We know corals as the colourful rock-like structures under the sea that host numerous species of sea life. But what are corals, really? Are they living creatures or just plain rocks? Are they animals or plants? Corals are in fact, animals. Corals are formed by coral polyps, animals with soft bodies and short, tentacle-like arms that pull their food and sweep it into their mouths. They are close relatives to sea anemones and jellyfish. Corals are also sessile, which means they attach themselves to the seafloor.
Figure 1: The similarities in the anatomy of a coral polyp and a jellyfish (medusa). See Footnote 1 for image credits.
Corals connect with each other and create a colony which grows over hundreds of thousands of years and becomes a reef. The base of corals is made of a protective limestone skeleton (calcium carbonate) called a calicle which forms the structure of these reefs.
Figure 2: The anatomy of a coral polyp. See Footnote 2 for image credits
Corals are indeed living creatures, but how do they obtain their nutrition? The bodies of polyps contain photosynthetic algae (plant cells) called zooxanthellae (ZOH-oh-ZAN-thell-ee). The coral and algae have a mutualistic relationship where the coral provides a protected environment for the algae and in return, the algae supply the coral with its photosynthetic products (i.e: glucose, glycerol, and amino acids). As high as 90% of the organic materials produced by the algae is transferred to the host coral tissue, and this supports the growth and productivity of coral reefs.
The life cycle of corals
Corals can undergo sexual reproduction in two ways: broadcast spawning and brooding.
Broadcast spawning
During broadcast spawning, mature corals will start to produce gametes (sperms and eggs) through meiosis. When the conditions are right, corals release their gametes into the water column. The positively buoyant gametes float to the ocean’s surface, and external fertilisation occurs between the sperms and eggs. When the eggs are fertilized (zygote is formed), their development begins. During this phase, zygotes drift in the current. Cell division (mitosis) begins while the zygote continues to drift until an embryo is formed.
These embryos become a larva called planula. A planula is a type of zooplankton that can manoeuvre using the cilia covering its body. Although the planula has the ability to swim, they are not strong enough to swim against ocean currents, so will look for an acceptable solid substratum to settle on. Metamorphosis occurs once the planula settles on a hard substrate and it develops from a juvenile to an adult. A calcium carbonate (CaCO3) corallite is then laid down in the juvenile polyp. If the coral is colonial, the polyp undergoes asexual reproduction to increase the number of polyps and expand the coral colony. When the adult polyp becomes sexually mature, the life cycle starts all over again.
Brooding
During brooding, corals invest more energy to create a fully-developed planula. Brooding corals release negatively buoyant sperms into the water column. Fertilisation takes place within the coral when the sperm finds unfertilised eggs in an egg-carrying coral. After fertilisation, the zygote undergoes mitotic cell division to form an embryo. The embryo then grows into the planula within the brooding coral.
Figure 3: The life cycle of corals during broadcast spawning & brooding. See Footnote 3 for image credits.
The importance of coral reefs
Corals are extremely important for four key reasons:
Coastal protection
Coral reefs are effective barriers against the forces of the ocean and are important protectors of ecosystems between the reefs and coasts. They can absorb wave energy and reduce the damage during storms, and even during natural disasters such as hurricanes and tsunamis.
Tourism
A beautiful coral reef ecosystem attracts tourists, especially divers, to visit. This is essential particularly for many small Pacific islands, where more than 90% of new economic development depends on coastal and reef tourism. If tourism is practised sustainably, it can boost the economy while preserving the health of coral reefs.
Protein source
Coral reefs provide protein for nearly 1 billion people as it can sustain a large fish and shellfish population. It also acts as a breeding and nursery ground for many commercially valuable species such as sweetlips, snapper, parrotfish, moray eel, and grouper.
Medicine
Sea sponges are one of the organisms that depend on corals, as they primarily live attached to coral reefs. These sponges are marine organisms that are considered very valuable in the pharmaceutical industry, as they “often contain diverse and abundant microbial communities - including bacteria, archaea, microalgae, and fungi. In some cases, these microbial associates comprise as much as 40% of the sponge volume and can contribute significantly to host metabolism (e.g: via photosynthesis or nitrogen fixation).” Approximately 20,000 bioactive compounds including terpenoids and alkaloids have been found in marine organisms, most of which have originated from sea sponges. Every year, many new compounds are discovered from sponges and these compounds are already in clinical trials as agents against cancer, microbial infections, inflammation, and other diseases.
What is coral bleaching?
