Australia Reef Ecosystems

This page is an extract of a script that is part of the video series
"The Living Landscape-an Australian Ecosystems Series".
Visit all ecosystems.

REEF ECOSYSTEMS

A coral reef is composed of calcium carbonate, or limestone, derived from the water by the reef organisms: colonies of coral polyps and coralline algae. Most of this structure, the underlying foundation of the reef, is dead, made up of layer upon layer of coral skeletons.

LIVING CORAL POLYPS
The living reef is just a veneer, but it's this living part that continually adds new limestone to the massive base structure. Coral is the building block for this reef construction. Though coral looks like a plant, it's really an animal: or rather, a colony of animals that belong to the cnideria (the same group as jellyfish and sea anemones). There are a multitude of different kinds of coral, about 350 species including both hard and soft varieties, on the Great Barrier Reef. Their shapes are very different, and their colours come in the hundreds.

SOFT CORAL

HARD CORAL

Coral animals are called polyps: tiny, primitive marine organisms. Each coral polyp is an individual organism, and the reef is made up of colonies of these organisms.

At certain times of the year, a strange event occurs on the reef - a synchronised mass spawning. Sperm and eggs are released by the parent polyps, and the fertilised eggs drift in the water and develop into larvae. Millions upon millions of larvae are produced.

CORAL LARVAE UNDER A MICROSCOPE
It takes this huge production of sperm and eggs to ensure that just a small percentage of larvae will survive. The predators are overloaded - but even so, almost all larvae are eaten. The timing of the spawning is crucial. Tide and water temperature need to be just right, to ensure larvae survive. Those larvae that survive attach themselves as quickly as possible to anchorages like rocks or dead coral, and start to build new colonies. The polyp builds its own skeleton by secreting calcium carbonate.

ALGAE INSIDE CORAL TISSUE
Corals are frequently the dominant group on the reef because they have evolved to overcome the problem of low energy levels. They draw most of their energy from photosynthesis, from the algae that live in the tissues of corals, and supplement by feeding on microscopic animals that they catch with expanding tentacles. The tentacles contain stinging cells that immobilise the prey, which is then drawn to the polyp's mouth. The coral polyps feed, and grow, and eventually they die, and become part of the base structure of the reef.

COLOURFUL FISH HIDE INSIDE STAGHORN CORAL
Most of the animals on a coral reef spend their entire lives in and under the water of the reef.

GREEN TURTLE LAYING EGGS AT NIGHT
The green turtle is one that does come ashore, but it's only the female that does so, and she does it for a very special reason. To lay her eggs.

FEMALE GREEN TURTLE
An adult green turtle weighs about 180 kilograms, and is around a metre long. Trekking up a sloping beach, probably quite close to the spot where she was hatched herself, is not easy. When she reaches her selected spot, she'll lay up to 120 eggs, and then go back to the sea.

Turtles TURTLES HATCHING AT NIGHT
Six to seven weeks later, when the eggs hatch, the babies are on their own. The hatchlings make a dash for the open sea, usually at night, to avoid gulls and other predators. Little is known about turtles between hatching and maturity. In 10 to 40 years, as few as one in 1000 of the hatchlings may return to the beach to breed. Female green turtles could be anywhere between 10 and 40 years old when they first breed, and they often return to their own island to lay their eggs.

BACK TO WILDLIFE PAGES