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 shrimp, Rimicaris exoculata, debunked the prevailing opinion that all animals living around deep hydrothermal vents were blind. Recent dives with DSV 'Alvin' recovered new bresiliid shrimp [?Chorocaris (Lucky Strike) and ?Chorocaris (Broken Spur)] from two new vent fields on the Mid-Atlantic Ridge. The Lucky Strike species has recently been named Chorocaris fortunata and may be the same as the Broken Spur species - the retinal 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 appear to have evolved from the stalked compound eyes typical of caridean shrimp, those of C. fortunata are forward-facing whilst that of R. exoculata is located dorsally. The massive array of photosensitive membrane (rhabdom) of C. fortunata Lies beneath a smooth cornea within a matt, white, reflecting matrix and occupies 80% of the available volume of the photoreceptors compared to the expected 10-15% of shallow water decapods. All screening pigment is located out of the light path at the bottom of the retina. There is no ultrastructural evidence for cyclic 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 species imaging optics have been sacrificed to achieve the increased visual sensitivity necessary to detect the very dim light emitted from the throats of the black smoker chimneys around which they live.