A coral reef is home to many marine animals. With corals creating complex habitats to sustain biodiversity, reefs are often called "rainforests in the sea."
Some may think that corals are plants, but each piece actually comprises a colony of tiny animals, called polyps. There are two major types of corals: soft and hard. The polyps of the former are connected by soft tissue, while those of the latter are joined by their limestone skeletons. When the polyps die, the skeletons remain and leave a foundation for other corals to grow. Over centuries, they stack together to form reefs.
Corals perform an important role in the ecosystem, supplying food and shelter for around a quarter of all marine species in the world. They share a symbiotic relationship with a type of tiny algae called zooxanthellae. In return for shelter, the algae gives corals different colors, and supply them with oxygen and food generated from photosynthesis.
Many fish are attracted to corals; some invertebrates, such as shrimps, crabs and octopuses, reside in the reefs as well. Some corals have tentacles to catch tiny prey such as plankton or small fish.
Coral reefs can be found in many different regions, but are mostly distributed in shallow waters in tropical and subtropical regions. In Hong Kong, corals inhabit in the east and northeast waters, including Hoi Ha Wan, Yan Chau Tong and Tung Ping Chau Marine Parks. According to a Hong Kong Reef Check conducted in 2019, more than 80 species of hard corals are found locally. They support life for about 350 types of fishes.
For sexual reproduction, fertilization of eggs and sperms are done across different colonies - enhancing the genetic diversity of the species.
Despite the wide variety of coral species in the sea, their survival is threatened by climate change.
Excessive emissions of greenhouse gases increase the global temperature, forcing the algae to leave the coral polyps. Corals lose their colors and their main source of food, resulting in a bleaching effect in which they turn white and eventually die.
Excessive carbon dioxide also acidifies seawater, which gradually dissolves the limestone skeletons of hard corals. Moreover, water pollution caused by different human activities and inappropriate disposal of fishing gear can also damage coral reefs.
On the International Union for Conservation of Nature Red List, six species are critically endangered, 25 others are endangered and 201 are vulnerable.
To protect corals, we can lower our carbon footprint by saving electricity, and adopt a green diet with less processing and packaging.
Explore the spectacular underwater world of corals at the Grand Aquarium in Ocean Park Hong Kong.