In researching structural colors, I recently learned something that surprised me. I always assumed the colorful jellyfish in aquariums were bioluminescencent. But the comb jelly (Ctenophore; in video above) is really something special and, in fact, not technically a jellyfish. Those rainbow like colors are created by tiny hair-like structures called cilia. The comb jelly constantly moves its cilia back and forth very quickly as a means to swim. Because those hair-like structures are so tiny, they diffract light as they move, causing effects that look like moving rainbows. Because this effect is not created by directly emitting different colors of light, it’s not bioluminescence, per se. It’s actually a kind of structural color. Since I brought up “bioluminescence,” let’s talk about its definition. This term means different things to different groups of people – same as “biomimicry.” One of my Facebook friends commented on this, saying: “all human problems are semantic,” meaning we use different terms to describe the same thing and vice versa. “Luminescence” means “emitting light.” There are many different types of luminescence including: photoluminescence (emitting light as a result of absorbing light), chemiluminescence (emitting light as a result of chemical reactions), mechanoluminescence (emitting light as a result of mechanical stimuli), etc. Photoluminescence can be further divided to fluorescence and phosphorescence, as shown in the figure below.
If “luminescence” means “emitting light,” then it would seem to follow that “bioluminescence” means “emitting light by living organisms.” But in the last 10 years, some people advocate that there should be a term called “biofluorescence”, defined as proteins that emit light by absorbing light first. Then the term “bioluminescence” would fall under the “luminescence” umbrella on the same level as chemiluminescence, mechanoluminescence, …etc, and be defined as light emitted by biochemical reactions assisted by enzymes.
As this figure shown here.
From this figure, you can probably see why putting “biofluorescence” and “bioluminescence” on the same level doesn’t really make sense. If we accept this new definition, can the following examples still be called “bioluminescence”?
It only glows when physically disturbed, right? Sounds like the definition for “mechanoluminescence”, doesn’t it?
However, it’s impossible to force definitions. Those “semantic” problems will always be there. At least we can try to understand what other people could possibly mean when they’re not using language in a way with which we’re familiar. I think this is a very important and fundamental skill for interdisciplinary and cross-cultural research, such as biomimicry.