What is the difference between an animal that glows and one that only fluoresces?

Bioluminescence: luciferin, luciferase and photoproteins

The general scheme is an oxidation. A light-emitting substrate, called a luciferin, reacts with oxygen under the control of an enzyme, a luciferase, and the product is formed in an excited state that relaxes by emitting a photon. 'Luciferin' and 'luciferase' are job titles, not single molecules: the chemistry has arisen many times, from bacteria to fish, and unrelated groups use unrelated compounds. In some animals the substrate, oxygen and protein are pre-assembled into a photoprotein that fires when it binds an ion. Aequorin, isolated from the jellyfish Aequorea in 1962, is triggered by calcium. Other animals make no light chemistry of their own and instead culture luminous bacteria; deep-sea anglerfishes house theirs in the lure, and most species surveyed share one bacterial species acquired from the surrounding water.

Fluorescence: borrowed light, shifted in colour

A fluorescent molecule absorbs a photon, loses a little of the energy as heat, and emits a photon of lower energy and therefore longer wavelength. Blue or ultraviolet in, green or red out. The same 1962 work on Aequorea noted a protein that fluoresced green; the jellyfish's blue-emitting photoprotein hands its energy to it, and the animal's light is green as a result. That protein, GFP, turned out to have relatives in reef corals that produce no light at all. Six were cloned from non-bioluminescent corals in 1999, including yellow and red emitters, establishing that GFP-like proteins are not always tied to light production. In corals they are simply fluorescent pigments in the host tissue.

A fluorescent object never emits more light than it absorbs. It can look brighter than its surroundings at the emitted wavelength only because it is converting light from wavelengths the surroundings reflect poorly.

What fluorescence has been shown to do

For corals there is physiological evidence. Fluorescent pigments were shown to dissipate excess light energy at wavelengths of low photosynthetic use and to reflect light in strong sun, and more fluorescent colonies resisted bleaching better during a heat-stress event — consistent with a protective role for the coral and its symbiotic algae, though the bleaching result is an association. For visual signalling the case is much thinner. A critical review of claimed uses across flowers, corals, spiders, mantis shrimp, fish, reptiles and parrots concluded that most observations of fluorescence lack enough evidence to suggest a role in visually driven behaviour. The usual gap is the same: nobody has shown that the emitted light is a meaningful fraction of what the surface reflects anyway in natural illumination.

Glowing mammals: what the reports establish

Since 2019 a series of papers has reported fur that fluoresces under ultraviolet lamps. Platypus fur, brown in daylight, appeared green or cyan with emission peaking near 500 nm — in three museum specimens. A museum survey using fluorescence spectroscopy then reported the phenomenon in 125 species spanning all 27 living mammalian orders, confirmed for model species that it was true fluorescence and not scattered lamp light, and found that preservation method changed its intensity. The same paper states that it remains unclear whether fluorescence has any biological role in mammals. Separate chemical work traced the reddish glow of springhare and hedgehog hair to porphyrins and argued that, since porphyrins are destroyed by light, the glow persists in nocturnal species as a by-product of ordinary physiology.

What would count as evidence of function

Three things have to be shown, and photographs under an ultraviolet torch show none of them. First, that natural light where the animal lives contains enough of the exciting wavelengths — moonlight and twilight contain little ultraviolet in absolute terms. Second, that the emitted light adds measurably to the surface's ordinary reflection in that light, which requires calibrated spectra and not a long-exposure image with the visible light filtered out. Third, that a relevant viewer has receptors tuned to the emission and behaves differently when it is removed. Bioluminescence clears this bar more easily because the light is its own illumination: fireflies are the best-studied case, with evidence that female Photinus choose mates on the characteristics of male flashes.

The data

What goes wrong, and what to do

What the evidence says

Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.

• FactStrong evidence

  Bioluminescence has evolved many times independently, from bacteria to fish, using chemically unrelated luciferins and luciferases or pre-charged photoproteins, and functions in the sea range from defence to reproduction.

  Source: Bioluminescence in the sea

• FactStrong evidence

  Six proteins homologous to green fluorescent protein were cloned from reef corals that are not bioluminescent, two of them emitting at yellow and red wavelengths, showing that GFP-like proteins are not always functionally linked to bioluminescence.

  Source: Fluorescent proteins from nonbioluminescent Anthozoa species

• FactModerate evidence

  A museum survey with fluorescence spectroscopy reported fluorescence in 125 mammal species representing all 27 living orders, found that specimen preservation affected its intensity, and stated that any specific biological role remains unclear.

  Caveat: Museum specimens under laboratory excitation. Presence of fluorescence in preserved fur does not show that it is visible, or seen, in living animals under natural light.

