Remarkable Fish Can See Color in Deep, Dark Ocean

The viperfish uses multiple rod opsins within their eyes and can see color even in the depths of the ocean. (Dr. Wen-Sung Chung / University of Queensland, Australia)

(CN) – In the dark world of the deep sea, being able to discern prey from predator can mean life or death for fish. Now researchers have learned how certain fish adapt and thrive in this harsh environment: They can see in color.

A research team announced their findings Thursday, shining new light on the evolution of vision in vertebrates – including humans. The research published in the journal Science involved the work of an international team of researchers, including evolutionary biologists from the University of Basel in Switzerland.

University of Queensland Brain Institute scientist Fabio Cortesi said humans and other vertebrates use two types of photoreceptor cells, called rods and cones, to see. Cones are used in bright-light conditions, while rods are generally used in dim light.

Rods and cones contain light-sensitive proteins called opsins, and each of the opsins absorbs light at specific wavelengths.

Our ability to see in color is because cones use around four different opsins, Cortesi said, and this gives vertebrates sensitivity to a broad range of colors.

However, about 99 percent of vertebrates have only one opsin protein in their optical rods, so most are color-blind in dim light, according to Cortesi.

Many organisms have adapted to life in the near-total darkness of the seemingly inhospitable deep sea.

“It’s very monochromatic,” University of Queensland ecologist Fanny de Busserolles said of the deep sea. “But we have discovered some spectacular exceptions.”

Fish that live in the deep sea – 200 to 1,500 meters below the surface – tend to be single-opsin vertebrates. But researchers have now discovered certain fish can see in color down deep, and being able to distinguish the kind of light coming from other aquatic vertebrates can mean life or death for deep-sea creatures.

“If you want to survive down there, you need to quickly decide if you are seeing a potential predator or potential prey,” Cortesi said.

Some organisms, such as the anglerfish, attract prey with their bioluminescent organs.

Researchers examined the genomes of 101 fish species and found 13 species have more than one optical rod opsin gene. The silver spinyfin fish has 38 of these opsin genes, which Cortesi said may have evolved as a survival mechanism for the silver spinyfin.

“Our findings help redefine the current paradigm of vertebrate vision in terms of the role of rod photoreceptors,” the researchers said in a statement, noting their work “presents yet another instance in which analyzing whole genomes led to new biological discoveries.”

The deep sea is the largest habitat on Earth but one of the least explored because its remoteness.

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