Light is a major external environmental factor that influences the circadian rhythm of photosynthetic organisms and various physiological phenomena, such as growth, maturation, and behavior. The number of light-reaching organisms changes depending on the season and atmospheric conditions, and the intensity and wavelength of light differ depending on the organisms inhabiting the environment. Altered light changes the circadian rhythm of fish, which is controlled by clock genes, such as period 2 (Per2), cryptochrome 1 (Cry1), and melatonin. In this study, we set the zeitgeber time (ZT; 14 light-10 dark, LD) based on the actual sunrise and sunset times and examined Per2 and Cry1 activities, levels of aralkylamine N-acetyltransferase (AANAT), and melatonin in Pholis nebulosa, a drifting seaweed species exposed to irregular light. Per2 and Cry1 levels increased during the daytime and decreased after sunset. The AANAT levels decreased during the daytime and increased during the night. Melatonin concentration was highest around midnight (ZT21, 23:30), but exhibited similar concentrations during the daytime. While the activity of Per2, Cry1, and AANAT levels exhibited a typical circadian rhythm observed in most vertebrates, melatonin concentrations did not show a significant difference between the daytime and nighttime. These findings provide insights into the circadian rhythm patterns of organisms exposed to irregular light environments, such as P. nebulosa, which differ from those of typical fish species.

Introduction
Materials and Methods
    1. Field sampling
    2. Experimental fish and sampling
    3. Analysis of Per2, Cry1 and AANAT in brain
    4. Analysis of AANAT and Melatonin in plasma
    5. Statistical analysis
Results
    1. Per2 activity in the brain
    2. Cry1 activity in the brain
    3. AANAT level in the brain
    4. Melatonin concentration in plasma
Discussion
Acknowledgment
References

• Jin A Kim(Department of Convergence Study on the Ocean Science and Technology, Korea Maritime and Ocean University, Busan 49112, Korea)
• Min Ju Kim(Division of Marine BioScience, Korea Maritime and Ocean University, Busan 49112, Korea)
• Young-Su Park(Department of Nursing, Catholic University of Pusan, Busan 46252, Korea)
• Jun-Hwan Kim(Department of Aquatic Life Medicine, Jeju National University, Jeju 63243, Korea) Corresponding author
• Cheol Young Choi(Department of Convergence Study on the Ocean Science and Technology, Korea Maritime and Ocean University, Busan 49112, Korea, Division of Marine BioScience, Korea Maritime and Ocean University, Busan 49112, Korea) Corresponding author