Comparative Studies of the Thyroid Function in Teleosts of Tropical and Temperate Habitats

The presence of thyroid gland tissue in teleosts was first reported by Simon (1844), and later its structure was shown to consist of follicular units similar to those in the thyroid gland of higher vertebrates. The teleostean gland is not discrete but consists of numerous scattered follicles. Marine & Lenhart (1910) observed a hyperplastic condition of the thyroid tissue in some trout in hatcheries which was correlated with sluggishness, altered pigmentation and fatness. This condition suggested an altered metabolism. Feeding experiments with mammalian thyroid tissue (Etkin, Root & Mofshin, 1940; Smith & Everett, 1943; Hasler & Meyer, 1942; Root & Etkin, 1937) failed to alter oxygen consumption of teleosts. Only a few individuals showed a rise in oxygen consumption when fed teleostean thyroid tissue (Smith & Matthews, 1947). Thyroid hormone production in mammals is depressed by thiourea. Matthews & Smith (1947) however, were unable to demonstrate a depression of oxygen consumption in Fundulus by the use of this drug. As growth and development of fish may be determined to some extent by the presence of thyroid hormone, experiments to elucidate this effect have been carried out. Some observers have reported a positive effect on giving thyroid hormone (Grobstein & Bellamy, 1939; Goldsmith, Nigrelli, Gordon, Charipper & Gordon, 1944; Hopper, 1952; Vivien & Gaiser, 1952), and others (Smith & Everett, 1943) the reverse. Using thiourea, Smith, Sladek & Kellner (1953) recorded a depressive effect on the growth of Lebistes, but they attributed this to a slight toxic effect on the drug. These contradictory results prompted the work reported below. If the thyroid mechanism regulates metabolism in the teleostean fish, it might be expected that a study of such fish at two temperature extremes would throw some light on the problem. A comparison of the effects of temperature change on thyroid activity, as exemplified by structural changes of the thyroid, and on growth and development afforded a promising approach. Fish from temperate and tropical climes were selected and these were studied under different environmental temperature conditions. As seasonal variations in the thyroid gland of many teleosts are known to occur it was necessary to extend observations over a whole year (Lieber, 1936; Hoar, 1939; Buchmann, 1940).