“Is it to see prey species? Or to find mates in a completely dark, or almost dark, environment? Or to avoid predators?” Salzburger said. Their most recent common ancestor dates to more than 100 million years ago, so the researchers think the additional genes evolved independently in each lineage. Although they found a dozen species with up to seven cone pigment genes, what really struck them was the discovery of 13 species that had more than one rod pigment gene.įour of those species stood out with five or more of the genes: the tube-eye ( Stylephorus chordatus), the glacier lanternfish ( Benthosema glaciale), the longwing spinyfin ( Diretmoides pauciradiatus) and the silver spinyfin ( Diretmus argenteus).Īll four of these fish live 1,000 meters to 2,000 meters below sea level. (At those depths, most of the light is produced by fish themselves through bioluminescence.)įor the new study, the researchers started by counting the number of genes for both rod and cone pigment proteins in the genomes of 101 species of fish living in a diverse array of habitats. ![]() Some developed large pupils and very long rods help them catch whatever light is around. Very little is known about fish that reside more than 1,000 meters below sea level. After all, if any fish stood to benefit by having more ways to see in dark conditions, it would be fish that live in water so deep that light barely reaches them. “We just thought if other fish are more variable in their visual system than previously thought, we should look at the deep-sea fishes,” said Walter Salzburger, an evolutionary biologist at the University of Basel in Switzerland who oversaw both the 2015 study and the new one. Their question was prompted by a 2015 study of mostly shallow-water fish that turned up several species with more genes for cone pigment proteins than scientists had expected. ![]() The rods can’t distinguish between colors because they all have the same pigment protein, which is why humans and most other animals are said to be colorblind at night.Ĭortesi and his colleagues wondered if they could find some exceptions among fish who lived in perpetually dark environments. A typical vertebrate eye has multiple types of cones that work in bright conditions - each capable of sensing a certain range of colors - and one type of rod that senses light when the environment is dim.
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