The coral reef is all around us: It serves as a water break so that our island isn’t pummelled with the full force of ocean waves. What’s more, it creates an enchanting underwater landscape that you can experience by snorkelling or SCUBA diving and it supports a diverse range of species, from damselfish to groupers. Just as lions wouldn’t be able to survive without the grassland they live on, marine shallow-water animals can’t survive without reefs.
Corals are animals, not plants. They’re tricky to conceptualise because they’re not animals in the way that we think of them. They’re closely related to stingy, faceless carnivores like jellyfish and anemones, and to survive they grow plants in their tissues. Here are some aspects that characterise corals:
1. Colonial – Corals are colonial organisms. This means that it’s hard to define where their “body” is. A human is just one body made up of millions of cells. A coral is one calcium carbonate skeleton, that’s shared by many individual animals called polyps.
2. Polyp – A coral polyp is the tiny tentacled animal that makes up a coral. They can wave outside of the coral’s skeleton to catch food or retract inside for protection. Each coral has many little bumps on its skeleton where a polyp lives, and each polyp is genetically identical.Have you ever seen a porous sea rod? This is a common coral that looks like many brown fuzzy tree branches sticking up in the water. It might have tickled your feet as you were swimming or snorkelling. If you look closely at the sea rod, you will find that the fuzzy look is caused by lots of small tentacled creatures on its surface. These are individual coral polyps.
Coral polyps range in size, but most can be seen with the naked eye. They are usually smaller than a fingernail. So, it’s the polyps that are the individual animals that makeup what we call coral. A coral colony might look like a rock underwater – that’s because most of what we see is the skeleton the coral has constructed itself.
3. Stinging Cells – Coral polyps have tentacles with stinging cells to help them capture food, but most coral stinging cells can’t be felt by humans. Coral polyps are soft and vulnerable. They capture food with their tentacles and digest it in their stomachs.
4. Symbiosis – This is where it gets complicated – most corals are symbiotic with zooxanthellae. That means that each polyp also hosts microscopic algae in its tissues. The algae are plants, and act like plants, taking in sunlight and converting it into sugars for its food. Zooxanthellae shares the food it creates with the coral itself. In return, the algae get a safe place to live within the coral’s hard skeleton.
How Does the Relationship Between Coral Polyps and Zooxanthellae Work?
Coral polyps and zooxanthellae are just like other symbiotic relationships in the animal community – as oxpecker birds spend their days on the hides of hippos and zebras, picking ticks off their fur, zooxanthellae live on coral. However, zooxanthellae make their own food and share it with the host in return for protection.
Zooxanthellae provide the lion’s share of food for the coral, and it needs a lot of nutrients if it is to stay competitive among the many other corals competing for space and sunlight on the reef’s surface. In most cases, zooxanthellae provide all the carbon needed for the coral to build its calcium carbonate skeleton.
A plant and an animal living so close together has its advantages – coral and zooxanthellae neutralise each other’s chemical processes. The polyps breathe oxygen and produce carbon dioxide, and the algae take up carbon dioxide and produce oxygen. Not bad for two organisms not simply of different species, but of completely different kingdoms!
Deep-sea exploration has revealed corals that grow without the presence of sunlight. These corals cannot host algae because they algae wouldn’t be able to photosynthesize, so their entire diet is made up of particles of food its polyps catch and gulp down. They grow much more slowly than their shallow-water relatives.
What is Coral Bleaching?
Sometimes shallow water corals must expel their zooxanthellae. In doing so, they lose their colour and stop growing. Corals can survive for a short time without zooxanthellae, catching food only with their tentacles, but not indefinitely.
It is the zooxanthellae algae that give coral its spectacular colour, and without it, the corals turn an eerie white. Corals lose their zooxanthellae when stressed, usually due to water temperature fluctuation, but zooxanthellae will return when conditions are favourable once more.
It might seem like corals are making a bad move to expel zooxanthellae, their food provider, in dire conditions. The reason corals expel zooxanthellae is because the algae get damaged and risk mortality in higher water temperature conditions. The coral wants to avoid accumulating damaged cells in its tissues, so it expels the lot.
Do Corals Get Any Other Diseases?
In Bermuda, corals can get black band or yellow band disease. Both diseases are caused by bacteria, and attack coral that is already weakened by stress. According to a BREAM report, coral disease decreased in general from 2015-2016.
What Threatens Corals?
1. Rising water temperatures and ocean acidification mean that more corals are bleaching the world over. The Great Barrier Reef in Australia has lost approximately half of its corals in the past 30 years.
2. Humans pose a direct threat to the reef by standing on it and vandalizing it. The oils in our skin disrupt coral mucus and can kill corals.
3. Sediments are a bigger problem than you might think for corals. In fact, dredging in Castle Harbour to build the airport in Bermuda killed off many brain coral colonies there because they couldn’t stand the sudden increase in sediment. All corals can remove sediments from their surfaces by trapping it in mucus and moving it to the side with small hair-like projections, but larger domed corals can usually only survive in water farther offshore, where sediments are less of a problem.
4. Runoff from land can fuel algal blooms that block sunlight from getting to corals
5. Overfishing can cause knock-on effects for coral reefs. For example, the overfishing of grouper has caused a boom in damselfish populations, which farm algae gardens in patches on the reef. Too many of these algae gardens threatens the room for coral to grow.
Corals Turn On Each Other
It might not seem like corals would have the faculties to be aggressive, but if two corals are growing in close proximity things can get ugly. Corals are constantly competing for growing space, and if two corals crash into each other as they spread, they will produce filaments that can kill the tissues of their foe. In some cases, a more aggressive coral may overgrow the neighbouring colony.
How Do Corals Reproduce?
Corals can reproduce asexually, that is, by one polyp splitting itself in half into two genetically identical individuals. Coral colonies grow in this way, but to colonise new areas, corals must reproduce sexually.
Once a year, when the lunar cycles are correct, corals release eggs and sperm into the water, gametes of the same species combine and form larva that is able to swim. These larvae can travel long distances and remain able to swim for long periods of time but are stimulated to settle on the bottom by detecting warm and shallow water. A new coral colony is born!