Jellyfish are ancient invertebrates found in every ocean. Their coloration is complex and far from uniform, ranging from near-perfect invisibility to vibrant hues and the ability to generate their own light. Understanding their color requires looking beyond passive pigment and into their unique anatomy and chemical processes.
The Dominant Appearance: Transparency and Clarity
The most common visual state for jellyfish is transparency, which serves as highly effective camouflage in the open ocean. This clarity is primarily due to the mesoglea, the gelatinous, non-living layer that makes up the bulk of the animal’s body. The mesoglea is largely acellular, consisting of 95% or more water interspersed with fibrous proteins like collagen for structural support.
Because the mesoglea is highly hydrated, it refracts light similarly to the surrounding seawater, allowing the animal to blend into its environment. This adaptation is prevalent in species like the Moon Jellyfish (Aurelia aurita) and the Crystal Jellyfish, which appear almost invisible to predators and prey alike. This transparent structure helps them avoid detection.
The Spectrum of Jellyfish Colors
Moving away from transparency, many jellyfish species exhibit a range of visible colors, including blues, purples, reds, and golds. The purpose of these colors is often tied directly to the animal’s habitat, serving as specialized camouflage or protection. Surface-dwelling species, such as the Portuguese Man-o’-War, frequently display blue or purple tones that help them disappear against the ocean surface or downwelling sunlight.
In the deep sea, where sunlight does not penetrate, many species exhibit deep red or reddish-brown coloration, such as the Blood-belly Comb Jelly. Red light is quickly absorbed by seawater, meaning red pigments appear black in the perpetual darkness, providing excellent camouflage. Other species, like the Sea Nettle (Chrysaora fuscescens), may display vibrant red, orange, or brown hues, making them conspicuous in coastal waters.
The Source of Pigmentation
The fixed colors seen in jellyfish tissues are not produced by the animal itself but are acquired through two primary biological mechanisms: diet and symbiotic relationships. Pigments from consumed prey are absorbed and incorporated into the tissues, directly influencing the hue. For example, some Moon Jellyfish can take on pink or purple tints after consuming small, pigmented larval crustaceans.
A second source of color comes from symbiotic algae, known as zooxanthellae, which live within the jellyfish’s tissues. These dinoflagellate symbionts contain pigments like peridinin and chlorophyll, which impart golden, green, or brownish colors to the host, as seen in species like the Upside-Down Jellyfish (Cassiopea andromeda). The algae provide the jellyfish with photosynthates—organic carbon products from photosynthesis—and in return, the host provides a stable environment and necessary nutrients.
Creating Light: Bioluminescence
Distinct from passive coloration, bioluminescence is the active production and emission of light through a chemical reaction, a trait shared by over half of all jellyfish species. This process involves two specialized compounds: the light-emitting molecule luciferin and the enzyme catalyst luciferase. The enzyme accelerates the oxidation of luciferin, which releases energy as cold, visible light.
In many jellyfish, the light production system is refined by proteins such as the Green Fluorescent Protein (GFP), first discovered in the Crystal Jellyfish (Aequorea victoria). Energy from the initial chemical reaction is transferred to GFP, which re-emits the light at a longer wavelength, converting blue light into a green glow. Bioluminescence serves various functions, most commonly for defense in the deep ocean. These functions include startling a predator, creating a glowing decoy, or using a “burglar alarm” effect to attract a larger predator to attack the initial threat.