Introduction: A Hidden Spectrum Beneath the Waves

Step into the dim blue twilight of the sea and an unseen show begins. Under specially filtered light, corals blush neon, sharks shimmer electric green, and quiet reef fish reveal intricate, secret messages scribbled in luminous lines. This phenomenon—biofluorescence—is not creatures making light, but creatures transforming light: absorbing high-energy blue photons and re-emitting them at longer, visible wavelengths such as greens, oranges, and reds.

Biofluorescence is easy to confuse with bioluminescence (where organisms produce their own light through chemical reactions) or with phosphorescence (which stores light and releases it slowly). Biofluorescence is different. It requires an external light source—especially the abundant blue light that penetrates seawater—and it switches on and off instantly as that light appears and disappears.

How Biofluorescence Works

Two intertwined stories power marine biofluorescence: the physics of light and the biochemistry of pigments and proteins.

The Physics

• Absorption: A molecule or protein absorbs a photon, typically blue light (around 450–490 nm).
• Relaxation: Some of that energy dissipates as heat in femtoseconds to picoseconds.
• Emission: The molecule re-emits a photon at a longer wavelength—green, yellow, orange, or red.

The Biochemistry

• Fluorescent proteins (FPs): Many marine animals, especially corals and cnidarians, use GFP-like proteins with an internal chromophore that forms spontaneously. Classic examples include the green fluorescent protein from a jellyfish and a family of red/orange proteins from corals and anemones.
• Small fluorescent molecules: Some fishes and sharks employ small-molecule fluorophores, including derivatives of amino acids, to generate striking patterns. These can be embedded in skin, mucus, or denticles.
• Optical filters and structures: Many fishes have yellow intraocular filters that block blue light, making dim green or red fluorescence stand out for those species. Skin layers, reflective plates, and microscopic structures can amplify or sculpt the glow.

Who Glows? A Tour of Biofluorescent Marine Life

Biofluorescence turns out to be widespread in the sea. Surveys over the last decade revealed hundreds of biofluorescent species across diverse lineages.

• Corals and anemones (Cnidaria): Reef-building corals often fluoresce green, cyan, and red. Juvenile corals can blaze especially bright, and fluorescence may change with light conditions and stress.
• Jellyfish and comb jellies: Some hydromedusae and ctenophores exhibit brilliant fluorescence in body rims, tentacles, or bell tissues.
• Fishes: Gobies, wrasses, scorpionfishes, lizardfishes, eels, and many others display fine tracings, spots, and bars that are nearly invisible in white light yet vivid under blue light with a yellow filter.
• Sharks and rays: Several catsharks and swell sharks glow a distinct green; patterns can be species-specific. In some species, specialized fluorescent compounds occur in the skin and are thought to also offer antimicrobial benefits.
• Sea turtles: The hawksbill sea turtle has been observed with spectacular green and red biofluorescence; some of the red glow in observations likely comes from algae on the shell, while the turtle itself displays green fluorescence.
• Nudibranchs and crustaceans: Sea slugs often carry fluorescent highlights, and certain shrimps and stomatopods show fluorescent patches used in displays or camouflage.
• Algae and microbes: Pigments such as chlorophyll and accessory pigments fluoresce naturally, shaping the landscape of color on reefs by providing background glows and reflections.

Colors range from emerald greens and icy cyans to fiery oranges and deep reds—red being especially dramatic at depth, where red sunlight no longer reaches.

Why Glow? Ecological Roles and Hypotheses

Biofluorescence likely serves multiple functions that vary across species, depth, and habitat. While research is ongoing, several leading roles have emerged:

• Communication and mate choice: Complex, species-specific patterns suggest private signaling channels, visible to those with the right visual filters. Fluorescent facial masks, fin edges, and body stripes could help individuals recognize their own species or court potential mates.
• Camouflage and concealment: “Fluorescent crypsis” may help organisms blend into a fluorescent background, particularly within coral gardens that already glow. Breaking up outlines with matching emission colors can foil predators.
• Prey detection and hunting: Some fishes appear to use fluorescent contrasts to detect prey that absorbs blue light but reflects or emits distinctive wavelengths under illumination.
• Photoprotection and light management: In corals, fluorescent proteins can redistribute or dissipate excess light energy, potentially protecting both coral tissues and their photosynthetic symbionts. In low light, fluorescence may also “recycle” photons into more useful wavelengths for photosynthesis.
• Stress signaling and wound response: Changes in coral fluorescence can flag stress, disease, or injury—sometimes weeks before whitening is visible—serving as an early-warning indicator for reef health.

Depth, Color, and the Underwater Light Field

Water rapidly filters sunlight. Reds vanish within a few meters, yellows fade with depth, and blues push the furthest. The deeper you go, the bluer the ambient world becomes. This spectral shift makes biofluorescent emissions especially conspicuous for organisms that can see them. In low-light and mesophotic zones (roughly 30–150 m), green and red emission can create visual channels unavailable in brighter, broadband surface waters.

How to See It: Diving and Photography Tips

• Blue excitation light: Use a “royal blue” dive torch or video light (around 450–470 nm). Ultraviolet can also excite fluorescence, but blue light is common, effective, and easier on eyes.
• Barrier filter: Wear a yellow filter over your mask, and place a matching filter on your camera lens or port. These block overwhelming blue light and reveal the emitted colors.
• Dark adaptation and angle: Let your eyes adjust. Sweep slowly and change the light’s angle to catch specular highlights and pattern edges.
• Safety first: Maintain excellent buoyancy. At night or in caverns, stay aware of partners, air, and navigation. Avoid shining bright lights into animal eyes and never touch living corals or wildlife.
• Camera settings: Start with high ISO and wider apertures; adjust shutter to control ambient spill. Shoot RAW to preserve subtle color gradients.

