Coral reefs are known as the rainforests of the sea as they contain more species than any other marine ecosystem. Hawai'i has approximately 66 species of hard corals, fewer than other archipelagos due its relative isolation. 18% of these are found nowhere else on earth as they have evolved from the original species into new and unique species-these are called endemic species.



Hanau ka Uku-ko ‘ako ‘a Hanau kana, he Ako ‘ako ‘a, puka
Born the coral polyp. Born of him the coral colony emerged
- Kumulipo Creation Chant.

Hard corals are essential to the reef and to humans.
Hard corals are the building block of coral reefs, providing a solid structure that creates food and shelter for other reef organisms. Corals also provide humans with food resources, income from tourism and protection from wave energy. In Hawaiian culture the Kumulipo creation chant begins with the coral polyp from which all other life originates.






Hard corals are living organisms made up of tiny animals called polyps. They are colonial organisms, sharing nutrients and growth. One coral can contain hundreds of thousands of polyps. Coral polyps deposit their own skeleton made of calcium carbonate which they extract from the water. Some corals grow as slowly as 5mm a year! The oldest living corals in Hawai'i are 400 years old. This is why we must take care when snorkelling or diving as the slightest touch can damage hundreds of years of growth.

Coral polyps are related to the jellyfish and anemone. They have a soft body with tentacles and stinging cells called nematocysts. They use their stinging cells in their tentacles to catch prey in the water such as zooplankton. Corals are voracious predators and even attack other corals when competing for light and space. Corals are most active at night.



Corals get up to 90% of their energy from a unicellular algae that lives inside their tissue called zooxanthellae. The coral’s color actually comes from this algae. The coral and algae have a symbiotic relationship: the coral provides shelter for the zooxanthellae while the zooxanthellae’s biproduct provides food for the coral host.



Coral Bleaching occurs when the coral becomes stressed and the algae is expelled. These stresses can be caused by increases in water temperature and high nutrient inputs. If this continues for an extended time period the coral may not survive since it depends on the nutrients provided by the zooxanthellae.

Corals need specific environmental conditions to survive and are therefore very vulnerable to environmental change. Corals are found in water temperatures of 70-85 F and may bleach if these temperatures increase. They need clear, nutrient-free water and sufficient sunlight so their zooxanthellae can photosynthesize. They cannot tolerate too much fresh water or sediment, hence few corals are found near river mouths or areas of high sediment run-off.

Corals can reproduce sexually and asexually. Broadcast spawning occurs where eggs and sperm are simultaneously released into the water column to fertilize at specific times of the year. Corals can be male, female or both. Cauliflower coral (Pocillopora Meandrina) has separate sexes and spawning occurs in April- May. Rice Coral (Montipora capitata) is hermaphroditic (both sexes) and spawning occurs in June- July. Corals also reproduce asexually through fragmentation or budding, whereby polyps split into more polyps.

Coral reefs are threatened by both human and natural impacts. Bad land use practices including overdevelopment, pollution, recreational overuse and the introduction of alien species threaten Hawaiian reefs. See Threats for more information. Global warming increases the risk of bleaching. A heavily impacted reef can take up to 20 years to recover. It is essential to minimize our impact on this unique and fragile ecosystem.

For information on managing coral reef areas see Management. See Guidelines for how to make sure you are minimising your impact on the reef in Puako.

Photographs: Robert Shallenberger and Andrew Walsh.