Next time you see a rainbow, be thankful that you are not a water bear. Although microscopic creatures, also known as water bears, are the main survivors in terms of radiation, space, or extreme temperatures (SN: 7/14/17), they may lack a way to appreciate the world they live in: Look at the color.
Arthropods, close relatives of tardigrades, can see color and ultraviolet light. But tardigrade animals do not have the same light-sensitive protein that arthropods have, called opsin. Researchers reported on Genome Biology and Evolution on July 13 that this means they may not be able to see visible or ultraviolet light.
While working at the Institute of Advanced Biosciences at Keio University in Yamagata, Japan, evolutionary biologist James Fleming and his colleagues cataloged opsins in tardigrade animals. The team then used genetic analysis to determine whether these opsins are active in two species: Hypsibius exelaris and Ramazzottius variornatus.
Despite the active opsin, R. variornatus has no eyes, which is a problem of seeing things. Nonetheless, “he is still doing something with [opsin],” said Fleming, who now works at the Museum of Natural History at the University of Oslo. What exactly it is is still unknown.
H. Exelaris has eyes, but it does not have opsins that can respond to multiple types of light. The research team found that this is a key feature for detecting different colors. Fleming said that the eyes of tardigrade animals are very simple, which means that even with the extra opsin, your vision may look like a silent black and white film, rather than a cloudy 19th-century photograph.
Many opsin genes are also more active when these organisms are eggs than when they are adults. Fleming said: “It is understandable that when you are in an egg, it does not have much ecological use to see.” But there may be other photosensitive processes that are important for egg development.
The results of the study did not completely rule out whether tardigrade animals can see color. “In general, color vision is a very complex subject,” Fleming said. Direct testing of the water bear worm’s eyes will help researchers determine.