Corals are little animals called polyps. The coral polyp is an invertebrate belonging to the group of Cnidarians (jellyfish and sea anemones belong to the same group). Roughly there are 2 kinds of corals. In hard corals the polyps live in calcium stone housing, these are called the reef builders. And we have the beautiful soft corals, that don’t have the rock-like calcareous skeleton. Soft corals have wood-like cores for support and fleshy rinds for protection.

Corals have developed a smart system of survival over millions of years. They formed a ‘collaboration’ (a symbiosis) with algae, the so-called Zooxanthella. The coral polyp consumes the Zooxanthella, but the algae stay alive and live inside the polyp. The algae feed the polyp with approximately 90% of its needs (it differs per coral species). It does so by photosynthesis: the waste of the polyp’s feeding process and CO2 in combination with sunlight produces O2 and nutrients. Apart from using the excrements of the polyp, the algae also finds protection inside the polyp. The other 10% of the polyp’s diet is caught by the polyp’s tentacles hanging out in the water fishing for plankton or small sea animals like fish.

The symbiotic relationship between the Zooxanthella algae (plant) and the polyp (animal) is quite unique but also complex. The algae gives the corals its colours. Most corals live in groups of hundreds to thousands of genetically identical polyps that form a ‘colony’. The colony is created by a process called budding, where the original polyp literally grows copies of itself. Another way of reproducing is sexual reproduction. Once or twice yearly corals ‘spawn’, releasing sperm and egg cells. The sperm and egg cells have to find each other in the water which is made possible by the extreme precise timing of the spawning all the colonies seem to follow (it appears to be related to the lunar cycle). When so, the egg can become a coral larvae floating around in the ocean until it finds a suitable substrate to form its own colony. Another form of reproduction is that the female coral keeps the eggs that get fertilized by the spawning of sperm cells.

Growth rate hard corals

In general, massive hard corals tend to grow slowly, increasing in size from 0.5 cm to 2 cm per year. However, under favourable conditions (high light exposure, consistent temperature, moderate wave action), some can grow as much as 4.5 cm per year.
In contrast to the massive species, branching colonies tend to grow much faster, and under favourable conditions, these colonies can grow vertically by as much as 10 cm per year.

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