Facts and Wonders: Did You Know?

• Private language: Many fishes have yellow ocular filters that enhance fluorescent contrasts. To predators lacking these filters, the same patterns may be much harder to see.
• Glowing sharks: Chain catsharks and swell sharks display green fluorescent patterns that differ by sex and species, hinting at roles in recognition and mating. Some fluorescent skin compounds in these sharks also show antimicrobial properties.
• Neon nurseries: Juvenile corals often fluoresce more intensely than adults, possibly aiding symbiont acquisition or acting as sunshades while tissues are thin and vulnerable.
• Fluorescent turtles: Hawksbill turtles have been filmed radiating green and red glows under blue light. At least some of the red is attributed to algae living on the shell, while the turtle’s tissues themselves fluoresce green.
• A toolkit for science: Fluorescent proteins from marine life revolutionized biology, enabling scientists to tag and visualize molecules inside living cells, and inspiring brighter, more stable variants in many colors.

From Reefs to Laboratories: Human Uses of Marine Fluorescence

The discovery and refinement of fluorescent proteins transformed life sciences. Researchers engineered variants that span the spectrum, from cyan through yellow to deep red, and tuned them for brightness, stability, and rapid maturation. These tools enable:

• Live-cell imaging: Watching proteins move, assemble, and signal in real time.
• Biosensors: Reporters that change fluorescence when calcium spikes, pH shifts, or voltage toggles.
• Developmental maps: Tracing cell lineages and neural circuits.
• Medical diagnostics and therapeutics: Labeling tumors, tracking drug delivery, and guiding surgeries.

Many of these breakthroughs trace to proteins from jellyfish and corals, and to new fluorescent systems discovered in marine fishes and invertebrates.

Threats, Clues, and Conservation

Climate change, warming seas, pollution, and ocean acidification stress reef ecosystems. Because coral fluorescence can shift under stress, it provides an early, noninvasive indicator for reef monitoring. Conservation actions that help include:

• Reducing greenhouse gas emissions to lower warming and acidification.
• Protecting water quality—limiting runoff, sediments, and nutrients that fuel algal overgrowth.
• Establishing marine protected areas and responsible tourism guidelines.
• Supporting restoration projects that monitor fluorescence as a health metric.

Myths vs. Facts

• Myth: Biofluorescent animals glow in complete darkness.
  Fact: They require an external light source. No light in, no light out.
• Myth: Fluorescence and bioluminescence are the same.
  Fact: Fluorescence re-emits absorbed light; bioluminescence creates light chemically.
• Myth: Only tropical corals fluoresce.
  Fact: Many fishes, some sharks, and invertebrates across varied habitats fluoresce.
• Myth: All red colors disappear at depth, so red fluorescence is useless.
  Fact: Red sunlight is filtered out, but red emission from fluorescence can still be seen by animals with suitable vision.

Milestones in the Science of Marine Fluorescence

• Mid-20th century: Early descriptions of fluorescent pigments in cnidarians and algae.
• Late 20th century: Characterization of jellyfish GFP and explosion of fluorescent protein technology in biology.
• 2010s: Broad surveys reveal widespread fluorescence in reef fishes and sharks; first reports of biofluorescent sea turtles.
• Today: Advanced imaging, genomics, and chemical analyses are uncovering new fluorophores, functions, and ecological roles.

Frequently Asked Questions

Do humans naturally see biofluorescence underwater?

Not usually. Our eyes are overwhelmed by scattered blue light. A yellow barrier filter is key—it blocks the blue and lets the emitted glow shine through.

Is UV light required to see it?

No. Blue light is commonly used, safe when applied responsibly, and very effective at exciting fluorescence in many marine organisms.

Is it safe for animals?

When done carefully—moderate power, limited exposure, respectful distances—fluorescence diving is considered low impact. As always, avoid touching or stressing wildlife and follow best practices and local regulations.

Why don’t we notice fluorescent patterns in daylight?

In broad-spectrum sunlight the emitted light is drowned out by reflections. Under controlled blue light and with filters, emission becomes the dominant signal.

What colors are most common?

Greens and cyans are widespread; reds and oranges are striking in corals and some fishes. Exact hues depend on the fluorophores present and local lighting.

Glossary

• Biofluorescence: Absorption of high-energy light and re-emission at a longer wavelength by living organisms.
• Bioluminescence: Light production by chemical reaction inside an organism.
• Fluorophore: A molecule or protein that fluoresces.
• GFP (Green Fluorescent Protein): A widely used fluorescent protein originally discovered in a marine jellyfish.
• Barrier filter: Optical filter that blocks excitation light and transmits emission light to the viewer or camera.
• Excitation light: The light used to stimulate fluorescence (often royal blue for underwater work).

Looking Ahead: New Colors, New Clues

Every season brings new reports of fluorescent species and new explanations for why they glow. Chemists are charting novel fluorophores in fishes and sharks; ecologists are testing how signals are seen by intended receivers; reef biologists are turning fluorescence into a sensitive stethoscope for coral health. The sea holds many more colors than we see at first glance— and with the right light, its living palette keeps getting richer.