Eye degeneration in the blind cave-dwelling fish Phreatichthys andruzzii

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Abstract
The blind cave-dwelling fish Phreatichthys andruzzii shows rapid eye development, and at a temperature of 29.5°C, maximal eye differentiation is reached 36 h after egg laying. Very rapid eye degeneration then occurs, and a rudimentary and very degenerated ocular cyst is all that remains 1 month later. This may explain why this species has so often been erroneously considered anophthalmic. Ocular degeneration starts with a decrease in the rate of mitosis in the neuroblastic cells of the retinal Anlage. Degeneration by means of lytic processes is clearly observed beginning 32 h after egg laying. Disappearance of the lens, transformation of the cornea into skin, and cryptic degeneration of the eyes are phenotypic variations of the fundamental mechanisms of eye development in vertebrates. These can be considered evolutionary adaptations by the ancestral fishes to the aphotic cave environment.