Tardigrades, those chubby multiple-legged organisms known for being virtually indestructible, apparently see the world in black and white - they have no color vision.
Also known as water bears, these organisms are related to arthropods, a group of invertebrates characterized by jointed legs, segmented bodies, and hard exoskeletons, such as insects, spiders, crabs, and centipedes. Additionally, arthropods perceive colors due to opsins, which are light-sensitive proteins that aid in these creatures' vision and circadian rhythms or their body's internal clock.
While it is understood that tardigrades also have opsins, little is known about them and the vision they provide. It led a team of researchers to conduct genetic analysis in two distinct species of water bears and understand how opsins affect their vision. They presented their findings in the accepted manuscript "New Tardigrade Opsins and Differential Expression Analyses Shows Ontogenic Variation in Light Perception," appearing in the Genome Biology and Evolution (GBE) journal.
Understanding Vision and Opsins in Tardigrades
"In general, vision in tardigrades is not particularly well understood," noted James Fleming, lead author of the study and a postdoc fellow at the Natural History Museum of the University of Oslo in Norway, in correspondence with online science portal LiveScience. He also explained that there had been previous efforts to examine the eyespots of these water bears, usually using simple structures containing a few cells, as well as few eyeless species. These structures were also tested for their response to light.
Visual response to light in tardigrades, Fleming added, varies from moving from dark to light or vice versa, and that some of them start moving when exposed to light and look for darker locations. Another unique trait among their eyes is the absence of lenses, which suggests that they do not form images. Instead of being image or spatial-based as in a human vision, the tardigrades' response to light might be leaning more toward the direction of source or intensity of light received.
Additionally, the superphylum Ecdysozoa, known as "molting animals" that include nematodes and tardigrades on top of arthropods, has r-opsins, or rhabdomeric opsins, the group of proteins mostly associated with vision. Animals with color vision usually have multiple copies of these classes of opsins, each copy corresponding to a specific range in the light spectrum.
The Lack of Color Vision Among Water Bears
In the new study appearing in the GBE journal, Fleming's team examined genetic data from two species of water bears: Hypsibius exemplaris and Ramazzottius variornatus. They searched for opsins embedded in the transcriptomes, the DNA information transcribed as RNA material, meaning they are due to translation into specialized protein depending on their intended function.
The National Human Genome Research Institute explained that through examining the transcriptome analysis, researchers can identify when the particular genes within them will be activated, as well as the time when they remain dormant.
Noting that both species of tardigrades have well-documented transcriptomes, researchers found multiple r-opsins in tardigrades also associated with vision, tracking opsin activity in both specimens across the stages of their life cycle: egg, juvenile, and adult water bears.
Researchers also found that the opsins in both species did not respond well to different wavelengths of light. They reported that only specific opsins associated with vision were activated at different points in life, in different amounts, including those during their egg stage, a point where vision is not expected to be used prominently. These patterns led the researchers to conclude that while tardigrades do have multiple opsins, it is highly unlikely that they have color vision.
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