The Moon Jellyfish (Aurelia aurita) is a widespread species, commonly encountered in coastal waters and often displayed in public aquariums. These mesmerizing creatures feature translucent, saucer-shaped bells, making their four horseshoe-shaped reproductive organs clearly visible. The source of the subtle glow sometimes associated with them is often misunderstood, leading to confusion about whether these animals generate their own light or merely reflect it. This involves a fundamental distinction in how living organisms interact with light in the marine environment.
Fluorescence Versus Bioluminescence
The direct answer to the question is that Moon Jellyfish are fluorescent, not bioluminescent, which involves two entirely different physical processes. Bioluminescence is the creation of light through a chemical reaction within an organism, making the creature its own light source. This process typically involves a light-emitting molecule, luciferin, reacting with oxygen and being catalyzed by an enzyme called luciferase, similar to how fireflies glow or deep-sea fish flash.
Fluorescence is the immediate re-emission of light after absorbing energy from an external light source. A fluorescent molecule, known as a fluorophore, absorbs light at a shorter wavelength, such as ultraviolet or blue light. The molecule then instantly releases that energy as light at a longer, visible wavelength, often appearing as green or red light. This process requires outside energy, meaning the organism only appears to glow when illuminated by the correct external light.
Because fluorescence depends on external illumination, the effect disappears the moment the light source is removed. Bioluminescent organisms, such as deep-sea jellies, produce light independently of any external source, making them self-illuminating. This difference in the mechanism of light generation is the defining factor distinguishing the two phenomena.
The Specific Mechanism of the Moon Jellyfish Glow
The characteristic green glow of the Moon Jellyfish is caused by the presence of Green Fluorescent Protein (GFP). This protein acts as the fluorophore, absorbing light energy and re-emitting it as visible green light. The GFP molecule forms a barrel shape that protects the central chromophore, which is responsible for the light-emitting reaction.
The chromophore within the GFP absorbs high-energy blue or ultraviolet light, wavelengths that penetrate well into the upper ocean layers. This absorption raises the energy level of the protein’s electrons to an excited state. When the electrons return to their stable ground state, they release the excess energy as a photon of green light.
While the Moon Jellyfish itself is fluorescent due to GFP, the protein was first discovered in the related crystal jellyfish (Aequorea victoria), which is both bioluminescent and fluorescent. In A. victoria, a separate protein called aequorin generates a blue bioluminescence upon binding calcium ions. The GFP then absorbs this blue light and converts it into a green fluorescence through a process called energy transfer.
Though Aurelia aurita may not utilize this full bioluminescence-fluorescence cascade, its fluorescence relies on the same light-conversion mechanism of the GFP molecule. The protein’s structure is efficient; the chromophore forms spontaneously without external enzymes, only requiring molecular oxygen. This robust process allows scientists to use GFP extensively as a biological marker in research.
Ecological Role of Moon Jellyfish Fluorescence
The exact function of GFP-driven fluorescence remains a topic of scientific investigation, though several roles have been proposed. One hypothesis suggests the fluorescent proteins offer photoprotection. By absorbing damaging ultraviolet light and re-emitting it as less harmful green light, the proteins could shield sensitive tissues, such as the reproductive organs, from sun damage in shallow waters.
Another possible function relates to species recognition or reproductive signaling, especially where fluorescence is concentrated in the gonads. This glow could potentially signal sexual maturity or reproductive status to other Moon Jellyfish. Since the exact function is not definitively known, fluorescence likely serves multiple purposes depending on the life stage and environmental conditions.
Compared to the clear functions of bioluminescence in deep-sea organisms—which often use light for attracting prey or startling predators—the ecological purpose of the Moon Jellyfish’s fluorescence is more subtle. The simple, translucent body plan of the Moon Jellyfish, which is primarily a passive, drift-feeding organism, suggests its light interactions may be a secondary adaptation rather than a primary tool for hunting.