Many species of jellyfish and their close relatives possess the remarkable ability to produce their own light, a phenomenon known as bioluminescence. This natural light show is widespread in marine environments, particularly in the dark depths where sunlight cannot penetrate. The soft, often blue or green glow is the result of a highly efficient chemical reaction occurring within their bodies, not heat. This adaptation serves various complex purposes, making the glow a tool for survival in the vast, dark ocean.
The Chemical Process of Light Production
Bioluminescence is a form of chemiluminescence, meaning the light is generated through a chemical reaction in a living organism. The process is remarkably efficient, producing light known as “cold light” because less than 20% of the energy is lost as heat. This reaction fundamentally involves the interaction of two main components: a light-emitting molecule and an enzyme catalyst.
The light-emitting molecule is called a luciferin, and the enzyme that speeds up the reaction is called a luciferase. In a typical bioluminescent reaction, the luciferase catalyzes the oxidation of luciferin, requiring oxygen. This oxidation creates an unstable, high-energy intermediate molecule that decays to a stable state. As the molecule drops from its excited state, it releases the excess energy as a photon, or visible light.
Many jellyfish utilize coelenterazine, a specific type of luciferin often bound to a specialized photoprotein like aequorin. Instead of a standard luciferase, the photoprotein-coelenterazine complex is triggered by calcium ions. The rapid influx of calcium ions causes a conformation change in the protein, initiating the oxidation of coelenterazine and producing a flash of blue light. Some species use a secondary protein to shift this blue light to a different color, often green, before emission.
Ecological Roles of Bioluminescence
The ability to generate light serves several functions for jellyfish and other marine life, primarily defense, camouflage, and communication. A common defensive strategy is the “burglar alarm” effect, where a threatened organism produces a bright flash to attract a larger predator. The sudden light reveals the attacker to its own potential predators, causing the initial threat to break off its attack. This acts as a decoy, allowing the jellyfish to escape.
Another function is counterillumination, which is a form of active camouflage used by organisms living in the mesopelagic or “twilight” zone. These animals emit light from their undersides to match the faint, filtered sunlight or moonlight coming from above. By adjusting the intensity of their glow, they effectively erase their silhouette, making them nearly invisible to predators looking up from below.
While less common than defense, some cnidarians use their glow for intraspecies communication or signaling. This can involve species-specific light patterns to attract mates in the deep sea. Precise control over the timing and location of the light allows for complex displays. A localized flash can also be used as a startle tactic, similar to a flare, to momentarily blind or disorient a potential threat.
Identifying Bioluminescent Cnidarians
The organisms in the phylum Cnidaria, which includes true jellyfish, sea anemones, and corals, show wide diversity in light-producing capabilities. The most famous example is the hydrozoan jellyfish Aequorea victoria, often called the crystal jelly, found along the Pacific coast of North America. This species is known for its natural blue flash and the secondary protein it contains.
The crystal jelly is the source of Green Fluorescent Protein (GFP), which absorbs the initial blue light produced by the photoprotein aequorin and re-emits it as a bright green glow. The discovery and isolation of GFP earned a Nobel Prize and revolutionized cellular biology, making this particular jellyfish one of the most scientifically significant organisms. The phenomenon is not limited to true jellyfish (Scyphozoa); many siphonophores, which are colonial hydrozoans, also exhibit spectacular bioluminescence.
Siphonophores, such as the genus Praya, can reach extraordinary lengths and produce massive, undulating displays of light when disturbed. Many comb jellies, or ctenophores, are also highly bioluminescent and often create a shimmering, rainbow-like effect due to the diffraction of light off their comb rows. These examples illustrate that the ability to glow is a widespread and varied trait among the gelatinous inhabitants of the world’s oceans.