Coral reefs are found in shallow tropical oceans and seas around the world. They are the most diverse and complex marine ecosystems, with an estimated 90% of the species in less than 7% of the total marine area. They are the rainforests of the marine world.
The Phylum Cnidaria contains the coral animals that proved the structure of the reef. Jellyfish and sea anenomes also populate the reef and belong to this phylum of simple animals that have tentacles and stinging cells, but it is the coral animals that build the calcareous structures that provide shelter and food for so many other species. Coral animals are upside down jellyfish with the bell at the bottom and their tentacles waving out of their tiny homes. Each piece of living coral is home to thousands of individual coral animals. They are filter feeders, but the waters of the reef are nutrient-poor so they also get food from their resident symbiotic algae, called zooxanthellae. These single-celled algae get protection from the corals and in return, make sugars by photosynthesis and as a sideline, provide the wonderful colours to coral reefs. Corals come in two basic types: hard and soft. Soft corals are pretty but not as important structurally as the hard corals. Hard corals are grouped by their shapes: staghorn corals, brain corals and plate corals are three common types.
Sponges, phylum Porifera, are also well represented in reef communities. These strange, primitive animals look more like plants but, like the corals, are communities of individual sponge organisms within the sponge structure. Sponges are also filter feeders.
The reefs are home to a multitude of marine worms, representing several phyla. Flatworms belong to the Phylum Platyhelminthes, have a flattened body and come in many beautiful colours. Polychaete worms are segmented and belong to the Phylum Annelida. Their larvae are free-living but they settle among the corals and make calcareous tubes. They often have brightly coloured and feather-like feeding tentacles which they quickly withdraw into the safety of they tubes when disturbed. Ribbon worms (phylum Nemertea), are also called gordian knot worms because they can tie themselves in knots. They are common but inconspicuous. The last group of worms found commonly on reefs are Acorn worms that burrow in sand and silt bottoms. They are a close relative of Chordates and belong to the phylum Hemichordata.
There are three major invertebrate phyla that contribute a large number of reef species: Mollusca, Crustacea (Arthropoda) and Echinodermata. The Molluscs are the sea shells, the nudibranchs and sea slugs and the squid and octopus. There is a mind-boggling diversity of shelled molluscs on the reefs. The most common types are gastropods, who have a single shell and a muscular foot. Limpets, turban shells, periwinkles, scorpion shells, moon shells, cowries, helmet and trumpet shells, murex shells and corals shells, dog whelks, volutes and cone shells are all gastropods. They are mostly herbivores, grazing on algae and sea grasses on the reefs. The cone shells deserve a special mention because they are poisonous. It's not a good idea to pick up beautiful shells on the reef as the sting of a cone shell is painful and potentially fatal. The second main group of molluscs are the bivalves: oysters, mussels and clame. They are burrowers and filter feeders. The most impressive reef bivalves are the giant clams, whose lips are brilliantly coloured by symbiotic algae. There are also primitive chitons on reefs, usually on hard surfaces in the intertidal zone. These molluscs are distinguished by having a shell of 8 overlapping plates and they are herbivores.
The cephalopod molluscs, squid and octopus are important predators on the reef. They depend on camouflage and quick movement to avoid being eaten themselves as they have lost their shells secondarily in the course of evolution. Octopi can be particularly difficult to spot as they blend into the corals while waiting for their prey.
The Crustacea are the insects of the oceans. They are Arthropods and so have jointed legs and an exoskeleton that must be shed before they can grow. Common reef crustacea include lobsters, shrimp, crabs, barnacles and many smaller organisms that float in the plankton. The crabs, lobsters and shrimp are predatory while the barnacles are filter feeders. Barnacles will attach to anything, even the heads of green turtles. They were originally classified as molluscs because of their shells but a closer look revealed that their 'tentacles' are actually jointed legs and their larval forms show their arthropod origins.
The last great invertebrate phylum that flourishes on the reefs is the Echinodermata: starfish, brittle stars, sea urchins and sea cucumbers. The starfish and brittle stars are slow motion predators, moving about on their tube feet and opening molluscs for their dinners. Sea urchins and sea cucumbers are herbivores. Sea cucumbers are particularly common on sandy inner reef areas, sometimes contributing a huge amount of biomass to that area. Sea urchins come out at night to graze on algae and other encrusting organisms on the reef structure such as sponges. They have horny teeth to scrape the food off. During the day they wedge themselves in rocks and crevices.
Much of the life and colour of a reef is provided by the vertebrates that live there, particularly the fish. Tropical reef fish have been described as the heart of the reef and they are one of the main reasons that reefs are so popular for diving and snorkelling. There are thousands of species grouped into ten main sorts of reef fish: damselfish (family Pomacentridae), wrasses (Labridae), angel and butterfly fish, cardinal fish, groupers, parrotfish, surgeionfish, blennies and gobies. My favourites are the parrotfish, beautifully coloured in irridescent shades of blue, green, yellow, pink and purple and equipped with parrot-like beaks for chewing coral up. They digest the hapless coral animals and poop out pure reef sand, that sparkles in the sunlight as it floats down to become part of the sandy spaces between the corals.
The fish are hunted by sharks and rays, the chondrichthyes or cartilaginous fishes. Many species of sharks hunt the reefs including tigers and hammerheads, blacktip and whitetip sharks and leopard sharks. Manta rays and eagle rays float over the reef, scooping up plankton and the sandy bays hide many types of bottom dwelling rays such as the shovel nose and the electric ray. Most people are more afraid of sharks than rays but Steve Irwin showed that rays, with their spiny tails, can also be dangerous.
The other vertebrates found on reefs include two types of reptiles: sea snakes and sea turtles. There are seven species of sea turtles, all now endangered. Some of the most moving experiences on a coral atoll are witnessing giant green turtle females dragging themselves ashore to lay their eggs or watching baby turtles make their mad, mostly doomed dash to the sea after hatching. Sea snakes are less obvious but potentially more dangerous. They are wonderfully adapted to life in the sea, brightly coloured to advertise their venomous nature and with flattened tails for swimming. Still they must surface regularly to breathe, just like the turtles.
Sea birds don't live under the water but they are an important part of the reef ecosystem. Reef herons hunt in the shallows. Gulls and terns rest on the beaches. Boobies and other birds nest on the islands. Their guano contributes nutrients to otherwise poor soils.
Last but definitely not least are the whales. Each year Humpback whales migrate from the feeding grounds in the Antarctic to the Great Barrier Reef to play, relax and give birth to their babies. Watching them frolic in the warm waters is another wonderful reef experience.
Reference: Allen and Steene, Indo-Pacific Coral Reef Field Guide