Comparative morphology of the eyes of postlarval bresiliid shrimps from the region of hydrothermal vents

The structure and ultrastructure of the eyes of postlarval vent shrimps provisionally designated 'Alvinocaris' and 'Chorocaris' are described. The eyes of the postlarval 'Alvinocaris' are cylindrical, borne on short stalks, and contain closely packed rhabdoms. The ommatidia lack dioptric apparatus and have rhabdoms extending almost to the cornea. The rhabdoms consist of orthogonal layers of microvilli typical of crustacean rhabdoms. The eyes of the 'Chorocaris' are similar, but the rhabdom layer extends back through the reduced eyestalks and covers some of the dorsal surface of the cephalothorax. The rhabdoms from both the anterior and the thoracic regions consist of layered microvilli. The eyes of a slightly smaller postlarval vent shrimp, termed 'Type A', differ. Although clearly related to the other vent shrimps, Type A has stalked eyes held at an angle to the head. The eye displays a gradient of ommatidial development, with the older ommatidia closely resembling those seen in the other postlarval types. Between the cornea and the rhabdom layer, the youngest ommatidia possess quadripartite crystalline cones similar to those seen in related families of caridean shrimps; these are absent in the more mature ommatidia. The external structure of the anterior and thoracic eyes of juvenile Rimicaris exoculata (after settlement at the vent site) is also described. Juveniles up to 9 mm in carapace length have anterior corneas similar to those seen in postlarvae, whereas in larger specimens the corneas are progressively replaced by an ocular plate.