Ultrastructural evidence for the existence of the distal retinal pigment light-adapting hormone in the sinus gland of the prawn Palaemon paucidens.

The position of the distal retinal pigments was measured in the eyes of Palaemon which had been exposed to various light intensities (0-2000 lux) for periods up to 7 days. Electron microscopical observations of sinus glands from these animals revealed that long-term illumination (1000 lux, 7 days) resulted in almost complete degranulation of the Type 2 axon terminals, reflecting intense discharge of the granules. On the other hand, the Type 2 terminals of dark-adapted prawns showed no signs of hormone release, such as exocytosis. The results indicate that the light-adapting hormone is liberated from Type 2 terminals. The Type C cell somata in the medulla terminalis, containing granules of similar size to those in the Type 2 terminals, may be the site of synthesis of this hormone.