Structural and ultrastructural comparison of photophores of two species of deep-sea fishes: Argyropelecus hemigymnus and Maurolicus muelleri

The structure of the photophores (luminous organs) of two species of deep-sea fishes from the Strait of Messina, Argyropelecus hemigymnus and Maurolicus muelleri, was examined. Although significant structural differences were identified between species, especially in photophore organization and distribution, ultrastructural comparisons indicated a marked similarity between them. The photocytes exhibited numerous secretory granules, of different electron density, embedded in an extremely developed rough endoplasmic reticulum. Numerous mitochondria were observed among the secretory granules. The lens appeared to be composed of tightly contiguous polyhedral cells. The reflector was made up of cells rich in guanine crystals embedded in an amorphous matrix and appeared to be surrounded by a layer of connective tissue full of melanocytes. Ventrally, every photophore was delimited by a thick cellular layer called the ‘gelatinous layer’ with dioptric properties. The results confirm that even though A. hemigymnus and M. muelleri differ widely phylogenetically, they exhibit adaptive convergence, involving similar morphology and physiology in these luminous structures.