Like many corals, staghorn corals such as Acropora granulosa have a special symbiotic relationship with algae, called zooxanthellae. The algae gain a safe, stable environment within the coral's tissues, while the coral receives nutrients produced by the algae through photosynthesis. While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from photosynthesis by zooxanthellae, the coral may also feed on zooplankton (3).
Acropora granulosa and its zooxanthellae are very sensitive to changes in water temperature and acidity. Any increase in water temperature greater than one or two degrees Celsius above the average can stress the coral and cause ‘bleaching’, a phenomenon in which the coral expels it zooxanthellae and turns white (3) (7).
Staghorn corals are reef-building, or ‘hermatypic’ corals, and are incredibly successful at building reefs for two main reasons. Firstly, they have light skeletons which allow them to grow quickly and out-compete their neighbouring corals. Secondly the corallite of a new polyp, is built by specialised ‘axial’ corallites. These axial corallites form the tips of branches and, as a result, all the corallites of a colony are closely interconnected and can grow in a coordinated manner. This means that by harnessing the sun's energy, staghorn corals are able to grow relatively rapidly and form vast reef structures, but are constrained to live near the water surface (3).
Very little is known about the specific reproductive biology of Acropora granulosa, although it is likely to be able to reproduce both sexually and asexually. Asexual reproduction occurs via fragmentation, when a branch breaks off a colony, reattaches to the substrate and grows (3). Sexual reproduction occurs via the release of eggs and sperm into the water. On the Great Barrier Reef, most staghorn corals appear to sexually reproduce simultaneously (8), an incredible event that usually occurs on just a few nights soon after the full moon, during one or two specific months of the year (8) (9). Streams of pinkish eggs are released from corallites on the sides of branches, to be fertilised 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. The eggs and sperm form slicks on the surface that can be up to a few kilometres in length, making them visible from the air (10). Some of the resulting larvae from these mass spawnings settle quickly on the same reef, whilst others may drift around for months, finally settling on reefs that may be hundreds of kilometres away (3).