THE MORPHOLOGY OF THE DORSAL EYE OF THE HYDROTHERMAL VENT SHRIMP, RIMICARIS-EXOCULATA

The bresiliid shrimp, Rimicaris exoculata, lives in large masses on th e sides of hydrothermal vent chimneys at two sites on the Mid-Atlantic Ridge. Although essentially no daylight penetrates to depths of 3500 m, very dim light is emitted from the hydrothermal vents themselves. T o exploit this light, R. exoculata has evolved a modified compound eye on its dorsal surface that occupies about 0.5% of the animal's body v olume. The eye's morphology suggests that it is extremely sensitive to light. The cornea of the dorsal eye is smooth with no dioptric appara tus. The retina consists of two wing-shaped lobes that are fused acros s the midline anteriorly. The rhabdomeral segments of the 7000 ommatid ia form a compact layer of photosensitive membrane with an entrance ap erture of more than 26 mm(2). Within this layer, the volume density of rhabdom is more than 70%. Below the rhabdomeral segments, a thick lay er of white diffusing cells scatters light upward into the photorecept ors. The arhabdomeral segments of the five to seven photoreceptors of each ommatidium are mere strands of cytoplasm that expand to accommoda te the photoreceptor nuclei. The rhabdom is comprised of well-organize d arrays of microvilli, each with a cytoskeletal core. The rhabdomeral segment cytoplasm contains mitochondria, but little else. The perikar yon contains a band of mitochondria, but has only small amounts of end oplasmic reticulum. There is no ultrastructural indication of photosen sitive membrane cycling in these photoreceptors. Vestigial screening p igment cells and screening pigment granules within the photoreceptors are both restricted to the inner surface of the layer of the white dif fusing cells. Below the retina, photoreceptor axons converge in a fan- shaped array to enter the dorsal surface of the brain. The eye's size and structure are consistent with a role for vision in shrimp living a t abyssal hydrothermal vents.