Hydra oligactis is a freshwater hydrozoan polyp (family Hydridae) found in cool, temperate lakes and streams across the northern hemisphere, notable for its seasonal senescence—undergoing rapid aging and death in autumn following sexual reproduction, a phenomenon absent in other Hydra species. Unlike many cnidarians, it lacks a medusa stage and reproduces asexually by budding during most of the year, regenerating from any body fragment. The species is an important model for cnidarian aging biology, contrasting with H. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.