Top 14 Fascinating Facts About Coral

1. Description Information

Coral, marine animals belonging to the Anthozoa class, exist in the form of small polyp entities resembling sea anemones. They often form communities with many identical individuals. These organisms secrete calcium carbonate to create a hard skeleton, forming coral reefs in tropical seas.

An actual coral 'head' is made up of thousands of genetically identical polyps, each with a diameter of only a few millimeters. After thousands of generations, these polyps leave behind a characteristic frame that defines their species. Each coral head develops through the asexual reproduction of polyps. Coral also undergoes sexual reproduction through gametes, released simultaneously during consecutive nights of a full moon.

Although corals can use stinging cells (nematocysts) to release toxins for prey capture, they primarily obtain nutrients from a symbiotic relationship with single-celled algae called zooxanthellae. Therefore, most corals rely on sunlight and thrive in shallow waters, usually at depths not exceeding 60 meters (200 feet). Corals significantly contribute to the physical structure of coral reefs that develop in tropical or subtropical regions, such as the Great Barrier Reef off the coast of Queensland, Australia.

Some coral types do not depend on algae and can live in deeper waters, like species in the cold-water genus Lophelia, found at depths of up to 3,000 meters in the Atlantic Ocean. Another example is the Darwin Mounds off the southwest coast of Cape Wrath, Scotland. Corals are also found off the coast of Washington and the Aleutian Islands in Alaska, USA.

2. Sexual Reproduction in Coral

Corals primarily undergo sexual reproduction, with 25% of corals relying on algae (hard corals) forming single-sex colonies, while the rest are hermaphroditic. About 75% of corals dependent on algae 'broadcast gametes' by releasing gametes (eggs and sperm) into the water to disperse coral populations. Gametes combine upon fertilization to form a tiny larva called a planula, usually pink and oval-shaped. On average, a medium-sized coral colony can produce several thousand planula larvae each year to overcome the low probability of forming a new colony.

Planula larvae swim towards light, exhibiting positive phototaxis, reaching the surface where they drift and develop for some time before swimming back down to the seabed to find a surface to attach to and build a new colony. Many stages of this process have a high failure rate, and although each coral colony releases millions of gametes, very few new colonies are formed. The time from gamete release to settled planula larvae is typically 2 or 3 days but can extend up to 2 months. The planula larvae develop into a coral polyp and eventually become a coral head through asexual reproduction creating new polyps.

Most coral species, excluding hard corals, do not broadcast gametes. These species release sperm but retain eggs, allowing the development of larger planula larvae that are ready to settle later. The planula larvae develop into coral polyps and eventually become coral heads through asexual budding and growth to create new polyps.

Synchronous gamete release often occurs and is typical in coral reefs; even with multiple species on a reef, all corals release gametes on the same night. This synchrony is crucial for male and female gametes to meet and form planula larvae. The guiding cues for gamete release are very complex but involve long-term environmental changes like daytime length, water temperature, and/or temperature change rates; and short-term cues, most commonly the lunar cycle, with gamete release controlled at sunset. About 75% of coral species are broadcast spawners, most of them relying on golden algae or reef-building corals. Gametes with positive buoyancy float to the surface where fertilization takes place to form planula larvae. The planula larvae swim towards the light on the surface before drifting in currents, staying for about 2 days, but can extend up to 3 weeks, and in one known case, 2 months, before sinking and transforming into polyps, eventually forming new colonies.

Corals need to rely on environmental cues, depending on the species, to determine the exact time for gamete release into the water.

There are two methods by which corals use sexual reproduction, differing in whether female gametes are released:

• Spawning corals, mostly those that reproduce massively, heavily rely on environmental cues. Unlike egg-brooding corals, they release both sperm and eggs into the water. Spawning corals use long-term signs such as daytime length, water temperature, and/or temperature change rates, and the most common short-term cue is the lunar cycle, with sunset controlling the release time. About 75% of coral species are spawning corals, mostly relying on golden algae or reef-building corals. Gametes with positive buoyancy float to the surface where fertilization takes place to form planula larvae. The planula larvae swim towards the light on the surface before drifting in currents, staying for about 2 days, but can extend up to 3 weeks, and in one known case, 2 months, before sinking and transforming into polyps, eventually forming new colonies.
• Egg-brooding corals, most commonly not dependent on golden algae (non-reef-building), or some corals relying on golden algae in areas with strong wave action or currents. Egg-brooding corals only release sperm, with negative buoyancy, and can store fertilized eggs for several weeks, reducing the need for synchronous mass-spawning events, although they can still occur. After fertilization, these corals release ready-to-settle planula larvae.

