The gas transfer system in alvinellids (Annelida Polychaeta, Terebellida). Anatomy and ultrastructure of the anterior circulatory system and characterization of a coelomic, intracellular, haemoglobin

Abstract

In Alvinella pompejana, A. caudata and P. grasslei, the vascular blood which contains an extracellular haemoglobin is propelled through the anterior gills by a branchial heart, in which a rod-like heart-body increases the pumping efficiency of the heart. Behind the heart, the dorsal vessel runs back to the major part of the body and contains an intravasal haematopoietic heart-body. Coelomic erythrocytes, not previously known in alvinellids, contain an intracellular haemoglobin. These erythrocytes are clustered together with granulocytes in a perioesophageal pouch which encloses also a well developed plexus of thin blood capillaries. In the pouch, the diffusion distance between the extracellular and the intracellular haemoglobins is short (0.5 µm) and the association of blood capillaries and erythrocytes represents in alvinellids a complex respiratory gas transfer system previously unknown in polychaetes. Histological observations of dark granules in the blood vessels' wall also suggest that, in P. grasslei, sulfide enters the body by diffusion across the branchial surface area, is transported by the blood and immobilized mainly in the coelomic epithelium lining the blood vessels. It is suggested that the respiratory gas transfer system of the perioesophageal pouch should also participate in sulfide detoxification and that it could have been selected in these species in relation with the varying physico-chemical conditions of the deep hydrothermal environment.