Yes, jellyfish do actually glow in the dark. However most do no not.
Many jellyfish are completely transparent or translucent in appearance. They are frequently displayed in public aquariums fading hypnotically from one color to another. Although watching these delicate creatures slowly fading form one color to another is admittedly mesmerizing, it is not the jellyfish itself that is glowing. This beautiful array of colors is simply a matter of colored light passing through the jellyfish’s membranes.
The species commonly referred to as comb jellyfish do not need an elaborate light set up to produce their very own disco display. They are not, however, actual jellyfish. Nor are they bioluminescent. They simply appear to be. Although they are closely related to one another, comb jellies are from an entirely different phylum (Ctenophora) than jellyfish. Jellyfish use what can most accurately be described as jet propulsion as a means of locomotion. Comb jellies have 8 rows of combs, thus their name. These combs are the equivalent to thousands of tiny oars. They move through the water by beating these combs in sequence. Their combs are highly reflective and produce a mesmerizing disco type light show as they move. Scientists speculate that this hypnotic light display acts to entice the natural curiosity of potential prey.
There is only one species of jellyfish that is bioluminescent,Aequorea spp. These jellyfish are more commonly referred to as Aequorea victoria or crystal jellyfish. Their bilochemical light emission is produced by a combination of two molecules; aequorin and green fluorescent protein. Together they produce flashes of green light. GFP (green fluorescent protein) has been isolated by scientist and is being used in cellular research. GFP was the proteinDr. Zhiyuan Gong and his colleagues at the National University of Singapore incorporated into the genome of zebra danio eggs to create the immensely popular freshwater aquarium fish, the Glofish. Further experimentation has progressed to the incorporation of this gene into mammals and most recently primates. It should be noted that none of these genetically engineered animals actually glow in the dark. They will, however, fluoresce under a backlight.
Moon jellyfish do not glow in the dark. But they do have translucent bodies. They look absolutely amazing with an array of fading LEDs shining through them. You can now buy a Jellyfish Fish Tank Aquarium to raise your own pet jellyfish in. You can even light them up just like they are in the big public aquarium jellyfish exhibits.