Structure and ultrastructure of the viper fish photophores, Chauliodus sloani Bloch & Schneider, 1801

The viper fish (Chauliodus sloani) belongs to the family Stomiidae, which is represented by deep-sea ray-finned fish, including the barbeled dragonfishes. They are considerable components of the micronekton in the oceans, found in mesopelagic waters and include approximately 277 species in 27 genera. These fish reach high levels of biomass in the ocean, and their diel vertical migration plays an important role in the marine ecosystem in transferring energy from shallow to deep-sea waters. Moreover, they are considered key species in the pelagic environment and are main components in the diet of several top marine predators. The viper fish, are well adapted to living in the twilight zone and possess several photophores and other light organs, that produce intrinsic bioluminescence. Photophores are glandular cutaneous organs, present in several species of marine organisms, devoted to the production of chemical light and common in mesopelagic and deep-sea fish belonging to several families [1]. These light organs are ventrally or laterally arranged and located in different parts of the fish body such as the tail peduncle and head. In general, the main function of photophores located in ventral and lateral surfaces of the fish body is to counterilluminate the silhouette of the fish, so providing camouflage [2]. A contribution to the knowledge of the photophore structure of the mesopelagic fish C. sloani is given by means of a structural and ultrastructural study, to better identify the anatomical structures constituing these light organs. The present study showed that photophores of C. sloani consist of a deep photogenic chamber containing photocytes and support cells and a set of annexes as filter lens, gelatinous body, reflector and pigmented layer with a dioptric function. Particularly, the photocytes are characterized by numerous vesicles, abundant sarcoplasmic reticulum and granules, different both in size and electron density. The photocytes are placed in a radiated laminar system which converge towards the centre of the photogenic chamber communicating, by an extracellular channel, with lens filter. The latter is composed of contiguous cords of irregularly shaped cells characterized by a homogeneous cytoplasm and severals vesicles. Evident desmosomes are present along the cells borders. A gelatinous body, distally located in the photophore, near the basal membrane of the lining epithelium, covers the lens filter and shows granules which confers a typical mucous consistence. Laterally to the lens and around the photogenic tissue a reflector is present formed by cells containing crystals-like guanine arranged in layers. The external surface of the reflector wall is surrounded by a thin pigmented layer composed by melanine granules. These data will be useful to provide a baseline to better understand the physiology, use and function of the photogenic system of this species in the mesopelagic environment.