Most jellyfish you see at the beach or in an aquarium are clear, milky, or faintly tinted, because their bodies let light pass almost straight through. But jellyfish color covers a wide range, from brown and gold to cobalt blue to deep crimson, and some species make their own light. Each color usually does a job, such as feeding the animal, shielding it from the sun, or hiding it. One jellyfish’s glow even ended up changing medical research worldwide.
Why Most Jellyfish Look Clear
A jellyfish’s body is roughly 95% water held in a soft jelly layer. Light only shows you an object when it bends or bounces off the surface, and the less an object differs from what surrounds it, the less light it bends. Even the gas float of a Portuguese man o’ war bends light almost exactly as much as seawater does,1PubMed Central. Optical properties of the jellyfish surface above the waterline: microvillar array in pleustonic hydrozoans so light passes through that float with little reflection.
In water, a moon jelly may be nearly invisible except for its four pinkish rings. Stranded on sand, it collapses into a glossy, colorless blob. The colored parts you notice are usually organs, such as the stomach or gonads, not the bell itself. Aquariums light jellies from the side or against dark backgrounds so their edges show.
Being see-through is a hiding strategy. In open water there is nowhere to hide, so a clear body keeps a jellyfish out of sight of predators and prey. How color looks underwater depends on the light around the animal and the eyes of whoever is looking.2Oceanography. Light, Colour, and Vision in the Ocean
A see-through body is still not invisible. A jellyfish is hardest to see when light comes evenly from all sides, as in open, bright water, and easier to spot when lit from one side or silhouetted against the surface. The stomach and gonads are denser or carry pigment, so they stay visible even when the bell almost vanishes. A small amount of light also bounces off the body, which is why a diver can catch a faint glint or outline.
Clear does not mean harmless. Transparent species still carry stinging cells in their tentacles, and those thin, nearly invisible tentacles can trail well beyond the bell.
Keep children and dogs away, because curious noses and bare feet are the usual way people get stung by beached jellyfish. If you want a closer look, a few feet of distance or a stick works better than a hand, and you can still see its shape and any colored organs. If you are stung, rinse the skin with seawater rather than fresh water, since fresh water can set off more stinging cells in some species.
Brown and Golden Jellyfish Get Color From Algae
In the upside-down jellyfish Cassiopea, tiny algae live in the tentacles and give them their color.3PADI Blog. Three Incredible Jellyfish You Probably Don’t Know About The spotted lagoon jellyfish Mastigias papua is brownish because of these algae and has white spots.4Plankton and Benthos Research. Umbrella of Mastigias papua (Scyphozoa: Rhizostomeae: Mastigiidae): hardness and cytomorphology with remarks on colors The algae use sunlight to make food and share some of it with the jellyfish, so these jellies often sit or swim where sunlight is strong.
That partnership shapes where they live. The algae need light, so these jellies stay in clear, sunlit shallows such as lagoons, mangroves, and sandy flats rather than deep water. Some pulse in sunny spots through the day, which keeps the algae exposed. Because the jelly gets part of its food from its partners, it can get by on less hunting than a jelly without algae, though it still catches tiny prey.
Upside-down jellyfish lie bell-down on sandy or mangrove shallows with their tentacles pointing up toward the sun. From above they look like a patch of brownish-green seaweed.
The shade shifts with light. In Cassiopea, color changes under different amounts of sunlight track changes in how many algae it holds.5Frontiers in Marine Science. Illuminating Cassiopea jellyfish: biochemical revelations from metabolism to coloration under ultraviolet A and photosynthetically active radiation Strong sun can reach about 100,000 lux in the shallow water where these jellies live, while shade cuts that to about a quarter.6Journal of Experimental Marine Biology and Ecology. Light exposure induces phenotypic plasticity of the upside-down jellyfish Cassiopea sp. and its endosymbiotic dinoflagellates So the same species can look paler or darker depending on how much light it gets.
