Melatonin is Involved in Sex Change of the Ricefield Eel, Monopterus albus Zuiew

The ricefield eel (Monopterus albus Zuiew), a burrowing eel-like synbranchoid teleost, undergoes a natural sex change from female to male during its life history. Since the teleost pineal gland and its melatoninergic output have been suggested as regulators in seasonal reproduction and sexual maturation in many fish species, it is reasonable to postulate that melatonin may play important roles in the ricefield eel’s sex-change process. This hypothesis was tested by examining secretional characteristics and reproductive effects of melatonin in the ricefield eel. Results indicate that serum melatonin (mainly secreted from the pineal complex, retinae and gastrointestinal tract) is involved in sex change of this species. It seems that, within a reproductive cycle, relatively lower mid-night serum melatonin (MNSM) levels are necessary for natural spawning, but relatively higher MNSM levels after spawning permit initiation of the sex-change process. A putative model is presented to clarify the involvement of melatonin in natural sex change of the ricefield eel, although the precise mechanisms are still under further investigation.