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The almost invincible tardigrades can see only in black and white

The next time you look at a rainbow, thank you for not being a tardigrade. Although microscopic creatures, also known as water bears, are the master survivors when it comes to radiation, space, or extreme temperatures (SN: 7/14/17), they may lack a way to appreciate the world in which they live. : the ability to see in color.

Nearby arthropod relatives of Tardigrades can see color and ultraviolet light. But tardigrades do not have the same light-sensitive proteins, called opsins, as arthropods. That means they may not be able to see visible light or UV, researchers reported on July 13 in Genology Biology and Evolution.

While working at Keio University Institute of Advanced Biosciences in Yamagata, Japan, evolutionary biologist James Fleming and colleagues cataloged what tardigrade options they have. The team then used genetic analysis to find out whether or not these opsins were active in two species: Hypsibius exemplaris and Ramazzottius variornatus.

Despite having active options, R. variornatus has no eyes: a problem to see things. Still, “he’s doing something with (the opsins),” says Fleming, now at the Natural History Museum at the University of Oslo. It is still unknown what exactly that is.

H. exemplaris has eyes but has no options that can respond to various types of light, depending on the equipment: a crucial trait for detecting different colors. And the belated eyes are pretty straightforward, Fleming says, which means that even with additional opsins his vision can resemble a dumb black-and-white film instead of a murky 19th-century photo.

Many of the opsin genes were also more active when the critters were eggs than when they were adults. “It’s understandable that there’s not much ecological use to be able to see while you’re inside an egg,” Fleming says. But there could be other light-sensitive processes important for egg development.

The findings do not completely rule out whether tardigrades can see the color. “Color vision in general is a very messy subject,” says Fleming. Directly testing the eyes of water bears would help researchers know for sure.



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