Glowing Shark Do-Do-Do-Do-Do-Doo!
Glowing marsupials are so last year. Research published in March 2021 has revealed the largest light-producing vertebrate yet: the kitefin shark. According to researchers, the discovery of bioluminescence in the 180cm-long predator changes our understanding of the adaptation and its role in the deep-sea.
Bioluminescence is the ability of organisms to produce light. While it may seem exotic, it’s a widespread phenomenon in the natural world (even humans faintly glow!). Bioluminescence is particularly common in marine ecosystems. Here it is displayed by several distinct groups, including fish, cephalopods and crustaceans.
The adaptation is highly prevalent in the mesopelagic zone, between 200 and 1000 metres below the surface. Only a small amount of Sunlight can penetrate to these depths; a feature which gives the region its nickname of “the twilight zone”. Scientists estimate that over 90% of animals here are able to luminesce.
While several shark species were already known to produce light, a team of researchers from the Université Catholique de Louvain in Belgium, and the National Institute of Water and Atmospheric Research (NIWA) in New Zealand, set out to better understand the diversity of the adaptation among shark species.
To achieve this, they trawled the mesopelagic zone, in an area to the east of New Zealand. As well as discovering bioluminescence in the kitefin shark, the researchers also discovered two smaller glowing species: the blackbelly lanternshark and the southern lanternshark.
Bioluminescence in sharks is possible thanks to small organs in the skin called photophores. The blue dots in the picture above show the distribution of these organs over the kitefin shark’s body. Each photophore has a layer of pigmented cells in the shape of a cup which contains 1-12 specialised cells called photocytes. In the photocytes, an enzyme called luciferin reacts to produce light. The photophore is topped by one or more lens cells which then refract the light outwards.
Curiously, while the nervous system controls these organs in most animals, shark photophores are controlled by hormones. This makes sharks the only multi-cellular organisms known to control their bioluminescence using the endocrine system. The photophores of all three species examined in this study responded to three hormones, with one triggering light production and the other two inhibiting it. These same hormones have previously been shown to control bioluminescence in other lanternshark and kitefin shark species. This suggests that these groups evolved from a common ancestor who also exhibited hormonally-controlled bioluminescence.
Previous findings have suggested that mesopelagic sharks use light to communicate with one another, to warn off predators, and for “counterillumination”. Counterillumination is a form of camouflage, similar to countershading, in which animals emit more light from their underside than from their dorsal (top) side. Marine animals are therefore camouflaged against light coming from above and against the darkness of the depths below. Counterillumination was previously considered to have evolved in mesopelagic animals as camouflage from predators.
While the scientists believed that this was likely to be the case for the two smaller lanternshark species, they were intrigued to discover that the kitefin shark, which has few or no predators, also displayed counterillumination. As a result, they proposed that kitefin sharks may use their bright underside to illuminate the ocean floor while searching for food or even as predatory camouflage, enabling them to stealthily approach prey species. The kitefin shark is also the first species found to have a fully luminous dorsal fin. This raises yet more questions about how bioluminescence is being utilised by the world’s largest glowing vertebrate.
According to the authors of the study, more research is required. They hope to observe the behaviour of live specimens in their natural environment in the future. For now, however, the discovery of three new bioluminescent shark species adds weight to a growing body of evidence which emphasises the importance of light-production in the planet’s largest ecosystem.
The paper which this article is based on can be found at: https://www.frontiersin.org/articles/10.3389/fmars.2021.633582/full