Corals

Corals are animals. Some species have separate sexes; others are both sexes in one. All produce eggs and sperm. During the annual spawning, corals release thousands of eggs and sperm, which rise to the surface forming slicks of pink goop. This occurs between sunset and 11pm, 4-6 days after the full moon in October, November and or December.

The floating slick is basically a soup of sperm and eggs, which hope to meet and successfully fertilise. Sperm will live several hours and eggs only 3-6 hours. The morning after, fertilised eggs have developed into embryos. The floating embryo slick will only last 12 hours before disintegrating. One day after fertilisation, the embryo develops into a little worm-shaped juvenile, a planula. Although ready to swim down to the reef in search of a home only four days later the planula can probably survive up to 20 days floating at sea.

The planula searches for a place to settle on the reef and probably chooses the site using a combination of small chemical cues, light intensity, water pressure and surface texture. The planula glues its bottom down to secure its new home and within hours of moving in transforms itself into the adult form, a polyp.

Some coral mums equip their eggs with zooxanthellae, or plant cells. Species without this "packed lunch" must pick up plant cells from the water around them. These are usually obtained within 14 days of settlement.

24 hours after settlement, the polyp, with the help of its plant cells, begins to lay down it's hard, calcium carbonate skeleton. After several weeks or months, this skeleton is a small 2-5mm cup shape visible to a searching human eye. The coral polyp will divide into two, genetically identical, polyps after just 14 days. The polyps continue to divide and at six months, the little colony could consist of at least half a dozen individual polyps and be 5-10mm in diameter.

A one year old colony can be anywhere from 4-25mm in diameter and contain between 1-50 polyps. Over the years, the polyps continue to divide, the colony continues in size and within 2-3 years begins to take on a shape that is representative of its species.

To identify the various coral species you only need to look at their shape. In most cases the common name predicts what shape they are. Corals, both soft and hard, come under a common family name, and there are many types with in that family group.

You will often hear the term Bommie used when discussing dive and snorkelling sites. The word is derived from the French bombax and is sometimes spelled bombie, but more commonly bommie. The term is not normally used in scientific papers but is usually understood to describe anything from a single coral boulder to a large reef structure. Technically it is a small patch reef in column form in, usually, a sheltered lagoon. It is attractive to divers because of the diversity and abundance of marine life that it attracts.