Six families of corals make up the vast majority of modern day reef composition and out of these six the Acropora coral is by far the most numerous, diverse and arguably also the most successful. This coral family has been the backbone of reef structures across the Indo-Pacific and Atlantic areas for over 60 million years, erecting huge calcium carbonate construction that support entire reefs.
The skeletal structure of the Acropora coral is that of an open “synapticular” framework supporting a polyp, facilitating rapid growth while at the same time minimizing the required amount of calcium carbonate to increase efficiency. This has allowed them to dominate in terms of species variety. The skeletons themselves are made up of the crystal aragonite form of calcium carbonate, in various shapes and sizes dictaded by the individual species.
Corals are colonial organism comprised of thousands of individual polyps. These polyps have a simple structure, with a “mouth” encircled by tentacles which capture plankton for food, clean away debris and protect them. Inside the polyp’s gastrodermal cells, most corals have symbiotic algae called zooxanthellae, which provides them with the building blocks (CaCO3) needed to contruct the reefs.
The species of the Acropora coral reproduce both sexually and asexually. Their polyps are hermaphrodites (meaning they are both male and female at the same time), and they develop gemetes (eggs and sperm) with fertilization taking place externally. The fertilized eggs drift through the water for some days until finding a suitable spot to establish a new coral colony. Asexually they reproduce through mesenteries, fragments of coral that break of and settle somewhere else, starting a new colony.
While corals generally exhibit a very wide range of shapes and sizes, most can be classified into then general forms:
1. Branching corals with a few primary branches with many secondary ones;
2. Digitate: several large finger-like clumps with no secondary branches;
3. Table corals;
4. Elkhorn corals, which have large, flattened branches;
5. Foliose corals have wide plates on several levels;
6. Encrusting corals, which grow as a thin crust over the substrate;
7. Submassives have several large wedges of columns rising form an encrusting base;
8. Massive corals with their ball-shaped forms of widely different sizes;
9. Mushroom corals;
10. Cup corals, which look like squashed, elongated cups.
Acroporidae have the largest number of species of all corals, and so exhibit most of the above forms. However, despite their incredible diversity, the Acropora coral is extremely succeptible to diseases and changes in its habitat, which have even led to localized extinction – threatening the very existence of coral reef across the globe.