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 active hydrothermal vents at the Snake Pit site on the Mid-Atlantic Ridge. Casual examination of animals collected with the DSV 'Alvin' suggests that, like other bresiliid shrimp from hydrothermal vents, the eyes of this species are adapted for vision in very dim light. However, examination of the structure and ultrastructure of eyes of animals collected and immediately fixed shows that the expected massive array of photoreceptors is partially or completely missing. The eye is enlarged, its dioptric apparatus has disappeared, its screening pigment is essentially gone, and its reflecting pigment cells have formed an enlarged mass of white diffusing cells behind the expected layer of photoreceptors. In half of the animals examined there were no recognizable photoreceptors in the retina, and in the remaining animals there were only scattered photoreceptors with poorly organized microvillar arrays of photosensitive membrane. We conclude that this species is blind despite some retinal adaptations for vision in very dim light. Apparently, the ambient light of this animal's environment is below the quit point (the minimum level that can be exploited) so that the retina has begun to degenerate by losing its photoreceptors.