The corals of the Great Barrier Reef: illustrated
THERE ARE MORE than 600 coral species in the Great Barrier Reef Marine Park, which covers 344,400sq.km of ocean. Corals are colonies of tiny polyps, which are animals related to sea anemones and jellyfish. A reef begins when a polyp attaches to a rock on the seabed and divides into clones. These connect, creating a colony that functions as a single organism.
Corals are either hard or soft. Hard corals, which have polyps that produce a calcium carbonate skeleton to protect and support them, are building blocks for reefs. Soft corals are flexible and lack a solid skeleton; they are instead supported by microscopic, spike-like ‘spicules’, designed to deter predators such as fish.
Hard corals have a symbiotic relationship with zooxanthellae algae, which provide them with food and colouring. High water temperatures cause this relationship to break down; the zooxanthellae are expelled by their host corals, and the corals starve and turn white in the phenomenon known as bleaching. Surveys carried out by the GBR Marine Park Authority and Australian Institute of Marine Science show that in 2016 more than 60 per cent of the Great Barrier Reef was severely bleached, leaving 22 per cent of corals dead. It was the worst damage to the reef yet recorded and raised questions about Australia’s environmental stewardship.
The coral species illustrated here are all found in the outer reef at Heron Island, at about 1–3m depth, as well as in other shallow reef zones of the Great Barrier Reef.
Organ pipe coral
Size: Up to 1m across
Most soft corals have limestone spicules that provide some structural support by slightly stiffening the soft tissues. In organ pipe coral, the spicules fuse to create a hard skeleton of pipe-like tubes, with each single polyp bearing eight feathery tentacles. Colonies of organ pipe coral can occupy large patches of reef. Inhabits shallow waters down to about 12m.
Size: Polyps can grow up to 10cm long and 2cm wide
Flowerpot corals are made up of many individual polyps joined together at the base of their skeletons to form branches, columns or dome-shaped colonies. The individual polyps are highly flexible and active, and constantly moving around and feeding. Their colonies can spread widely, growing many metres across. Found in upper reef slopes with low wave action, 5–25m deep.
Smooth cauliflower coral
Size: Up to 30cm across
Common in shallow reef areas, particularly those that are exposed to strong wave action, although it can occur to a depth of about 15m. It is a hard, branching coral with blunt, slightly flattened ends. Colour ranges from cream, pink or blue to greens.
Size: Up to 15cm across
Common in shallow inner reefs, but also found less frequently on the outer reef. This small, stony coral grows in a rounded hump shape. It prefers to grow in the absence of other species, although it can occasionally be found near algae or other corals. It uses symbiotic algae to photosynthesise by day and filter feeds on plankton by night.
Size: Up to 40cm across
A stony, reef-building coral that grows in either hand-like or tree-like colonies, with blunt, upright branches. It is covered in very small corallites that give it a rough, sandpaper-like texture. Like most other hard corals, it gets its energy from both the zooxanthellae that live within its tissues, as well as active carnivorous feeding. Found in shallow reef environments and mud flats.
Size: Domes of 1m or more
Like many other hard corals, honeycomb coral has a common name that describes exactly what it looks like. It grows in a large dome shape and is covered in a skeleton of corallites – the individual calcium carbonate cups in which polyps sit. These are packed closely on the surface in a honeycomb pattern. Inhabits all reef environments to a depth of 20m.
Common mushroom coral
Size: Up to 28 cm across
Unlike most corals, does not form colonies. Mushroom corals are large, free-living, solitary polyps that aren’t attached to the substrate. they are flat or dome shaped with wide, slit-like mouths. Young mushroom corals begin life on stalks and bear a striking resemblance to actual mushrooms. Found in shallow areas on the reef crest and flat, but mostly within cavities on the reef flat.