Biology deep sea Evolution Genetics genomics

Fish with supervision? See for yourself!

Article: Musilova Z, Cortesi F, Matschiner M, Davies WIL, Patel JS, Stieb SM, De Busserolles F, Malmstrøm M, Tørresen OK, Brown CJ, Mountford JK, Hanel R, Stenkamp DL, Jakobsen KS, Carleton KL, Jentoft S, Marshall J, Salzburger W (2019) Vision using multiple distinct rod opsins in deep-sea fishes. Science (80- ) 364:588–592. DOI: 10.1126/science.aav4632

Surviving in the deep

Take a dive into the deep, dark ocean and you will see a diverse world of sea creatures that light up, use chemicals for energy, and now even see in the dark. An international group of scientists recently discovered that deep-sea fish can see in the dark ocean because their eyes have specially evolved over time. It was previously thought that the deeper a fish lives in the ocean, the simpler its eyes are, and that these deep-sea fish were essentially color blind. But, this trend does not continue for the bottom dwellers of the ocean. This increased vision in deep-sea fish aids in their ability to capture food in an otherwise desolate environment and sheds light on how deep-sea animals survive in the dark depths of the ocean. Without these special adaptations, it would be nearly impossible for life to live in the deep-sea.

Sunlight can’t penetrate farther than about 350 feet deep in the water, and so much of the ocean is pitch black. One way deep-sea animals adapt to the dark is by producing their own light called bioluminescence. While most fish and humans can barely detect the subtle shimmer produced from bioluminescence, the scientists discovered that the additional eye genes found in some deep-sea fish enable them to see more of the colors of bioluminescence better and capture the few rays of light available below a depth of 1000 meters (3300 feet). This visual adaptation is important for deep-sea fish to navigate, feed, and find friends in the dark ocean.

Deep-sea evolution

The tube-eye fish, Stylephorus chordatus, was found to have five different eye genes. The long cylindrical shape of its eyes increases light capture and also enables the fish to move the eyes from a horizontal to a vertical position. Image credit: Dr. Wen-Sung Chung, University of Queensland, Australia.

This vision discovery was made by comparing the genes found in the eyes of over 100 deep-sea fish. Most fish have one or two genes designed to receive light and detect color, but the scientists found that the lanternfish, the tube-eye fish, and two spinyfin fishes all have at least five genes. The silver spinyfin (Diretmus argenteus), which thrives down to 2000 meters (6500 feet) deep, has 38 eye genes — the most ever recorded in a fish! This result means that the genes in some deep-sea fish have diversified to capture all types of light possible and that they can actually see light and color in the dark.

 

 

A lanternfish, one of the deep-sea fish identified as having specially evolved eyes that enable to see light and color in the dark ocean (image source).

By comparing the eye genes of different deep-sea fish, the scientists also discovered that the ability of these fish to see in the dark evolved separately among the three different fish types (lanternfish, tube-eye & spinyfin). This is an important finding because these fish live in different parts of the world’s ocean so the fact that they all developed this deep-sea vision separately is an impressive example of evolution. Given that we know more about the moon than we do the deep ocean, these findings are important for understanding how life survives in the darkness of the deep.

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