trap the remaining algal cells in the exhalant
water. It is said that this screen filter can
remove algae of 10 µm (1000 µm = 1 mm) in
diameter from the water.
Single celled green algae are usually the most
abundant pelagic algae, and these are usually
the most digestive forms. Diatoms may also be
pelagic, but are mainly sedentary, and are an
important component of the diet. Diatoms are
algae that are all characterized by a siliceous
outer case or frustule and may be single celled
units or aggregate collections. The inert silica
frustule is perforated by a network of small
holes and, although the frustules themselves
are resistant to the digestive enzymes,
these enzymes can penetrate the pores and
thus digest the inner cell content. The blue
green algae, a group particularly prevalent
in eutrophic water, differ from algae in that
they have cellulose “cell walls” surrounded by
a mucus coating. Cellulose itself cannot be
broken down by the digestive enzymes but the
strong stomach acids (pH 1.2) aided by some
mechanical damage caused by the pharyngeal
teeth can in fact damage this resistant outer
coating sufficiently to release the cell contents.
Herbivorous tilapia can also feed extensively
on periphyton (the algal mats that grow
attached to submerged objects) when this is
AgriKultuur |AgriCulture
available. When feeding on this algal flora, the
fish rasp the algae off the substratum using
their chisel shaped mandibular teeth and the
food is then processed in a similar manner to
that previously described.
Substrate feeding or feeding on sedimented
planktonic algae and episalmic diatoms (those
found in the sand), follows the same general
principles previously described, but the
mechanism of collection differs somewhat.
Feeding on sedimented detritus formed
from either algae or other decaying plant
matter, may be an important source of food
for Oreochromis mossambicus when either
planktonic or periphytic algae are scarce.
The ability of tilapia to feed on bacteria is
a tremendous innovation because during
certain periods of the year or in certain areas,
the bacteria present in decaying sediments
are not only a good source of protein, but
some populations may rely almost exclusively
on these food sources. This form of feeding
however, is generally not enough by itself to
maintain good growth rates in tilapia.
If algal populations were large, fish would not
need to feed on detritus, but in the absence
of reasonable quantities of algae, detritus
becomes an acceptable alternative.
7