Agri Kultuur December / Desember 2018 - Page 7

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