Not all hydras are immortal: asexual Hydra oligactis undergoes senescence

Abstract The freshwater cnidarian Hydra vulgaris has been shown to exhibit extremely low rates of senescence when maintained under laboratory conditions, with a potential maximum lifespan extending to hundreds or even thousands of years. A congeneric species, H. oligactis, which undergoes senescence following sexual reproduction, has also been suggested (based on anecdotal evidence) to display negligible senescence during its asexual phase, similar to H. vulgaris. However, a proper evaluation of age-dependent mortality patterns in asexual H. oligactis is lacking. Here we analyzed age-dependent mortality and reproductive patterns in H. oligactis individuals derived from parental polyps collected from a pond in Southern France. Throughout their lifespan, experimental animals were maintained in the laboratory under two feeding regimes: high food availability (five feedings per week) and low food availability (three feedings per week). We recorded asexual reproduction and spontaneous tumours and used Bayesian Weibull survival models to relate food treatment, reproductive investment and tumour development to survival rates. Our results provide clear evidence of actuarial senescence in H. oligactis: mortality rates increased substantially over time, with a maximum observed lifespan of approximately two years. Survival was higher in the low-food group, indicating a dietary restriction effect in this non-bilaterian species. Asexual reproduction rate was positively associated to survival probability, while tumour development significantly reduced it. These findings show that H. oligactis, contrary to its congener H. vulgaris, undergoes senescence in the asexual stage, revealing previously unrecognized diversity in aging patterns within the genus Hydra.