Largest Bioluminescent Vertebrate Discovered Off New Zealand

The lateral view of the kite shark's luminescent pattern pictured with a daylight view (Image: Mallefet et al/Frontiers in Marine Science)

By Mark 'Crowley' Russell

The largest bioluminescent vertebrate known to science has been discovered by scientists studying sharks off the coast of New Zealand. The findings, published in the online journal Frontiers in Marine Science, confirmed that three known species of deepwater shark are capable of glowing in the dark, including the kitefin shark (Dalatias licha, pictured above), which can reach up to 180cm in length.

The scientists collected the specimens of kitefin shark, blackbelly lantern shark (Etmopterus lucifer) and southern lantern shark (Etmopterus granulosus) during a fish survey off Chatham Rise, a relatively shallow – and bountiful – body of water off the eastern coast of New Zealand's South Island. The 1000m deep underwater plateau is New Zealand's primary fishing grounds, and an important habitat for a range of species, including sharks and whales.

The three species of shark studied by the scientists are all found in the mesopelagic zone, also known as the 'twilight zone', as it ranges from 200m to 1000m deep, where light penetration is minimal at all times. While there have been many other documented examples of biofluorescence in deep-sea sharks and bony fish, most are relatively small in size. Furthermore, other light-emitting species, such as anglerfish, appear to have a clear purpose behind their glowing appendages, which serve either as a lure to attract prey, or a distraction to avoid predators. In other examples, bioluminescence appears to be a form of communication, and perhaps a mating signal.

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Dorsal view of the kite shark's luminescent pattern. The red arrow highlights a bright glow on the shark's second dorsal fin, the purpose of which is unknown. (Image: Mallefet et al/Frontiers in Marine Science)

In the case of the larger shark species involved in the study, however, the purpose of their glowing skin is unclear. Due to the location of the glow and lack of a particular pattern of light, the scientists suggest that its most likely function is as a form of counter-illumination, whereby the underside of the animal glows to reduce the outline of its shadow when seen from below, and therefore minimising the likelihood of mid-water predation.

The scientists also propose that, in the case of the large, slow-moving kite shark, which appears to have no natural predators, the light may be used to illuminate the seafloor in search of prey.

Both theories, however, are somewhat confounded by the fact that the sharks appear to have a bioluminescent dorsal surface, where it would serve no purpose as either camouflage or for the illumination of prey. The kite shark, in particular, has a pronounced glow on its second dorsal fin, for reasons that remain unknown.

Further study is required but the depth and size of the habitat in which the animals are located makes research extremely difficult. 'Bioluminescence has often been seen as a spectacular yet uncommon event at sea,' write the report's authors, '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 in structuring the biggest ecosystem on our planet.'

The complete study, by Jérôme Mallefet and Laurent Duchatelet, of the Marine Biology Laboratory, Earth and Life Institute, Université Catholique de Louvain Belgium and – UCLouvain, Louvain-la-Neuve, Belgiumo; and Darren W Stevens, of the National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand, is published in full atwww.frontiersin.org/articles/10.3389/fmars.2021.633582/full