Color can also change with age and where the animal grows. Young Rhizostoma luteum jellyfish turn from light mustard to dark golden yellow as they develop.7PubMed Central. First description of the life cycle of the jellyfish Rhizostoma luteum (Scyphozoa: Rhizostomeae) Yet lab-raised medusae of this species had violet bells, while wild ones were milky bluish-white.8PubMed Central. First description of the life cycle of the jellyfish Rhizostoma luteum (Scyphozoa: Rhizostomeae) A jellyfish’s color is not fixed for its species and can depend on its diet and surroundings.
Blue and Red Jellyfish Use Pigment for Protection
By-the-wind sailors (Velella velella) drift at the ocean surface with a rich cobalt-blue color.9Los Angeles Times. What are the blue blobs washing up on SoCal beaches? Welcome to Velella velella Valhalla That blue pigment provides both camouflage and protection from the sun’s UV rays on the open ocean.10Los Angeles Times. What are the blue blobs washing up on SoCal beaches? Welcome to Velella velella Valhalla The blue dragon sea slug uses the same kind of blue pigment as a sunscreen.11Oregon Marine Reserves. Strange Creatures Cast Ashore: Velella Velella
On the beach, Velella are small oval floats with a clear triangular sail on top. Mass strandings can turn stretches of beach blue in spring and summer on the US West Coast. Velella washed up for a while lose their rich blue color, so only fresh strandings show it.12Los Angeles Times. What are the blue blobs washing up on SoCal beaches? Welcome to Velella velella Valhalla Over days they dry out, fade, and turn papery and clear. Their sting is mild for most people, but wash your hands after handling them and avoid touching your eyes.
Red works the opposite way. Red light fades out quickly in seawater, so little of it reaches the depths. The red deep-sea jellyfish Atolla looks black wherever there is any light, which makes it hard to see.13Oceanus Magazine. Shedding Light on Light in the Ocean It lives from about 500 meters down to 4,500 meters, far below where sunlight reaches.14Oceanus Magazine. Shedding Light on Light in the Ocean The giant phantom jelly appears as a billowing crimson curtain only when a submersible shines its lights on it.15MBARI. Giant phantom jelly Its red-orange color shows only in visible light, so in the dark deep sea it is effectively invisible.16Wikipedia. Stygiomedusa Most deep animals see blue best, so a red body gives them little to detect.
Red also helps hide a meal. Many deep jellies eat glowing prey, and a red or dark stomach keeps that swallowed light from showing through the clear body. Without it, the jelly would act like a lantern for predators.
Which Jellyfish Glow and Which Do Not
Bioluminescence, an animal making its own light, is widespread among jellyfish. But the near-surface layer is dominated by jellyfish that do not glow, including moon jellies and sea nettles.17PubMed Central. Quantification of bioluminescence from the surface to the deep sea demonstrates its predominance as an ecological trait Deeper down, the most abundant jellyfish are crown jellies, which have dramatic light displays.18PubMed Central. Quantification of bioluminescence from the surface to the deep sea demonstrates its predominance as an ecological trait The jellies you see at the beach usually will not glow.
Sparkling blue waves at the beach usually come from tiny single-celled plankton instead. Most glowing jellies live far offshore and deep.
Some jellyfish carry ready-made light proteins that flash blue the moment calcium reaches them, without needing oxygen.19PubMed. Shining the light: the mechanism of the bioluminescence reaction of calcium-binding photoproteins The same light-making chemical is shared across many jellyfish and their relatives, such as sea pansies.20PubMed Central. Chemical nature of bioluminescence systems in coelenterates The Aequorea jellyfish relies only on this calcium-triggered kind of light.21PubMed Central. Chemical nature of bioluminescence systems in coelenterates The glow is blue because blue light travels farthest in seawater and most deep-sea animals see blue best, so a blue flash reaches more eyes.
