Three deepwater shark species (Dalatias licha, Etmopterus lucifer, and Etmopterus granulosus) found off the coast of New Zealand, seem to glow in the dark. They were collected off an area of ocean known as Chatham Rise, in January 2020. The three species are the kitefin shark (now the largest known luminous vertebrate), which can reach up to 180cm (5ft 11in) in length, the blackbelly lanternshark and southern lanternshark. Although scientists were already aware of the sharks, this is the first time that bioluminescence has been observed in all three.
Bioluminescence is present everywhere in nature and is a cool phenomenon, whereby organisms emit their own light through a chemical reaction, or the light is produced by symbiotic bacteria that live within the animals themselves. It occurs widely in marine vertebrates and invertebrates, more than 75 species of fungi, many microorganisms including some bioluminescent bacteria (that live in marine sediment, gut of marine animals and even on the skin of decomposing fish), and terrestrial arthropods such as fireflies and glow worms. However, they had never been found in large vetebrates, especially sharks, compelling the scientists to refer to the kitefin shark as “giant luminous shark”.
According to the study published in the Frontiers in Marine Science journal, the bioluminescence is produced through thousands of light-producing cells known as photophores located in the sharks’ skin. The researchers, from the Université Catholique de Louvain in Belgium and the National Institute of Water and Atmospheric Research in New Zealand, said the findings are important to increase understanding of life in the deep sea; among the least-studied ecosystems on the planet.
The three species of sharks inhabit the mesopelagic zone (or the twilight zone) that ranges from 200m to 1,000m depth. Beyond this depth sunlight does not penetrate, which means that the sharks’ sillouettes can be seen by predators below, as they are exposed to sunlight from above. The study suggests that the sharks therefore have no place to hide, so they use this counterillumination on their underbellies as camouflage. In the kitefin shark, (the dorsal fin of which also glows) has no or few predators, the bioluminescence is probably used to illuminate the ocean floor as it searches for food, or to disguise itself as it looks for prey. But still the purpose of this light show in the deep has not been confirmed. Further studies are needed to confirm the hypothesis, as well as to understand how the bioluminescence funtions.
“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 researchers wrote.
Image and study: Mallefet Jérôme, Stevens Darren W., Duchatelet Laurent, Bioluminescence of the Largest Luminous Vertebrate, the Kitefin Shark, Dalatias licha: First Insights and Comparative Aspects, Frontiers in Marine Science