An investigation of filter-feeding in the tilapia Oreochromis miloticus (L.)

Various aspects the feeding ecology of the tilapia, Oreochroims niloticus (L.), were investigated. An ultrastructural and histological survey of the pharyngeal apparatus associated with filler feeding, in laboratory-held fish, demonstrated that the development was complete by approximately 40 mm S.L. The filter apparatus would appear not to work as a passive sieve ant the presence of large numbers of mucus cells substantiate a theory of mucus entrapment of algae. Three mucosubstances were demonstrated to be produced by the pharyngeal apparatus. Neutral and sialyated mucus were produced by the mucus cells of the gill rakers and were proposed to be involved in die filtering process. The mucus cells of the tooth sockets in the pharyngeal pads produced a sulphated mucosubstance and this was proposed to act as a ‘heavy’ lubricant during the raking action of the pads. A SEM study revealed ultrastructural differences in the mucus related to function. A quantitative investigation of filter feeding revealed the ingestion and filtration rate dynamics of O. niloticus grazing on two species of blue-green algae, Anabaena cylindrica and Microcystis aeruginosa. Both 40 mm and 85 mm S,L fish were demonstrated capable of filter feeding on the above phytoplankton. The results substantiated a universality in die fundamental regulatory mechanisms of suspension feeders. During fieldwork in Thailand the feeding ecology of small (- 35 mm S.L.) and large (- 8C mm S.L.) O. niloticus was investigated. The results demonstrated the diverse dietary habits of both size classes of fish, including detritivory, herbivory and carnivory. The proposed switch in diet at approximately 60 mm T.L., from omnivorous, particulate feeding to phytoplanktivorous filter feeding, was shown not to be an obligate event. The ultrastructural and histological investigation revealed similar findings to the laboratory-held O. niloticus. The histological results indicate continual mucus production from the pharyngeal apparatus and some other mechanism must be capable of preventing ingestion. In conclusion, the work has elucidated further the feeding mechanism of O. niloticus and demonstrated the diverse feeding habits of both small and large fish.