Understanding rhythms in the digestive functionality of fish gut

Many living organisms are exposed to the predictable environmental cycles of nature (e.g. day-night alternation, lunar phases, seasons and tides). Like all other organisms, fish have developed internal time-control systems to anticipate these natural fluctuations, tuning internal physiology to external conditions. These systems are synchronised by external signals such as light and temperature. Practically all behavioural activities and physiological processes exhibit rhythms directed to optimize the success of the species while minimising energy expenditure. The feeding process, considered in a broad sense from food detection to absorption of nutrients in the intestine, is also dependent on both external and internal rhythms. However, feeding time may also function as synchroniser of the digestive function. The gastrointestinal tract and accessory glands form a complex system involved not only in the processing of ingested food but also in hormonal and immunological responses. This Review highlights whether cyclic rhythms are driving different crucial aspects of the fish digestive process at different stages of development (from larvae to adults). We discuss this topic at different levels of organisation, from cells (i.e. molecular networks) to organs (i.e. luminal conditions, gut functionality and microbiota). Gathering the current knowledge on rhythmic regulation of digestive and related tissues is crucial to design future research directions and improve the understanding of fish metabolic responses and health. The biological variety of fish offers an excellent comparative framework which allows for comparisons with other zoological groups.