Coral

Most people know about coral reefs, but many don’t know that coral is actually an animal. They are often mistaken for plants because they are sessile, meaning they permanently attach to the ocean floor. Coral are not plants because they can’t make their own food, like plants can. Coral, like sea anemones, have polyps. Coral polyps are a small cylindrical-ish organism. On one side there is a mouth surrounded by venomous barbed tentacles that let the coral capture zooplankton to eat. Inside the body structure is digestive and reproductive tissues. At the base of a polyp there is a protective mineral skeleton called a calicle, which is what differs coral from sea anemones. Polyps attach themselves to the sea floor and then divide into thousands of clones. The calicles connect and act as one organism called a colony. Colonies grow over hundreds of years and join with other colonies, creating reefs. Coral reefs are very old; some have been growing for 50 million years. Coral reefs can be found in tropical waters, usually near the surface. Most coral also get their food from algae called zooxanthellae, which grow on the coral. The algae gives some of the food it makes photosynthesizing to the coral in exchange for nutrients. Coral reefs create the habitat for about 25% of marine animals. Because of this biodiversity, they are called the rainforests of the ocean.

The main reasons coral is endangered is warming ocean temperatures and ocean acidification. Overfishing, pollution and invasive species also play a part. Warming ocean temperatures and other changes to the coral's natural environment cause coral to go through a process called coral bleaching. Coral bleaching is caused when coral lose their zooxanthellae, causing the coral to turn white. Coral bleaching doesn't always kill the coral, but it puts them under extreme stress, makes them vulnerable to other threats, and sometimes does kill them. Ocean acidification makes acidic seawater, which makes it hard for coral to build their skeleton, and if it gets really bad it may break the skeletons that already are there. By 2085 it is predicted that oceans will be so acidic that corals will begin to dissolve. In Hawaii, one reef has already started to dissolve. Overfishing disrupts the balanced diverse ecosystem the coral reef creates. Fish are important to the reef because they keep organisms that can be harmful to coral in check; for example, some eat seaweed that strangles coral and some eat predators of coral. Pollution feeds algae that smothers the reef, and deforestation quickens erosion that clouds water and strangles coral. Invasive species like the lionfish, which is native to Indo-pacific waters but is spreading to areas without it’s natural predators, eat and compete with natural reef animals and ultimately affect the reef. Because of all this, some reefs have been destroyed, and some are shrinking.