Endocrine control of freshwater tolerance in teleosts

The teleosts Xiphophorus maculatus, Xiphophorus hellerii, Xiphophorus milleri, and Xiphophorus variatus xiphidium are native to freshwater streams. Following hypophysectomy their ability to survive in fresh water is abolished and they must be maintained in 1/3 sea water. When hypophysectomized fish are returned to fresh water they succumb within 16 days. The mean survival time of these fish in fresh water is: X. maculatus, 8.7 days; X. hellerii, 8.6 days; X. milleri, 5.5 days; X. v. xiphidium, 4.3 days. Fish in osmotic shock, when returned to dilute sea water, generally show gradual recovery. Injections of saline, neurohypophysial hormones (vasopressin and oxytocin), growth hormone, TSH, and ACTH failed to maintain hypophysectomized X. maculatus in fresh water. Only prolactin (ovine) was effective. Platyfish, injected with 10 μg of prolactin per gram weight of fish, survived the duration of hormone administration. However, following the withdrawal of hormone, all completely hypophysectomized fish died within 16 days. Prolactin in quantities of less than 1.0 μg is totally unsuccessful in promoting freshwater survival of fish without their pituitary glands. Compared with uninjected controls, the administration of saline or ineffective hormones considerably shortens the mean survival time of hypophysectomized fish in fresh water. In a few fish, hypophysial tissue in amounts sufficient to support them in fresh water, regenerated in four to six weeks. In some of these individuals the glands regenerated without connection to the hypothalamus.