Hydra viridissima

Hydra viridissima is a species of cnidarian which is commonly found in still or slow-moving freshwater in the Northern temperate zone. Hydra viridissima is commonly called green hydra due to its coloration, which is due to the symbiotic green algae Chlorella vulgaris which live within its body. Recent genomic research has shown that Hydra viridissima suppresses parts of its innate immune system to allow its symbiotic Chlorella algae to survive within its cells. This adaptation provides insight into the evolution of intracellular symbiosis in early metazoans. These creatures are typically 10 mm long and have tentacles that are about half of their length. They are strictly carnivorous and typically feed on small crustaceans, insects and annelids. Hydra are normally sessile and live on aquatic vegetation. They secrete mucus to attach to substrate using their basal disc.

"Green hydra" more broadly refers to a clade containing H. viridissima and closely-related photosymbiotic species. There are at least 4 species in this clade: this species, H. hadleyi, H. plagiodesmica, and H. sinensis.