Morphological and Immunohistochemical Study of Ventral Photophores of <i>Ichthyococcus ovatus</i> (Cocco, 1838) (Fam: Stomiidae)

Photophores are light-producing organs found in many fish species living in the mesopelagic, bathypelagic, and abyssal layers of the ocean. They function to attract prey, confuse predators, and communicate with other individuals of the same species. Understanding the structure and function of photophores is crucial to exploring bioluminescence and the ecological adaptations of marine life in deep-sea environments. The present study is the first to investigate the photophore anatomy of the mesopelagic fish <i>Ichthyococcus ovatus</i> (Cocco, 1838), using specimens naturally stranded along the coast of the Strait of Messina. The morphology of the ventral photophores of <i>I. ovatus</i> includes four functional parts: a tank containing photogenic cells, a lens filter, a reflector surrounding the entire organ, and a pigmented layer. An immunohistochemical assay was conducted using anti-nNOS and anti-S100p antibodies. The presence of nNOS/NOS type I immunolabeling the pigmented layer surrounding the photophores and the nerve fibers reaching the lens suggests a potential role of neuronal nitric oxide signaling in modulating light shielding by the pigment sheath, controlling light exposure, and adjusting light focusing though the lens-associated nerves. S100p immunostaining was observed in the nerve fibers reaching the photophores, highlighting its potential involvement in regulating neuronal calcium levels and, consequently, influencing signal transmission to control bioluminescence output. A sensory feedback pathway from the photophore to the CNS is suggested. Within the lens and in the irregularly shaped cells located in the photophore’s lens, S100p immunolabeling could indicate active signaling and differentiation processes. These findings expand our understanding of light-emitting systems in mesopelagic fishes and offer a valuable foundation for future studies on the functional and evolutionary significance of photophores.