PITUITARY CONTROL OF ADAPTATION TO FRESH WATER IN THE TELEOST GENUS FUNDULUS

¹ Bingham Laboratory and Department of Biology, Yale University, New Haven, Connecticut 06520

1. Eight species and one subspecies of Fundulus native to brackish-water environments fail within 1-10 days following abrupt transfer to fresh water and fail between 1-4 when subjected to gradual declines in salinity following hypophysectomy. Intact specimens of most species survive indefinitely in fresh water.

2. Of fourteen species of Fundulus and one species of Profundulus typical of freshwater habitats, none fail before one month and some survive for periods of up to one year following hypophysectomy and transfer to fresh water.

3. One inland species of Fundulus characteristic of saline and freshwater environments demonstrates a failure pattern intermediate between the brackish-water and fresh water species of the genus.

4. In those species tested at two or more different temperatures, low temperature generally prolongs survival in fresh water after hypophysectomy.

5. Although hypophysectomy has only minor effects on serum electrolytes (sodium and chloride) in the three species of Fundulus tested in sea water, it results in marked declines in serum sodium and chloride in fresh water in the five species tested. Extremely low serum electrolytes are associated with failure: the levels at failure do not differ between species but the rate of decline is much slower in freshwater species than in brackish-water forms.

6. Hypophysectomized F. heteroclitus fail even at salinities iso-osmotic with serum (12), although survival in dilute sea water is much prolonged over that in fresh water and fish survive indefinitely in sea water (29). Failure in dilute sea water is not associated with very low serum chloride and sodium, suggesting that factors other than the regulation of these electrolytes may be involved in the process of failure.

7. We conclude that the relative importance of the pituitary in adapting to fresh water is closely correlated with normal environmental salinity in the genus Fundulus; the hypophysis being most critical to those species which are subjected to both marine and freshwater conditions in nature.