Newly hatched Convoluta convoluta (Abildgaard) are always without symbionts. They acquire the organisms, which later become their zooxanthellae, with their food. In the field this is an automatic process, since C. convoluta without zooxanthellae have never been reported. The main diet of the young C. convoluta are various diatoms and spores of red algae. The symbionts of C. convoluta originate from diatoms of the genus Licmophora. This fact has been established in young specimens both in the sea and in laboratory cultures. Freshly hatched C. convoluta were fed with Licmophora hyalina(Kütz.) Grunow and Licmophora communis(Heib.) Grunow. Licmophora cells, capable of infesting C. convoluta, slip out of their silica shells and later occur between the cells of the peripherical parenchyma. High population densities of zooxanthellae, produced by numerous divisions of the naked diatom cells, are only possible in the host's parenchyma if the young C. convoluta continue to feed. Adult turbellarians with a high population density of symbionts feed less than the young, but never stop food uptake completely. Artificial infestation with symbionts can be brought about by making sub-adult turbellarians take up isolated zooxanthellae as food. Symbionts outside their host neither propagate nor regenerate silica shells in the culture medium employed. The obligatory nature of the mutual interrelation between C. convoluta and zooxanthellae has been proved by the great difficulties in rearing symbiontless individuals, and by starvation experiments both with sub-adults containing only a few symbionts and adults coloured brownish due to a compact layer of zooxanthellae. © 1969 Springer-Verlag.
