32 Australian GeographicTHERE ARE MORE than 600 coral species in the
Great Barrier Reef (GBR) 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 relation-
ship 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 car-
ried out by the GBR Marine Park Authority and Austral-
ian Institute of Marine Science show that in 2016 more
than 60 per cent of the GBR 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 GBR.NATURE WATCH
Coral reef scene
Three-quarters of the world’s 798 coral species can be found on Australia’s
Great Barrier Reef, and they come in a huge variety of shapes, sizes and colours.STORY BY SOFIA CHARALAMBOUS ILLUSTRATION BY ROBERT KAYGANICH1TABLE CORAL
Acropora cytherea
Size: Up to 3m across
Table coral is a hard, stony
coral that grows horizon-
tally in wide, flat, table-like
formations that are thin
and finely structured. The
‘tables’ are made up of
lots of short, cylindrical
branches that grow
outwards and link together
in the centre of the plate,
forming a mass of solid,
joined branches. Found
on shallow reef slopes
and lagoons in tropical
environments at
depths of 3–25m.HONEYCOMB
CORAL
Diploastraea sp.
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 environ-
ments to a depth of 20m.FINGER CORAL
Montipora digitata
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, sandpa-
per-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.232341