The Gastrointestinal Tract of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 3

ST

PC

F

M

HC B

Fig. 1.1 The alimentary tract of Atlantic cod ( Gadus morhua L.). ST, stomach; PC, pyloric caeca; F,
proximal intestine; M, mid intestine; B, distal intestine; HC, fermentation chamber. (Source: Lisbeth Løvmo
Martinsen.) For colour detail see Plate 1.

and gall bladder) provides the functions of digestion, osmoregulation, immunity, endocrine
regulation of GI tract and systemic functions, and elimination of environmental contaminants
and toxic metabolites. The GI tract is basically a tube that courses through the body. The GI
tract in Atlantic cod ( Gadus morhua L.) is shown in Figure 1.1. This tract is divided into the
following characteristic regions: mouth, gill arch, oesophagus, stomach, mid intestine, distal
intestine and fermentation chamber.

1.3 STOMACH AND INTESTINAL BULB

Two main groups of fish are commonly distinguished on the basis of presence or absence of
stomach. The most remarkable feature of the digestive system of lampreys, hagfish, chimaeras,
and many herbivorous fishes belonging to Cyprinidae, Cyprinodontidae, Balistidae, Labridae,
Scomberesocidae and Scaridae, is the lack of a true stomach. In cyprinids, for example mrigal
( Cirrhinusmrigala ), the anterior part of the intestine becomes swollen to form a sac-like struc-
ture called the intestinal bulb or pseudogaster (Figure 1.2). In the absence of a stomach, the
anterior intestine performs the function of temporary storage of ingested food (Sinha 1983). In
stomachless fish the intestinal bulb apparently secretes mucus, and histologically the mucosa
resembles closely that of the intestine and is devoid of any digestive components (Horn et al.
2006; Manjakasy etal. 2009). The mucosa of the intestinal bulb is thrown into prominent folds
or villi (for lack of a better term; strictly speaking they are not true villi due to the absence
of lacteals) that are lined with absorptive and mucus-secreting cells. The absence of stomach
in many stomachless fish is compensated by the presence of pharyngeal teeth or gizzards for
grinding food (Suyehiro 1942; Fänge and Grove 1979). Wood-eating fishes have specifically
adapted spoon-shaped teeth for efficiently rasping wood (Nelson et al . 1999). The lack of a
stomach in some species of fish raises questions regarding its significance. Several hypotheses
have been put forward to explain the absence of a stomach which are often contradictory and
speculative (for review, see Wilson and Castro 2011). The shape, size and structure of the
stomach, when present, are related to the duration between meals and the nature of the diet
(Suyehiro 1942; Smith 1989; De Silva and Anderson 1995). A stomach is defined as a portion