The roundworm Caenorhabditis elegans is among the most studied species on Earth, but it retains the capacity to surprise, or even astonish, scientists. The most recent discovery is the worms can avoid eating blue bacteria. That is wise since blue identifies the microorganisms as poisonous. It's also completely weird because these worms lack not only eyes but cells for processing light.
C. elegans feeds on microorganisms in decomposing organic matter. Some of these microbes use chemical warfare to exact revenge on anything that eats them, but that's not much use if their poisonous status isn’t recognized until after they've been consumed. Just as red signals macroscopic animals might carry toxins, the bacterium Pseudomonas aeruginosa accompany their dangerous chemicals with the blue pigment pyocyanin.
That's a smart defence against anything that can see them, but C. elegans can't. Nevertheless, biologists have noticed the worms still avoid pyocyanin-producing bacteria, greatly to the benefit of their health. Most biologists assumed C. elegans must be detecting something else about P. aeruginosa besides the color, most likely sensing chemicals through smell.
Professor Michael Nitabach of Yale University saw no reason to doubt that assumption, but fortunately for science one of his former students, Dipon Ghosh did. Ghosh thought the worms might have a way of detecting color. “I told him he was being ridiculous,” Nitabach said in a statement.
Ghosh persisted and persuaded Nitabach to help put his extraordinary hypothesis to the test. In Science they report C. elegans can indeed distinguish short-wavelength visible radiation (what the rest of us call blue light) from the rest of the spectrum, despite lacking any of the organs normally thought required to do so.
The pair found harmless bacteria dyed with pyocyanin were avoided by the worms – at least when there was enough light for them to detect it. Just in case the worms were sniffing out the pyocyanin, the authors tried another blue dye, and found C. elegans avoided that as well. However, when optical filters were used to remove the blue light, the worms were keen to feed again.
The prior assumption did have some truth, however. The authors tried 59 wild strains of C. elegans that have evolved in different conditions. Some appeared to rely heavily on blue-light sensitivity, while others were more influenced by chemical detection, suggesting the worms have adapted to their environments to prioritize one sense or the other.
Nevertheless, knowing why the worms can spot blue is different from knowing how. Some flatworms and deep-sea creatures possess patches of specialized photoreceptor cells that allow them to differentiate light from dark, but lack lenses to focus an actual image. However, C. elegans lacks even this level of optical development. They don’t even have the opsin photoreceptor genes thought to be responsible for light detection in other animals.
However, Ghosh and Nitabach were able to identify two proteins that allow C. elegans color to sense short-wavelength light and the genes that produce them. This makes it possible to expand the study of the differences between C. elegans strains.
It’s not the first time C. elegans have over-achieved. After all, this is the worm that performs calculus, despite lacking a brain.
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