Multiple ecological axes drive molecular evolution of cone opsins in Beloniform fishes

Beloniformes, the order including needlefishes, flying fishes, halfbeaks, and allies, comprise over 200 species occupying a wide array of habitats—from the marine epipelagic zone to tropical rainforest rivers. These fish also exhibit a variety of diets, including piscivory, herbivory, and zooplanktivory. The diversity of these species makes them an excellent model group for studying the relative impact of different ecological axes on the molecular evolution of visual transduction genes. 

Katherine D. Chau and colleagues have investigated how diet and habitat have affected the molecular evolution of Beloniform cone opsins, visual proteins that play a key role in bright light and color vision. Via codon-based clade models of evolution, they identified evidence for positive selection in medium-wavelength opsins for piscivores and long-wavelength opsins for zooplanktivores. Medium-wavelength opsins, sensitive to the visual spectrum’s blue/green region, likely enhance prey fish detection against dark backgrounds. In contrast, long-wavelength opsins (sensitive to red light) are thought to help zooplanktivores detect pigment molecules present in surface-dwelling zooplankton. 

Although marine/freshwater habitat transitions also affect opsin molecular evolution, Chau and colleagues found that diet plays a more critical role in Beloniform visual adaptation. Overall, the study suggests that evolutionary transitions along ecological axes produce complex adaptive interactions that affect selection patterns on genes that underlie vision. 

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