Presents a study which showed that in the gutless marine oligochaete worm Olavius algarvensis, endosymbiotic sulphate-reducing bacteria produce sulphide that can serve as an energy source for sulphide-oxidizing symbionts of the host. Method used in determining the phylogenetic identity of symbionts; Relationship of the symbionts of Olavius algarvensis to free-living sulphate-reducing bacteria.