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• Most complex and diverse marine systems
• Formed by biological activity of corals belonging to the phylum Cnidaria (relatives of jellyfish)
• Oldest marine communities, more than 500 million years (different species, though) 
• Cover ~600,000 km² in water >20°C (23-29°C opt.)
  
• Salinity requirement 32 - 42‰
• Light is required for coral growth due to symbiontic, photosynthetic zooxanthellae (dinoflagellates); < 25 m water depth
• Turbidity by sediments or phytoplankton affects coral growth and health
• Corals are related to benthic sea anemones, class Anthozoa, phylum Cnidaria
• Reefs are built of tiny individuals called polyps
  
• Polyps possess tentacles containing batteries of nematocysts to capture and paralyze prey
• Colonies are created by asexual division called budding, and all polyps remain connected by tissue
• Polyps secret calcium carbonate exoskeleton of 1-3 mm diameter, which forms the base of the reef
• Individual colonies can range in size up to some hundred tons, and colony form depends on species and environment (wave exposure)
• Reef-building (hermatypic) corals possess symbionts, which are already given in the planktonic larvae
  
• Non-reef-building corals lack symbionts, can be solitary and live deeper (no light required)
• Polyps reproduce sexually, producing planktonic larvae (called planula), which settle again for new colonies; planulae possess already symbionts
  
• Symbionts (dinoflagellates) provide O₂ and organic carbon from photosynthesis; also enhance CaCO₃ deposition by corals (experimentally shown)
• Animals provide shelter and nutrients for zooxanthellae – close nutrient cycling in the oligotrophic sea (f-ratio <0.1)
• All corals are mixotrophs, and polyps also feed on zooplankton, preferably at night
 
Coral Diseases and Coral Bleaching
 
• Coral deseases are rapidly spreading worldwide
• Temperature changes such as related to El Nino are made responsible for lowered resistance of corals to infections
• Deseases without pathogen include Tissue Bleaching (temperature >32°C), White Band Desease (temperature-controlled, triggered by settlement of cyanobacteria), White Plague, White Pox
• Deseases with pathogen include Black Band Desease, Red Band Desease, Black aggressive Bands, Lethal organe desease, Yellow Band Desease, and more. Most are caused by bacteria or triggered by overgrowth by cyanobacteria
• Worldwide average: 16% of reefs lost, 27% of reefs in acute threat; in certainareas, the share of affected reefs can besubstantially higher