Marine sponges discriminate between food bacteria and bacterial symbionts: electron microscope radioautography and in situ evidence

Abstract

This study considered whether marine sponges are selective particle feeders, and whether they are capable of distinguishing between sponge bacterial symbionts and other bacteria. Four species of marine sponges ( Aplysina aerophoba, A. cavernicola, Pericharax heteroraphis and Spongia sp.) were fed in situ with tritium-labelled bacteria, either symbionts or other bacteria isolated from seawater. A large proportion of bacterial symbionts passed through the sponge and were expelled in the exhalant current, whereas seawater bacteria disappeared from the incubation water and were retained within the sponge. The seawater bacteria were observed in choanocytes and occasionally in pinacocytes after 30-40 min, whereas symbiont bacteria were rarely observed. Although sponges do have mechanisms to ingest particles such as food, they appear to be unable to consume bacterial symbionts, probably because their identity is masked by capsular sheaths. A specific mechanism for recognition and rejection of self-particulate matter is proposed for sponge epidermal cells.