A moving ribbon of rainbow colour can make a comb jelly look electrically illuminated. Before calling every bright marine display bioluminescence, ask whether the animal is producing light or changing the light that reaches it.
The Smithsonian Ocean account of jellies explains that comb jellies move using rows of fused cilia. The rainbow effect along those rows comes from light scattered by the moving structures, rather than being that effect’s own biological light production.
Read the lighting conditions
When watching a documented aquarium or research video, look for information about illumination. A lamp outside the animal can be part of what makes the visible colours possible. A dark background does not prove that no external light was used.
Pause the video and sketch where the coloured rows appear, then play it again to follow their movement. Keep the location of the effect separate from an explanation of its mechanism until you have read the source.
This does not mean a comb jelly can never produce light; it means the rainbow effect described here and bioluminescence are different phenomena. Identify the particular effect instead of assigning one explanation to everything bright in the scene.
The reward is a better question for the next ocean image: what is the light doing, and what evidence tells us where it came from?
Editorial illustration from this site’s image library; not documentary evidence of the example or object discussed.