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Coral Reef

Called “rainforests under the sea,” coral reefs support 25% of all known marine species, including fish, plants, and invertebrates.

Coral Reef photo, James Watt
James Watt/NOAA

A Reef from Polyps

People often think coral reefs are plants but in fact they're made of thousands of miniature animals! Each coral animal is called a polyp, which is a type of invertebrate that is similar to jellyfish. A polyp has a sac-like body and a mouth encircled by stinging tentacles that extend to feed at night.  Coral polyps use calcium carbonate from sea water to build protective limestone skeletons (hard coral).  Thousands of polyps make up a coral colony or coral tree.  

A Growing Reef

In general, reefs grow very slowly—less than 1 inch a year. Branching and Staghorn corals can grow up to 8 inches a year.  Coral polyps produce eggs, which are fertilized inside or outside the polyp body.  Once in the sea, the fertilized eggs or larvae are attracted to light and swim to the ocean’s surface.  After a few days, the larvae fall back to the ocean floor to create new coral colonies where they land.

A Hungry Reef

For coral polyps, there are only two things on the menu: zooplankton or zooxanthellae.  Some species use their tentacles to capture zooplankton (tiny aquatic animals) at night.  Others enjoy a symbiotic relationship with algae called zooxanthellae.  Inside the polyp, the algae performs photosynthesis, the process of using sunlight to make energy-rich sugar.  The polyp provides the algae with carbon dioxide and a safe home; the algae gives the polyp nutrients and oxygen, the byproducts of photosynthesis.

Because so many polyps depend on zooxanthellae (algae), they need sunlight to survive.  For this reason, coral reefs are found in calm, clean, shallow waters. 

Hawai`i's Reefs

Hawai‘i’s coral reefs are very different from other reefs in tropical areas.  First, they are less ancient and well-developed.  The main Hawaiian Islands are surrounded by small, fringing reefs rather than large, barrier reefs. Second, they have fewer filter-feeding animals like sponges and bivalves and more corals because Hawaiian waters are comparatively nutrient-poor.  This is great for scientists studying corals!  Cauliflower coral, lobe coral, and finger coral are three common coral types in Hawai‘i.  Of the ~50 coral species in Hawai‘i, about 20% are unique (endemic) to the Islands.

Lobe coral photo by Donna Loudon
Donna Loudon


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