Fig. 1: Siboglinidae is a symbiotic annelid group.

a Siboglinidae is a diverse clade of annelid worms that evolved chemosynthetic symbioses (left side). There are four main lineages within Siboglinidae, namely Frenulata, Osedax, Sclerolinum and Vestimentifera. Chemolithoautotrophy occurs in Frenulata, Sclerolinum and Vestimentifera, which associate with gammaproteobacteria that employ sulphur or methane to produce organic compounds in an array of marine ecosystems, from reducing sediments to methane seeps and hydrothermal vents (right side of the panel). Differently, Osedax worms (e.g., O. frankpressi; b, c) have secondarily evolved a heterotrophic association with Oceanospirillales to exploit decaying vertebrate bones. The genomic basis for the evolution of these nutritional symbioses in Siboglinidae is unclear (question marks on the left) because genomic information only exists for Vestimentifera hosts (green circles on the right). The species herein studied are highlighted in boldface. b, c Photographs of O. frankpressi in a whale bone (b; arrowheads point to O. frankpressi) and a mature female adult (c). O. frankpressi settles and colonises decaying vertebrate bones (b). There, the posterior part of the body becomes stably infected with environmentally acquired Oceanospirillales bacteria. This body part (the so-called roots) harbours the bacteria and grows to penetrate the bone, dissolving the organic components. These nutrients are absorbed and transported towards the bacteriocytes containing the endosymbionts, which will proliferate and act as food for the worm. Anterior to the root tissue there are the reproductive ovisacs and the head bears two pairs of palps.