scabbardfish Deletion of single molecule makes fish switch to violet vision Researchers from Emory University have identified the first fish to have switched from ultraviolet vision to violet vision, i.e. the ability to see blue light. This fish in question – a type of scabbardfish – is also the first example of an animal where a deleted molecule has resulted in a change in visual spectrum.

Many species, including humans, have violet vision but our common vertebrate ancestor had UV-vision and could not sense the blue colour spectrum.

All fish studied before the scabbardfish have been found to have UV vision. The scabbardfish is believed to have switched from UV vision to violet vision by deleting the molecule at site 86 in the chain of amino acids that makes up the opsin protein.

“Normally, amino acid changes cause small structure changes, but in this case, a critical amino acid was deleted,” Yokoyama explains.

Vision is of particular interest to evolutionary geneticists since it is a comparatively straight-forward sensory system with a low number of genes involved. Human vision is for instance made possible by no more than four genes.

It’s amazing, but you can mix together this small number of genes and detect a whole color spectrum,” says evolutionary geneticist and research team leader Shozo Yokoyama. It’s just like a painting.”

In their study, the Emory researchers linked molecular evolution to functional changes and the possible environmental factors driving them.

This multi-dimensional approach strengthens the case for the importance of adaptive evolution,” says Yokoyama. “Building on this framework will take studies of natural selection to the next level.”

The Scabbardfish spends most of its life at a depth of 25-100 meters and at these depths UV light is less intense then violet light, something which may have prompted the change in vision. Living deep down in the ocean will however not necessarily make you benefit from a vision switch; the Lampfish has for instance retained its UV vision – most likely because it swims up to the surface at night to feed on translucent crustaceans that are easier to locate if you have UV vision.

The finding implies that we can find more examples of a similar switch to violet vision in different fish lineages,” says Yokoyama. “Comparing violet and UV pigments in fish living in different habitats will open an unprecedented opportunity to clarify the molecular basis of phenotypic adaptations, along with the genetics of UV and violet vision.”

The article has been published in the October 13 issue of Proceedings of the National Academy of Sciences.

In addition to evolutionary geneticist Shozo Yokoyama, the research team also included post-doctoral fellow in biology Takashi Tada and post-doctoral fellow in biology and computational chemistry Ahmet Altun.