Hydra viridissima, the green hydra (family Hydridae), is a freshwater hydrozoan notable for its obligate endosymbiotic relationship with unicellular green algae (Chlorella-like), which give it its distinctive emerald green coloration and provide photosynthetically derived nutrition. Found in ponds, lakes, and slow-moving streams globally, it is a small solitary polyp with typically 5–6 tentacles bearing nematocysts for prey capture. Unlike other Hydra, its photosymbiosis with intracellular algae makes it a valuable model for studying freshwater cnidarian-algal symbiosis distinct from the Symbiodiniaceae-based systems of marine corals. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.