Use of radioactive labelled silt to show depletion of food supply by upstream colonies of Flustrellidra hispida (Fabricius)

Colonies of the large ctenostome bryozoan Flustrellidra hispida (Fabricius) were placed in a chamber with a slow flow through it and were offered a mixture of food. This consisted of a suspension of the single celled alga Tetraselmis suecica mixed with fine particles of clay silt, which had been allowed to acquire a bacterial coating. The bacteria had been previously labelled with radio isotope so that if they were eaten off the ingested silt particles, the colonies would show labelling. The colonies did take up the label and in all cases the upstream colonies showed higher, often much higher, levels of radioactivity. If bacterial inhibitor was used during labelling, significantly reduced levels of label were found in the Bryozóa. It is concluded that the bryozoans are removing the bacteria from the silt and that the upstream colonies depleted the food supply to the downstream colonies.