Melatonin acts mainly via high-affinity receptors coupled to heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins). Three high-affinity melatonin receptor subtypes have been cloned so far and classified as MT1a (also called MT1), MT1b (or MT2), and MT1c . The MT1a receptor subtype is believed to mediate major neurobiological functions. In our study, melatonin receptor gene (Unigene23893_All) was significantly downregulated. This means that melatonin has a negative effect on the development of AG and thus influences the development of testis in E. sinensis.