Abstract
To examine the biological significance of thyroid hormones contained in eggs, the hormones were depleted from eggs by treating the mother fish with thiourea. Constantly spawning medaka (Oryzias latipes) were reared in fresh water containing 0.03% thiourea. The plasma thyroxine (T4) and triiodothyronine (T3) concentrations in the mother fish were about 8 and 5 ng/ml, respectively, before the treatment, and decreased to 1/5 of the initial levels within 1 day. Initial concentrations of T4 and T3 in eggs just after fertilization were about 4 and 1 ng/g, respectively, and decreased to less than 1/4 when the mother was treated with thiourea for 7 days. Minimum effective concentration of thiourea to decrease the hormone concentrations in mother's plasma and the eggs was 0.003%. During the course of normal development, egg thyroid hormones are maintained at the initial levels during the first 4 days, and continuously decrease toward hatching (10 days after fertilization). There was no difference between the normal eggs (derived from the mother fish without thiourea treatment) and hormoneβdeficient eggs (hormone concentrations were less than 1/10 of the control) in hatchability, the time of hatching, and survival under starvation. No difference was seen between the two groups in body weight, body length, condition factor, and survival rate 16 days after hatching. These findings indicate that more than 90% of the hormones contained in eggs are not essential for early development in medaka.