What are corals/types of coral?

What are corals/types of coral?

Corals are two-layered invertebrates that live in groups (i.e. they are colonial) and are related to jellyfish and sea anemones.

Corals are made up of tiny individuals called polyps. Each polyp is like a fl uid-fi lled bag

with a ring of tentacles surrounding its mouth, and looks like a tiny anemone. Polyps within a colony are linked by living tissues and can share their food (Allen & Steene, 1994). In

some corals, the polyp extracts calcium carbonate from the sea and secretes it as a cup of calcium carbonate from the bottom half of its body. These cups provide anchorage

for the polyps but when threatened, the polyp can retreat into the safety of the hard cup. When the calcium carbonate cups of many billions of these polyps fuse together, they

form coral reefs (Veron, 2000).

There are two main types of corals (1) Stony Corals and (2) Soft Corals.

(1)Stony (Hard) Corals:

Some stony corals obtain their food from one-celled organisms called zooxanthellae.

Zooxanthellae are single-celled organisms that use sunlight for photosynthesis and transfer 95% of the food they produce to coral polyps. Both coral and the zooxanthellae benefit from this association. The zooxanthellae receive protection from currents and herbivores, as well as some nutrients from waste produced by coral polyps. This kind of association - where two different kinds of organisms benefi t from each other - is called a mutualistic association. These corals are called hermatypic corals. Individuals polyps of hermatypic corals secrete calcium carbonate (limestone) skeletons which, in time form coral reefs. Therefore, hermatypic corals

are also known as reef building corals.

Because of this association with zooxanthellae that need sunlight to produce food, hermatypic corals are dependent on sunlight and only grow in clear shallow waters less

than 60m deep, which have a temperature range between 25° and 30°C. Hermatypic corals prefer narrow salinity and low turbidity ranges. Therefore, hermatypic corals need

(a) a particular range of temperature;

(b) sunlight;

(c) generally clear water (low turbidity); and

(d) a narrow range of salinity (Allen & Steene, 1994).

There are about 845 species of reef-building corals (Global Marine Species

Assessment, 2008).

There also are some stony corals which do not have zooxanthellae and do not build reefs.

These are called ahermatypic corals and can live in both shallow and deep water (some up

to 6,000m deep).

Stony corals have different shapes and forms. Some selected shapes are shown on these

facing pages.

Foliaceous (forming a whorl)

Massive

Tabular

Branching

Columnar

Digitate (like fingers)

Encrusting

Mushroom

             (2) Soft corals:

Soft corals lack a calcium carbonate skeleton, hence their common name. However, in their bodies are tiny hardened calcium particles called spicules that provide support.

Some selected soft corals are shown below.

Black or Thorny corals

Fire and Lace Corals

Sea fans

Sea whips