The phenomenon of bleaching is described as the loss in colour in symbioses between zooxanthellae and marine benthic animals of the phylum Cnidaria, which includes sea anemones, zoanthids, and of course, corals.
In corals, bleaching occurs when drastic changes in environmental factors lead to the intracellular algal pigments (zooxanthellae) being partially or completely eliminated, due to expulsion from the cell or cell death. Hence, corals lose their main source of food and turn into the dull white colour of their underlying skeleton (calcium carbonate, CaCO3). When a coral is bleached, the symbiosis might recover over a period of weeks to months. Alternatively, they may die due to increased vulnerability to starvation and disease (Figure 4). Corals on certain reefs have suffered from mass mortality as a consequence of bleaching events.
Figure 4: The regression in the health of a coral upon stress exertion by external factors Image Source: modestfish.com
Coral bleaching was first described by Glynn in 1984, who linked high-temperature stress with coral mortality through their observations in areas across the Pacific Ocean. Since then, coral bleaching has occurred in various parts of the Caribbean, Indian, and Pacific Oceans regularly, with scientists in French Polynesia witnessing bleaching through 1983-1996. The gradual, yet dramatic rise in incidence and severity of bleaching over the last two decades has led to the perception that it is a fundamentally unnatural phenomenon, and the ecological collapse of the world’s reefs is being predicted by simulation models using available data on the incidence of bleaching and projected climate change.
Causes of coral bleaching
Despite the considerable amount of research constantly being established on the topic of coral bleaching, there is yet to be a simple explanation of what causes the bleaching. A simple framework was constructed to divide such causation into three different elements:
Factors that trigger bleaching
Mechanisms that define the response of the symbiosis to the triggers
Resulting observed symptoms
Figure 5: An explanatory framework that describes the causes of bleaching through three different progression elements.
Large-scale bleaching events have been attributed primarily to increased seawater temperatures and solar radiation, which were further linked to long-term climate changes. In laboratories, scientists discovered other factors that could also trigger bleachings, such as prolonged darkness, the presence of heavy metals (especially copper and cadmium), and pathogenic microorganisms, but these do not exert much pressure comparatively. While not much has been explained on the mechanisms of bleaching, certain information has been confirmed, such as:
Damage in the D1 protein of photosystem II reaction centre of zooxanthellae
Inhibition of photosynthesis in response to a toxin produced by the pathogen Vibrio shiloi
Elements of programmed cell death pathways mediating lysis
These phenomena are not necessarily mutually exclusive and may vary in importance depending on the trigger involved. Such mechanisms tend to give rise to the expulsion of zooxanthellae, although it is still uncertain as to the extent to which the external factors triggering bleaching determine its symptoms.
The economic impact of coral bleaching
Aside from the effects on underwater ecosystems, the degradation of coral reefs has severe economic impacts as well - primarily through the losses in fishing income, protein sources, and tourism revenue.
The fishing industry: a vital source of income and food
The fishing industry indirectly provides jobs for thousands of people who are involved in processing, marketing, boat building, net making, and other support services. In the Caribbean, the industry is predominantly small-scale and artisanal, employing more than 120,000 full-time fishers and many part-time workers.
Furthermore, coral reefs are home to millions of aquatic organisms. This supports the fishing industry as they can harvest the reef's products which are a key source of food for the locals in the area. For example, in the Caribbean, the reef fisheries provide a vital source of protein for the millions of people living in the region.
Based on the reasons above, there is no doubt that corals play a vital role to ensure the survival of the fishing industry and the people who work within it. Therefore, coral bleaching would no doubt negatively impact the fishing industry. People would not only lose their jobs but would also lose their source of protein due to the loss of corals.
Coral reefs in tourism
Studies on the economic value of coral reefs within the Caribbean have suggested that the total annual economic benefits from coral reefs have ranged from US$100,000 to US$600,000 per square kilometre of coral reef, with tourism and recreation contributing to the largest share of this income. A useful way to examine the situation is to take a close look at one of the main sources of tourism revenue that is directly linked to coral reefs: scuba divers. Making up about 10% of all visitors but generating 17% of tourism revenue, the average diver spends about US$2,100 per trip to the Caribbean, compared to US$1,200 for tourists in general. The study estimated net benefits to the local economy by estimating costs such as transportation, fuel, and boat expenses (65% of expenditure.) A multiplier of 25% is then applied to account for expenditures rippling through the local economy.
