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Bioluminescence, i.e. the ability for living organisms to emit light, is a widespread phenomenon in deep-ocean, due to valuable ecological advantages it provides to organisms endowed with this capability. Six percent of deep-sea cartilaginous fishes (sharks, rays, skates and chimaeras) are luminous. Among them, the lantern shark Etmopterus spinax Linnaeus, 1758 is one of the most commons and can be easily caught. For this reason, it has recently become a model species for studying bioluminescence in shark. Although various aspects (functions and control) of this complex phenomenon are now better understood in this species, the photocyte (i.e. the light-emitting cell) mechanism of light emission remains unsolved. This is the subject addressed in this work. In the first chapter, photophore ultrastructure is explored since it is a key step in investigating an unknown bioluminescent mechanism. By using transmission electron microscopy (TEM), we were able to describe photophore fine organization (Paper 1): a new structure, a reflector, which was believed to be absent in shark photophores, was highlighted. Photocytes were found to be divided into three distinct areas, of which the apical one, oriented toward the photophore center and containing numerous granular inclusions, was proved to be the light-emitting area (Paper 2). By investigating photocyte ultrastructural changes during the course of a lightemission event, we proved granular inclusion to represent shark microsource (i.e. the intracellular light-emitting site, where the chemiluminescent reaction takes place), and we proposed the term “glowon” to characterize this novel particle (Paper 2). In the second chapter, we addressed E. spinax feeding habits in order to identify putative luminous preys and analyze whether the shark could obtain a known luciferin (i.e. the luminous substrate) through its diet and use it for its own light production. The results we obtained (Paper 3) suggested E. spinax does not use a know, (SC - Sciences) -- UCL, 2014

Many midwater animals emit ventral light to hide their silhouette in the water column. This phenomenon known as counterillumination typically requires fine control over light emission since it needs a luminescence that closely matches the properties of downwelling light (intensity, angular distribution and wavelength). Here we provide evidence that, although lacking complex structures of counterilluminating animals, the deepwater luminescent shark Etmopterus spinax could, in Norwegian fjords, efficiently cloak its silhouette from downwelling ambient light to remain hidden from predator and prey. This represents the first experimentally tested function of luminescence in a shark and illustrates how evolution can take different routes to converge on identical complex behavior. (C) 2010 Elsevier B.V. All rights reserved.