How do corals reproduce?

Plate Acropora spawning Photo: AIMS

On the Great Barrier Reef, many coral species spawn once each year in the annual mass spawning that takes place in spring or early summer. In this event, many corals release eggs and sperm into the water on a few nights of each year, linked to the phases of the moon. The most common timing is 3-5 days after the full- moon in late spring/early summer (November or December).

The eggs are fertilised in the water to produce embryos or larvae. A smaller number of coral species produce larvae at other times of the year. Larvae are about 1-2 mm long and can swim short distances, but are usually moved by water currents.

Coral spawning Photo: AIMS

Coral larvae are carried in the water from a few hours up to several weeks before they are mature enough to settle onto the reef and start a new coral colony. In that time, some species of corals may settle within metres of the parent corals, but most species will travel long distances, from hundreds of metres to tens of kilometres.

When they are ready to settle, coral larvae move down through the water column until they touch the reef surface. They are susceptible at this stage to being eaten by one of the many hungry reef organisms, including other corals! If they find a suitable empty space that is not occupied by another animal, they will attach to the reef surface and begin to lay down the base of their skeleton. At this time, they are still about 1 - 2 mm across, and will have just one polyp. This single polyp will continue to divide throughout the life of the coral to produce a colony- all derived from the single settling larva.

Newly settled corals are 1-2 mm in diameter Photos: Vicki Harriott