Tiny coral polyps secrete limestone skeletons. Over generations, colonies and other calcifying organisms build reef structure, while symbiotic algae supply much of the energy that lets many tropical corals grow in clear, nutrient-poor water.
Three things to know
- Coral is
- An animal
- Most visible reef builders are colonies of many connected polyps.
- Framework
- CaCO₃
- Calcium-carbonate skeletons accumulate into complex three-dimensional habitat.
- Key partnership
- Algal symbiosis
- Photosynthetic dinoflagellates exchange energy and nutrients with coral hosts.
The reef is built slowly
A polyp adds skeletal material beneath its living tissue. Colonies grow upward and outward, while storms, grazers and boring organisms break material into sediment. Reef growth is the balance between construction, erosion and sea-level change.
Diversity through structure
Branches, crevices, overhangs and rubble create many microhabitats. Fish, crustaceans, mollusks, worms, sponges and microbes occupy different niches. The reef’s physical complexity is therefore inseparable from its biological diversity.
A narrow environmental window
Many tropical reef corals thrive in warm, shallow, clear water but are vulnerable when temperature stays above their local tolerance. Acidification, disease, polluted runoff and destructive fishing can also reduce growth or recovery. Deep and cold-water corals live under very different conditions.
What people usually ask
Are all corals reef builders?
No. Many soft corals and solitary corals do not build massive calcium-carbonate reef frameworks.
Do reefs only occur in the tropics?
Warm shallow reefs dominate the familiar image, but cold-water corals form important deep habitats too.
Here’s where the answer came from
Open the original government science or research-institution source behind this page.