Mycosporine-like amino acids in the fish ocular lens: Biochemistry, evolution, ecology and function

The lens of the vertebrate eye often contains pigment molecules that attenuate short- wavelength light. It has been hypothesized that these pigments may protect ocular tissues from UV damage and/or increase visual acuity by removing scattered light, preventing chromatic aberration and improving contrast vision. The most common lenticular pigments found in fishes are the mycosporine-like amino acids (MAAs). The purpose of this dissertation is to determine how MAAs accumulate in the lens, what controls variation in lenticular MAA content between species and how pigmentation changes during ontogeny. Lens pigmentation was analyzed primarily with spectrophotometry.; The flatfishes (order Pleuronectiformes) were used as a model group for this study. MAAs occurred in the lens and humors of the fish eye, but were found at higher concentrations in the lens. Concentrations were higher in the outer cortex of the lens than in the nucleus. Cultured lenses actively transported MAAs, but did not significantly alter the relative proportions of MAAs found in the culture medium. It appears that MAAs may share the same transport system with amino acids. Individuals of the same species contained similar pigmentation. The relative proportions of lenticular MAAs in a species reflected that species' evolutionary history. The total concentration of MAAs in a species reflected both its evolutionary history and its environment. Species living in deeper water were constrained to containing low levels of MAAs. Pigmentation in the lens of the California halibut, Paralichthys californicus, began to accumulate near the time of settlement and continued to increase in concentration as the fish grew. The timing of the accumulation of MAAs differed between environments.; The various data from this dissertation show that the accumulation of MAAs in the lens is influenced primarily by the metabolism of the fish, and secondarily by diet and the environment. MAAs come from the diet, but are differentially broken down and excreted before being transported to the eye. MAAs do not function as UV light protectants in flatfishes, but may serve visual functions. Any real function of these pigments may be a fortuitous byproduct of fish metabolism.