Zooanthellae

Zooxanthellae is a general term for single-celled dinoflagellate algae that live symbiotically within the tissues of many marine invertebrates. These algae are most famously associated with corals, where they are crucial for the coral's survival and the vibrant colors often seen in reef ecosystems. The term "zooxanthellae" itself is not a formal taxonomic classification but rather a descriptive one, referring to algae that live within animal tissues.

The relationship between zooxanthellae and their hosts is a form of mutualism. The algae, through photosynthesis, produce energy-rich compounds such as glucose and amino acids, which they transfer to their host. In return, the host animal provides the zooxanthellae with a protected environment and access to sunlight, as well as essential nutrients like carbon dioxide and nitrogenous waste products. This symbiotic partnership allows corals and other host organisms to thrive in nutrient-poor tropical waters.

The expulsion of zooxanthellae from their host tissues is known as coral bleaching. This phenomenon occurs when corals are stressed by environmental changes, such as elevated sea temperatures, increased light intensity, or pollution. Under stress, the host animal ejects the algae, causing the coral to turn white. While bleached corals are not dead, they are severely weakened and can die if the stressful conditions persist and the zooxanthellae do not return. The health of zooxanthellae populations is therefore a critical indicator of coral reef health.