Earthworms harbor in their nephridia (excretory organs) symbiotic bacteria which densely colonize a specific part of the nephridia, called the ampulla . The symbiosis is species-specific and the symbionts form their own monophyletic genus Verminephrobacter (β-proteobacteria)  and are vertically transmitted . For these reasons we hypothesized that the earthworm-Verminephrobacter association evolved by co-diversification. This hypothesis was investigated by a comparison of earthworm and symbiont phylogenies. The earthworm phylogeny was based on Cytochrome c oxidase subunit I (COI) and Histone H3 and the symbiont phylogeny was based on 16S rRNA gene sequences and RNA polymerase β subunit (rpoB). The phylogenies were found to be largely congruent, however, leaving possibility for horizontal symbiont transfer events. In addition symbiont losses have occurred. The overall congruency suggests that the symbiosis has been stably maintained over evolutionary time dating back to the last common lumbricid earthworm ancestor. How this evolutionarily stable association is maintained is unknown; symbiont-free worms can be reared in lab culture and therefore the symbionts are not essential to the survival of the worms, but at the same time the symbionts are consistently found in almost all Lumbricid worm species. The symbionts have been hypothesized to be proteolytically active during excretion, thereby enhancing the earthworm’s absorption of nitrogenous compounds otherwise lost in the urine; such protein recycling should therefore increase worm fitness under nitrogen-limited conditions . To test this hypothesis we conducted a comparative fitness study of worms with and without symbionts; the worms were grown in soil and fed with either a nitrogen-rich or a nitrogen-poor diet. The experiment showed no significant differences in growth rate and fecundity between symbiotic and aposymbiotic worms. Thus the symbionts do not appear to have an effect on worm fitness, under growth conditions tested. The underlying functional and maintaining mechanisms of this symbiosis remain a conundrum.  Knop,J. 1926. Z Morph Ökol Tiere, 6(3):588-624.  Schramm,A. et al. 2003. Environ Microbiol 5(9):804-809.  Davidson,S.K. & Stahl,D.A. 2006. Appl Environ Microbiol 72(1):769-775.  Pandazis,G. 1931. Zentralbl Bakteriol 120:440-453.