The shell-eyes of the chiton Acanthopleura granulata (Mollusca, Polyplacophora) use pheomelanin as a screening pigment

Certain species of chiton (Mollusca, Polyplacophora) have hundreds of small (<100 mu m) eyes embedded in their dorsal shell plates. These eyes each contain a retina, a layer of screening pigment, and a lens. Previously, we demonstrated that the eyes of chitons provide spatial vision. As in other camera-type eyes, the screening pigments in the eyes of chitons absorb off-axis light in order to preserve the contrast of images formed on the retina. Our results indicate that the red-brown, alkali-soluble screening pigment associated with the eyes of the chiton Acanthopleura granulata (Gmelin, 1791) is pheomelanin. Using high-performance liquid chromatography (HPLC) and MALDI-TOF mass spectroscopy, we find that degrading A. granulata's screening pigment with alkaline hydrogen peroxide produces 6-(2-amino-2-carboxyethyl)-2-carboxy-4-hydroxybenzothiazole (BTCA), a diagnostic marker of pheomelanin. Chitons are the first molluscs demonstrated to use pheomelanin as a screening pigment in their eyes. Our results suggest that the image-forming eyes of chitons may have evolved separately from eyes that employ different types of screening pigment, such as those of most other invertebrates. Further, we hypothesize that change in the expression pattern of tyrosinase - an enzyme responsible for melanin synthesis in many other metazoans - may have contributed to the origin of screening pigments in chitons, a critical step in the evolution of their eyes.