Like many corals, staghorn corals have a special symbiotic relationship with algae, called zooxanthellae, which provide the coral with energy through photosynthesis. The zooxanthellae live inside the tissues of the coral and, in return for providing the coral with energy, the coral provides the algae with protection and access to sunlight. This limits the depth at which Acropora horrida can occur as sunlight does not pass down through water below around 20 metres. Zooxanthellae are very sensitive to changes in temperature and pH (a measure of how acidic or alkaline the water is); significant changes in either can result in the algae being expelled from the polyps and the coral dying.
Like all staghorn corals, Acropora horrida is a reef-building species and is incredibly successful at this task. One of the reasons for this is its light skeleton which allows it to grow quickly and out-compete neighbouring corals (5).
Staghorn corals reproduce sexually or asexually. Sexual reproduction occurs via the release of eggs and sperm into the water. Most staghorn corals on the Great Barrier Reef sexually reproduce simultaneously, an incredible event that occurs soon after the full moon, from October to December. Streams of pinkish eggs are released from polyps on the sides of branches, to be fertilized by sperm released from other polyps at the same time. The water turns milky from all the eggs and sperm released from thousands of colonies. Some of the resulting larvae settle quickly on the same reef, whilst others may drift around for months, finally settling on reefs hundreds of kilometres away (5). Asexual reproduction occurs via fragmentation, when a branch breaks off a colony, reattaches to the substrate and grows (6).