Moon jellyfish (Aurelia aurita), commonly found in coastal waters, often appear to emit a soft, ethereal glow, leading many to wonder if they produce their own light. While they seem to shine, moon jellyfish are not truly bioluminescent like some deep-sea creatures. Their visible luminescence stems from fluorescence, a distinct biological process from chemical light production. This article will explore the specific mechanisms behind the moon jellyfish’s apparent glow and differentiate it from true bioluminescence, clarifying why these invertebrates appear to illuminate their surroundings.
Understanding Bioluminescence
Bioluminescence is the ability of living organisms to generate light through a chemical reaction within their bodies. This natural light production is a form of “cold light,” meaning less than 20% of the energy released manifests as thermal radiation, making it highly efficient. The core of this process involves a light-emitting molecule, luciferin, and an enzyme called luciferase, which catalyzes the reaction. Oxygen typically plays a crucial role, allowing the luciferin to oxidize and release energy as visible light.
Many organisms exhibit bioluminescence, from fireflies to deep-sea fish, fungi, and bacteria. These organisms actively produce and control their light, utilizing it for a range of functions such as attracting mates, luring prey, deterring predators, or communicating within their species. This internal light generation, independent of any external illumination, is a defining characteristic of true bioluminescence.
Moon Jellyfish and Their Apparent Glow
Moon jellyfish (Aurelia aurita) are frequently observed with a subtle glow, leading to the assumption that they are self-illuminating. Despite their appearance, moon jellyfish do not possess the biochemical machinery to produce their own light through bioluminescence. Unlike organisms such as fireflies, they do not undergo a luciferin-luciferase reaction to generate light from within their cells.
The luminous effect seen in moon jellyfish stems from fluorescence. This distinction highlights a fundamental difference in how light is generated. The common misconception arises from the visual similarity of their glow to true bioluminescence, especially in dim environments. Their visible light emission is entirely dependent on absorbing energy from an external light source, which they then re-emit.
The Science of Moon Jellyfish Fluorescence
The apparent glow of moon jellyfish is attributed to specific fluorescent proteins within their bodies, most notably Green Fluorescent Protein (GFP). These proteins absorb light at a particular wavelength and then re-emit it at a longer, different wavelength. In moon jellyfish, GFP typically absorbs higher-energy light, such as ultraviolet (UV) light or blue light, which is often abundant in their shallow coastal habitats.
Upon absorbing this higher-energy light, electrons within the GFP molecule temporarily jump to a higher energy state. As they return to their ground state, they release absorbed energy, re-emitted as light of a lower energy and longer wavelength, typically appearing green. This process requires a continuous external light source; without it, moon jellyfish would not appear to glow.
The presence of GFP and similar fluorescent proteins is widespread among various marine organisms, serving diverse biological roles. For moon jellyfish, their fluorescence might provide a form of camouflage in certain light conditions, help with light sensing, or even offer protection from harmful UV radiation. The discovery and study of GFP, originally isolated from the crystal jellyfish Aequorea victoria, revolutionized molecular biology, earning a Nobel Prize for its utility as a biological marker in research. This protein allows scientists to track biological processes and visualize structures within living cells.
Jellyfish That Are Truly Bioluminescent
While moon jellyfish exhibit fluorescence, numerous other jellyfish species actively produce their own light through true bioluminescence. These organisms utilize the luciferin-luciferase system to create internal light, often with striking visual effects. One notable example is the Atolla jellyfish (Atolla wyvillei), a deep-sea species that emits a bright, pulsating blue light, sometimes described as a “burglar alarm” display. This rapid flashing is thought to deter predators by attracting larger predators to the scene.
The crystal jellyfish, Aequorea victoria, is another well-known example that possesses both fluorescence and true bioluminescence. While its Green Fluorescent Protein (GFP) is famous for its fluorescent properties, Aequorea victoria also produces blue light through a distinct bioluminescent reaction involving the protein aequorin. This blue light can then excite the GFP, leading to a green glow, creating a complex light display.
Comb jellies, or ctenophores, represent another diverse group of marine invertebrates that are predominantly bioluminescent. Unlike true jellyfish, they move using rows of cilia, which refract light to create iridescent rainbow effects, but many also produce their own light through chemical reactions. Species like Mnemiopsis leidyi emit flashes of blue or green light when disturbed, a common defensive mechanism to startle or distract potential threats. These diverse bioluminescent displays serve various ecological purposes, including attracting prey, signaling to mates, or evading detection.