Coral reefs are estimated to cover only 0.1-0.5% of the ocean floor, yet coral reefs are among the most productive and biologically diverse ecosystems on Earth. Almost one third of all marine fish are found on coral reefs, and approximately 10% of fish consumed by humans are caught on coral reefs, which have been called rainforests of the ocean.
Corals are generally divided into hard and soft corals, with hard corals building the structure of the reef. Corals are small invertebrate organisms called polyps belonging to the phylum Cnidaria which also includes sea anemones, jellyfish, and hydroids. Polyps are tiny, soft-bodied organisms which produce a hard exoskeleton to protect their bodies.
Coral polyps are translucent, and get their colour from zooxanthellae algae which they host within their bodies. This is a symbiotic relationship where the polyps receive food from the algae’s photosynthesis and the algae gains protection and carbon dioxide from the polyp.
While most corals gain their nutrients from the algae’s photosynthesis, some have barbed venomous tentacles which they stick out of the exoskeleton to grab zooplankton and even small fish. Coral polyps are mostly colonial and amass into spectacularly diverse reef communities.
There are four main types of coral reefs: fringing reefs, barrier reefs, atolls, and platform reefs. Coral reefs supply a large number of goods and services such as seafood, ecotourism, coastal protection, aesthetic and cultural benefits.
Unfortunately, many coral reefs around the world are in decline. There are many factors that contribute to coral reef decline including increases of nutrient and sediment loads caused by deforestation, agriculture, and urbanization; overharvesting of reef species; destructive fishing methods; uncontrolled and unregulated ecotourism; and global climate change. One of the consequences of these, besides physical damage of the coral reefs, is coral bleaching. Polyps under stress will evict their zooxanthellae which results in their starvation and death.
Written by: Minke Witteveen
For further reading:
- Moberg, F. and Folke, C. 1999. Ecological goods and services of coral reef ecosystems. Ecological Economics 29: 215-233.
- National Geographic. 2017. Coral. Accessed: 2017-05-01. URL: http://animals.nationalgeographic.com/animals/invertebrates/coral/
- The Ocean Portal Team. 2016. Corals and Coral Reefs. Smithsonian National Museum of Natural History. Accessed: 2017-05-01. URL: http://ocean.si.edu/corals-and-coral-reefs