Coral reefs are highly sensitive to changes in the natural environment. Scientists have predicted that by 2030, over 50% of the world's coral reefs could face destruction[15]; hence, they are often protected by environmental laws. A coral reef can easily be overwhelmed by algae if there is an excess of nutrients in the water. Corals will also die if the water temperature deviates more than 1-2 degrees beyond the normal range or if the water salinity decreases. Early signs of environmental stress include corals expelling golden-brown algae; without their symbiotic algae, coral tissues turn white, revealing the white color of the calcium carbonate skeleton, a phenomenon known as coral bleaching.

Many governments now prohibit the harvesting of coral from reefs to reduce damage caused by divers using air tanks. However, coral reefs still suffer damage from ship anchors or fishing activities. In places where local fishing practices harm coral reefs, awareness programs have been implemented to educate the public about the ecosystem and the need to protect coral reefs.

Narrow ecological niches dominated by coral, and the dependence of stony coral species (Scleractinia) on the precipitation of calcium carbonate, mean they are highly sensitive to changes in water pH. As atmospheric CO2 levels rise, ocean acidification, where CO2 dissolves in water causing a decrease in pH, is occurring in surface waters of the oceans. Low pH impairs coral's ability to form calcium carbonate skeletons, and in extreme cases, these skeletons may completely dissolve. Scientists are concerned that without significant and early reduction in human CO2 emissions, ocean acidification will cause severe damage or extinction of coral species and coral ecosystems.

A combination of temperature changes, pollution, exploitation by scuba divers, and jewelry manufacturers has led to the destruction of many coral reefs worldwide. This has elevated the importance of coral biology as a field of study. Climate changes alone can induce temperature variations sufficient to destroy corals. For example, during the warming period in 1997-98, all Millepora boschmai coral populations near Panama turned silver and died within 6 years - this species is now considered extinct.

In a recent study, scientists revealed new insights into the biological life of coral discovered in the past few years, helping explain why coral reefs worldwide are gradually vanishing and what needs to be done for them to survive climate change and ocean acidification.

Scientists state that the increasing sea temperature caused by climate change leads to coral bleaching, along with other factors such as overexploitation of coral and ocean pollution, which have killed many coral reefs over the past decade. They predict the 'worst-case scenario' for coral reefs is possible extinction within the next 30-50 years.

According to scientists' statistics, in 1998, up to 16% of coral reefs globally died from coral bleaching, and 2010 is considered one of the 'worst' years for this phenomenon, with reports indicating 100% coral bleaching in many reefs in the Indonexia region.

It seems that coral, with a complex genetic system equivalent to humans and a sophisticated biological information system, is threatened by global climate change, surviving only through a balanced symbiotic relationship with algae living inside coral bodies - according to a recent report in the journal Science.

One of the findings of the recent study is the immense complexity of coral biology, and even similarity to other life forms. For example, the gene that controls bone development in humans is remarkably similar to the gene that helps develop an expansive skeletal structure in coral – illustrating that certain common features are maintained across species over hundreds of millions of years since they diverged along different evolutionary paths from a common ancestor.

Researchers state that there is still much to discover about this process, accompanied by immense diversity. For example, there are 1,000 different coral species, and perhaps thousands of algae species forming various symbiotic combinations with coral. Experts believe that this diversity provides hope for reasonable combinations to help corals better adapt to changing conditions of ocean temperature, acidity, and other threats.

Coral reefs are increasingly facing significant challenges. They are currently under pressure from changes in ocean temperature, pollution, overfishing, sedimentation, water acidification, oxygen depletion, and various diseases simultaneously, which could destroy coral reefs even though corals can cope well with each issue individually. Some estimates indicate that up to 20% of coral reefs worldwide have died, and an additional 24% are seriously endangered.

