Coral Reef Ecosystems

Coral Reefs—Part 1

Coral reefs are one of the most diverse and beautiful natural environments on earth. The world’s first coral reefs occurred about 500 million years ago, and the first close relatives of modern corals developed in the southern Europe about 230 million years ago. Coral reefs are extraordinary living geological structures. They are highly diverse and productive, but grow best in oceanic waters with very low levels of nutrients. Coral reefs can form only in relatively clear, shallow, warm waters, where the water temperatures range between 18-33c.

What makes a coral reef?

Corals belong to the group of animals (Cnidarians) that also include anemones, jellyfish, bluebottles and hydroids. This group of animals is quite simple, with a nervous system but no brain. The basic structure shared by all corals is the coral polyp. This is the basic building block of a coral colony. The polyp is a small tube-like structure, with a central space opening via the mouth. The mouth of the polyp is surrounded by six (or a multiple of six) tentacles. A coral polyp resembles a small anemone. A coral colony is made up of thousands of tiny little animals. A coral reef is therefore made up of millions of these animals.

Other types of animals and plants also contribute to the structure of the reef. Many types of algae, seaweed, sponge. Sediment and even molluscs like giant clams and oysters add to the corals’ growth at different rates, depending on water temperature, salinity, turbulence and the availability of food. The massive corals are the slowest growing species, adding between 5 to 25 millimetres (.2 to 1 inch) per year to their length. Branching and staghorn corals can grow much faster, adding as much as 20 centimetres (8 inches) to their branches each year.

How do corals get their colour?

Most coral polyps have clear bodies and their skeletons are white, like human bones. Most corals get their colour from the zooxanthellae inside them. Several million zooxanthellae live in just one square inch of coral and produce pigments. These pigments are visible through the clear body of the polyp and give the coral its beautiful colour.

Why are some corals hard?

In hard corals the polyp sits in a small limestone cup (corallite) that secretes and protects the soft polyp tissue. The limestone cup is in fact made by coral polyps extracting calcium from seawater. When the polyp grows upwards, it is divides into two. Each new polyp secretes a new corallite. Thus as the coral grows upward and outwards, the coral tissue lifts itself up and lays down new skeleton, forming something similar to tree rings. In most coral species, the new polyps remain joined together as they grow and food can be moved between the polyps through the coral colony.

How do corals eat if they cannot move around?

Inside a coral polyps clear outer tissues live microscopic algae called zooxanthellae. This is called a symbiotic relationship, where both parties benefit from their partnership. The algae transform sunlight and use polyp’s waste to make sugars through photosynthesis. The coral helps itself to some of the sugars enabling it to grow and the algae gain protection from predators inside the coral polyp’s tissues. The sugars can provide up to 98% of the polyps dietary requirements. This is why corals are found in shallow waters, as they require sunlight to survive. The zooxanthellae give the corals their brilliant colours of pinks, greens, yellows and browns.

How do corals reproduce?

Corals reproduce in two ways: asexually and sexually. Some corals divide to form individuals. This is known as asexual reproduction. Sexual reproduction takes place as a mass-spawning event, where polyps release millions of eggs and sperm, all at the same time and on the same night. This timing is linked to moon cycles. Polyps are either male or female or both male and female. After the eggs and sperm are released, they float to the surface. The fertilised eggs that escape predation by other animals hatch into larvae and drift with the plankton and currents. The tiny per cent that survives settles on the reef and begins new coral colonies.

The Coral Reef in Fiji

Perhaps the most sought after resource by resort goers. The Mamanucas boasts of coral reef systems that have grown to be resilient over the years, despite the many major impacts they receive from anthropogenic causes in the form of pollution, overfishing, destructive fishing practices, and natural causes in the form of cyclones, crown of thorns outbreak and high sea temperatures.

This is part 1 of 4 in the Coral Reefs series.