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 shrimp s provisionally designated 'Alvinocaris' and 'Chorocaris' are describe d. 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. T he rhabdoms consist of orthogonal layers of microvilli typical of crus tacean rhabdoms. The eyes of the 'Chorocaris' are similar, but the rha bdom 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', di ffer. Although clearly related to the other vent shrimps, Type A has s talked 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 crystallin e 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 (af ter settlement at the vent site) is also described. Juveniles up to 9 mm in carapace length have anterior corneas similar to those seen in p ostlarvae, whereas in larger specimens the corneas are progressively r eplaced by an ocular plate.