New data on morphology and ultrastructure of skin photophores in the deep-sea squid Histioteuthis bonnellii (Ferussac, 1834), Cephalopoda: Histioteuthidae

Morphology and ultrastructure of skin photogenic organs in the deep-sea squid Histioteuthis bonnellii from the Strait of Messina (Mediterranean Sea) was investigated. The photophore is formed by a glandular complex, made up of photogenic cells involved in the light production process, and a set of annexes (filter, reflector, pigmented layer and gelatinous body), with suggested dioptric function. The photocytes have an elongated shape, often arranged in an approximately radial series within the chamber, and show eccentric rounded euchromatic nucleus. The filter shows a membranous tissue made up of rows of cells, rich in smooth endoplasmic reticulum. The analysis on reflector's cells highlighted the presence of several iridosomal platelets in the cytoplasm, having irregular and sinuous shape. The pigmented layer and the gelatinous body provide protection to the organ and may have dioptric functions too. The photogenic organs were found to be innervated by nerves, and it was possible to confirm the strictly glandular nature of the photophores of this species. The detailed anatomical descriptions of the photophores of H. bonnellii extend our current knowledge and provide a scientific basis for future studies aimed to relate the evolution of photogenic system to the ecology of this squid.