A quantitative study of the retention of radioactively labeled E. coli by the freshwater sponge Ephydatia fluviatilis

In this method designed to quantify the amount of bacteria retained by sponges, young Ephydatia fluviatilis raised in vitro were provided with E. coli grown in the presence of ³H-labeled thymidine. The maximal retention capacity was reached after 15-24 h and was followed by a loss of radioactivity at a steady rate. The nonspecific adhesion of bacteria to the substratum remained close to 10% for 15 h and increased to as much as 43% by 24-48 h. Sponges provided repeatedly with bacteria showed a higher retention capacity, suggesting the efficiency of the bacterial nutrition. Transmission electron microscopy and scanning electron microscopy showed that, after a preliminary adhesion to the choanocytes and exopinacocytes, bacteria were engulfed in individual phagosomes, which later fused together. The quantitative method established in this paper should prove useful for further studies of the retention of any particulate matter by sponges.