Comparative studies in the light sensitivity of blind characins from a series of Mexican caves. Bulletin of the AMNH ; v. 89, article 5

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New York : [American Museum of Natural History]
"One. Five caves in a Mexican valley in the state of San Luis Potosi contain related blind characins differing slightly in morphology and reactivity to light. 2. The deriving river form and the blind cave forms still somewhat pigmented near the river connection are light negative, while the unpigmented are light positive. 3. The unpigmented forms in the cave nearest the river became light negative after having been kept in light for a time, concomitantly developing a heavy layer of guanine, and some of those from the next cave in a similar manner developed melanophores, while those from the remaining three caves showed no such changes in behavior or pigmentation following exposure to light. 4. The phototaxis exhibited, either positive or negative, is mediated through the remnant optic cyst, since both types become light indifferent on removal of the cysts. However, some fish in all the more advanced populations are light indifferent and have no nervous connection between the cyst and the brain. 5. The sign of the phototaxis is determined by whether the pineal complex is exposed to light or not, the fish in which it is exposed being light positive. Thus with the elimination of pigmentation and the reduction of other overlying tissues, these fishes, in an evolutionary sense, pass from a condition of being light negative to that of being light positive. 6. The sign of the light reaction may be reversed experimentally by covering the normally exposed pineal area or exposing the normally covered area in the two types of fishes. 7. Hybrids between light positive and light negative fishes are found to be intermediate in morphology and reactivity to light. 8. All these fishes have a secondary 'pineal opening,' which is well covered with pigment and guanine in the river fish and the less advanced cave forms. However, these structures are progressively reduced in the more advanced cave forms, thus accounting for the exposed area. 9. Genetic and morphologic data suggest that these populations worked up the now dry valley under ground and did not simply 'sink' into the ground from local surface populations when the caves were formed to develop independent but parallel blind fish populations. 10. The survival value of a light negative reaction of blind fishes near a cave outlet is obvious, and it would appear that the reversal of sign in the more advanced forms can be tolerated only because it occurs in caves too remote from surface outlets to permit the fishes from draining out into an unsuitable lighted environment"--P. 349.
p. 323-351 : ill., map. ; 27 cm.
Includes bibliographical references (p. 350-351).