Adaptation of the Physiological, Endocrine, and Digestive System Functions to Prolonged Food Deprivation in Fish

Abstract

Most of the theory about starvation physiology in fish is based on observations of birds and mammals. Such observations have given rise to the idea that starving animals undergo three distinct physiological and/or morphological phases. These phases are typically defined by the type of physiological fuel (e.g. carbohydrates, lipids, or proteins) that is being utilized. Transitions from one phase to another may be indirectly identified by changes in hormone levels, enzyme activities, blood metabolites, or body mass. Similar to birds and mammals, we notice three distinctive transitions in the sequential compositional changes during long-term starvation of fish. The first phase is a short transient one where both protein tissues and fat reserves are mobilized, and where the concentration of several hormones (such as ghrelin and growth hormone levels) deviates significantly from the normal steady-state levels. The second phase appears to be a (usually long) steady state, with mobilization of fat as the main source of energy. During this phase the change in concentration of endocrine factors is minimal and protein breakdown is nearly constant. When the primary lipid source which was utilized as energy during the second phase reaches a critical value, a transition to the third stage occurs, in which proteins are being mobilized as the primary energy source. It appears that various fish species use different lipid sources (e.g. liver, viscera, muscle) and exhibit transition to Phase III at different critical values. Hormonal levels also change significantly at this final stage and they may facilitate the transition to alternative energy source (e.g. muscle protein). We also notice changes in composition and structure of the gut system of fish during these stages. A loss of vacuolization of the mucosa cells and the transformation to finely granular cytoplasm is already noticed after 2 days and is prominent after 7 days of starvation (Phases I and II, respectively). Changes in the mucosa folds can be observed after a short fasting period, at Phase II. Generally, the cells kept their regular cylindrical form and the number of goblet cells increased during the second stage of starvation. To conclude, data on starvation in fish suggest three distinctive phases during prolonged starvation periods, with transitions that are triggered by hormonal changes and are strongly effected by temperature.