RETINAL ANATOMY OF NEW BRESILIID SHRIMP FROM THE LUCKY STRIKE AND BROKEN SPUR HYDROTHERMAL VENT FIELDS ON THE MID-ATLANTIC RIDGE

The 1989 discovery of a large dorsal eye on the hydrothermal vent shri mp, Rimicaris exoculata, debunked the prevailing opinion that all anim als living around deep hydrothermal vents were blind. Recent dives wit h DSV 'Alvin' recovered new bresiliid shrimp [?Chorocaris (Lucky Strik e) and ?Chorocaris (Broken Spur)] from two new vent fields on the Mid- Atlantic Ridge. The Lucky Strike species has recently been named Choro caris fortunata and may be the same as the Broken Spur species - the r etinal morphologies reported here are very similar. Like R. exoculata, C. fortunata has a visual apparatus adapted to the very dim light of its environment. Although in both species enlarged, non-imaging eyes a ppear to have evolved from the stalked compound eyes typical of caride an shrimp, those of C. fortunata are forward-facing whilst that of R. exoculata is located dorsally. The massive array of photosensitive mem brane (rhabdom) of C. fortunata Lies beneath a smooth cornea within a matt, white, reflecting matrix and occupies 80% of the available volum e of the photoreceptors compared to the expected 10-15% of shallow wat er decapods. All screening pigment is located out of the light path at the bottom of the retina. There is no ultrastructural evidence for cy clic rhabdom shedding or renewal. Thus the cellular organization of C. fortunata is remarkably similar to that of R. exoculata and distinct from its surface-dwelling relatives. This suggests that in these speci es imaging optics have been sacrificed to achieve the increased visual sensitivity necessary to detect the very dim light emitted from the t hroats of the black smoker chimneys around which they live.