Ref ID : 646
Tom Fenchel; Suspension feeding in ciliated protozoa: Structure and function of feeding organelles. Arch.Protistenk 123:239-260, 1980
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The function of mouth organelles in suspension feeding ciliates has been investigated. Ciliary membranelles propel water in a direction parallel and posterior to the individual membranelles (perpendicular to the membranellar band). In most oligohymenophores (hymenostomes and peritrichs) the feeding currents are forced through the ciliary paroral membrane which acts as a sieve, retaining food particles. In polyhymenophores (spirotrichs), the membranellar band propels water out of the buccal cavity and the band itself also functions as a filter, retaining particles which are larger than the free space between two adjoining membranelles. Due to the torsion of the band, the retained particles are concentrated in the vicinity of the cytostome. The filtering activity of the mouth apparatus may concentrate suspended particles by a factor of several thousand; in the newly formed feeding vacuoles, ingested particles are further concentrated by a factor of 3-4 due to the absorption of water from vacuole. On the basis of simple physical considerations and observational data, it is concluded that most retention mechanisms previously suggested to be important for filter feeders cannot be of significance in the case of ciliates. Quantitative estimates of the amounts of water transported through the ciliary filters, in conjunction with data on the clearing rate for suspended latex beads show that for a certain range of particle sizes, retention is nearly 100%. Further, there is a correlation between the minimum particle size retained and the free space between adjoining cilia of the ciliary filters. It is therefore concluded, that in suspension feeding ciliates, particle retention takes place by a sieving mechanism. The evolution of suspension feeding in ciliates is discussed. Primitive ciliates are predominantly raptorial macrophagous. The mouth of advanced (oligo- and polyhymenophoran) ciliates evolved as an adaptation to microphagy via some Vestibulifera-like ancestor. Many hymenostomes and spirotrichs have secondarily reverted to feed on large food particles, but they have remained suspension feeders and employ the same basic mechanisms for the concentration of food particles as do their microphagous relatives.