Thus selection is primarily based on particle size, avoiding the ingestion of sand particles and also, on the preferential selection of particles associated with organic material, within the fine (silt-clay) fraction of the sediment. On the basis of these results, we hypothesise that this species exhibits two levels of selectivity in its feeding behaviour. Measurement of the organic content of faeces (measured as % loss on ignition) showed that they had a consistently higher organic content than the sediment, considered as whole sediments or the <63 μm sieved fraction.
tubifex fed selectively on the organic rich particles of the sediment, and this feeding was independent of particle size. A more detailed analysis of faeces revealed that about 75%, by volume, was composed of particles with a mean diameter < 25μm, and the mode was < 10μm. This suggests that this species actively selects the silt-clay fraction, avoiding larger sand particles. The faecal material from worms cultured in a range of sediments was composed of particles with a mean diameter of less than 63 μm, and only a few isolated larger particles were found by microscopic analysis. Among the natural feed organisms Tubifex tubifex is one of the best candidates for the following reasons: (i) it has a short generation time (42 days) on a single medium containing organic wastes like cow dung and coconut mesocarp and thus helps to abate the organic pollution of the medium, (ii) it occurs in a wide range of habitats and tolerate. The particle size distribution of faecal pellets produced by the tubificid worm Tubifex tubifex in laboratory culture, was measured with a Coulter® Multisizer.
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