The widespread vulnerability of Hydra oligactis to tumourigenesis confirms its value as a model for studying the effects of tumoural processes on the ecology and evolution of species.

Tumoural processes, ubiquitous phenomena in multicellular organisms, influence evolutionary trajectories of all species. To gain a holistic understanding of their impact on species' biology, suitable laboratory models are required. Such models are characterised by a widespread availability, ease of cultivation, and reproducible tumour induction. It is especially important to explore, through experimental approaches, how tumoural processes alter ecosystem functioning. The cnidarian Hydra oligactis is currently emerging as a promising model due to its development of both transmissible and non-transmissible tumours and the wide breadth of experiments that can be conducted with this species (at the individual, population, mechanistic, and evolutionary levels). However, tumoural hydras are, so far, only documented in Europe, and it is not clear if the phenomenon is local or widespread. In this study we demonstrate that Australian hydras from two independent river networks develop tumours in the laboratory consisting of interstitial stem cells and display phenotypic alterations (supernumerary tentacles) akin to European counterparts. This finding confirms the value of this model for ecological and evolutionary research on host-tumour interactions.