The largest known shark that can glow in the dark has been discovered by researchers off the coast of New Zealand.
Scientists discovered that three species, the kitefin shark, the blackbelly lanternshark, and the southern lanternshark, emit a luminescent glow in their habitats deep below the surface of the ocean.
All three, deep-sea sharks which live at depths between 200m and 1,000m, were known to science already, but their ability to glow was not documented.
The three species were collected during a fish survey in eastern New Zealand in January last year, and observed in tanks before being dissected and analysed.
One of them, a kitefin shark which can reach up to 1.8m long, is now the world's largest known luminous vertebrate, said the paper published in the journal Frontiers in Marine Science, and also the first documented shark with fully luminous dorsal fins.
The researchers believe that kitefin sharks, which have few if any predators, use their glowing ability to camouflage themselves from prey and to illuminate the ocean floor while hunting, but said more evidence was needed to confirm this theory.
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"The use of counterillumination for this giant luminous shark is here suggested to be co-opted for a camouflage-type approach as a predatory tool," the researchers said.
Lead author Jérôme Mallefet, of the University of Louvain-la-Neuve, Belgium, said the sharks used a different mechanism to glow than other bioluminescent animals.
"The sharks discovered off New Zealand in January 2020 control their light production system by hormones, while most of the bioluminescent organisms seen to date use nerve control to trigger their light," he told Belgian broadcaster RTBF.
The researchers concluded that bioluminescence plays a greater role than previously thought in deep-sea ecosystems.
"Considering the vastness of the deep sea and the occurrence of luminous organisms in this zone, it is now more and more obvious that producing light at depth must play an important role structuring the biggest ecosystem on our planet," the paper said.
Other luminous marine creatures include some species of algae, crustaceans and jellyfish, and the ability serves different purposes for different species.
Some prey animals, such as jellyfish, use the function to startle predators and attract other creatures which prey on their predators.
Other non-marine creatures, such as glow worms, use their light to attract prey.
There are also more than 75 known species of bioluminescent fungi, which only glow at night time, attracting insects which land on them and pick up spores which then spread in other areas.
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