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.
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Hydra viridissima in the context of Green algae
The green algae (sg.: green alga) are a group of chlorophyll-containing autotrophic algae consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyta) have emerged deep within the charophytes as a sister of the Zygnematophyceae. Since the realization that the Embryophyta emerged within the green algae, some authors are starting to include them. The completed clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid (spherical), and filamentous forms, and macroscopic, multicellular seaweeds. There are about 22,000 species of green algae, many of which live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.
A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in dinoflagellates of the genus Lepidodinium, euglenids and chlorarachniophytes were acquired from ingested endosymbiont green algae, and in the latter retain a nucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridissima and in flatworms. Some species of green algae, particularly of genera Trebouxia of the class Trebouxiophyceae and Trentepohlia (class Ulvophyceae), can be found in symbiotic associations with fungi to form lichens. In general, the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus. Trentepohlia is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae. Also the macroalga Prasiola calophylla (Trebouxiophyceae) is terrestrial, andPrasiola crispa, which live in the supralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.