Coral reefs: Exploring the rainforest of the seas

Coral reefs are one of the most diverse ecosystems on the planet, and thousands of sea creatures call it home, from microscopic organisms to enormous predators. In fact, the coral is an entire colony of living animals in itself, because unlike plants, corals do not make their own food. They spread across the ocean floor and provide shelter and food for the reef’s other residents.

Tiny plantlike organisms called phytoplankton sit at the bottom of the reef food chain, getting their energy from the sunlight that penetrates the shallow water. They serve as food for larger zooplankton, which are in turn eaten by corals, sponges, sea urchins and a whole host of other creatures.

Several species of fish, mollusc and crustacean then take advantage of this buffet, before they become lunch for the sharks, rays and the marine mammals at the top of the chain. Each creature plays an important role in this delicate ecosystem, which requires a specific set of characteristics to survive.

As well as serving as a home and restaurant for an incredible array of creatures, coral reefs provide huge benefits for humans too. The underwater structures help to protect shorelines, absorbing 90 per cent of the energy from wind-generated waves. This significantly limits the damage caused by storms and erosion.

The fishing industry is also reliant on the reefs, as its inhabitants are a major food source for over a billion people worldwide and capturing them provides vital income for those living in remote areas. The trade of ornamental aquarium fish is also a big industry, and other creatures and plants found on the reef are important sources of medicines, used to treat a range of conditions from arthritis to cancer.

However, tourism is perhaps the biggest benefit as the millions of snorkelers and scuba divers that visit the reefs each year inject an estimated £6.2 billion ($9.6 billion) into the global economy.

Coral reef facts

Shallow reefs

Reefs typically grow best at depths of 18-27m (60-90ft), shallow enough for sunlight to reach the coral for photosynthesis.

Deep-sea coral

Some coral reefs can be found at depths up to 6,000m (20,000ft) but all their energy comes from consuming prey instead of photosynthesis.

Clear waters

Warm water between 21-29°C (70-85°F) is best for coral, as it contains fewer plankton and is clearer, allowing sunlight to penetrate.

Salty home

Coral can only grow in saltwater, so you’ll never find a reef in areas where freshwater rivers flow out into the sea.

Coral colours

Most coral species are yellow or brown, the colour of their symbiotic algae, but others contain colour pigments that help protect them from UV light.

Soft corals

Some coral polyps are joined together by a jelly-like tissue containing spiny skeletal elements instead of a hard skeleton.

types of coral reef

Types of coral reef

Protecting coral reefs

There are a number of factors threatening the world’s coral reefs. Global climate change is causing sea temperatures to rise and the water to become more acidic, and coastal agriculture, deforestation and urbanisation is resulting in an increase in sediment and other pollutants seeping into the ocean. Both of these factors are altering the coral’s habitat, causing it to expel its symbiotic algae and lose its main source of food in a process known as coral bleaching. Over- fishing is also disrupting the delicate reef ecosystem and nets and boats can often damage the coral too.

Many coral reefs now fall within marine protected areas (MPAs), with coastal and fishing-management strategies implemented in order to prevent further damage. Attempts are also being made to try to restore sick reefs with a process called mineral accretion. It involves submerging a metal structure that has a low-voltage electric current running through it into the water. The current causes natural minerals in the seawater to adhere to it and crystallise forming hard shells similar to that of coral.

These structures soon become home to fish and other marine life. Some scientists are also trying to crossbreed hardier species of coral and introduce tough new species into reefs with the hope that they can survive the effects of climate change.

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Plus, take a look at:

5 things you should know about the ocean

How do fish gills work?

Why do fish live in schools?