Predictions suggest that ocean acidification in the next century will reduce 50% of coral reefs and increase the coral skeleton decomposition process, as stated by researchers in a report.

Some coral types are creating a 'sunscreen layer' to defend themselves against the increasing sea temperatures, according to a study by scientists at the University of Southampton (UK).

Many coral types incorporate algae into their cells to aid mutual growth. However, even a 1-degree Celsius temperature increase can disrupt the balance, causing the algae to be expelled. This exposes the coral's skeleton to sunlight, often leading to coral death. This phenomenon is known as coral bleaching.

Coral bleaching occurs when the water temperature reaches 30-31 degrees Celsius due to climate change. The coral expels symbiotic algae inside its tissues, which serves as a food source. If the warm condition persists for more than a few weeks, the coral will die.

Scientists from the University of Southampton's coral reef laboratory recently published a report in the journal Current Biology. They observed some corals creating a 'sunscreen layer' through a color layer during short periods of increased water temperature.

The research team believes that these bright neon colors are emitted to encourage the algae to return. Professor Jorg Wiedenmann explains: 'Our study found that creating these colors involves a self-regulating mechanism, called optical feedback, involving symbiotic partners (coral and algae).'

In healthy corals, most sunlight is absorbed by the photosynthetic pigments of symbiotic algae. When corals lose this symbiosis, excess light enters the tissue, reflected by the white coral skeleton. However, if coral cells can continue to perform some normal functions despite environmental stress causing bleaching, the increased internal light will stimulate the production of vibrant photosynthetic pigments. This sunscreen layer will promote the return of symbiotic organisms.

According to The Guardian (UK), coral reefs are the most diverse marine ecosystems globally, often regarded as the rainforests of the ocean. Although they occupy only about 0.2% of the ocean floor, they provide a habitat for approximately one-third of marine species. According to scientists' statistics, in 1998, up to 16% of coral reefs globally died from coral bleaching, and 2010 is considered one of the 'worst' years for this phenomenon, with reports indicating 100% coral bleaching in many reefs in the Indonexia region.

Facing this alarming situation, the EDGE project prioritizes the protection of 10 tropical coral species selected from famous reefs worldwide. These include the largest Great Barrier Reef (Australia), the coral reefs surrounding the Chagos Archipelago (UK), the 'coral triangle' around the Philippines, the coral reefs west of the Indian Ocean around the Mozambique Channel, and those in the Caribbean Sea.

Among the 10 EDGE project coral species, some are quite unique, such as the pearl bubble coral Physogyra lichtensteini, a food source for the hawksbill turtle Eretmochelys imbricata, and the mushroom coral Heliofungia actiniformis, considered a 'home' for at least 15 shrimp species.

Here's a close look at the tropical coral species in the EDGE project:

• Brain coral Catenella chagius, endemic to the Chagos Archipelago.
• Coral Catalaphyllia jardinei.
• Elkhorn coral Acropora palmata.
• Pillar coral Dendrogyra cylindrus.
• Grooved brain coral Dichocoenia stokesii in the Caribbean Sea.
• Star coral Horastrea indica.
• Mushroom coral Heliofungia actiniformis.
• Stony coral Parasimplastrea sheppard.
• Pearl bubble coral Physogyra lichtensteini.

Corals, tiny animal polyps with genetically identical individuals, can eat, defend themselves, and eliminate nuisance creatures for food. In the process, they also produce calcium carbonate, forming the foundation for the expansive skeletons they sit upon.

Over time, these calcified layers grow into massive structures, creating coral reefs—one of the most valuable ecosystems in the world, nurturing over 4,000 fish species and numerous marine life forms.

However, corals are not truly self-sufficient. Inside their bodies, corals host a beneficial symbiotic algae—a type of marine plant capable of absorbing carbon, using solar energy for photosynthesis, and producing sugar.

“Some of these algae living inside coral bodies are remarkably beneficial, and in some cases, they provide 95% of the energy production for the coral,” said Weis. “In return, the algae obtain nitrogen, a nutrient scarce in the ocean. It's truly a beautiful symbiotic relationship.”

