Occurrence and morphological variability of Hydra (Hydroida: Hydridae) in freshwater bodies of Southeast of Buenos Aires Province, Argentina

Hydra constitutes a monophyletic clade of sessile and solitary polypsthat occur on all continents except Antarctica. Despite its widespreadpresence in freshwater habitats, Hydra requires reasonably unpollut-ed conditions for survival, making its presence a reliable indicator ofgood water quality and a certain degree of ecosystem conservation.Since their discovery, these polyps have become valuable biologicalmodels due to their morphological simplicity, global distribution, andlow maintenance requirements in the laboratory conditions, enablingexperiments on regeneration, reproduction, senescence, cellular differ-entiation, among other processes. All Hydra species are clustered intofour morphological groups: viridissima (green), vulgaris (common), oli-gactis (stalked), and braueri (gracile). Species in the viridissima groupare green due to the presence of intracellular symbiotic algae, whereasthe brown hydras (braueri, oligactis, and vulgaris) lack such endosym-bionts. The oligactis and braueri groups are restricted to the Northern Hemisphere, while the viridissima and vulgaris groups include cosmo-politan species. To date, only five published records documenting thepresence of Hydra in Argentina are available in the scientific literature.In the present study, we report six new occurrences from four wetlandsin the southeastern region of Buenos Aires Province: two each for H.viridissima, H. vulgaris, and H. vulgaris pedunculata. The study includesa quantitative morphological analysis (body measurements, cnidome,and reproductive structures) to assess intraspecific variation among pop-ulations from different freshwater bodies. These findings expand theknown distribution of Hydra in South America and provide referencedata for future ecological and taxonomic studies.