The influence of light and darkness on thyroid and pituitary activity of the characin Astyanax mexicanus and its cave derivatives. Bulletin of the AMNH ; v. 93, article 7

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New York : [American Museum of Natural History]
"One. The eyed, river-dwelling characin, Astyanax mexicanus, can live for apparently indefinite periods in total darkness, but under such conditions develops hyperplasia of the thyroid, reduction in the relative number of pituitary basophiles, reduction in the size of the gonads, and an alteration in body form caused by the accumulation of subdermal fat. 2. The blind, cave-dwelling derivatives of Astyanax thrive in either light or darkness, and under either condition show no reduction in size of the gonads nor any change in body form. The form from La Cueva Chica may exhibit a slightly increased thyroid activity in the light, and deposits additional quantities of subdermal guanin. The form from Cueva de los Sabinos shows neither of these reactions. 3. When either river or cave fish that have been raised in the light are placed in darkness, no reduction in guanin takes place. 4. For comparison, young Ameiurus were similarly treated and showed a slight rise in thyroid activity in darkness. 5. Blinded river fish kept in the light show no morphological changes like those exhibited by individuals kept in the dark. Therefore enforced activity caused by the impossibility of optical fixation in the river fish in the dark cannot be the cause of increased thyroid activity, since the blinded fish kept in the light are forced into the same activity without subsequent increase in the height of the thyroid epithelium. 6. Hyperplasia of the thyroid in the normal river fish in light may be produced by treatment with thiourea, and a decrease in thyroid activity in river fish kept in the dark may be produced by feeding desiccated mammalian thyroid. In the first case the condition approximates that found in the untreated fish living in the dark, and in the latter the decrease may exceed that of normal fish living in the light. 7. The development of aberrant features in river fish kept in the dark and the lack of these features in their cave-dwelling derivatives may indicate that the latter have developed some modification of the endocrine system that enables them to function normally in the total darkness of caves. 8. Because of these reactions of the river fish to a totally dark environment, the present extensive cave populations may have been established by a few fish from the river that were able to make the appropriate endocrine adjustment, or from the eggs of river fish spawned in the cave before the influence of darkness had disturbed the endocrine balance sufficiently to make reproduction impossible. None of the present experimental animals was able to make such an adjustment. The physiological behavior of fish produced from eggs hatched in darkness is yet to be determined"--P. 528.
p. 501-531, [6] p. of plates : ill. ; 26 cm.
Includes bibliographical references (p. 529-531).