  Source: All-a-glow: spectral characteristics confirm widespread fluorescence for mammals

• FactStrong evidence

  A review of fluorescence in visual communication concluded that most observations of fluorescence lack enough evidence to suggest they are used in visually driven behaviours.

  Source: Fluorescence as a means of colour signal enhancement

• FactModerate evidence

  The reddish ultraviolet-induced glow of some mammal hair is caused by free-base porphyrins, and its concentration in nocturnal and crepuscular species has been attributed to the light-sensitivity of porphyrins, not to a signalling function.

  Caveat: The by-product interpretation is the authors' argument from chemistry and distribution; it was not tested behaviourally.

  Source: Fur glowing under ultraviolet: in situ analysis of porphyrin accumulation in the skin appendages of mammals

• FactStrong evidence

  Deep-sea anglerfishes from six genera mostly share a single species of luminous bacterial symbiont, and the lack of matching host and symbiont phylogenies together with symbiont genes recovered from seawater indicate that the bacteria are acquired from the environment.

  Source: Diverse deep-sea anglerfishes share a genetically reduced luminous symbiont that is acquired from the environment

• Colourwise interpretationStrong evidence

  A photograph of an animal fluorescing under an ultraviolet torch is evidence that a fluorescent compound is present and nothing more; it should not be captioned as showing how the animal appears to others of its kind.

  Based on: Follows from the review's criteria for a visual function and from the survey's own statement that no role has been established for mammals.

  Source: Fluorescence as a means of colour signal enhancement; All-a-glow: spectral characteristics confirm widespread fluorescence for mammals

Related questions

Why is most light made by sea animals blue?

Blue-green light travels furthest through seawater, and most marine animals' eyes are most sensitive to it, so light of that colour is the most visible at a distance. The dominance of blue emission across unrelated groups is associated with that transmission window.

Is the green of GFP produced by the jellyfish's own light?

In the living jellyfish, yes. Its photoprotein would emit blue on its own, but the energy is passed to green fluorescent protein nearby, which emits green. Extracted GFP makes no light and fluoresces only when lit with blue or ultraviolet.

Does a scorpion's glow under a black light help it in any way?

Scorpion cuticle fluoresces strongly, and several functions have been suggested. The general position of critical reviews applies: a visible effect under a lamp is not evidence of use, and a function needs behavioural tests under natural light levels.

Do anglerfish make their own light?

No. The light in the lure comes from luminous bacteria living in it. Genetic work indicates that most anglerfish species share one bacterial species and pick it up from the water instead of inheriting it from a parent.

Related topics

Go further

Sources

• Bioluminescence in the sea — Annual Review of Marine Science 2, 443–493 (Haddock, Moline, Case) (Academic)
• Fluorescent proteins from nonbioluminescent Anthozoa species — Nature Biotechnology 17, 969–973 (Matz, Fradkov, Labas and others) (Primary research)
• All-a-glow: spectral characteristics confirm widespread fluorescence for mammals — Royal Society Open Science 10, 230325 (Travouillon, Cooper, Bouzin and others) (Primary research)
• Fluorescence as a means of colour signal enhancement — Philosophical Transactions of the Royal Society B 372, 20160335 (Marshall, Johnsen) (Academic)
• Fur glowing under ultraviolet: in situ analysis of porphyrin accumulation in the skin appendages of mammals — Integrative Zoology 18, 15–26 (Toussaint, Ponstein, Thoury and others) (Primary research)
• Diverse deep-sea anglerfishes share a genetically reduced luminous symbiont that is acquired from the environment — eLife 8, e47606 (Baker, Freed, Easson and others) (Primary research)
• Fluorescent pigments in corals are photoprotective — Nature 408, 850–853 (Salih, Larkum, Cox, Kühl, Hoegh-Guldberg) (Primary research)
• Extraction, purification and properties of aequorin, a bioluminescent protein from the luminous hydromedusan, Aequorea — Journal of Cellular and Comparative Physiology 59, 223–239 (Shimomura, Johnson, Saiga) (Primary research)
• The Nobel Prize in Chemistry 2008: popular information — Nobel Prize Outreach (Museum or institution)
• Understanding bioluminescence in dinoflagellates – how far have we come? — Microorganisms 1, 3–25 (Valiadi, Iglesias-Rodriguez) (Academic)
• Flash signal evolution, mate choice, and predation in fireflies — Annual Review of Entomology 53, 293–321 (Lewis, Cratsley) (Academic)
• Biofluorescence in the platypus (Ornithorhynchus anatinus) — Mammalia 85, 179–181 (Anich, Anthony, Carlson and others) (Primary research)

Reviewed 6 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.