Thyroid hormone stimulates the shift of erythrocyte populations during metamorphosis of the flounder

Hormonal control of changes in circulating erythrocytes was studied in the metamorphosing flounder (Paralichthys olivaceus). Premetamorphic flounder larvae had large, round erythrocytes with small, round and pyknotic nuclei. At metamorphic climax, many smaller erythroid cells with larger nuclei appeared in the circulation. These cells developed into elliptical, adult-type erythrocytes in fully metamorphosed juveniles. Larval erythrocytes were not observed in juvenile fish. Biochemical analysis of hemoglobins are different between larvae and juveniles. L-thyroxine (T₄) treatment at 100 ppb in ambient seawater clearly stimulated a precocious morphological shift in the erythrocyte populations within 15 days, while the predominant blood cells in control fish were the larval erythrocytes throughout the experiment. The erythrocyte shift found in T₄-treated larvae coincided with metamorphic changes in external appearance and behavior of the fish. These results indicate the presence of the two distinct populations of erythroid cells, larval type and adult type, in developing flounder larvae, and suggest that thyroid hormone suppresses larval erythropoiesis and stimulates establishment of adult, or definitive erythropoiesis.