Gastric acid is similarly important in digestion of some eukaryotic cells.
The efficiency of diatom digestion increases with lower pH to pH 2.5 (Bowen
197613). Tilapias that consume macrophytes also have low gastric pH when
feeding and there is some indication that gastric acid may facilitate digestion
of their food as well (Caulton 1976).
Details of gastnc digestion have been studied for S. mossambicus in Lake
Valencia, Venezuela (Bowen 1981). Comparison of food and stomach
contents for juveniles fed periphytic detrital aggregate (PDA) in aquarium
experiments showed that gastric acid decomposed much of the mineral
component of the diet (Table 2). In effect, this increased the concentration
of organic matter available for the fishes' nutrition by a factor of 1.5.
Measurable protein values increased by a factor of 1.6. Although a small
fraction of this increase may be gastric enzymes, in vitro simulation of
gastric acidification showed most of the increase was due to the effect of
acid in facilitating protein extraction for subsequent assay (Table 3). Exper-
imental data indicate that about 20% of the dietary carbohydrate is decom-
posed by gastric acid.
Table 2. Results of aquarium experiments to assess effects of gastric digestion on periphy-
tic detrital aggregate ingested by Sarotherodon mossambicus. Samples for which the coef-
ficient of variation (CV) is not given were pooled. Total organic values are % of sample
weight; protein and carbohydrate values are % of organic weight.Food Experiment Food Stomach Stomach (%)
Component number intake (7%) contents (%) CV (%) food (%)Total Organic 1
2 3 4 5 6 7ProteinCarbohydrate* 4
5
6*Mean of three determinations