The Gastrointestinal Tract of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 4

DI IB

PI

MI

Fig. 1.2 Alimentary tract of the mrigal ( Cirrhinus mrigala ). IB, intestinal bulb; PI, proximal intestine; MI,
mid intestine; DI, distal intestine. Relative intestinal length (RIL) is 14-15. (Source: Arun K. Ray.) For colour
detail see Plate 2.

of the digestive tract with distinctive cell lining, where acid is secreted, usually along with some
digestive enzymes like pepsin (Olsen and Ringø 1997). In his early study, Suyehiro (1942)
classified stomachs of fish into five categories according to their morphological appearance:
(a) straight tube (Pleuronectidae, Esox ), (b) U-shape (Salmonids), (c) V-shape (Plecoglossi-
dae, Mugilidae, Salmonidae, Sparidae), (d) Y-shape (Mugilidae, Clupeidae), and (e) I-shape
(Carangidae, Gadidae, Scombridae, Serranidae). The highest degree of modifications of the
pyloric stomach have been reported in several members of Clupeoidei, Channidae, Mugili-
dae, Acipenseridae, Coregoninae and Chanidae (milkfish, Chanos chanos ) where it acts as a
'gizzard' for trituration and mixing (Fänge and Grove 1979; Kapoor et al . 1975; Buddington
1985; De Silva and Anderson 1995). This development of a 'gizzard' has been attributed to
microphagy, and is thought to partly compensate for poor dentition (Pillay 1953). The ante-
rior part of the stomach (cardiac or fundic region) is characterized by the presence of gastric
glands (Figure 1.3A) and the musculature is also usually more prominent (De Silva and Ander-
son 1995). The stomach mucosa is lined with columnar epithelium and studded with minute
depressions, the gastric crypts or pits that lead into the tubular or alveolar gastric glands. Gas-
tric glands are present in abundance throughout the cardiac stomach, so much so that they
occupy the entire mucosal layer beneath the superficial epithelium (Figure 1.3A). This part
of the stomach is secretory in nature and is responsible for storage and initial physical and
enzymatic breakdown of the diet; readers with special interest in this topic are referred to the
comprehensive review of Bakke et al . (2011). The mucosa of the posterior part of the stom-
ach (pyloric stomach) contains many mucus-producing tubular mucus glands or pyloric glands
(Figure 1.3B). The number of these glands decreases considerably in the middle region and
they are completely absent in the posterior region. The pyloric stomach is completely devoid