Having a daily cycle, processes or rhythms related to the daily rotation of the earth (tides, vertical migration). Many marine species, in particular fish, exhibit diurnal vertical migration, resulting in a shallow distribution at night and a deeper distribution during daytime. The trigger for the migration is the daily light cycle. Diurnal migration behavior may serve as a protection against predation or conserving energy as it involved moving to deeper colder and darker water- layers during daytime, resulting in lower metabolic rates and reduced visibility.