A couple of friends and I visited the Natural History Museum on Sunday, we went to see the Coral Reefs exhibition. Without a doubt, this was one of the most beautiful and interesting exhibitions I have ever visited. Beautiful, but quite frightening too, in many ways. I knew that coral reefs were endangered but didn’t really know why.

I also didn’t know until Sunday that the coral reef is one of the most biologically diverse ecosystems on the planet, providing a home to at least 25% of marine species. The Great Barrier Reef is the largest living structure on the planet, and the coral triangle in South-East Asia has the highest diversity of marine life in the world! Corals are of immense use to humans in a number of ways – for example, they absorb the impact from waves hitting the shoreline; they can take in up to 97% of their energy, serving to protect both the coastline and the people living there. It really is worth our while to look after them.

Coral reefs are made up of thin layers of calcium carbonate which is secreted over animals known as coral polyps, which are quite simple organisms. They seem to form these layers in tiny hexagonal shapes, something that appears quite a lot in nature as it’s strong. Not only are they invertebrates, they don’t have any bone structure at all, nor any of the complexities that are associated with so-called higher organisms.


There are two kinds of corals – hard and soft. The hard corals have a rigid exoskeleton, the soft ones don’t, and these are the ones that we see swaying with the ocean currents. Both forms are sessile, ie, they do not move about, and tend to have tentacles that they use to catch prey. Every gap between different parts of the coral is inhabited, which serves to make a more stable structure. They may be considered lower life forms, in evolutionary terms, but they have complex relationships with the other creatures that exist around them, forming symbiotic associations with algae – algae produce food and carbon by photosynthesis and the coral provide protection. It’s these algae – zooxanthellae – that give the beautiful colours to the reefs.

As well as the algae, there are many other creatures that live in the reefs: oysters, clams, shrimps, crabs, sea worms, star fish, sea urchins, jelly fish, sea anemones and turtles, to name a few. There are fish known as Gardener Species such as Parrot Fish play a part in keeping the reefs clean and tidy, and Clown Fish (as in Finding Nemo) forms a symbiotic association with sea anemones – both protect each other from their specific parasites, the sea anemone provides food for the Clown Fish and the Clown Fish helps the sea anemone with its parasites. Far too many different fish and organisms to mention here.


Many species of coral reproduce during mass spawning events, when the adults release thousands of eggs and sperm pretty much simultaneously on just one day of the year. The sperm are guided by moonlight and once they have fertilised an egg, they attach to a suitable place on the reef and start to build their limestone skeleton. Who said romance was dead?

Like most ecosystems that are left to their own devices, corals are very resilient to natural events such as hurricanes and the like, and can recover pretty quickly. Many coral reefs are in trouble though, largely through man’s interventions or, some would say, interference. Corals are intolerant of wide ranges in water temperature, salinity or pH of the water. Any change outside of a narrow range can stop growth, for example, too much sunlight will cause the coral to expel the algae (coral bleaching), and if the coral can’t find more food, within a few weeks it will starve. This is when we are left with the barren, skeletal type structures with no fish or other organisms swimming nearby. Conversely, too little light prevents growth. More carbon dioxide in the atmosphere is causing ocean acidification which slows down limestone secretion.

Industrial run-off, human sewage and other toxic waste, be they from pesticides or other sources can increase the amount of nitrogen and phosphorous in the water, which can in turn affect the natural balance of organisms living in the reefs, or even just poison them. Removal of trees near to the water’s edge can increase the amount of sunlight reaching the reefs. Land clearance can cause soil erosion which can then bury corals.


Fortunately, all is not yet lost. A number of countries with coral reefs nearby are taking proactive steps to protect the reefs, eg the United States has a Coral Reef Task Force and there are coral reef initiatives such as the Coral Triangle Initiative. These are all aimed at addressing the threats to coral reefs, and if we work together, we can give them a chance to repair themselves and/or grow again.

If you are interested in learning more about protecting coral reefs, check out
© Susan Shirley 2015


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