A comparative light and electron microscopic study of the pineal complex in the deep‐sea fishes, Cyclothone signata and C. acclinidens

The pineal complexes of the two closely related deep‐sea fishes Cyclothone signata and C. acclinidens were compared both qualitatively and quantitatively. Photoreceptor and supportive cells were identified in both species. The deeper‐dwelling species, C. acclinidens , had a significantly greater number of photoreceptor‐cell outer segment saccules and a higher ratio of receptor cells to nerve fibers in the pineal stalk. It was suggested that these indicate increased photosensitivity of the pineal. Supportive cells were sometimes seen to contain arrays of undulating tubules. The functional significance of these tubules is not understood. A prominent dorsal sac is closely associated with the pineal end‐vesicle. Both structures appear to have a common vascular supply suggesting that they are functionally related. Dorsal sac cells contained abundant mitochondria, glycogen, and large filament‐like inclusions.