Coral reefs are geological structures by accumulation of dead hermatypic corals and other organisms. Many corals live on the reefs and form the coral reefs. Thus, coral reefs ecosystems are characterized by biological and geological presences. Their characteristics are 1) highly productive because corals have small algae (zooxanthellate), 2) clear and warm water with very low levels of nutrients, 3) highly diversity because there are lots of houses given by coral calcium skeletons. It is important not only for creatures living there but also for human life as a living place, fisheries, truism etc.

However, recently, coral reefs have been under direct threat from human activities and global changes. In Ryukyu Islands, coral reefs have been damaged by outbreaks of Acanthaster planci (crown-of-thrown of starfish), red-soil runoff, and heavy bleaching due to the higher temperature occurred. Thus, conservations and managements of the reefs are needed immediately for such environmental problems.

Terms

  • 1) What are corals?

    Corals look like plants, but they are animals. They belong to Phylum “Cnidarians” that includes jellyfish and sea anemone. Their grope has needles with poisons in their body and simple body structure like a bag. Corals formed coral reefs has small phytoplankton named zooxanthellate (hermatypic coral). They live in the shallower and warmer area because zooxanthellate needs light for photosynthesis. That is why we find them in tropical and subtropical oceans. On the other hand, corals living in the deep sea do not have them (ahermatypic coral).

  • 2) Reproduction of corals

    Corals reproduce asexually and sexually (see figure). In the sexual reproduction, many corals spawn their eggs and sperm (i.e., their gametes) simultaneously in the water column at night at almost predictable times of the year. This type of reproductive mode is classified as broadcast spawning. Some of corals, on the other hand, are called brooders, and they release their planulae larvae directly into the water column after fertilization. After dispersal, larvae settle on the sea bottom, and metamorphosis to coral polyps. Then, they divide or make other polyps with skeleton asexually and growth to adult corals.