The Gastrointestinal Tract of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 2

and other organ systems to adapt to changes in the contents of the GI tract (amounts and types
of nutrients, pH, ionic composition etc.) and environmental conditions (Holst et al . 1996).
The key feature of the alimentary tract is its ability to digest foodstuffs to make them
suitable for absorption by various transport mechanisms in the wall compartments of differ-
ent GI sections (Bakke et al . 2011). Besides the hydrolytic reactions catalysed by endogenous
enzymes secreted by the pancreas and cells in the gut wall, which are considered to play the
major roles in digestion, fermentation plays key roles in digestive processes in many mono-
gastrics. The role of fermentation in fish is less clear due to a lack of knowledge, but it is
considered to be of minor quantitative importance for nutrient supply in cold water species.
However, qualitative importance may be significant regarding specific nutrients and immune
stimulating processes.
The anatomy and physiology of the GI tract are important determinants for the estab-
lishment and for the quantitative as well as the qualitative aspects of its microbiota. The
microbial communities may seem to be assembled in predictable ways (Rawls et al . 2006). In
this study the authors showed that microbial communities transplanted from mice to gnoto-
biotic zebrafish ( Danio rerio ) alter quantitatively in the direction of the normal biota of the
zebrafish species and vice versa. This indicates that environmental conditions of the intestine,
determined by species-specific parameters along the GI tract such as anatomy, endogenous
inputs of digestive secretions, pH, osmolality, redox potential, compartment size and structure,
passage rate and residence time, help to define and shape the GI tract microbiota. However,
diet composition is also an important environmental condition for fish development. Diet com-
position is ideally species specific regarding available essential nutrients, but supplies variable
amounts of unavailable material depending on the feedstuffs used in the diet formulations. The
gut microbiota is also probably inevitably linked to digestion by the production of exogenous
enzymes and vitamins produced which might aid host digestive function (Ray et al . 2012).
This chapter summarizes the current state of knowledge highlighting the morphological and
histological variations in the lower GI tract of fish associated with digestion and absorption;
comprehensive reviews on the gut microbiota are presented in Chapters 4-6 .
1.2 ANATOMY OF GI TRACT
The structure and functional characteristics of the GI tract vary widely among species
(Suyehiro 1942) and seem, to a great extent, to match the wide diversity of feeding habits
and environmental conditions exploited by fish. The structure of the alimentary canal varies
in different species of fish, and is generally adapted in relation to the food and feeding habits.
Depending on feeding habits and diet, fish are generally classified as carnivorous (eating
fish and larger invertebrates), herbivorous (consuming mainly plant material), omnivorous
(consuming a mixed diet) and detritivorous (feeding largely on detritus) (De Silva and
Anderson 1995; Olsen and Ringø 1997; Ringø et al. 2003), together with the genera Panaque
and Chochliodon which are capable of digesting wood. However, such division may not
always be correct since most species consume mixed diets or their feeding habits may change
through the life cycle (Olsen and Ringø 1997). The variation becomes obvious by comparing
the GI tract characteristics of carnivorous and herbivorous fish and those from freshwater
and seawater. The mucosal lining of the GI tract represents an interface between the external
and internal environments, and in conjunction with the associated organs (e.g. pancreas, liver