Light can work as an alarm. When attacked, Atolla flashes a spinning ring of blue light around the rim of its bell.22深海éšå›³é‘‘. Atolla Jellyfish: The Deep-Sea Jellyfish That Sounds a “Burglar Alarm” With Blue Light The flash can draw a bigger predator that targets the attacker instead, a trick compared to a burglar alarm.23深海éšå›³é‘‘. Atolla Jellyfish: The Deep-Sea Jellyfish That Sounds a “Burglar Alarm” With Blue Light
Making light costs energy, so jellies glow in short, useful bursts rather than constantly. Many flash only when touched, bumped, or attacked, which is why a calm jelly in a net or tank can look dull until it is disturbed. A flash can serve as a warning, a distraction, or a signal, depending on the species.
To see glowing jellies yourself, some aquariums run dark tanks with deep-sea species, and deep-sea research videos online show real displays filmed by submersibles.
Why Comb Jellies Flash Rainbow Colors
Comb jellies look like jellyfish but belong to a separate group. They swim with rows of tiny beating hairs, and those tightly packed hairs split light into colors as they beat.24PubMed. Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora) The rainbow shifts as the animal swims and the angle of light changes.25PubMed. Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora) The colors are highly saturated and even reach ultraviolet that people cannot see.26PubMed. Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora)
The hairs sit in tight, regular rows, and light bouncing between them adds some colors together and cancels others, so the color comes from the shape of the surface rather than from a dye. Soap bubbles and oil slicks look rainbow-colored for the same reason, and a peacock feather gets its shimmer partly this way.
Unlike true jellyfish, comb jellies have no stinging cells, so they do not sting. In an aquarium, bright white or full-spectrum lamps bring out the strongest shimmer, while dim or colored lighting makes the rows look dull.
That outside light is what separates shimmer from true glow. Rainbows need lamps or sun, so if the lights are switched off the rainbow disappears even though the animal is still swimming. True bioluminescence shines in darkness and is usually blue or green. Some comb jellies can do both at once.
The Green Glow That Changed Medicine
The crystal jelly Aequorea victoria makes blue light that a second protein, GFP (green fluorescent protein), turns into green. When the two proteins sit together, GFP shifts to soak up the blue light better and passes the energy on efficiently.27PubMed. Fusion of Aequorea victoria GFP and aequorin provides their Ca(2+)-induced interaction that results in red shift of GFP absorption and efficient bioluminescence energy transfer Purified GFP shines green only under a blue or UV lamp, because fluorescence needs outside light, while the jelly’s own glow is self-made. Most wild Aequorea look clear in daylight.
Osamu Shimomura isolated the protein from hundreds of thousands of jellyfish.28PubMed. GFP: from jellyfish to the Nobel prize and beyond His work showed that the glow was a real, separable chemical. In 1994, Martin Chalfie showed that GFP glowed inside bacteria and worms, which many had believed would not work.29Angewandte Chemie International Edition. Green Fluorescent Protein: Discovery of Green Fluorescent Protein (GFP) (Nobel Lecture) That turned it from a curiosity into a tool. Roger Tsien then created a palette of fluorescent proteins in other colors,30PubMed. GFP: from jellyfish to the Nobel prize and beyond letting researchers track several things at once. The green glowing protein is now a standard marker to see where genes switch on and where proteins go in living cells.31PubMed. The green fluorescent protein This chain of steps won the 2008 Nobel Prize in Chemistry.
The protein can be attached to a protein a scientist wants to follow, so wherever that protein goes, a green glow goes with it. The glow forms by itself without added chemicals, so cells can be watched alive. Scientists can also fuse it to a gene’s control region so cells glow green only when that gene is switched on. Researchers tag proteins and watch them move under a microscope, and it has helped track how cancer cells spread in lab studies.
Proteins of this type are not unique to jellyfish. They are found in jellyfish and corals, in copepods, and in lancelets, a small fish-like animal.32PubMed Central. Sea as a color palette: the ecology and evolution of fluorescence In reef corals they come in many colors and have repeatedly switched between them.33PubMed Central. Diversity and evolution of the green fluorescent protein family