Growth hormone (GH) is an essential polypeptide required for the normal growth and development of vertebrates. We have studied the effects of light-emitting diodes (LEDs) emitting different spectra (red, green, and blue) on the GH of yellowtail clownfish Amphiprion clarkii. Full-length GH cDNA from the pituitary of the yellowtail clownfish was first cloned and then the expression of GH mRNA under different light spectra was measured. GH mRNA expression was significantly higher under green and blue light than under red light spectra. These results indicate that in yellowtail clownfish, short-wavelength LED enhances growth more than long-wavelength LED, and that LED lights are more effective for enhancing growth than white fluorescent bulbs. Injection of melatonin resulted in significantly higher expression levels of GH mRNA compared to the control. We therefore conclude that green and blue light enhance GH levels and that melatonin plays a role in modulating growth of the yellowtail clownfish.[PUBLICATION ABSTRACT]