Bacterial Symbionts in Pogonophora

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Abstract
Prokaryote organisms, with characteristics of Gram-negative bacteria, occur intracellularly in Pogonophora, as described here for seven small species. The tissue containing the bacteria lies between the two longitudinal blood vessels in the posterior part of the trunk and has a special blood supply. This tissue is probably homologous with the so-called trophosome tissue of the much larger vestimentiferan pogonophores, which also contains bacteria, and the term can be applied to all pogonophores. The presence of such bacteria-containing trophosome tissue may be a characteristic of the phylum. In both large and small species examined the bacteria appear to be chemoautotrophs and probably assist the nutrition and/or metabolism of their hosts. It is not yet certain if the bacterium-containing cells do originate from mesoderm or endoderm, but, if the latter, then the trophosome represents the remains of the missing gut. The trophosome tissue situated internally, and transfer of bacteria must take place early in the life history, in the egg or embryo.