Organ of Bellonci and sinus gland in three decapods from Atlantic hydrothermal vents: Rimicaris exoculata, Chorocaris chacei, and Segonzacia mesatlantica

Crustaceans of the hydrothermal vent field of the Mid-Atlantic Ridge present graded variations of eye structure. In the shrimps Rimicaris exoculata and Chorocaris chacei, external eyestalks are missing or reduced and conventional compound eyes are lacking. The crab Segonzacia mesatlantica has small eyestalks. Our objective was to localize and describe the organ of Bellonci and the sinus gland usually included in the eyestalks. In Rimicaris exoculata and Chorocaris chacei, the organs of Bellonci are well developed and contain onion body-like structures. They are located in front of the cerebral ganglia close to the hemiellipsoid bodies, a position unusual among decapods. Each organ of Bellonci extends to a complex cuticular structure which is probably a sensory pore. In the two shrimps, the optic lobes are located within the brain. The four optic neuropile layers are present. The sinus gland is small and cupula-shaped, with usual histological features. It is located between the interna and externa medullae, adjacent to a hemolymph lacuna. In Segonzacia mesatlantica, the eyestalks contain massive nervous masses with numerous and complex chiasmata from which only the lamina ganglionaris is slightly separated. The organ of Bellonci and the sinus gland are both located inside the eyestalks on opposite sides. The organ of Bellonci contains onion bodies but no distinguishable sensory pore. The sinus gland is large with visible patches of neurosecretory secretions and is surrounded by a thick layer of connective tissue.