Is the "Coral Spiral" in "Return to the South Sea" mysterious? The real coral reef is eye-opening!

Not long ago, the online drama "Return to the South Sea" was a hit, and the mysterious and dangerous "coral spiral" in the drama attracted the audience. Although there is no "coral spiral" in the real world, in the ocean of the real world, coral reefs can play many tricks!

1 From the shallow sea to the deep sea, wherever I am, that is home

The coral reefs in the ocean are built by tiny coral polyps. Generations of coral polyps grow, reproduce, and die on the same seabed with a suitable environment. New coral polyps continue to grow on the skeletons of dead corals. Over the years, spectacular coral reefs are formed. From shallow seas to deep seas, there are many places where coral reefs can make their home.

(1) Fringing reefs

Today, most of the coral reefs we see were formed after the end of the last ice age. A large amount of glacial meltwater flowed into the ocean , causing sea levels to rise, forming a large area of ​​warm shallow sea on the submerged continental shelf , which is a very suitable environment for the growth of most coral reefs.

Some coral reefs grow closely around the continent or islands. These coral reefs that grow along the coast and in shallow water near the coast are called fringing reefs. The largest fringing reef in the world is the Ningaloo Reef, which stretches about 260 kilometers along the coast of Western Australia, which is almost equivalent to the distance of 6 marathons.

Figure 1: Aerial view of Ningaloo Reef in Western Australia, the world’s largest fringing reef and a UNESCO World Heritage Site (Source: Wikipedia)

(2) Barrier Reef

Some continental shelves where coral reefs grow sink due to geological movements, forcing these coral reefs to grow on the edge of the continental shelf farther offshore. These coral reefs are called barrier reefs.

The most famous barrier reef is the Great Barrier Reef off the coast of Queensland, Australia. It is the largest coral reef in the world, consisting of more than 2,900 independent coral reefs and about 900 islands of various sizes. It is more than 2,300 kilometers long and has a total area of ​​more than 300,000 square kilometers. If compared with the Vatican, the smallest country in the world, the area of ​​the Great Barrier Reef is equivalent to more than 700,000 Vatican cities.

Figure 2: Part of the Great Barrier Reef near Queensland, Australia, taken from the International Space Station (Source: Wikipedia)

Figure 3: Aerial view of Arlington Reef, one of the main coral reefs of the Great Barrier Reef (Source: Wikipedia)

(3) Atoll

Some coral reefs growing around islands, due to weathering or other geological movements, the part of the island that is exposed above the water sinks underwater to form reefs, leaving only the circular coral reefs growing around the island on the sea surface, called atolls, and the waters in the middle of the atolls are called "atoll lagoons." The "ring" of an atoll (i.e. the outline of the atoll) is not only circular, but also elliptical or even other irregular shapes.

Generally speaking, reef-building can only occur in tropical or subtropical waters. Therefore, atolls can generally only occur in tropical or subtropical waters. However, the famous "devil's" area - Bermuda, is sometimes called the "northernmost atoll" at 32°18′ north latitude. At this latitude, coral polyps cannot grow, but the strong Gulf Stream flows through here, which promotes the formation of coral reefs. However, Bermuda is called a "pseudo-atoll" because although its overall shape is similar to an atoll, its cause is completely different.

Figure 4: Satellite image of Bermuda (Source: Wikipedia)

( 4 ) Deep sea coral reefs

In addition to these coral reefs that grow in warm shallow seas, there are also a few corals that can grow in the dark and cold deep sea, with the deepest living depth reaching 3,000 meters. Due to the relatively harsh living environment, the growth rate of these deep-water coral reefs is much slower than that of coral reefs in warm shallow seas. A coral reef with an area of ​​several square meters may have a growth period of thousands of years.

Figure 5: Bubble Gum Coral at a depth of 1,257 meters in the waters of California (Source: Wikipedia)

Deep-sea coral reefs were first discovered by fishermen about 250 years ago. With the maturity of manned mini-submarine technology, the mystery of deep-sea coral reefs has been slowly revealed. In July 1982, the Norwegian National Petroleum Corporation captured the first real-time video of a large deep-water coral reef in history. This coral reef is located at a depth of 280 meters near Fuglei Island, north of the Arctic Circle in northern Norway, and is 15 meters high and 50 meters wide.

2 All that's gold on the outside, all that's gold on the inside: The powerful coral reef

(1) Coral reefs are one of the most biodiverse and important habitats on Earth.

The area of ​​coral reefs in the world only accounts for about 0.1% of the global ocean area, but they support at least 25% of the world's marine species and are known as the "tropical rainforests in the ocean." Many edible fish species with important economic value in fishery production and their bait animals need to grow and fatten in coral reefs.

Figure 6: Coral reefs are home to a wide variety of organisms (Source: Wikipedia)

(2) Coral reefs are important “helpers” in alleviating global warming .

Coral polyps fix the carbon dioxide dissolved in seawater in the form of calcium carbonate in their own calcareous skeletons. There are about 600,000 square kilometers of coral reefs in tropical waters around the world, and the amount of calcium carbonate fixed in them each year is as high as 640 million tons.

(3) The reefs growing along the coast can also resist the erosion of the coast by waves and play a role in protecting the coast .

3 Protecting coral reefs is urgent

However, due to the destruction of human activities, 50% of the world 's coral reefs have disappeared since the 1950s . The main reasons are as follows:

(1) Global warming causes the sea water to warm up, which reduces the number of symbiotic zooxanthellae, leading to the death of coral polyps due to lack of food. At the same time, the acidification of sea water will also corrode the skeleton of coral polyps.

Figure 7: Coral bleaching event in the Great Barrier Reef (Source: Wikipedia)

(2) Soil erosion and water pollution on land have led to reduced seawater visibility, which has seriously affected the photosynthesis of zooxanthellae, destroyed their symbiotic relationship with coral polyps, and even caused the coral polyps to be covered by sediments or algae blooms, leading to suffocation and death;

(3) The large-scale capture of predatory animals such as giant conches and rippled lip fish has caused the number of crown-of-thorns starfish, which feed on coral polyps, to increase dramatically in recent years due to the lack of natural enemies, and they have eaten a large number of coral reefs;

(4) Illegal mining of corals by unscrupulous traders and overfishing of other marine animals also pose a serious threat to the survival of coral reefs and the marine animals that live there;

(5) For coral reefs growing in the deep sea, the disaster caused by deep-sea trawling is devastating. Coral reefs that have grown for thousands of years may be destroyed in a few minutes.

Figure 8: (A) Coral communities on an untrawled seamount. (B) Exposed bedrock on a trawled seamount. Both are located 1,000–2,000 m below the surface (Source: Wikipedia)

Coral reefs are humanity's precious marine treasure, and their future is in the hands of each and every one of us.

Author: Hu Kai, science research instructor, CCTV-9 documentary channel science consultant

Reviewer: Liu Qinghua, Researcher, Institute of Oceanology, Chinese Academy of Sciences

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.