The Gastrointestinal Tract of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 6

ST

PC

DI

PI

Fig. 1.4 Alimentary tract of murrel ( Channa punctatus ). ST, stomach; PC, pyloric caeca; PI, proximal
intestine; DI, distal intestine. Relative intestinal length (RIL) is 0.5. (Source: Arun K. Ray.) For colour detail
see Plate 3.

present, number from a few as in murrel Channa punctatus (Figure 1.4) to several hundred
as in Atlantic cod (Figure 1.1). The caeca of different species vary considerably in size, state
of branching and connection to the gut (Suyehiro 1942; Olsen and Ringø 1997; Ringø et al.
2003). Histologically, they closely resemble the intestine (Figure 1.3D), and possibly serve to
increase the absorptive surface of the gut (Bergot et al. 1975). The pyloric caeca are always
absent in stomachless fish (Barrington 1957; Kapoor et al . 1975). Although the presence or
absence of the pyloric caeca has no apparent correlation with the nature of the food or with
feeding habits (Khanna 1961; Mohsin 1962), the caeca are typically absent or much reduced
in omnivorous and herbivorous species (Rust 2002). There is also no clear correlation between
the number of caeca and the length of the gut, and feeding habits (Harder 1975; Hossain and
Dutta 1996). Pyloric caeca have been reported to increase the surface area for digestion and
absorption but do not have any role in fermentation or storage (Buddington and Diamond
1987). In salmonids, the pH of caeca and caecal intestine is 7.0 and 7.5, respectively (Ringø
et al. 2003). Compared to the numerous studies evaluating the finfish gut microbiota (e.g.
Cahill 1990; Ringø et al. 1995; Ringø and Gatesoupe 1998; Hansen and Olafsen 1999; Ringø
and Birkbeck 1999; Austin 2006; Kim et al . 2007; Merrifield et al . 2011; Lauzon and Ringø
2012), fewer studies have investigated the microbiota of pyloric caeca (Lesel and Pointel 1979;
Gildberg et al . 1997; Gildberg and Mikkelsen 1998; Navarrete et al . 2009; Zhou et al . 2009b).

1.5 INTESTINE

In fish, the intestine is the main organ for digestion/absorption. In addition to digesting and
absorbing feedstuffs, the intestine is critical for water and electrolyte balance, endocrine reg-
ulation of digestion and metabolism, and immunity (Ringø et al. 2003). The intestine shows
considerable variation in its length and arrangement in different species of fish (Kapoor et al.
1975; Fänge and Grove 1979; Stevens 1988). Some fish have a relative intestinal length (RIL
= length of intestine/length of body) less than 1, while some fish species have an RIL of 10
to 20 times their body length (Suyehiro 1942; Olsen and Ringø 1997). The highest RIL gen-
erally occurs in herbivorous and detritivorous species (Figure 1.2), while the lowest is found
in strictly carnivorous and predatory species (Figures 1.1 and 1.4). The intestine in Cyprinids
and Loricariids exhibits a wide range of looping and coiled arrangements (Figure 1.5), while