UV-protective sunscreen compounds and their differential accumulation across ocular tissues in fish eyes from Baltic Sea

Many fish species achieve ultraviolet radiation (UVR) protection by accumulating mycosporine-like amino acids (MAAs) from their diet. High concentrations are often found in the eyes, but the localization of MAAs in specific fish eye tissues is unknown. In the present study we dissected the eyes of six fish species, and prepared cornea, lens, vitreous body and a mixed tissue sample consisting of retina, choroidea and sclera (RCS). Furthermore, to test if MAAs are transferred further up the food web, we investigated the occurrence of MAAs also in eyes of warm-blooded piscine predators. Most, but not all, fish eye tissue samples contained the MAAs palythine, asterina-330, usujirene and palythene, but in different quantitative patterns. The highest MAA concentrations were found in the cornea, lens and vitreous body, while the innermost RCS layer showed a much smaller content. All data indicate that most MAAs are mainly located in the cornea and/or lens and vitreous body, thereby providing an improved photoprotection of the RCS layer, and in particular of the innermost, UVR-sensitive retina. No MAAs were detected in the eyes of cormorants and piscivorous mammals, suggesting that the trophic transfer of MAAs from the primary producers via zooplankton apparently ends in the fish.