Corals

Corals are marine invertebrates in the phylum Cnidaria and the class Anthozoa. They include hard corals, which build calcium carbonate skeletons and form most tropical reefs, as well as soft corals and hydrocorals. Most reef-building corals are colonial polyps that live together as a single organism.

Many corals have a symbiotic relationship with photosynthetic algae called zooxanthellae that live in their tissues. The algae provide energy through photosynthesis, contributing to coral growth, while the coral provides a protected environment and nutrients. This symbiosis limits most corals to sunlit, clear, warm waters.

Coral skeletons are primarily calcium carbonate and form complex reef structures that protect coastlines, provide habitat, and support high biodiversity. Reefs are built by hermatypic corals, which deposit calcium carbonate as they grow, while soft corals contribute to reef structure in other ways.

Corals reproduce sexually by releasing eggs and sperm into the water column, a process that can be synchronized among colonies, or asexual ly by fragmentation. Larvae (planulae) settle on suitable substrates to form new colonies. These reproductive strategies maintain genetic diversity and allow rapid colonization of new areas.

Coral distributions are mostly tropical and subtropical, thriving in clear, shallow waters with ample light, typically within 30 meters of the surface. They occur worldwide in the Indo-Pacific and Caribbean, among other regions, but are sensitive to temperature, salinity, sedimentation, and pollution. Climate change is causing widespread bleaching and mortality, threatening reef ecosystems.