Harboring at least 25% of all marine species, yet taking up just 1% of the ocean floor, coral reefs are critical to both our planetary health and global economy. Reef ecosystems support rich biodiversity, provide food for our communities, hold a wealth of natural products with potential medicinal or agrochemical applications, and protect our coastlines from wave damage.
So how can we protect these vital structures? Our marine scientists share their thoughts.
While they might look like plants, corals are actually animals. They are colonies of tiny coral polyps, which can collect in their thousands, growing and dividing on a shared surface. These marine invertebrates are related to jellyfish or sea anemones and have lots of stinging tentacles.
Corals can be split into two main types: hard and soft corals. It’s hard corals who construct reef structures. Each hard coral polyp secretes calcium carbonate, which builds up over time into the large reefs that we recognize today.
These calcium carbonate structures are white. Coral polyps are generally very pale or see-through. So how do we get colorful corals? The answer lies in colorful symbiotic single-celled organisms called zooxanthellae that live inside the polyps’ tissues. Photosynthesis from these algae produces glucose which feeds the polyps and helps the corals grow.
Of course, these reef systems are not just home to coral and algae. They shelter wide varieties of fish, invertebrates and microbes, acting as some of the most diverse ecosystems on our planet.
There are several different ways that corals reproduce, but the most beautiful and bizarre are the mass spawning events, known as broadcast spawning.
Often tied to the lunar cycle and sea temperatures, these mysterious annual events see corals releasing countless tiny bundles of reproductive cells into our oceans. This only happens at night, and tends to last just a few days, with different corals spawning on different days. The mass approach helps increase the likelihood of fertilization as different cells drift together.
When fertilized, coral eggs develop into little swimming larvae called planulae. These drift in the ocean until one day they find a suitable surface to attach to, developing into a coral polyp.
Zooxanthellae live within certain cells that line the stomach of the polyps. When corals get stressed, they eject their colorful companions from these cells in a process called bleaching. Losing their main energy source means these corals die, revealing the white calcium carbonate reef structures.
Environmental factors such as light and heat are the main culprits of environmental stress, with rising sea temperatures playing a big part in mass coral bleaching. Such events, like the 2014-2017 global bleaching, wipe out thousands of kilometers of corals, with devastating knock-on impacts on other reef creatures. The US National Ocean and Atmospheric Administration assessed that the 4th global coral bleaching event took place between 2023-20251. So, what can we do to keep our corals healthy?
Recent bleaching events have affected almost 85% of the world’s coral reefs2. But it’s not all doom and gloom. Some corals, like Acropora tenuis, appear resistant to bleaching. OIST researchers are involved in long term cultivation projects and study of this intriguing species. By exploring the genomics and gene expression of these creatures, scientists hope to unlock the secrets of their durability, paving a path towards healthier future reef systems.
Coral-algae symbiosis is the key to bleaching. By understanding the relationship between these organisms, scientists can get new insights into why bleaching events might occur.
OIST researchers are studying this symbiosis in a variety of ways, including examining the impact of warming waters on symbionts, and exploring how the structure of corals supports symbiosis.
The Coral Project, launched by Prof. Noriyuki Satoh and Prof. Tim Ravasi, aims to protect corals with genomic information. The team are planting and nurturing corals at various sites, monitoring restoration using eDNA methods to ensure the thriving health and biodiversity of coral species and the other marine creatures that make up reef ecosystems.
If you’d like to learn more about these efforts, ongoing research projects or make a donation to support such research, visit OIST’s Coral Project website.
When we think of a cold-blooded killer, we’re probably not picturing a starfish. But Crown-of-Thorns starfish (CoTS) outbreaks are a massive threat to corals. These venomous starfish can gather in their thousands, eating coral tissue and decimating reef-building coral populations.
OIST’s collaborative research aims to understand the driving force behind CoTS outbreaks, to discover new strategies to mitigate these swarms. By understanding how the starfish communicate and swarm, our researchers hope to develop efficient strategies to deal with CoTS outbreaks.
While some species eat our corals, others are essential to their survival. Overfishing disrupts the careful balance within reef ecosystems, making them less resilient against stress and bleaching.
