Abstract. The gutless marine oligochaete, Olavius crassitunicatusfinogenova 1986 (Tubificidae), from suboxic to sulfidic sediments off Peru, consistently harbored 3 structurally distinct types of extracellular bacterial symbionts. Large, oval bacteria were labeled immunocytochemically, proving their nature as autotrophs. Spectroscopical analysis documented storage of sulfur in this numerically dominant morphotype. Small, rod-shaped bacteria attained a more peripheral position adjacent to the cuticle. The third bacterial type was represented by long, filamentous forms which were often in close contact to the oval bacteria. With their curved or undulate cells, these filiform bacteria resembled spirochetes. They were clearly distinguishable and consistently found in all investigated host specimens.
While molecular analyses could not be performed, structural and immunocytochemical evidence indicated that the oval bacteria seemed equivalent to the γ-proteobacteria in related gutless oligochaetes. On the basis of morphological similarity and indications from closely related symbiotic tubificids, the possible relationship of the two other morphoptypes must remain unsolved and needs further molecular analysis. The three bacterial morphotypes live in a consistent, elaborate, and apparently obligate coexistence with a host that has completely reduced its digestive and excretory organs.