What Actually Are Coral Reefs?
A coral reef in Egypt, 2023. Photo by: Charlotte Bedford
As a Marine Biologist and Scuba Instructor, I often get asked questions about coral reefs. What exactly are they? What is coral bleaching? Why are coral reefs important to us?
So, let’s dive into my first post…
What Actually Are Coral Reefs?
While many of us are familiar with the beauty of coral reefs from snorkelling or scuba diving, how many really understand what coral reefs are and how they are created?
Coral reefs are large underwater structures formed by numerous smaller colonies of corals. These corals belong to the phylum Cnidaria, classifying them as a type of invertebrate (an animal that lacks a backbone) which are related to Jellyfish and Anemones.
Each coral colony consists of hundreds of thousands of tiny coral polyps. The photo below gives you a brief overview of the anatomy of an individual polyp. These organisms can reproduce both sexually and asexually, with some species using both methods at different stages of their life cycle.
Photo by: NOAA
Sexual Reproduction in Corals
Sexual reproduction in corals occurs through two methods: broadcast spawning and brooding. In broadcast spawning, parent polyps simultaneously release their eggs and sperm into the water, where fertilisation occurs. This synchronised mass spawning typically happens just once a year, with the timing varying between different colonies. Scientists are still exploring how entire coral colonies can coordinate the release of gametes so precisely, but it’s believed that the lunar cycle and water temperature are crucial to the timing of this event. Witnessing this incredible phenomenon has always been on my bucket list!
During brooding, male corals release sperm into the water which is then taken in by female corals for internal fertilisation. Once the planulae (baby corals) are fully developed and environmental conditions are favourable, the parent polyp releases its offspring. This reproductive method is advantageous for certain species as it increases the chances of survival for the planulae.
Asexual Reproduction in Corals
Corals also reproduce asexually through two main methods: budding and fragmentation. In the budding process, a portion of the parent polyp is ‘pinched’ off to form a new, genetically identical polyp known as a bud. Budding can be further categorised into intra-tentacular budding, where the bud develops within the parent polyp’s tentacle ring and extra-tentacular budding, where the bud develops outside of the tentacle ring. I know… confusing… so make sure you refer to the diagram of the coral polyp above for a clearer visualisation and refer to the image below which shows intra-tentacular budding on the left and extra-tentacular budding on the right.
Photo by: Indo Ocean Project
Fragmentation happens when a piece of a parent coral is broken off due to natural processes like storms or animal activities. If conditions are favourable and the coral fragment encounters a suitable surface, it will attach to the substrate and begin developing into a new, genetically identical coral colony. The process of fragmentation is now a key method for coral reef restoration. Coral fragments are typically transplanted onto coral nurseries or artificial reefs, and once they are ready, they can be reintroduced to natural reefs.
With each of these processes, new coral colonies are developed and sometimes join together, creating these huge coral reef ecosystems we admire while exploring the ocean. A fact that really amazes me is that coral reefs can take thousands of years to form. It was estimated that the famous Great Barrier Reef in Australia began to form around 600,000 years ago. Isn’t it crazy that these beautiful and diverse marine structures have been there far longer than we can even imagine!
The Relationship Between Corals and Zooxanthellae
Another fascinating aspect about corals is their mutualistic relationship with photosynthetic algae called zooxanthellae, in which both organisms benefit from each other. The corals offer shelter within the polyp’s surface tissue and supply inorganic nutrients that zooxanthellae require for photosynthesis and growth. In return, the zooxanthellae provide oxygen and energy as products of photosynthesis, supplying over 90% of the coral’s metabolic requirements.
Additionally, zooxanthellae are responsible for the vibrant colours of most coral species. These algae cells contain various types of pigments at different densities, which results in the rich diversity of colours found in coral reefs. However, when ocean temperatures rise, corals become stressed and expel the zooxanthellae from their tissues. Without this type of algae and their vital food supply, corals are left extremely vulnerable and often die from starvation or disease. As a result, they lose their colour and turn white, a phenomenon known as coral bleaching.
Photo by: people4ocean
Unfortunately, in 2024, we experienced our fourth and largest global mass bleaching event, impacting coral reefs in over 74 countries and territories. With scientists predicting that sea surface temperatures will continue to rise due to global warming, we must act urgently and make significant global changes if we want to see coral reefs survive.
References
GBR Biology (2023) Great Barrier Reef Timeline. GBR Biology. Available at: https://www.gbrbiology.com/knowledge-and-news/great-barrier-reef-timeline/
Porter, J.W. and Tougas, J.I. (2001) ‘Reef Ecosystems: Threats to their Biodiversity’, Encyclopedia of Biodiversity, 3, pp. 73-95. doi: https://doi.org/10.1016/B0-12-226865-2/00229-7
Miththapala, S. (2008) ‘Coral Reefs’, IUCN Coastal Ecosystem Series, 1, pp. 1-36. Available at: https://portals.iucn.org/library/sites/library/files/documents/CES-001.pdf
Island Expeditions (n.d.) What Are Coral Reefs and Why Do We Need Them. Available at: https://www.islandexpeditions.com/belize-vacations-blog/what-are-coral-reefs-and-why-do-we-need-them
Sharma, P.V., Shah, C.N., Singh, M. and Bhadauriya, M. (2019) ‘THE IMPORTANCE OF CORAL REEF AND THE CONSEQUENCE OF THEIR POSSIBLE EXTINCTION’. Available at: https://www.researchgate.net/publication/341049389_THE_IMPORTANCE_OF_CORAL_REEF_AND_THE_CONSEQUENCE_OF_THEIR_POSSIBLE_EXTINCTION
SECORE International (n.d.) Coral reproduction. Available at: https://www.secore.org/site/corals/detail/coral-reproduction.15.html
Khaled bin Sultan Living Oceans Foundation (n.d.) Coral Reproduction. Available at: https://www.livingoceansfoundation.org/education/portal/course/reproduction/#asexual-reproduction
Coral Disease and Health Consortium (n.d.) Coral Reproduction. Available at: https://cdhc.noaa.gov/coral-biology/coral-reproduction/
Withers, A. and Dickie, G. (2024) Exclusive: Global coral bleaching event expands, now the largest on record. Available at: https://www.reuters.com/business/environment/global-coral-bleaching-event-expands-now-largest-record-2024-10-17/
Dunleavy, B. (2023) Why Are Some Corals More Colorful Than Others? Available at: https://reefcoaquariums.com/blogs/news/why-are-some-corals-more-colorful-than-others?srsltid=AfmBOoqHeRZlXF-XrpGB9wnhM9GWgQGq18pL34TlR0CWpiSU98wrRrnv
NOAA Coral Reef Conservation Program (n.d.) Coral Facts. Available at: https://coralreef.noaa.gov/education/coralfacts.html
Braden, H. (2019) ‘The Negative Effect of Temperature on the Relationship Between Coral Reefs and Zooxanthellae’. doi: 10.131040/RG.2.2.15099.36643
Hoek, L. S. and Bayoumi, E. K. (2017) ‘Importance, Destruction and Recovery of Coral Reefs’, Journal of Pharmacy and Biological Sciences, 12(2), pp. 59-63. doi: 10.9790/3008-1202025963