The Gastrointestinal Tract of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 9

omnivorous species show an intermediate condition. There are also differences in RIL within
the same species. For example, in kalbasu ( Labeo calbasu ) the RIL of the detritivorous adult
is higher (2.1 to 13.0) than that of fry feeding on zooplankton (0.5 to 1.0) (Sinha 1976). It is
assumed that the long intestinal length of herbivorous compared to carnivorous fish is due to
the requirement for digesting and absorbing the portion of the plant food which they normally
ingest in the adult stage (Sinha and Moitra 1975). The greater length and mass of the intestine
in herbivores relative to carnivores have also been thought to allow for additional processing
of relatively difficult-to-digest items (Horn 1997; Clements and Raubenheimer 2005). How-
ever, it is also possible that herbivorous and/or detritivorous fish consuming plant fibres and
detritus depend on extended intestines in order to increase the utilization efficiency, which
is not directly related to the surface area (Olsen and Ringø 1997). On the contrary, Harder
(1975) opined that there are no clear relationships between intestinal morphology and feeding
type and it is not possible to draw any conclusion in this regard. Histologically, the intestine
in fish contains simple, columnar absorbing epithelium lined with brush border of microvilli,
which is typical of absorptive tissue (Figure 1.3C; De Silva and Anderson 1995) and gob-
let cells (mucus producing cells). In some fish species regional variations in the brush border
formations have been observed (Figure 1.6). The numbers of goblet cells are more numer-
ous in the posterior region than in the anterior and middle regions (Ray and Moitra 1982).
The posterior part of the intestine is considered to be the main site for intestinal absorption
of macromolecules in salmonids and some other fish species (for review, see Dalmo et al.
1997; Figure 1.7). The midgut starts immediately posterior to the pylorus and the hindgut is
an extension of the midgut with gradually diminishing digestive and absorptive functions and
increased level of mucus production (Ringø et al. 2003).
1.6 ENDOGENOUS INPUTS OF DIGESTIVE SECRETA
Different enzymes, bile acids and pancreatic enzymes are constantly secreted or leaking into
the GI tract from the wall tissue and from the liver and pancreas, respectively. These flu-
ids contain a great range of compounds that may affect the growth and composition of the
intestinal microbiota. Besides macromolecules such as a great number of proteins, for example
digestive enzymes and muco-polysaccharides, these fluids contain phospholipids, bile acids,
antioxidants such as glutathione, minerals, waste products eliminated from the body through
the faeces (e.g. bilirubins giving colour to the faeces) and bicarbonate to stabilize the pH of
the luminal contents. Although our knowledge is limited for fish, it can be suggested that these
fluids vary greatly in quantity as well as composition between intestinal segments and within
species under different conditions. To our knowledge, no information has been reported in the
scientific literature regarding quantities of water and material entering the GI tract of juvenile
or adult fish. However, alterations in composition have been observed, and information is avail-
able that alterations are observed in activities of digestive enzymes within the gut contents of
salmonids by incorporation of plant material in the diet (Romarheim et al . 2006; Gatlin III
et al . 2007; Santigosa et al . 2008) as well as alterations in content of bile acids caused by
dietary fibre (Romarheim et al . 2006). Various dietary components may serve as substrates
for the gut microbes, and enzymes such as proteases and lipases, bile acid and antimicrobial
components will also probably modulate the gut microbiota.