Moonlight affects nocturnal <i>Period2</i> transcript levels in the pineal gland of the reef fish <i>Siganus guttatus</i>

<jats:p><jats:bold>Abstract: </jats:bold> The golden rabbitfish <jats:italic>Siganus guttatus</jats:italic> is a reef fish with a restricted lunar‐synchronized spawning cycle. It is not known how the fish recognizes cues from the moon and exerts moon‐related activities. In order to evaluate the perception and utilization of moonlight by the fish, the present study aimed to clone and characterize <jats:italic>Period2</jats:italic> (<jats:italic>Per2</jats:italic>), a light‐inducible clock gene in lower vertebrates, and to examine daily variations in rabbitfish <jats:italic>Per2</jats:italic> (<jats:italic>rfPer2</jats:italic>) expression as well as the effect of light and moonlight on its expression in the pineal gland. The partially‐cloned <jats:italic>rfPer2</jats:italic> cDNA (2933 bp) was highly homologous (72%) to zebrafish <jats:italic>Per2</jats:italic>. The <jats:italic>rfPer2</jats:italic> levels increased at ZT6 and decreased at ZT18 in the whole brain and several peripheral organs. The <jats:italic>rfPer2</jats:italic> expression in the pineal gland exhibited a daily variation with an increase during daytime. Exposing the fish to light during nighttime resulted in a rapid increase of its expression in the pineal gland, while the level was decreased by intercepting light during daytime. Two hours after exposing the fish to moonlight at the full moon period, the <jats:italic>rfPer2</jats:italic> expression was upregulated. These results suggest that <jats:italic>rfPer2</jats:italic> is a light‐inducible clock gene and that its expression is affected not only by daylight but also by moonlight. Since the <jats:italic>rfPer2</jats:italic> expression level during the full moon period was higher than that during the new moon period, the monthly variation in the <jats:italic>rfPer2</jats:italic> expression is likely to occur with the change in amplitude between the full and new moon periods.</jats:p>