Absence of cospeciation in deep-sea vestimentiferan tube worms and their bacterial endosymbionts

Molecular evolutionary relationships among tube worm hosts and their sulfur-oxidizing chemoautotrophic bacterial endosymbionts were deduced from sequences of the mitochondrial cytochrome oxidase I gene and the small subunit rDNA gene, respectively. Evolutionary relationships among host taxa reflect traditional morphological taxonomic species boundaries. Cold seep tube worms were more closely related to each other than to tube worms from hydrothermal vents. There is a single rDNA-type of hydrothermal vent symbiont that occurs in Oasisia alvinae, Ridgeia piscesae, Riftia pachyptila, and Tevnia jerichonana. This type is distinct from the lineage of symbionts found within seep tube worms. Within the diversity of symbionts found in tube worms from cold seeps, there were three groups, which reflect, with a single exception, the geographic locations. One group of symbionts was exclusive to Lamellibrachia sp. nov. 2 and an 'escarpid-like' tube worm of the shallow Gulf of Mexico (500-700 m), while the second was found in Lamellibrachia sp. nov. 1 and Escarpia laminata from Alaminos Canyon in the Gulf of Mexico (2,020 m). The third group of cold-seep symbionts was found in Escarpia laminata from the Florida Escarpment (3,700 m) and Lamellibrachia barhami from Middle Valley in the Northeast Pacific (2,420 m). The lack of congruence between phylogenies of host tube worms and their symbionts supports the hypothesis that the endosymbionts of tube worms are acquired de novo each generation from environmental sources.