Ultrastructural organization of lantern shark (Etmopterus spinax Linnaeus, 1758) photophores

Etmopterus spinax Linnaeus, 1758 is a deepsea lantern shark that emits blue light thanks to thousands of tiny cup-shaped organs made of a pigmented sheath enclosing light-emitting cells topped by an iris-like structure and a lens. In this study, we investigate the ultrastructure of these photophores in order to improve our understanding of the light emission process. The presence of a novel layer, a putative reflector upholstering the pigmented sheath, is highlighted. The intracellular organization of the photocytes is addressed. They appear as regionalized cells: their basal area is occupied by an ovoid nucleus, their medial area is highly vesiculated and their apical area, oriented toward the photophore center, displays small granular inclusions. We hypothesize this granular area to be the intracellular site of photogenesis in E. spinax, as it is also the most fluorescent part of the photocyte