Near-invincible tardigrades may see only in black and white

Next time you’re looking at a rainbow, be thankful you’re not a tardigrade. While the microscopic creatures, also known as water bears, are master survivors when it comes to radiation, space or extreme temperatures (SN: 7/14/17), they may lack one way to appreciate the world they live in: the ability to see in color.

Tardigrades’ close arthropod relatives can see color and ultraviolet light. But tardigrades don’t have the same light-sensing proteins, called opsins, that arthropods do. That means they might not be able to see either visible or UV light, researchers report July 13 in Genome Biology and Evolution.

While working at Keio University Institute of Advanced Biosciences in Yamagata, Japan, evolutionary biologist James Fleming and colleagues cataloged which opsins tardigrades have. Then the team used genetic analysis to figure out whether these opsins were active or not in two species: Hypsibius exemplaris and Ramazzottius variornatus.

Despite having active opsins, R. variornatus doesn’t have eyes — a problem for seeing things. Still, “it’s doing something with [the opsins],” says Fleming, now at the University of Oslo Natural History Museum. What exactly that is remains unknown.

H. exemplaris have eyes but don’t have opsins that can respond to multiple types of light, the team found — a crucial trait to detect different colors. And tardigrade eyes are fairly simple, Fleming says. Even with additional opsins, they can’t make out images. In a more complex eye, those additional opsins might create vision that resembles a black and white silent film instead of a murky 1800s photo, he says.

Many of the opsin genes also were more active when the critters were eggs than when they were adults. “Understandably, there is not a lot of ecological use for being able to see whilst you’re inside of an egg,” Fleming says. But there might be other light-sensitive processes important for the egg’s development.  

The findings don’t entirely rule out whether tardigrades might see color. “Color vision in general is a very messy topic,” Fleming says. Directly testing water bears’ eyes would help researchers know for sure.