Plasticity of diel and circadian activity rhythms in fishes

Abstract

In many fish species, some individuals are diurnal while others are nocturnal. Sometimes, the same individual can be diurnal at first and then switch to nocturnalism, or vice-versa. This review examines the factors that are associated with such plasticity. It covers the breakdown of activity rhythms during migration, spawning, and the parental phase; reversals of activity patterns during ontogeny or from one season to the next; effects of light intensity, temperature, predation risk, shoal size, food availability, and intraspecific competition. Case studies featuring goldfish (Carassius auratus), golden shiner (Notemigonus crysoleucas), lake chub (Couesius plumbeus), salmonids, sea bass (Dicentrarchus labrax), and parental sticklebacks and cichlids illustrate some of these influences. It is argued that most species have a circadian system but that having such a system does not necessarily imply strict diurnalism or nocturnalism. Rigidity of activity phase seems more common in species, mostly marine, that display behavioral sleep, and for these animals the circadian clock can help maintain the integrity of the sleep period and ensure that its occurrence takes place at that time of day to which the animal's sensory equipment is not as well adapted. However, in other fishes, mostly from freshwater habitats, the circadian clock seems to be used mainly for anticipation of daily events such as the arrival of day, night, or food, and possibly for other abilities such as time-place learning and sun compass orientation, rather than for strict control of activity phase. In these species, various considerations relating to foraging success and predation risk may determine whether the animal is diurnal or nocturnal at any particular time and place.