Most reef-building corals can survive to a maximum depth of about 50 meters. Their depth range is limited mainly by sunlight, because many reef-building corals depend on symbiotic algae called zooxanthellae to produce energy through photosynthesis.
As depth increases, less light reaches the seafloor. Coral growth and reef construction therefore become increasingly difficult, even though some specialized corals can live deeper. Mesophotic coral ecosystems commonly occupy roughly 30 to 50 meters, and certain coral species have been recorded below that range.
The 50-meter figure is a useful general benchmark, not an absolute wall. Water clarity, latitude, species, temperature, sediment, and local conditions all affect how deep corals can persist. In unusually clear water, some zooxanthellate corals may occur considerably deeper, while cloudy or sediment-rich conditions can restrict corals to much shallower water.
This is also why deep-sea corals should not be confused with ordinary shallow-reef builders. Some deep-water corals live without sunlight and without the algal partnership that powers most tropical reef growth, but they generally do not form the same sunlit tropical reef systems.