Variable vision in variable environments: the visual system of an invasive cichlid (Cichla monoculus, Agassiz, 1831) in Lake Gatun, Panama

An adaptive visual system is essential for organisms inhabiting new or changing light environments. The Panama Canal exhibits such variable environments due to its anthropogenic origin and current human activities. Within the Panama Canal, Lake Gatun harbours several exotic fish species including the invasive peacock bass ( Cichla monoculus , Agassiz, 1831), a predatory Amazonian cichlid. In this research, through spectral measurements and molecular and physiological experiments, we studied the visual system of C. monoculus and its adapative capabilities. Our results suggest that (1) Lake Gatun is a highly variable environment where light transmission changes throughout the canal-waterway, and that (2) C. monoculus has several visual adaptations suited for this red-shifted light enviroment. C. monoculus filters short-wavelengths (∼400 nm) from the environment through their ocular media and they tune their visual sensitivities to the available light through opsin gene expression. More importantly, based on shifts in spectral sensitivities of photoreceptors alone, and on transcriptome analysis, C. monoculus exhibits extreme intraspecific variation in the use of vitamin A 1 /A 2 chromophore in their photoreceptors. Fish living in turbid water had higher proportions of vitamin A 2 , shifting sensitivities to longer-wavelengths, than fish living in clear waters. Furthermore, we also found variation in retinal transcriptomes where fish from turbid and clear-waters exhibited differentially expressed genes that vary greatly in their function. We suggest this phenotypic plasticity has been key in C. monoculus ’ invasion.