The significance of Calcium and Phosphate in ovarian maturation in the teleost fish Oreochromis Mossambicus.

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University of Dar es Salaam
The importance of exogenous and endogenous calcium and phosphate for reproduction in Oreochromis mossambicus was studied. Calcium and phosphate are essential for reproduction since most of the yolk proteins in fish as well as in other vertebrates are calcium binding phospholipoproteins. The effects of changing concentrations of calcium and phosphate in the water and the food on ovarian development were investigated. The study included determination of calcium and phosphate concentrations in bone during ovarian development, under normal feeding conditions and when the fish were put on a calcium and/or phosphate deficient diet. It was found that under normal feeding conditions calcium and phosphate were not mobilized from bone during ovarian growth. Neither did the absence of dietary calcium affect bone calcium, probably because the water rather than the food is the main source of calcium for the fish. However, temporary reduction of phosphate from food caused the fish to selectively extract phosphate from bone. This enabled the fish to sustain reproduction during this period, although the interspawning period was lengthened. Thus prolonged phosphate deficiency in the food may be a real limiting factor to oocyte growth. To investigate the role of the corpuscles of Stannius (CS) during reproduction, the effects on the size and the ultrastructure of the CS of different concentrations of calcium and phosphate in water and food were investigated. In addition the glands were studied in relation to sexual maturation and plasma calcium levels. Increasing Ca2+ concentration in the water increased the size of the CS. Changes in the calcium concentration of the food had no effect on the CS. High phosphate concentrations in the water did not influence the activity of the CS although a slight hyperphosphatemia was induced. Phosphate deficient food caused a slight hypophosphatemia but had no effects on the CS. Thus, there was no evidence for direct involvement of the CS in the control of phosphate metabolism. In sexually mature female fish, CS were larger than in males, and the size increased modestly with ovarian growth. Total plasma calcium was significantly higher in breeding female fish than in males. This was mainly caused by an increase in the non-filtrable calcium fraction, representing protein-bound calcium. It was suggested that activation of CS during ovarian development in females was a response to prevent the rise of the ultrafiltrable calcium fraction. This is a physiologically very important fraction since it consists mainly of ionic calcium. The ultrastructure of the gonadotropic cells was studied during ovarian growth, after ovariectomy and in different concentrations of calcium and phosphate in water and food. The granular endoplasmic reticulum was greatly dilated to form large "vacuoles". The vacuoles were more frequently observed in fish with large ovaries than in fish with small ovaries. In ovariectomized fish, the cytoplasm of the gonadotropes was dominated by vacuoles and was heavily degranulated. In the cytoplasm of some cells, dark electron dense rod-like crystals were observed. No solid evidence was found during the present study to suggest two types of gonadotropic cells in the pituitary of O. mossambicus. The cells did not respond to changes in the calcium or phosphate concentrations of the ambient water or the food.
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Urasa, F. M. (1989). The significance of Calcium and Phosphate in ovarian maturation in the teleost fish Oreochromis Mossambicus. Doctoral dissertation, University of Dar es Salaam. Available at (