Biological rhythms and photoperiod in fish

Biological rhythms are defined as any event that repeats on a regular basis in an organism; they are cyclical events, synchronized by an environmental cue. When fish are submitted to the daily light/dark cycle of the photoperiod, they show a pattern of locomotor activity that can be classified as diurnal, nocturnal or crepuscular. This cycle of light/dark has been considered as one of the most important environmental factors which synchronize biological rhythms, particularly as a key factor for the rhythmic synchronization of activity in fish. The feed cycle also acts as a potent synchronizer on the control of locomotor activity. This is demonstrated by feed anticipation activity as a biological rhythm, i.e., the capacity with which fish can skillfully predict and anticipate the recurring event of regular feeding. These anticipatory responses to feeding probably work under an endogenous control, where fish need to optimize the capture of food as well as the digestive and metabolic processes required in order to concentrate the feed intake in a shorter period of time, thus improving the use of nutrients. The biological clock in fish is generally considered to be controlled by a multiphotorreceptor and multioscillator system in which pineal gland, brain and retina are the main structures involved in the photic signal transduction which establishes a circadian rhythm in fish. The coupling between these structures can vary between species and individuals according to physiological and environmental conditions, resulting in a plasticity of the circadian system in teleost fish. Manipulation of the photoperiod in order to enhance fish growth has become increasingly common in the production of several commercial species. The photoperiod, among other environmental factors, presents the greatest influence on the biological clock of fish, affecting weight gain, feed intake, feed efficiency, energy expenditure, locomotor activity, repro-duction, and other physiological parameters related to stress. Therefore, improving our knowledge on the physiology of biological rhythms is essential to optimize the production of fish. Os ritmos biológicos são definidos como qualquer evento que se repete de maneira regular em um organismo, sendo um evento cíclico caracterizado por um ambiente do qual o animal pode se adaptar. Os peixes, quando submetidos ao ciclo diário de luz/escuro do fotoperíodo demonstram um padrão de atividade locomotora que os podem classificar como diurnos, no