Unsurprisingly, scuba divers prioritise the availability of a healthy, high-quality reef habitat with clear waters and a diverse range of coral and marine species. Therefore, loss of these habitats will heavily impact their value to these major economic contributors. The Reefs at Risk Threat Index is used as a proxy for future reef conditions. Through this, it is predicted that marine tourism will suffer a decline of 1 to 10%, leading to a loss in revenue for reefs at a medium or high level of threat on the Index.
How you can help
Hopefully, we have all reached the consensus that our corals need to be protected and rehabilitated. As of now, the best options to do so are coral repopulation and addressing climate change. However, as students reading this article, it can seem daunting. However, here are some simple and practical solutions you can practice to help our corals.
Reduce your meat consumption
One of the main causes of coral bleaching is an increase in ocean temperatures due to global warming. Although the government has the biggest responsibility in reducing greenhouse gas emissions through renewable energy, carbon tax, and carbon capture and storage (CCS), these solutions take a long time to implement. A quicker solution that we as a society can practice is reducing our red meat consumption, especially beef.
A study by the University of California found that if a person reduces their piece of steak from 170g (6 oz) to 113g (4 oz), they cut their carbon footprint by half. According to the UN IPCC Fifth Assessment Report in 2014, global greenhouse gas emissions from agriculture, forestry, and land use accounted for 24% of emissions. Thus, we must try to reduce our meat intake as much as possible. Switching to a Mediterranean diet which consists mainly of vegetables, fish, and poultry is the most effective meat-based diet in reducing carbon emissions. Even better options are vegetarian and vegan diets, however, not everyone can commit to adopting a plant-based lifestyle. A more realistic approach would be to adopt challenges like “Meatless Mondays” which is a great start to slowly reducing meat consumption.
Be water and energy conscious
Being conscious of water and electricity usage is essential in tackling our warming climate. The water in our households has been treated and pumped to us for our convenience. However, many people don’t realise how energy intensive the processes of producing clean drinking water or heated showers are. An estimate by the United States Environmental Protection Agency (EPA) predicted that if just one in every 100 American homes had water-efficient fixtures installed, it would save about 100 million kilowatt-hours of electricity per year. This converts to 80,000 tons of prevented global warming. It is important that we also conserve water by taking shorter showers, promptly fixing leaks, and turning off faucets. Our energy consumption can be radically reduced by switching incandescent bulbs to LED bulbs which are 80% more energy-efficient and turning off electrical appliances when not in use.
Cancel chemicals
To take things beyond reducing carbon footprint, we can take an active approach in reducing or even eliminating our use of chemical pesticides and fertilisers. This is because these substances seep into the soil and eventually end up in runoff into the ocean. These chemicals cause serious harm to corals as an estimated 20% of coral reefs are threatened by toxic substances. For example, a fungicide MEMC can inhibit coral fertilization and metamorphosis of the coral polyps even in low concentrations. Copper is another element that has serious effects on coral reefs. Copper found in fertilisers or anti-fouling paints has been found to disrupt the reproduction of corals.
Get involved
Volunteering is another great way to contribute to these efforts. Do some research on your local coral conservation centres and lend a helping hand. Activities include: beach and reef cleanups, learning more about corals, and participating in their coral replanting efforts.
The announcement of the Great Barrier Reef losing half its corals since 1995 was a jarring reminder that the culmination of mass production and human activity over the centuries has taken its toll on the planet. However, with a heightened awareness and a deep sense of responsibility paired with vigorous action, we can turn it around for the Earth and our corals.
Footnotes
Lyne Chahine-Böhme. "Difference between Polyp and Medusa." DifferenceBetween.net. February 26, 2018 <http://www.differencebetween.net/science/nature/difference-between-polyp-and-medusa/>.
Encyclopedia Britannica. 2019. Labradorite. [online] Available at: <https://www.britannica.com/science/labradorite> [Accessed 17 December 2020].
Coral Life Cycles by Spaully is licensed under CC BY-SA 1.0
Authors
First Steps for Wildlife (FSW) is a grassroots student organisation which aims to develop a sense of environmentalism and volunteerism amongst the Malaysian student community. We at FSW hope that through our events and community-based Instagram, we will develop a community where like-minded individuals can come together and feel inspired to start something bigger for our Malaysian wildlife and ecosystems.
Ng Eu Keat, MEng Biomedical Engineering (Imperial College London)
Audrey Au Yong, BSc Biological Sciences (Imperial College London)
Euan Thum, BSc Economics (University of Warwick)
Nabilah Ariffian, Masters of Environmental Studies (University Putra Malaysia)
Ashwinie Punniamoorthy, BSc Accounting and Finance (University of Bath)
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