iridescence assumed to warn of repugnant secretions or stinging sequestered cnidocytes. But Batesian mimics are usually less numerous than their models, as predators learn from the more numerous example and apply their learning to all. Iridescent nudibranchs are usually much less numerous than P. pellucida, so are unsuitable as models to mimic. . Adults concealed in holdfasts have little need for camouflage or warning colouration, and are often coated with epizoic growths 37 Patella pellucida. Length 10.1mm. Growth Stage 5 holdfast dweller (GS5h). March 2017. West Anglesey, Wales. . The use of the rays for sexual attraction in a non-copulating species is unlikely, especially as the rays become much less obvious at maturity, and their primitive eyes, almost certainly, cannot distinguish the rays.

Concise
Part 1, SHELL FORMS: 02 Patella pellucida. Growth Stage 2 (GS2). July 2012. Littoral. North Yorkshire, England. © P. Lightfoot.
Part 2, BODY & ANATOMY: 03 Patella pellucida. Growth Stage 3 (GS3). 2009. Sublittoral, Farne Islands, Northumberland, England. © P.Lightfoot.
Part, 3 HABITS & ECOLOGY: 04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.