The Great Barrier Reef has so many animals that it is impossible to cover them all here. Fifteen hundred fish species live at the northern end of the Reef in the warmest tropical waters,. Even at the bottom end of the reef there are still over five hundred species of fish. The number of invertebrate species, animals without backbones, is still unknown and must conservatively be in the tens of thousands. Besides the fish and the invertebrates there are also a number of other vertebrate species, including many birds, reptiles and a few mammals.
The basis of the reef ecosystem is the almost invisible coral animal. The coral structures are obvious but the tiny animals that inhabit the plates, staghorns and brain corals are not. These almost microscopic jellyfish-like animals live in the holes in the coral structures, where they feed, divide and create the coral structures by secreting limestone. Within their tiny bodies live the symbiotic algae that create the incredible colors of the corals. It takes thousands of coral animals to make one coral so the total number of coral animals needed to create the entire Great Barrier Reef is mind bogglingly large. Overall there must be billions of the little creatures for this structure to exist and billions more that have lived and died in the past to create the entire structure.
At the opposite extreme are the giants of the ocean, the Humpback whales, who come to the tropical waters to give birth to their young. Other large animals that are sometimes seen in these waters are the salt water crocodile and the gentle dugong or sea cow. None of these species are true reef animals. Crocs inhabit estuaries and salt water rivers inland of the reef. Dugongs feed on sea grass beds closer to the mainland and humpbacks do not feed on the reef because the water is nutrient poor and thus does not support the great plankton blooms found in temperate and polar waters.
When diving on the reef, one has the impression that there are almost no plants. The reef is teaming with animals but what is the basis of the food chain? There have to be producers somewhere. The answer is in the bodies of so many reef animals but in particular the coral animals and the clams. There reside the zooxanthellae, the symbiotic algae that keep the corals alive by feeding them sugars produced by photosynthesis. The little corals do their bit too, filtering the water for waste products and building the shelters that help keep them both safe, but without the unseen algae this great reef system could not exist..
The coral animals are fed upon by many of the reef's most beautiful fish such as the parrot fish, which have a parrot like beak that they use to scrape the coral animals from their burrows. The fish swallow both coral and animal, digest the soft bits and defecate pure coral sand.,thus contributing to the production of the sand that lie between the corals and eventually build up into coral cays.
The coral cays are breeding grounds for sea birds and sea turtles. Green and loggerhead turtles come ashore each year to lay thousands of eggs. The hatchlings form an important if seasonal food source for fish, sharks, birds and even crabs but a few survive to return to the beaches as adults and continue the cycle. Watching the ponderous females pull themselves ashore to dig great holes and lay their eggs, or watching the swarms of hatchlings rushing down to the sea are among the special experiences that a visit to the Great Barrier Reef allows.
The Reef has its own special bird: the Reef Egret, also mistakenly called a heron, which comes in two color morphs: white and steely-grey. They look different enough to be separate species but are not and they live together in harmony. They can be seen hunting small fish on the inner reefs surrounding coral cays such as Heron and Lady Musgrave Island. Another common bird on these islands is the Buff banded Rail, a funny little chicken-like bird which is almost flightless. On the mainland it is a shy and secretive bird. On the islands they have taken a liking to human foods and turned into cheeky thieves. If there are no humans around to rob, they live in the forests and scratch a living from the leaf litter.
Terns and gulls, shearwaters and boobies can be found on and around the Barrier Reef Islands, as can great white-breasted sea eagles. The terns and shearwaters spend most of their lives at sea but come ashore on the coral cays to nest: terns in the trees and shearwaters in burrows beneath the tree roots.
However the life on the islands is much less diverse than on the surrounding reefs. A dive on a reef reveals a bewildering variety of fish. The butterfly fish alone come in scores of varieties, colors and patterns. Parrot fish and butterfly fish, rabbit fish and blennies, wrasses, eels and clown fish, damsel fish, sea horses and angelfish, cardinal fish, groupers and basslets, surgeon fish, gobies and trigger fish: The fish of the reef are like the butterflies of the sea.
Why are they so colourful? There are numerous answers to this question. First of all is species recognition. With so many species of fish around it is necessary to be able to pick the right mate. Mating with the wrong fish could result in no offspring. Colors also tell male from female and immature from adults in prime breeding condition. They indicate healthiness and strength to a potential mate and can be used to scare off rivals as well. Poisonous fish use their colors and patterns to warn potential predators. Some patterns act as camouflage among the brightly colored corals, sponges and anemones. The simplest answer is because they can see color and respond to it so it forms an important aspect of reef fish lives.
As well as the bony fish, the reef is also home to many species of sharks and rays. The inner reefs are feeding and breeding grounds for many species of rays, which are social animals and often come together in large groups. Shovelnose rays and cow tailed rays are among the most common. Further out one may be lucky enough to see manta rays scooping up what plankton can be found in the clear blue waters. Grey reef sharks, black tips and white tips are all commonly found on the reef as well as the occasional more dangerous tiger shark. Scary as they can be, it is a real thrill to see a shark while snorkeling on the reef and know that this is their home while we are only visitors.
A lot of smaller creatures call the reef home too. There are numerous species of sponges, worms, jellyfish and anemones, starfish, sea urchins and sea cucumbers. The sea cucumbers are especially numerous on the inner reef areas of coral cays and are a dominant life form on the sands between the corals. They come in a variety of shapes and colors and spend their life gobbling up sand, digesting whatever is edible in it and pooping out long strings of waste material. Starfish are the slow motion predators of the reef, crawling over the surface to eat corals in the case of the crown of thorns or, more commonly, the many types of mollusks that live on the reef.
There are as many beautiful shelled molluscs as there are fish on the Barrier Reef. Some, the cone shells, charge a high price of people who pick them up as they have a poison dart which can inject a lethal dose into the offending hand. For this reason alone it is recommended that snorkelers look but don't touch. As well, these shells, including the large clams, are protected within the park boundaries. Over-collecting in the past has led to the disappearance of many beautiful species as well as contributing to imbalances in the system. Crown of thorns starfish were not a problem until their main predator, the giant triton, was over-fished for the shell collectors market. Seeing a large shelled mollusk steaming along the bottom with its mantle outspread and its tentacles waving is so much nicer than seeing one dead on a shelf.
The best thing about a reef is that the animals are so visible. Rainforests are as diverse but it is hard to see much of the life in the dense vegetation. Not so on the Barrier Reef. Here, the fish sport in front of the beautiful corals while sharks and sea turtles glide lazily by the fascinated snorkeler. It is one of the most accessible ecosystems in the world for studies of animal behaviour and certainly one of the most beautiful.