Development of the pituitary, thyroid and interrenal glands and applications of endocrinology to the improved rearing of marine fish larvae

Abstract

A key to success in the culture of marine fish is the mass production of high quality fry, a process largely dependent on successful first feeding and normal development and growth of fish larvae. In this regard it is important to examine the structural and functional development of the endocrine system during early ontogeny of marine fish. To characterize early endocrine development a comparative study was conducted with various marine fish. The age when the pituitary gland differentiated, assessed by immunohistochemical detection of GH and PRL, varied among species, but generally coincided with the time of eye pigmentation, when yolk absorption is very rapid in most marine fish larvae. Thyroid follicles and pancreatic islets first appeared at about the same time, while interrenal cells appeared later. Tissue concentrations of thyroid hormones decreased during embryonic development to nearly undetectable levels at the completion of yolk absorption. During early larval life thyroid hormone levels increased gradually and were significantly elevated during the transformation to the juvenile stage, especially in flatfish. Tissue concentrations of cortisol showed nearly the same developmental pattern, but increased prior to thyroid hormone levels. GH and PRL exhibited different developmental patterns. GH increased continuously during larval growth while PRL exhibited nearly the same pattern as cortisol in Japanese flounder. Based on these findings it is concluded that the endocrine system becomes functional before the completion of yolk absorption in most marine fish, when the switch from a maternal (yolk) to an endogenous larval hormone source occurs. A preliminary experiment on T 4 supplementation indicated that intact protein ingestion by pinocytosis in the rectal epithelium and fat absorption in the intestinal epithelium were enhanced. Potential benefits of exogenous hormone treatment are discussed particularly in relation to digestive functions.