THE INTESTINES OF CARNIVOROUS FISH - STRUCTURE AND FUNCTIONS AND THE RELATIONS WITH DIET

The intestine is a complex multifunctional organ. In addition to diges ting and absorbing feedstuffs, the intestine is critical for water and electrolyte balance, endocrine regulation of digestion and metabolism , and immunity. The intestines of carnivorous fish have evolved for pr ocessing a highly digestible, nutrient dense diet that is high in prot ein and low in carbohydrate. Correspondingly, abilities to digest prot ein are well developed, but carbohydrate digestion is low compared to omnivorous and herbivorous fish. Furthermore, the evolutionary stable diet is associated with a lack or reduced abilities to adaptively modu late digestive functions to match changes in diet composition. Despite similar selective pressures, intestinal structure is highly variable among carnivorous fish, reflecting phylogenetic diversity. Due to econ omic considerations, diets for cultured species often have varying pro portions of plant-based ingredients. Although such feeds are effective for raising omnivorous and herbivorous species, they have provided li mited success for carnivores, and this has been attributed to digestiv e limitations. Recent evidence of inflammatory responses to as yet uni dentified components of some plants suggest involvement of the enteric immune system. Changes in temperature and salinity alter intestinal s tructure and functions, and therefore processing of dietary inputs. A relatively unknown component of the intestine involves the resident mi crobiota and its role in the health and disease of carnivoraus fish.