Endosymbionts escape dead hydrothermal vent tubeworms to enrich the free-living population

For horizontally transmitted, facultative symbionts, cycles of infection and escape from the host are crucial for the persistence over host generations. The hydrothermal vent tubeworm Riftia pachyptila is entirely nourished by its thiotrophic endosymbiotic bacteria, which are acquired horizontally in settled larvae; however, release back into the environment has not been demonstrated. We show experimentally that viable symbionts are released upon host death. Moreover, observations of turnover of tubeworm clumps after a volcanic eruption provide evidence for rapid colonization, growth, and death. The observed connectivity of host-associated and free-living symbiont populations helps to explain the stability of this mutualism over ecological and evolutionary timescales.