zoosantellids

Zoosanthellids are single-celled dinoflagellate algae that live symbiotically within the tissues of marine invertebrates, most notably corals. These algae are also known as symbiotic zooxanthellae. They possess photosynthetic pigments, including chlorophylls and carotenoids, which allow them to convert sunlight into energy through photosynthesis. In return for shelter within the host's tissues, zoosanthellids provide their host with a significant portion of their metabolic energy requirements, often up to 90%. This symbiotic relationship is crucial for the survival of many reef-building corals, as the algae supply the energy needed for calcification, the process by which corals build their calcium carbonate skeletons.

The presence of zoosanthellids is responsible for the vibrant colors seen in many corals and other symbiotic invertebrates. When corals expel their zoosanthellids, a phenomenon known as coral bleaching occurs, leaving the coral white and vulnerable. This expulsion can be triggered by environmental stressors such as elevated sea temperatures, increased light, or pollution. While corals can sometimes recover their zoosanthellids if the stress is removed, prolonged bleaching can lead to starvation and death. Zoosanthellids are not a single species but rather a diverse group of genetically distinct clades, with different clades potentially influencing the physiology and environmental tolerance of their coral hosts.