Corals are fascinating living creatures. You can categorise corals in two main groups; hard corals and soft corals. Soft corals are flexible and sway with ocean currents, while hard corals, as the name suggests, are rigid and sturdy. In this article, we will focus on hard corals.
Corals are primarily made from a skeleton of calcium carbonate which is white. However, they have a symbiotic relationship with a type of algae called zooxanthellae, this algae provides corals with their vibrant colours, which will vary between species. A mutualistic symbiotic relationship is where both organisms coexist and both of them benefit from the relationship. The algae will photosynthesize and produce oxygen, it will also remove waste from the corals and in return the coral will provide protection from the elements.
Within the hard coral’s skeleton are “cups” and within these cups are polyps. The zooanthellae’s colour shows through the skeleton and polyps. Polyps are from the cnidarian family, which also contain jellyfish and anemones, they all have stinging tentacles. Each coral is a colony made up of countless polyps. During the day, polyps use the coral for protection, and at night, they extend their tentacles to capture passing nutrients, plankton, and small fish for food.
Types of Coral Reefs
Coral reefs consist of different groups of corals growing together and cover just 1% of the ocean’s floor, yet they host 25% of all marine life. Coral reefs are also known as “The Rainforests Of The Sea”. Fish and inverts rely on coral reefs to stay alive and so do we. Even though they are underwater, they play an important role on our planet.
There are three different types of coral reefs:
- Fringing Reefs
- Barrier Reefs
- Atoll Reefs
Fringing Reefs – These reefs grow directly around an island and are primarily composed of sloping reefs.
Barrier Reefs – Running parallel to an island, these reefs are separated from land by deep water or lagoons.
Atoll Reefs – These circular reefs form around a sunken island or volcano, creating a lagoon in the centre.
How do corals reproduce?
Depending on the species, corals will reproduce once or twice a year. One way that the corals reproduce is by spawning. There are male and female polyps and they will release their eggs and sperm into the water. The egg and sperm come together and will form a larva called planula. The planula then floats through the water until it finds a hard surface that it will call home.
Corals seem to plan their reproduction with luna cycles. The water temperature and acidification also plays a small role, however the main contributor to the timing is the moon. Without any communication between each other, they all somehow know when is the right time to release their eggs and sperm into the water. This can be a huge event to witness in some parts of the world, the event occurs on a grand scale.
Another way for corals to reproduce is through brooding. The male coral will release its sperm into the water and a female coral will catch the sperm and internally fertilise her egg. Once the cells have formed a planula, she will release it through her mouth where it will float in the ocean until it finds a suitable surface.
When the polyp starts to grow, it also has the ability to clone itself.
What is coral bleaching?
Coral bleaching is a pressing concern for our oceans, highlighted by several global mass bleaching events. When sea temperatures rise—sometimes by as little as 2-3°C – zooxanthellae become stressed and leave the coral. Since these algae are responsible for the corals’ colour, their absence leads to the coral turning white, or “bleaching.”
A coral can last a couple of weeks without the algae, however it will need new algae to settle within those couple of weeks otherwise the coral will slowly starve and eventually die. But this has a knock on effect for the rest of the reef. Creatures that rely on the reef will leave, but some fish will stay behind as dead coral will benefit them. For example, parrot fish will scrape and consume dead coral.
What happens if more and more corals are bleaching? This ruins more reefs around the world. When the fish leave the reefs, this will have an impact on the local community. They rely on the reef for food, tourism and protection. Local fishermen will no longer be able to provide food or money for their families. Fringing reefs offer protection, big waves are broken before they hit shorelines, which could save lifes. There have been many tsunamis that could have created more of a devistating effect on coastal towns if the waves were never broken further out at sea.
Human Impacts On Coral Reefs
Coral reefs not only have rising sea temperatures to deal with, we are causing a huge negative impact on our reefs. We are consuming too much fish from our oceans, the reefs need the fish and the apex predators, it is a delicate system. Removing apex predators like sharks can lead to an increase in parrotfish, which can over consume coral, further harming the ecosystem.
Scuba divers and snorkellers must exercise caution around reefs, as even minor contact can severely damage corals, which grow only a few centimetres a year. Activities such as anchoring, dredging, and the introduction of plastics pose significant threats to these ecosystems.
Fortunately, many organisations worldwide are working to protect our oceans. Marine Protected Areas (MPAs) are effective strategies for safeguarding coral reefs. These areas implement rules to reduce human impact, such as installing mooring lines to avoid anchoring, limiting the number of divers and snorkellers in a site, and prohibiting fishing. These small but vital steps contribute significantly to the preservation of coral reefs.
Many organisations are creating their own artificial reefs. These are helpful for local marine life and can attract snorkelers and beginner divers. Artificial reefs can take years to evolve due to the slow growth rate, but it is a step in the right direction.
