EFFECTS OF FOOD-RESTRICTION ON THYROID ACTIVITY IN THE GUPPY LEBISTES RETICULATUS (PETERS)

The growth and histological appearance of the thyroid gland of paired underfed and well-fed female guppies, Lebistes reticulatus ( Peters), have been described. Measurements were made of the height of the follicular epithelial cells and of thyroid length in relation to body length. An estimate was obtained of the percentage of "inactive" follicles in the gland. Consistent results were obtained with four pairs of fish, those of 6 months and 9 1/2 months of age. In each of these four pairs the thyroid gland of the underfed fish was more active than its paired, well-fed control. The pair of fish kept for 19 months gave contrasting results: in this pair, the thyroid of the underfed fish was generally inactive. The results obtained from the fish aged 6 months and 91/2 months are the converse of those observed in mammals, tadpoles, and trout, where starvation resulted in hypotrophied inactive glands. It has been suggested that the lowered activity of the thyroid gland in mammals during starvation results from a fall in the level of thyrotrophic hormone secreted by the pituitary. It is suggested here that the differences between thyroid activity in Lebistes and in mammals, tadpoles, and trout resulted from different feeding regimes; in the guppy experiments the degree of undernutrition was less extreme, and it is possible that acute starvation of these fish might lead to the same final effect as in the other animals. In fact, some indication of this happening was obtained from the pair of fish aged 19 months, where the thyroid of the underfed fish had become less active, and it is suggested that this prolonged period of undernourishment had reduced pituitary, and hence, thyroid activity. Fish with experimentally induced active thyroids have a greater efficiency of food conversion than fish whose thyroid activity has been experimentally reduced (Gross et aI. 1963), and these authors suggested that a relationship might exist between this efficiency factor and the level of thyroid activity. It is here suggested that the more active gland of the underfed guppies is related advantageously to a greater efficiency of food conversion.