However, what scientists are researching is that this relationship relies on a sophisticated information process from algae to coral, signaling that the algae belongs to the coral's body, and everything is harmonious. Otherwise, the coral will treat the algae as a parasite or an intruder, seeking ways to eliminate them.

Geneticists from the United States, Japan, South Korea, and Saudi Arabia affirm that modern coral polyps appeared during the Jurassic period and have thrived to this day through symbiosis with zooxanthellae algae.

According to the journal Current Biology, geneticists argue that modern coral polyps emerged during the Jurassic period when dinosaurs still roamed the Earth. Corals have survived even after mass extinctions and global climate changes, continuing to exist today.

Most modern coral polyps exist with the help of symbiotic creatures - zooxanthellae algae. Coral polyps, in turn, provide these algae with some organic compounds and share carbon. The vibrant colors of coral polyps are also thanks to the contribution of zooxanthellae algae.

Previously, it was believed that this symbiotic relationship appeared on Earth not more than 65 million years ago. However, a recent study by geneticists from the United States, Japan, South Korea, and Saudi Arabia shows that this estimation is far from the truth. In fact, the combination of these two species occurred 165 million years ago, at the beginning of the Jurassic period.

The authors studied the DNA of various algae species belonging to the Symbiodiniaceae genus using various methods, from computer simulation to electron microscopy. Scientists reached two conclusions: first, there are more Symbiodiniaceae algae species, and second, they are older than previously estimated. Researchers believe that different algae species originated from a common ancestor. Then, by tracking mutations in the DNA of Symbiodiniaceae, scientists could determine the time when coral polyps and algae formed a symbiotic organism.

According to the research results, modern coral reefs appeared simultaneously with the first symbiosis of polyps and algae. The authors believe that this is not a random coincidence but evidence that the symbiosis with algae helped coral polyps spread, survive through the dinosaur era, and continue to exist to this day.

Just like some plants and bees, even pressured fish change their gender. Professor Yossi Loya from the Department of Zoology at Tel Aviv University was the first worldwide to discover that Japanese coral reefs also undergo gender changes.

His research could offer insights into the survival of delicate marine corals that play a vital role in supporting diverse ocean life. Coral reefs are currently under serious threat due to global warming. During stressful periods, such as extremely high temperatures, fungiid corals undergo gender changes, leading to mostly male coral populations. According to the renowned coral researcher, the advantage of this is that male corals can quickly cope with increased pressure when resources are limited. Clearly, when times get tough, nature favors the 'gentlemen.'

One evolutionary strategy that some coral species employ is the ability to switch from female to male. As males, they can endure challenging times, then, when conditions become more favorable, they transform back into females. As females, they expend more energy. Having the ability to switch genders at different stages allows these corals to maximize their reproductive efforts.

Despite being members of the animal kingdom, corals can act like plants. Both males and females have a sedentary lifestyle, unable to move when living conditions become harsh.

Statistics reveal that only 1% of the 1,300km2 coral reefs along the coast of our country are in very good condition; 26% of the coral reefs are in good condition; 41% are in fair condition, and the remaining 31% are poor coral reefs.

Worryingly, 96% of coral reefs across the country's waters are negatively impacted by human exploitation activities, with nearly 75% of the reefs facing high and very high risks due to destructive exploitation.

For instance, in Da Nang, in recent years, the use of certain types of nets by fishermen to eradicate marine life in coastal areas and coral reefs in the southern Son Tra Peninsula has seriously threatened and devastated coral reefs in this area.

What is alarming is that this is not limited to Da Nang but is also observed in other provinces and cities such as Khanh Hoa, Ba Ria-Vung Tau, where individuals not only illegally exploit coral reefs using rudimentary tools but also employ heavy machinery such as excavators, dredgers, and cars to 'exterminate' coral reefs.

Throughout the country, over 50 tons of coral reefs are lost each year due to destructive exploitation and business-driven extraction, not to mention the loss of black coral in provinces like Quang Ninh, Hai Phong, Quang Tri, and Quang Binh.

Given the current rate of coral destruction, experts also suggest that in about 20 years, coral reefs may cease to exist in Vietnamese waters.