A triple bacterial endosymbiosis in a gutless oligochaete (Annelida): ultrastructural and immunocytochemical evidence

The gutless marine oligochaete, Olavius crassitunicatus FINOGENOVA 1986 (Tu bificidae), from suboxic to sulfidic sediments off Peru, consistently harbo red 3 structurally distinct types of extracellular bacterial symbionts. Lar ge, oval bacteria were labeled immunocytochemically, proving their nature a s autotrophs. Spectroscopical analysis documented storage of sulfur in this numerically dominant morphotype. Small, rod-shaped bacteria attained a mor e peripheral position adjacent to the cuticle. The third bacterial type was represented by long, filamentous forms which were often in close contact t o the oval bacteria. With their curved or undulate cells, these filiform ba cteria resembled spirochetes. They were clearly distinguishable and consist ently found in all investigated host specimens. While molecular analyses could not be performed, structural and immunocytoc hemical evidence indicated that the oval bacteria seemed equivalent to the gamma -proteobacteria in related gutless oligochaetes. On the basis of morp hological similarity and indications from closely related symbiotic tubific ids, the possible relationship of the two other morphoptypes must remain un solved and needs further molecular analysis. The three bacterial morphotype s live in a consistent, elaborate, and apparently obligate coexistence with a host that has completely reduced its digestive and excretory organs.