The role of animal found in the diets of tilapias is, at present, an enigma.
In some studies, animal remains are rare or absent from gut contents even
when hundreds of specimens were examined. In most cases animal remains
are present in low numbers and investigators have concluded these were
ingested incidentally, either as whole invertebrates or as fragments of dead
invertebrates, while the fish fed on other more typical foods. But in some
cases, invertebrates clearly make up a significant proportion of the diet and
are probably ingested intentionally (Abdel-Malek 1972; Spataru and Zorn
1978; Whitfield and Blaber 1978). Inclusion of invertebrates in the diet may
be an important variable in the feeding strategy of tilapias, but until more
quantitative data are available to describe circumstances under which animal
prey are selected and the significance of such selection for nutrition, this
aspect of the diet will remain an enigma.
The Process of Digestion in TilapiasDigestion in tilapias is a two-step process with distinct gastric and intestinal
components. The mechanism for gastric digestion found in tilapias appears
to be unique among animals. In other animals, the pH of fluids in an actively
digesting stomach ranges from about 2.0 to 2.2 (Barrington 1957). This is
the pH at which vertebrate gastric digestive enzymes, including those of
tilapias (Fish 1960; Nagase 1964; Moriarty 1973) show maximum activity.
In contrast, the pH of stomach fluid in actively digesting tilapias is frequently
as low as 1.25 (Moriarty 1973; Bowen 1976; Caulton 1976) and values as
low as 1.0 have been recorded (Payne 1978). Moriarty (1973) was the first
to describe the role of gastric acid in digestion by tilapias. He studied the
population of S. niloticus in Lake George, Uganda, which feeds on phyto-
plankton dominated by colonial and filamentous blue-green species. As
the algae pass from the esophagus they may travel either of two routes
through the stomach. Peristaltic movement carries the food in a posterior
direction along the ventral wall, up the posterior wall and back along the top
to the pyloric sphincter. Algae that travel this route are exposed to progress-
ively lower pH as HC1 is secreted by the gastric mucosa. Exposure to acid at
pH 1.8 or lower decomposes the algal chlorophyll to phaeophytin and thus a
gradual change in color from green to brown is seen as the algae move along
the gut wall. Some algae take a shorter route passing across the anterior of
the stomach from the esophagus directly to the pyloric sphincter and are not
exposed to acid concentrations below pH 2.0. These cells remain green.
Moriarty demonstrated that acid not only decomposes chlorophyll but
actually lyses blue-green cell walls. This makes subsequent intestinal digestion
possible by providing intestinal enzymes access to the algal cytoplasm.
Following Moriarty's discovery, it was found that the same mechanism
allows S. mossambicus to digest bacteria associated with detritus in its diet
(Bowen 1976a). These observations are significant since vertebrates lack
gastric enzymes capable of attacking the prokaryotic cell wall. Development
of a special mechanism for lysis of blue-green algae and bacteria allows the
tilapias special access to a relatively protein-rich (about 50%) food resource
for which there is little, if any, vertebrate competition.