Tardigrades May See Only in Black & White

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. Scientists 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.

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. 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.

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. 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.

CITATIONS

J.F. Fleming, D. Pisani and K. Arakawa. New tardigrade opsins and differential expression analyses shows ontogenic variation in light perception. Genome Biology and Evolution. Published online July 13, 2021. doi: 10.1093/gbe/evab164.