Symbioses between marine invertebrates and autotrophic sulfur-oxidizing bacteria have evolved from multiple lineages within the Gammaproteobacteria in a striking example of convergent evolution. These GammaSOX symbionts all perform the same basic function: they provide their hosts with nutrition through the fixation of CO(2) into biomass using reduced sulfur compounds as an energy source. However, our review of recent -omics based studies and genome mining for this study revealed that the GammaSOX symbionts diverge in many other metabolic capabilities and functions, and we show how these divergences could reflect adaptations to different hosts and habitat conditions. Our phylogenetic analyses of key metabolic genes in GammaSOX symbionts revealed that these differed markedly from 16S rRNA phylogenies. We hypothesize that horizontal gene transfer (HGT) would explain many of these incongruencies, and conclude that HGT may have played a significant role in shaping the metabolic evolution of GammaSOX symbionts.