Conditional Senescence and Longevity Mechanisms in Early-Branching Metazoans: Insights from Hydra.

Organismal aging is not inevitable in multicellular animals, as early-branching metazoan lineages, such as cnidarians, display negligible senescence under defined conditions, despite conserved cellular pathways. Contrasting longevity phenotypes in Hydra species reveal distinct regulatory mechanisms underlying aging. A comparison between Hydra vulgaris and Hydra oligactis suggests that sustained telomerase activity, autophagy regulation, and microbiome stability in H. vulgaris are associated with stem cell renewal and long-term tissue homeostasis, whereas cold-induced stress in H. oligactis is accompanied by a disruption of these pathways, leading to rapid somatic decline, where autophagy dysfunction and microbiome dysbiosis may act as contributing or amplifying factors. Environmental sensing via conserved pathways may integrate these regulatory mechanisms. These observations suggest that aging in early-branching metazoans is a regulated, context-dependent process characterized by substantial lineage-specific variation rather than an inevitable, universal consequence of cellular senescence. Further mechanistic studies may provide evolutionary insights into longevity mechanisms and identify potential targets for modulating aging, although direct experimental validation remains necessary.