For example, parrotfish or surgeonfish are considered prize catches for fishermen. But these fish eat algae which grows on the reef, ensuring it doesn’t take over and block all the space, sunlight and nutrients that corals need to grow.
Of course, fishing is essential for our economy and food security. By working together alongside local fisheries, we can help ensure the health and longevity of Okinawa’s reef systems.
What we do on land greatly impacts our seas. A key threat to coral is pollution. This could be from agricultural runoff, industrial sewage, plastic pollution and many other sources — even your suncream can have an impact.
After a particularly stormy day, it’s sadly quite common to look out at the Okinawan coast and see swaths of red streaking the ocean. This is due to red soil erosion. As rain saturates soil, sediment is washed into our oceans, settling on our reefs. This blocks sunlight for photosynthesis. The additional nutrients from fertilizers can also cause algal blooms, again competing with corals.
OIST researchers previously ran a project with local communities to explore ways to reduce runoff, with a little help from honeybees.
Plastic waste is a common problem in our oceans. Beyond the bottles and bags that we easily see, tiny fragments of plastics known as microplastics have been found everywhere. These are known to impact human health when we ingest them and pose risks for other organisms too.
OIST researchers have been monitoring microplastics within both Okinawan waters and some of our local sea creatures.
Explosive fishing techniques like blast fishing can physically damage and break apart reefs. Likewise, damage from boats, anchors, and even tourists snapping off a quick souvenir are all examples of physical reef damage.
Given the slow rate of coral growth (often only a few milli- or centimeters per year), even small damage can take decades to recover from.
With rising CO2 levels, seawater is becoming more acidic. This upsets the chemistry of the oceans, reducing the accessible carbonate ions that are essential for corals to build their characteristic calcium carbonate reef structures.
The Marine Climate Change Unit is investigating how ocean acidification might affect our underwater ecosystems by studying natural analogues, like underwater volcanoes or vents, where CO2 levels are high.
When water and CO2 combine, carbonic acid forms, which exists as hydrogen and bicarbonate ions.
Coral reef structures are made of calcium carbonate. However, hydrogen ions can preferentially bind to carbonate ions to form more bicarbonate ions, which corals can't use for reef-building.
If the levels of hydrogen ions in water are high enough, it can break apart existing calcium carbonate, impacting both coral and the shelled inhabitants of reefs, such as clams, mussels and oysters.
Just like humans, corals can suffer from diseases too, such as bacterial or fungal infections. These diseases are known to damage tissue, reduce growth or even kill coral colonies.
The Marine Structural Biology Unit has recently started investigating black band disease, which is known to affect some local Okinawan corals.
To protect our ecosystems, we first need to know what they’re made up of. Genomic methods can help us identify which animals are present in a particular area.
The Marine Genomics Unit were the first researchers to sequence a full hard coral genome. Together with other marine scientists, they’ve been steadily building a bank of genomic data on our corals, and developing a new way to use this data to identify the corals present in a particular region, just through small seawater samples.
The method, known as eDNA (environmental DNA) metabarcoding, can be used to study genus composition over time, tracking the changing biodiversity of our reef systems. Similar methods can work for studying fish and other reef creatures too.
The team have now published an eDNA Coral Atlas, documenting the state of coral reefs across the Ryukyuan islands. By making their data publicly available, they hope to support coral reef conservation efforts.
Environmental DNA is just DNA shed by organisms that’s made its way into the environment. For corals, eDNA originates in either tiny bits of coral debris or from the mucus secreted by corals into surrounding waters. This light debris floats at the surface of the sea, meaning it’s found within surface seawater.
To sample eDNA, scientists:
OIST scientists are zooming in on coral to get atomic-level insights into their biology. The Marine Structural Biology Unit is using cryo-electron microscopy (cryo-EM) to examine corals and their symbionts. Professor Oleg Sitsel, head of Marine Structural Biology Unit, says,
We’re interested in how symbioses start, are maintained, and break down — like in bleaching events. How do corals feed? What initiates bleaching cascades? What happens to the symbionts? We study these processes from a structural biology perspective, often looking at model organisms like anemones under our microscopes, as well as at corals themselves.”
Work with us! Find collaborations, new career opportunities and more information on how to help OIST advance coral research below.
