Coral Reefs

Coral Reefs (Cnidarians)

Hawaiian Name: Kohola

Indigenous (75%); Endemic (25%).

It is often as hard as rock, yet it spawns and reproduces, budding and growing into a wonderland of odd-shaped and colorful habitats for fish, eels, invertebrates and other ocean life.  This is the Hawaiian coral reef.

[Illustration: Coral reef with eel]



Coral is an animal, and belongs to a group of animals called Cnidarians (the “c” is silent).  The Cnidarian family is a group that also includes sea anemones and jellyfish. 

All Cnidarians have a single body opening, with a mouth at the top surrounded by feeding tentacles.  These tentacles have stinging cells called nematocysts that catch drifting animals (microscopic plankton) for food. 

Coral starts off as free-floating larvae, called planulae, which are carried by ocean currents.  Once a planula finds a hard bottom, it attaches itself and then changes into a polyp. 

Polyps are usually smaller than a pea and grow by a process called budding, in which a single coral polyp splits in two, making an identical copy of itself.  A thin layer of skin connects each polyp to its neighbors and allows the whole colony to benefit from nutrients obtained by individual polyps.

Though coral is an animal, it has in its tissues microscopic algae called zooxanthellae.  The coral and the zooxanthellae have a symbiotic relationship, which means they both help each other survive.  The zooxanthellae use photosynthesis to convert sunlight to energy, making the oxygen and food the polyps need.  In turn, the coral produces the carbon dioxide, nitrates, and phosphates the algae needs to survive.

Corals secrete a hard skeleton of calcium carbonate.  The coral reef is the built up layers of all of these old skeletons.  New corals are constantly growing on the surface of a coral reef.  Coral may grow from ½-inch (13 mm) to 4 inches (10 cm) per year.  Some coral colonies are hundreds of thousands of years old. 

Coral polyps use tentacles to capture prey, including small planktonic animals.  When the prey brushes up against the coral’s nematocysts, a spear-like projection shoots out and gets the animal.


Types of Coral Reefs

There are three main types of coral reefs: fringing reefs, barrier reefs, and atoll reefs.

Ø                  Fringing Reefs—Form close to shore; may be separated from land by a lagoon.

Ø                  Barrier Reefs—Form at the edge of continental shelves and partially submerged islands separated from land by lagoons less than 500 feet (152 m) deep. (e.g., Australia’s Great Barrier Reef is more than 1,200 miles (1,931 km) long.

Ø                  Atoll Reefs—Usually originate as fringing reefs of volcanic islands, first becoming circular barrier reefs as the island sinks (or the sea level rises).  When the island becomes completely submerged, what is left is an atoll reef.


Coral Species of the Hawaiian Islands

The most common corals in Hawaiian waters are lobe corals (which comprise most of the reef structure around the Hawaiian Islands, along with coralline algae), rice coral, blue coral, and cauliflower coral. 

These four types of coral are found in active shallow waters (e.g., where there is surf).  Delicate, branching corals such as shelf and finger coral are the most common Hawaiian corals growing in calmer, deeper waters.

[Illustration: Lobe coral, rice coral, blue coral, cauliflower coral]


In all, there are at least 150 different coral species in Hawaiian waters, and about 25% of these are endemic to the Hawaiian Islands. 

These native Hawaiian coral species include 47 hard, stony corals, which are the Hawaiian Islands’s most abundant coral (e.g., lobe, finger, and cauliflower coral).  There are also 93 species of sea fan coral, a type of soft coral that grows from 500 to 2,000 feet (150 to 600 m) deep, and 13 species of other soft corals (e.g., black coral).

Coral species in Hawaiian waters help support many of the 860 documented native algae species, which include at least 80 endemic algae species.  There are also at least 1,143 fish native to the Hawaiian Islands, including 149 endemic (unique) fish species. 

About 536 of these fish species are inshore fishes found near reefs and other nearshore areas to a depth of about 200 feet (61 m).  About 25% of these 536 inshore species are endemic to the Hawaiian Islands. (See Hawaiian Reef Fish, Chapter 6.)

Coral reefs around the Hawaiian Islands also help support native invertebrate species, such as crabs, sea urchins, and shrimp.  At least 9,270 native Hawaiian invertebrates have been documented, including at least 7,239 endemic species.



Coral species rejuvenate themselves through a process called spawning.  Many Hawaiian corals span just once or twice a year, and not all of Hawai‘i’s coral spawn at the same time (as occurs in many other places).  Rice coral spawns in June and July two days after a full moon between 8 and 10 p.m.

Mushroom coral, (also called razor coral) spawns one two four days after a full moon, between 5 and 7 a.m., between the months of June and September.  Both the female and male of the species emit clouds of sperm and eggs.


Importance of Coral Reefs around the Hawaiian Islands

Most corals only grow about ½-inch (13 mm) per year, and substantial coral reefs take hundreds of years to grow.  Coral reefs are living communities that provide shelter, food, and living space for thousands of native plant and animal species. 

Coral reefs are considered the aquatic equivalent of a tropical rainforest, since both support an incredible biological diversity of species. 

An ancient Hawaiian proverb states, “He pūko‘a kani ‘āina” (“A coral reef that grows into an island”), meaning, “A person beginning in a small way gains steadily until he becomes firmly established.”[i]

Hawaiian coral reefs provide important habitat for monk seals, green sea turtles, and more than 6,500 other species of animals and plants. 

Among the great diversity of organisms on Hawai‘i’s reefs are species that hold potential for natural medicines.  For example, a chemical found in Hawaiian sponges has recently shown promise in fighting off tumors and malaria.

Medical researchers have also found that ground-up coral works well for bone grafts in humans.  It turns out that coral is the first good artificial substance that human blood vessels can penetrate, and that the body accepts.  Corals are also providing researchers with insights into improving sunscreens.  Many corals have evolved unique compounds that naturally shield them against UV radiation.

[Photograph: Coral reef]

[i]p. 100, Pukui, Mary Kawena.  ‘Ōlelo No‘eau: Hawaiian Proverbs & Poetical Sayings.  Honolulu: Bishop Museum Press, 1983.  Proverb 932.