Clock genes are essential for the daily functions of vertebrates and invertebrates and participate in a wide variety of biological functions including biochemical, physiological, and behavioral processes. Circadian clock disorders are detrimental to both humans and fish, and therefore research on the regulation of genes involved in the circadian clock has attracted increasing attention. Period 2 (per2) is an important gene whose expression is closely related to melatonin secretion, in addition to being affected by the photoperiod length. To gain more insights into the potential biological function of per2, we have cloned and expressed per2 in Amur minnow (Phoxinus lagowskii). We also analyzed melatonin content and AANAT2 gene expression under different short or long-term photoperiod conditions. In the present study, the secretion of melatonin in P. lagowskii was highest in the dark phase under short-term experiments. In the pituitary, the expression of the per2 gene was highest during the dark phase in both the 16L: 8D and 8L: 16D groups. The expression of the per2 gene in the brain was also highest during the dark phase in the 12L: 12D group. Though the expression of the per2 gene was not the highest in the dark phase for the other groups, there were no significant differences compared to the highest value. In addition, in the brain and pituitary, AANAT2 expression was highest during the light phase, except in the 8L: 16D group. Our findings suggested that Amper2 may participate in the feedback regulation of melatonin by regulating the expression of AANAT2. Fluorescence in situ hybridization analyses confirmed that per2 mRNA was expressed during the dark phase, especially in 16L: 8D group. Immunohistochemistry analyses confirmed that PER2 protein was expressed higher in 8L: 16D group. The dual luciferase experiment first validated the promotion of Wnt/β-Catenin expression by the per2 gene. Our results provide novel insights into the expression and location of per2, which contributes to a better understanding of the role of clock genes in the circadian rhythm and seasonal responses of high-latitude fish and might provide useful information of practical interest for high-latitude aquaculture. [ABSTRACT FROM AUTHOR]