MORPHOLOGY OF THE EYE OF THE HYDROTHERMAL VENT SHRIMP, ALVINOCARIS-MARKENSIS

The bresiliid shrimp Alvinocaris markensis is a predator that inhabits the base of sulphide mounds built by the black smoker chimneys of act ive hydrothermal vents at the Snake Pit site on the Mid-Atlantic Ridge . Casual examination of animals collected with the DSV 'Alvin' suggest s that, like other bresiliid shrimp from hydrothermal vents, the eyes of this species are adapted for vision in very dim light. However, exa mination of the structure and ultrastructure of eyes of animals collec ted and immediately fixed shows that the expected massive array of pho toreceptors is partially or completely missing. The eye is enlarged, i ts dioptric apparatus has disappeared, its screening pigment is essent ially gone, and its reflecting pigment cells have formed an enlarged m ass of white diffusing cells behind the expected layer of photorecepto rs. In half of the animals examined there were no recognizable photore ceptors in the retina, and in the remaining animals there were only sc attered photoreceptors with poorly organized microvillar arrays of pho tosensitive membrane. We conclude that this species is blind despite s ome retinal adaptations for vision in very dim light. Apparently, the ambient light of this animal's environment is below the quit point (th e minimum level that can be exploited) so that the retina has begun to degenerate by losing its photoreceptors.