Extraocular photoreception and photophore ontogeny during embryogenesis of a luminous shark from Dalatiidae family, Squaliolus aliae

Bioluminescence, the emission of visible light by a living organism through a chemical reaction, is a common phenomenon in marine organisms, especially in deep water where light is very weak or non-existent. Bioluminescence can have a variety of uses for the organisms that possess it. However, there are three main strategies using bioluminescence: i) food research; ii) the ability to protect oneself or avoid predator attacks; iii) intraspecific communication, especially when searching for a sexual partner. Among the Elasmobranchs, there are two families of sharks that have acquired the ability to emit light, the Etmopteridae and the Dalatiidae. These luminous sharks have thousands of luminous organs called photophores that allow them to produce light. One of the main functions of bioluminescence is to camouflage their silhouette by imitating the ambient light on their ventral surface in order to camouflage themselves from potential predators coming from the bottom. This technique is called counterillumination. For this camouflage to be as effective as possible, it is necessary to be able to adjust the light emitted according to the ambient light. For this adjustment to be well managed, the shark must be able to perceive not only the ambient light, but also the luminescence emitted by itself in order to coordinate the two. Recent studies have shown the presence of an extraocular opsin, encephalopsin (Opn 3), in the photophores of Etmopterus spinax, a luminous shark of the family Etmopteridae. This extraocular opsin allows the perception of light at the level of luminous organs, suggesting that this shark can perceive its own bioluminescence. Furthermore, it has been shown that in E. spinax, encephalopsin, located in a structure acting as an iris in the photophore (the ILS)), acts on the pigment cells of the ILS to increase light emission when the opsin is excited by light. This phenomenon suggests that there is indeed a feedback between the perception of light and the emission of light. During this thesis, the presence of this same extraocular opsin in Squaliolus aliae, a species of luminous shark in the Dalatiidae family, was demonstrated through immunohistology. Encephalopsin is also present in the ILS of the photophores of this species, suggesting that it acts in the same way as in E. spinax on the regulation of light emission. The appearance of this opsin during the shark's embryonic development, while still in its mother's uterus, suggests that this opsin does allow the shark to perceive its own luminescence. The ontogeny of the luminescent organ during embryonic development has also been studied for S. aliae and appears to be approximately identical to that already shown for E. spinax.