The Feeding Apparatus and Digestive Tract in TilapiasIt is important to begin with consideration of the feeding apparatus and
digestive tract since these structures limit the range of potential food items
that can be consumed and digested efficiently. Compared to the haplo-
chromis cichlids, the feeding apparatus of tilapias is simple and unspecialized
(Fryer and Iles 1972). The jaw teeth are small unicuspid, bicuspid or tricuspid
structures that occur in one to five rows (Plate 1). In those species for which
descriptions are available, the jaw teeth are flattened distally to form blades
that appear to be useful as scrapers (Fryer and Iles 1972; Lanzing and
Higginbotharn 1976). Neither the gill rakers nor the buccal cavity appears to
be specialized for feeding, but considerable specialization is evident in the
dentition of the pharyngeal bones- (Plate 2). These teeth range from fine,
thin, hooked structures on the pharyngeal bones of Sarotherodon esculentus,
a phytoplankton consumer, to the coarse, robust structures on the pharyn-
geal bones of T. rendalli, a macrophyte consumer (Caulton 1976). Mechanical
and myological details of the cichlid pharyngeal apparatus are described by
Fryer and Iles (1972) and Liem (1973).
The role of the pharyngeal apparatus is to prepare food for digestion. In
many species this is done by breaking or cutting the food into smaller sized
units. In S. esculentus, filamentous and large colonial phytoplankton may be
broken into smaller units. In S. mossambicus and S. melanotheron, detrital
aggregate is broken into finer fragments (Bowen 1976a; Pauly 1976) and in T.
sparrmanii long filamentous periphyton is shredded to short segments of
uniform length. The advantages of reduced particle size include a greatly
increased surface to volume ratio that facilitates enzyme-substrate interaction,
and reduced resistance to peristaltic mixing. In addition, mechanical dis-
ruption of macrophyte cell walls by the pharyngeal apparatus in T. rendalli
increases the efficiency with which this food is digested (Caulton 1976).
The esophagus is short with a small diameter and leads to a small sac-like
stomach. Some investigators have questioned the identification of this latter
structure (Kamal Pasha 1964; Man and Hodgkiss 1977b) but its separation
from the intestine by a sphincter, the low pH of the fluid it contains (Moriarty
1973; Bowen 1976b; Caulton 1976) and the pH optima of proteases ex-
tracted from its mucosa (Fish 1960; Nagase 1964; Moriarty 1973) all attest
to its gastric function. Immediately behind the pyloric sphincter, the intes-
tine receives a common bile dud. The first, short intestinal segment is
thin-walled and of greater diameter than the remainder. Perhaps the most
striking feature of the digestive tract of tilapias is the exceptional length of
the intestine. Quantitative data for T. rendalli, S. melanotheron and S.
mossambicus show that the ratio of intestinal length to fish standard length
is between 7 :1 and 10:l (Caulton 1976; Pauly 1976; Bowen, unpublished).
Other nonquantitative observations reported in the literature suggest these
ratios are representative of the tilapias as a group. By comparison, Fryer and
Iles (1972, p. 41) report that only _three of 106 cichlids collected from Lake
Tanganyika and its affluents had gut length: total length ratios greater
than 6:l and these three may have been tilapias. The intestine ends in an
anal sphincter.