Table 3. Effect of gastric acidon the food of Saro herodon mossambicus in Lake Valencia,
Venezuela simulated in vifro (a% sample weight, '9. organic weight).PH organica proteinb ~arbohy drateb carbonateaThe importance of gastric enzymes in digestion by tilapias is not clear and
may vary with species or with diet for a single species. Although a protease
with pH optimum around 2.1 is present in the gastric mucosa of S. mossam-
bicus (Fish 1960; Bowen 1976a) and S. niloticus (Moriarty 1973), it has not
been detected in the gastric fluids (Moriarty 1973; Bowen 1976a). This
enzyme has a greatly reduced reaction rate at pH values below 1.5, and it is
possible that it is not secreted when gastric acid is used to lyse cells.
Secretion of gastric acid stops at the end of the daily feeding period and
stomach fluids return to pH 5 to 7. In S. mossambicus (Bowen 1976b) and
S. niloticus (Moriarty 1973) the stomach is completely empty during this
resting phase but in T. rendalli a small amount of food remains (Caulton
1976). At the start of the next day's feeding, acid secretion begins only as
the stomach is filled with the result that the first food to pass into the
intestine is not exposed to strong acid. In those species whose diet is rich in
chlorophyll, the food ingested at the start of a feeding period remains green
and undigested as it moves along the length of the intestine (Moriarty 1973;
Caulton 1976). Similarly the first diatoms to be ingested by S. mossambicus
remain undigested (Bowen 1976b). This explains why early workers who
, compared stomach and posterior intestine samples sometimes reported that
algae were not digested.
The second step in the digestive process occurs in the intestine. The
common bile duct which opens into the intestine on the back of the pyloric
sphincter adds bile salts that maintain the pH between 6.8 and 8.8 (Fish
1960; Nagase 1964). Trypsin, chymotrypsin, amylase and esterase activity
have all been identified in intestinal fluid (Fish 1960; Nagase 1964; Moriarty
1973).
Details of intestinal digestion have been studied for S. mossambicus in
Lake Valencia, Venezuela. Contents of digestive tracts removed from freshly
caught specimens were divided into five subsarnples: stomach contents, and
the contents of the first, second, third and fourth quarters of the intestine.
Each subsample was analyzed for organic matter, carbohydrate, protein and
total amino acids (Bowen 1980b, 1981). Values from these analyses were
expressed in units of weight per weight of hydrolysis resistant sample ash
(HRA). This indigenous component of the diet is used as an undigested
reference compound relative to which digestion and assimilation are measured
(Conover 1966; Bowen 1981). The results show total organic matter and
carbohydrate are digested rapidly in the first and second quarters of the
intestine with no evidence of digestion in the third and fourth quarters