Axes of visual adaptation in the ecologically diverse family Cichlidae

The family Cichlidae contains approximately 2000 species that live in diverse freshwater habitats including murky lakes, turbid rivers, and clear lakes from both the Old and New Worlds. Their visual systems are similarly diverse and have evolved specific sensitivities that differ along several axes of variation. Variation in cornea and lens transmission affect which wavelengths reach the retina. Variation in photoreceptor number and distribution affect brightness sensitivity, spectral sensitivity and resolution. Probably their most dynamic characteristic is the variation in visual pigment peak sensitivities. Visual pigments can be altered through changes in chromophore, opsin sequence and opsin expression. Opsin expression varies by altering which of the seven available cone opsins in their genomes are turned on. These opsins can even be coexpressed to produce seemingly infinitely tunable cone sensitivities. Both chromophore and opsin expression can vary on either rapid (hours or days), slower (seasonal or ontogenetic) or evolutionary timescales. Such visual system shifts have enabled cichlids to adapt to different habitats and foraging styles. Through both short term plasticity and longer evolutionary adaptations, cichlids have proven to be ecologically successful and an excellent model for studying organismal adaptation.