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Zooxanthellae (; zooxanthella) is a colloquial term for photosynthetic single-celled organisms capable of symbiosis with diverse marine invertebrates such as corals, jellyfish, demosponges, and nudibranchs. Most known zooxanthellae are in the dinoflagellate genus Symbiodinium, but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. The genus name "Zooxanthella"(meaning literally "little yellow animal") was originally given by Karl Brandt in 1881 to Zooxanthella nutricula (a mutualist of the radiolarian Collozoum inerme) which has been placed in the Peridiniales. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.
Zooxanthellae are photoautotrophs that contain chlorophyll a and chlorophyll c, as well as the dinoflagellate pigments peridinin and diadinoxanthin, which give the yellowish and brownish colours typical of many of the host species.
Morphology and classification
Zooxanthellae can be grouped in the classes of Bacillariophyceae, Cryptophyceae, Dinophyceae, and Rhodophycaeae and of the genera Amphidinium, Gymnodinium, Aureodinium, Gyrodinium, Prorocentrum, Scrippsiella, Gloeodinium, and most commonly, Symbiodinium. Zooxanthellae of genus Symbiodinium belong to a total of eight phylogenetic clades A-H, differentiated via their nuclear ribosomal DNA and chloroplast DNA.
Zooxanthellae are autotrophs containing chloroplasts composed of thylakoids present in clusters of three. The DNA possesses ribosomal RNA (rRNA) that is folded and of similar morphology to rRNA in archaeobacteria. This indicates that RNA is important for DNA packaging in zooxanthellae. In zooxanthellae of the genus Gymnodinium, one possible life cycle of the cell begins as an immature cyst which reaches maturity then divides to form an immature cyst once more. Once growing into an older cell, it becomes no longer useful. In the life cycle of a motile zooxanthellae cell, its youngest stage is known as a zoosporangium, which matures into a zoospore capable of motility. This motile cell produces and releases gametes for reproduction. Many different species of zooxanthellae are present in host organisms, each species with its own adaptive capabilities and degree of tolerance of varying environmental factors. Zooxanthellae provide nutrients to their host cnidarians in the form of sugars, glycerol, and amino acids and in return gain carbon dioxide, phosphates, and nitrogen compounds. However, clams discard zooxanthellae that are still alive and have been observed being able to recover them. They are also found in the eyes of clams like Tridacna where they act as a lens. Different clades of zooxanthellae have an impact on clam morphology. Clade E1 of zooxanthellae seems to influence or favor smaller offspring from clams when compared to clams harboring other clades and all five clades appear to be needed in order for larval settlement to occur. Many different types of zooxanthellae have been observed forming relationships with jellyfish across many different phylogenetic branches, and the roles they play will change throughout the jellyfish's life cycle. Although, jellyfish bleaching events have been documented during extreme heat events. Light availability can affect the lipid production of zooxanthellae that the jellyfish then utilize.