Bioluminescence is light generated by a chemical reaction within an organism and is visible in complete darkness; fluorescence is the re-emission of absorbed light at a different (usually longer) wavelength and requires an external light source, typically becoming visible as a glow under UV or blue light.
A living organism produces its own light through an internal chemical reaction, rather than reflecting light from another source.
A material absorbs light at one wavelength (often invisible ultraviolet) and re-emits it almost immediately at a longer, visible wavelength.
Bioluminescence is produced by a chemical reaction (typically involving a light-emitting molecule called luciferin and an enzyme called luciferase) inside an organism, and does not require any external light source — it is common among deep-sea marine species, fireflies and some fungi.
Marine life · confidence: high
Fluorescence occurs when a substance absorbs light at one wavelength and re-emits it almost immediately at a different, usually longer, wavelength — unlike bioluminescence, it requires an external light source (commonly UV or blue light) to be visible.
Marine life · confidence: high
Fireflies are a well-known example of bioluminescence used for communication and mate attraction, with flash patterns that vary distinctly between species.
Insects · confidence: high
Biofluorescence has been documented in a growing range of species, including some corals, fish, amphibians and even a few mammals, though the specific function is not conclusively established for every documented case.
Marine life · confidence: medium
Documented occurrence is broader than confirmed adaptive function.
Frequency and rarity claims above are scoped to a specific domain, geography or taxonomic group — none are presented as a single universal ranking. See methodology.