Like many coral species, Acanthastrea echinata is zooxanthellate, which means that its tissues contain large numbers of single-celled algae called zooxanthellae. The coral and the algae have a symbiotic relationship, in which the algae gain a stable environment within the coral’s tissues, while the coral receives nutrients produced by the algae through photosynthesis. By harnessing the sun’s energy in this way, corals are able to grow rapidly and form vast reef structures, but are constrained to live near the water surface (2).
While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from zooxanthellae photosynthesis, the coral may also feed on zooplankton (4). The polyps’ tentacles, which contain stinging cells called “nematocysts”, trap the drifting zooplankton, directing it into the central mouth, which also acts as an anus to excrete waste products after digestion (2) (4). Neighbouring polyps within an Acanthastrea echinata colony are linked by small tubes that distribute water and nutrients evenly, creating a similar rate of growth, and preventing competition for space (2).
Acanthastrea echinata is capable of both asexual reproduction, whereby the polyps proliferate through the production of clones, and by sexual reproduction, in which the polyps spawn large numbers of sperm and eggs. The fertilised eggs develop into planktonic larvae, which travel through the water column, before settling and metamorphosing into a sessile polyp (5).