Comb Jelly's 8 Cilia Rows Scatter Rainbow Light off Cornwall

Q: Why did a jelly-like creature in Cornwall glow with rainbow colors if it wasn’t bioluminescent—or even a true jellyfish?

It glowed with rainbow colors because it was a comb jelly, not a true jellyfish, and the colors came from light scattering off its moving comb rows rather than from the animal producing light. The Cornwall animal belonged to the phylum Ctenophora, a separate group from jellyfish. Comb jellies swim using eight rows of tiny hair-like cilia, called “combs,” that beat in sequence like miniature oars. When sunlight or a camera light hits those moving cilia, the light is split and scattered into shifting rainbow colors. Scientists describe this as structural iridescence. That means the display Katie Maggs saw was not bioluminescence. Bioluminescence is a chemical process in which an animal makes its own light, usually more visible in darkness. The rainbow shimmer on comb jellies is different: it is an optical effect created by the structure and motion of the cilia under external light. Comb jellies often fool people because they look jellyfish-like, but they are biologically distinct and generally harmless to humans because they do not sting. Instead of stinging cells, many use sticky tentacles to catch prey such as crustaceans and larvae. The sighting was striking partly because comb jellies are almost transparent, so the flashing comb rows can appear especially vivid as they pass through clear, sunlit water.

3 articles · Updated · The Dodo · Jul 31

A comb jelly filmed by snorkeler Katie Maggs off Cornwall appeared to glow in rainbow colors as it swam past her face.
Eight rows of hair-like combs create the effect by scattering light as their cilia move through the water, not by producing bioluminescence.
Comb jellies are distinct from jellyfish, using sticky tentacles rather than stinging cells to catch crustaceans, larvae and other jellies.
Fossils dating back 525 million years make comb jellies among the oldest animals, though most seen by people are only a few inches long and can reach 5 feet.