Crustaceans from Atlantic hydrothermal vent fields (Segonzac et al., 1993) present graded variations in eye structure. In the shrimps Rimicaris exoculata and Chorocaris chacei, external eyestalks are lacking or reduced and conventional compound eyes are lacking (Williams & Rona, 1986). The anterodorsal part of the cephalothorax presents an "ocular plate" (Van Dover et al., 1989), a structure containing pigment cells and photoreceptors (O'Neill et al., 1995; Nuckley et al., 1996). In the crab Segonzacia mesatlantica the eyestalks are small (Guinot, 1989). Our objective was to localize and describe the organ of Bellonci and the sinus gland usually included in the eyestalks (Charmantier-Daures & Segonzac, submitted). In the three species, the organs of Bellonei are well developped and contain onion body-like structures. In the two shrimps, they are located in front of the brain close to the hemicilipsoid bodies, a position unusual among decapods. Each organ of Bellonci extends toward a cuticular and complex structure which is probably a sensory pore. In Rimicaris exoculata and Chorocaris chacei, the optic lobes are internal and related to the brain dorso-laterally. The four classical medullae are present. In each optic lobe, the sinus gland is small, massive, with usual histological features and is located between the interna and externa medullae, adjacent to a haemc lymph lacuna. In Segonzacia mesatlantica, each eyestalk contains massive nervous masses with numerous and complex chiasmas from which only the lamina ganglionaris is slightly separated. The sinus gland is large, with visible patches of neurosecretory secretions, and surrounded by a thick layer of connective tissue. |
