No. 14: Winter 1981 Supplement
The sunlight that filters down into the depths of the sea is exceedingly weak. It is so dark down there that one would expect deep-sea fish to be blind like many cave-dwelling animals. They are not blind; rather many have eyes of fantastic size and novel construction. An unusual feature of some deep-sea eyes is a layered retina. In the conger eel, five layers of photoreceptors are plastered on top of one another. Yet, experiments with conger eel eyes reveal that only one layer of photoreceptors is active at any one time. R. Shapley and J. Gordon, who carried out these experiments at the Plymouth Lab., surmise that the extra retinal layers are being held in reserve, much like the rows of spare teeth found in sharks' mouths. If so, deep-sea fish are the only animals that have evolved spare stores of visual pigments.
(Anonymous; "The Mystery of the Non-Functioning Receptors," New Scientist, 88:366, 1980.)
Comment. Why haven't cave-dwelling fish taken the same evolutionary route?