Symbiont Reintroduction Alters Tumor Progression and Life‐History Traits in the Tumor‐Bearing Freshwater Cnidarian Hydra oligactis

ABSTRACT Environmental changes can disrupt long‐standing host–symbiont associations and influence tumor dynamics; however, how these two aspects interact remains poorly understood, particularly when previously co‐evolved symbionts are reintroduced into tumor‐prone hosts. We experimentally reintroduced a native commensal ciliate symbiont ( Kerona pediculus ) into two long‐term cultured symbiont‐free lines of the freshwater cnidarian, Hydra oligactis , differing in tumor affliction: one harbors a transmissible tumor, and one has historically low spontaneous tumor incidence. Unexpectedly, spontaneous tumors emerged at high frequency in the latter, independently of ciliate acquisition, fundamentally reshaping the experimental framework and enabling comparisons of how symbiont reintroduction affects hosts with either transmissible or de novo tumors. While ciliate infection did not alter tumor incidence, it slightly accelerated tumor onset, increased the likelihood of supernumerary tentacle formation, and reduced asexual reproduction (particularly at high symbiont densities) across tumor contexts. Spontaneous tumors appeared later than transmissible tumors, were less often associated with supernumerary tentacles, and induced an earlier reproductive burst. Our findings show that symbiont reintroduction and tumor context shape tumor dynamics and life‐history traits in tumor‐bearing hosts, emphasizing the potential role of symbiotic history and tumor evolutionary context when assessing the outcomes of such pressures in vulnerable host populations.