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coral reef



Coral reefs are primarily composed of the calcium carbonate skeletons of corals. These corals are essentially colonies of minuscule (1-2 millimetres in size) but identical organisms known as polyps, firmly interconnected. 
Within each coral colony, there is a symbiotic relationship with single-celled algae called zooxanthellae. These microscopic algae make their home inside the coral and provide both colouration and up to 90% of the coral's nutritional needs through photosynthesis. In return, the coral provides a secure habitat for these algae to thrive.
During daylight hours, the zooxanthellae use sunlight to produce energy for themselves and the coral. When the sun sets and night falls, the zooxanthellae rest, and the coral polyps come to life. They extend their tentacles into the water, catching particles, including plankton, in a graceful dance. This continuous activity sustains the coral reef, making it a vibrant and bustling ecosystem that operates 24 hours a day.


Coral reefs are often named the "rainforests of the sea" due to their unparalleled biodiversity. Despite covering less than 1% of the ocean floor, coral reefs are home to approximately 25% of all marine species. Their intricate physical structure provides numerous hiding places, shelters, and feeding grounds for various organisms.

Fish, in particular, benefit enormously from coral reefs. From tiny gobies and colourful reef fish to larger predators like groupers and barracudas, coral reefs offer abundant food sources, protection from predators, and suitable breeding grounds. Many fish species rely on reefs for shelter during their juvenile stages before venturing into open waters as adults.

Iconic examples include the clownfish, which seek shelter among the protective tentacles of specific coral species. In return, these fish help defend the coral by driving away potential predators. Additionally, fish contribute to the growth of coral by promoting it through their waste.

coral grouper

In a study published in Nature Communications in 2013, it was discovered that the red coral grouper utilizes specific signalling behaviours to communicate with other fish species. This unique behaviour allows the grouper to seek assistance in catching prey and protect itself from predators. By performing a distinctive headstand, the grouper signals to potential collaborators the presence of prey, facilitating cooperative hunting. Conversely, when a threat approaches, the grouper employs a head-shaking movement to warn other fish, prompting them to seek shelter.

Sebae anemone (Heteractis crispa)

Coral reefs also support numerous other marine species. Invertebrates such as sponges, sea anemones, and sea stars find habitat within the complex structure of reefs. Crustaceans, including crabs and shrimp, utilize reefs as havens and food sources. Molluscs like clams and snails thrive in reef environments, while sea turtles depend on reefs for food and nesting grounds.

harlequin shrimp (Hymenocera picta))

Harlequin shrimp (Hymenocera picta) eating Necklace starfish (Fromia monilis)
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The feeding behaviour of the Harlequin shrimp is specialized and highly efficient.

It primarily consumes the tube feet of the Necklace starfish. This mutualistic symbiosis benefits both species, as the shrimp gets a readily available food source, while the starfish benefits from the removal of harmful ectoparasites by the shrimp.

Porcelain Anemone crab (Neopetrolisthes maculatus) on sea anemone
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The relationship between Porcelain Anemone crabs (Neopetrolisthes spp.) and sea anemones is an example of mutualism in nature. Mutualism is a type of symbiotic relationship where both species benefit from their association. In this case, the Porcelain Anemone crabs and the sea anemones rely on each other for survival and protection.

hawksbill sea turtle (Eretmochelys imbricata)

Hawksbill sea turtle (Eretmochelys imbricata) in the reef
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Coral bleaching is a stress response exhibited by corals, where they expel the colourful zooxanthellae that live within their tissues. 
When corals face stressors such as elevated sea temperatures, pollution, or increased UV radiation, they expel these zooxanthellae, revealing the white calcium carbonate skeletons beneath.

This loss of pigmentation results in the bleached appearance of corals, hence the term 'coral bleaching'.

After coral bleaching, various types of algae can grow on the exposed coral skeletons. This includes macroalgae, filamentous algae, coralline algae, and microscopic algae. While some, like coralline algae, can aid coral recovery, others, such as macroalgae, may hinder it by competing for space and resources. 

Mass bleaching events are causing large coral mortality, which is endangering the ecosystem.

​​While bleaching events were previously localized and infrequent, the occurrence of marine heatwaves in the past two decades has caused widespread bleaching. Alarmingly, estimates suggest that by the end of this century, up to 99% of existing coral reefs could be affected by bleaching.

Close-up of  bleached coral

Close-up of  bleached coral
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Pocillopora bleached on reef flat

Pocillopora bleached on reef flat
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Coral bleaching
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Coral bleaching is primarily caused by several factors:


_The primary cause of coral bleaching is the increasing sea temperatures driven by climate change. Even small temperature increases of just 1-2 degrees Celsius above the normal summer maximum can trigger bleaching events.

_The increased concentration of carbon dioxide (CO2) in the atmosphere leads to global warming and results in higher CO2 levels in the ocean. This causes seawater to become more acidic, reducing the availability of carbonate ions that corals require to build their calcium carbonate skeletons. Weakened skeletons make corals more susceptible to bleaching during stress events, as they struggle to maintain their structure.

_Pollution from agricultural runoff, sewage discharge, and industrial activities introduces excess nutrients and toxins into the ocean. These pollutants disrupt the delicate balance of coral ecosystems. For example, nutrient-rich runoff can lead to overgrowth of algae, which competes with corals for space and resources. This competition and the presence of toxins can stress corals and contribute to bleaching.


_Human-released chemicals have depleted the ozone layer, allowing higher levels of harmful ultraviolet (UV) radiation to penetrate the water. UV radiation can directly damage the DNA of corals and their zooxanthellae, further adding to stress and bleaching. 

The loss of coral reefs impacts marine biodiversity and has far-reaching consequences on our global ecosystem and economy. Beyond the immediate threats to marine life and livelihoods, the decline of coral reefs can exacerbate climate change-related challenges. 

One critical aspect to consider is the role of coral reefs in carbon sequestration. These underwater wonders are not just a hub for marine life but also serve as vital carbon sinks, capturing and storing carbon dioxide from the atmosphere. This function helps mitigate the effects of climate change by reducing the levels of greenhouse gases in the atmosphere. As coral reefs decline, this natural carbon sequestration capacity diminishes, further contributing to the challenges posed by global warming. Additionally, the destruction of coral reefs can disrupt the delicate balance of ocean ecosystems, leading to cascading effects on the food security and livelihoods of millions of people who depend on fishing as their primary source of income and sustenance.

marine biologist measures bleached coral

It is imperative to preserve the breathtaking underwater world of coral reefs; doing so is crucial for safeguarding the fundamental pillars of our planet's health and prosperity.

Great Barrier reef - aerial view
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