The polychaete annelids Alvinella pompejana, Alvinella caudata, and Paralvinella grasslei are strictly associated with deep sea hydrothermal vents. Each species possesses an extracellular hemoglobin, Hb, which has been studied and compared to that of a common intertidal polychaete, the lugworm Arenicola marina. The four Hbs exhibit very similar quaternary structures and spectral properties, and only small differences appeared in the gross polypeptide compositions after reduction and sodium dodecyl sulfate denaturation of the native molecules. Conversely, by a comparison of the effects of pH (6.6-7.6) and temperature (10-40{deg}C) on their intrinsic O affinities, Bohr factors, cooperativities, and apparent heats of oxygenation, lugworm Hb can be differentiated from that of the alvinellids, and the Hb of A. pompejana from that of A. caudata. The known biology of the lugworm and a further analysis of the data suggest several hypotheses concerning the in vivo O2 transport function of the alvinellid Hbs, the in vivo blood pH value in the two alvinellid species, their respective range of optimal temperature, and their ability to create a differentiated and stable external microenvironment.