Marine colouration ranges from the countershading seen in many open-water fish, to the vivid warning colours of some reef species, to bioluminescence in the deep ocean where sunlight never reaches — with colour serving very different purposes depending on depth and habitat.
A molecule in the tissue absorbs some wavelengths of light and reflects others — the reflected wavelengths are the colour seen. Chlorophyll, carotenoids and melanin are the most common examples.
Common misconception: Not every vivid natural colour is a pigment — several of the most striking blues and greens in animals are structural colour instead, which looks similar but works completely differently.
Microscopic physical structures — ridges, layers or particles smaller than the wavelength of light — interfere with light waves so that only certain wavelengths are reflected back. No pigment is involved at all.
Common misconception: A structurally-coloured feather or scale ground into powder loses its colour completely, because grinding destroys the microscopic structure that produced it.
A living organism produces its own light through an internal chemical reaction, rather than reflecting light from another source.
Countershading — a darker back and paler underside — is common among open-water fish and marine mammals, helping to disguise their silhouette from predators or prey viewing them from above or below.
Marine life · confidence: high
Many coral reef fish display vivid colour patterns linked to species recognition and communication in the visually complex reef environment, though the specific function of colour varies by species.
Marine life · confidence: medium
Bioluminescence — light produced by a chemical reaction within an organism — is common among deep-sea marine species, where sunlight does not reach, and is used for purposes including attracting prey, communication and, in some species, camouflage against faint downwelling light.
Marine life · confidence: high
Ocean water itself appears blue primarily because water absorbs red light more strongly than blue light, so blue wavelengths are scattered back more than others — a physical absorption effect, not a pigment in the water.
Water · confidence: high
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.