Jellyfish are ancient marine invertebrates belonging to the phylum Cnidaria, populating every ocean depth. Many species possess the remarkable ability to create their own illumination. The glowing phenomenon observed in these creatures involves two fundamentally different ways of producing and manipulating visible light.
The Chemistry of Bioluminescence
Bioluminescence is the chemical creation of light, often called “cold light” because it generates almost no heat. The reaction is catalyzed by an enzyme or photoprotein acting upon a light-emitting substrate called luciferin. Oxygen is required to oxidize the luciferin, which elevates the resulting molecule to an energized state.
In many marine organisms, the luciferin is a compound known as coelenterazine. For species like the crystal jelly, Aequorea victoria, the reaction uses a photoprotein called aequorin, a complex containing coelenterazine. A sudden influx of calcium ions triggers the oxidation of coelenterazine, releasing energy as a photon. This typically emits a blue wavelength, which travels farthest through seawater.
The photoprotein system allows for a rapid, controlled flash of light, unlike a continuous glow. This chemical self-illumination is an active process, requiring the jellyfish to expend energy to generate light at will.
The Physics of Fluorescence
Fluorescence is a physical process of light re-emission, not chemical generation. It requires an external source of illumination to power the glow. Light-emitting chemicals absorb a high-energy photon and immediately re-emit a lower-energy photon.
The most famous example is the Green Fluorescent Protein (GFP), isolated from Aequorea victoria. When GFP is exposed to shorter wavelengths, such as blue light from bioluminescence or ultraviolet light, the protein absorbs that energy. It then instantly releases the energy as a longer, visible green wavelength.
The ability to fluoresce is due to a chromophore, a light-absorbing component, embedded within the GFP structure. The blue light from the bioluminescent reaction is often absorbed by neighboring GFP and re-emitted as green light. This explains why many of these jellyfish appear green to the human eye.
Survival Functions of Jellyfish Light
The ability to emit or re-emit light provides jellyfish with specialized tools for survival in the dark marine environment.
One primary function is the defensive “burglar alarm effect.” When attacked, the jellyfish releases a bright, flashing display of light. This intense flash attracts larger, secondary predators from the surrounding water. The goal is for the larger animal to prey upon the original attacker, allowing the jellyfish to escape. Deep-sea species such as the Atolla wyvillei use this dramatic light show when disturbed.
Light also serves as camouflage in mid-water depths where faint sunlight penetrates. Some species use counter-illumination, producing a soft glow on their underside that matches the low levels of downwelling sunlight. This emission hides the jellyfish’s silhouette from predators swimming below.
Other functions of light include:
- Communication with potential mates.
- Startling or distracting a predator with a sudden burst of illumination.