Three species of sharks have been found to glow in the dark while swimming in deep waters off the coast of New Zealand.
While glow-in-the-dark creatures are not uncommon, the phenomena had never been seen in kitefin shark, the blackbelly lanternshark and the southern lanternshark.
The sharks were already known to marine biologists but they had never been known as luminous vertebrates.
The discovery of their bright underbelly came about during a study of the Chatham Rise, an area of ocean floor to the east of New Zealand, in January last year.
According to the study by the Université Catholique de Louvain in Belgium and the National Institute of Water and Atmospheric Research in New Zealand, the bioluminescence is achieved thanks to thousands of photophores (light-producing cells) located within the sharks' skin.
The findings have now made the kitefin shark, referred to in the study as the "giant luminous shark", the largest known luminous vertebrate.
According to the researchers, the kitefin can grow up to reach 180cm (5ft 11in).
While it can be seen around the world at depths ranging from 50 to 1800 metres, it is usually found below 300m.
All three species were found in the mesopelagic, also known as the "twilight zone" of the ocean as the area, which sees no sun as it had depths ranging from 200 to 1,000 metres.
While the reason for the sharks' bioluminescence is yet to be identified, the study suggests it may be to help them marine creatures hide from their predators or other menaces beneath them.
But it may also serve to disguise themselves as they approach their prey.
Researchers said the findings could help gain a better understanding of the "biggest ecosystem of our planet".
They said: "Bioluminescence has often been seen as a spectacular yet uncommon event at sea, but 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".
