Interactions between appendicularians and ciliates were observed over the life span of Oikopleura dioica in laboratory cultures and clarified with the use of mathematical modeling and microscopic observations. Complex interactions including competition, parasitism, predation, and histophagy occurred simultaneously, resulting in apparent mutualism. The large ciliate Strombidium sp. entered the inlet filters of appendicularian houses (larger than 500 mu m body size) by distorting the mesh. Once inside, Strombidium fed on particles concentrated on the filters. When appendicularians were larger than 900 mu m, both the high flow rate in the buccal tube and their esophagus width allowed the "host'' appendicularian to capture and ingest ciliates. Thus, ciliates seem to be sequentially competitors, then parasites or commensal in appendicularian houses, and finally prey for appendicularians. Appendicularian rates of somatic growth and reproduction were enhanced when ciliates were ingested. This additional food supply could be essential in oligotrophic environments. Reciprocally, appendicularians support higher ciliate growth rates, allowing ciliates to survive and grow in food-limited environments. Appendicularians thus modify the size spectrum of the microbial food web both by removing small organisms (0.2-30 mu m) and enhancing the growth of mid-sized and large ciliates.