Plankton: The nighlight of the sea
How do plankton do this?
Bioluminescence occurs in 14 marine phyla, many of which include numerous luminescent species. All oceanic habitats, shallow and deep, pelagic and benthic, include bioluminescent species, but the phenomenon is most common in the upper 1000m of the pelagic environment (not near to the bottom, or ‘benthos’ of the ocean).
(tl;dr if chemistry gives you a headache!) Bioluminescence involves the oxidation of a substrate (luciferin) in the presence of an enzyme (luciferase). The distinctive feature of the reaction is that most of the energy generated is emitted as light rather than as heat. There are many different, and unrelated, kinds of luciferin, and biochemical and taxonomic criteria indicate that bioluminescence has been independently evolved many times. Marine animals are unusual, however, in that many species in at least 7 phyla use the same type of luciferin. This compound is known as ‘coelenterazine’ because it was first identified in jellyfish (coelenterates) and its molecular structure is derived from a ring of 3 amino-acids (2 tyrosines, and a phenyl alanine). Nevertheless many other marine organisms use different luciferins. In some animals (e.g. jellyfish) the luciferin/luciferase system can be extracted in the form of a stable “photoprotein” which will emit light when treated with calcium.
Sometimes the bioluminescent plankton are responsible for dramatic surface phenomena. Luminescent wave crests are particularly beautiful and common, but occasionally the sea may appear uniformly glowing white. This ‘milky sea' phenomenon has been described as like 'sailing through a field of snow' and is particularly common in the NW Indian Ocean at the time of the SW monsoon. It is probably the result of luminous bacteria growing on an oily surface scum. Other luminous phenomena include erupting balls of light exploding at the surface (probably fish schools coming up through dense luminous plankton and scattering at the surface) and, most dramatic of all, ‘phosphorescent wheels’. These appear first as parallel bands of light racing across the sea surface and then change to become vast rotating wheels whose spokes may appear to extend to the horizon and which travel past the vessel at 50-100 km hr-1! They occur only in less than 200m of water and are most frequent in the Arabian Gulf. Explanations include the stimulation of the surface bioluminescent plankton either by the ships engines or by seismic activity in the region. Neither alternative is wholly convincing…..
Read more about this at Faquatics
The top gif is of an organism that has been swallowed by a small fish. Bioluminescence is a defense mechanism caused by stress (edit-some people have been thinking that the fish is giving off bioluminescence!).