Coral Mystery pt IX Biology by Dott.ssa Margherita Superchi

They also studied the “foods” preferred by the polyp colonies. And do you know what they like to eat? Shrimp! Naturally, tiny, microscopic shrimp contained in the plankton!
Today we can say that we know much more. But let’s get back to the structure of the polyp.
The polyp is the structural unit of the branch: viewed in its entirety, it is shaped like a delicate white flower, transparent, with its eight petals swaying in the current and indicative of how and why its animal nature has for so long been ignored.
It is the polyps that provide nutrients to the colony, using their long tentacles to capture tiny particles of plankton fluctuating in the water and by swallowing dissolved substances also found in the water.
These substances circulate internally by internal secretion and cause the axial skeleton to grow. In fact, if you cut through a branch you can actually see the concentric circles of growth.
The times of opening/closing of the polyps have been studied and apparently they respond to the demand for food that comes from the colony: the hungrier it is the longer the polyps remain open.
The growing times of a colony vary from zone to zone, subject to subject and are closely linked to the quantity of food present in the water as well as the number of other colonies present within the same area.
Just think that in the zone of Portofino, on a single bank, in a surface area of just one square meter, we counted more than one thousand colonies.
Given the crowding it is clear that it would be highly difficult for any of the coral branches to reach any great size. To give you an idea, think of a forest: wherever the trees are overly abundant they rarely grow tall and robust. But if there are only a few, and if they manage to feed well, then they can become splendid corms.
Since the causes of growth are too numerous and too diversified, I must confess that studies of such aspects have yet to provide universally valid results. Considering the numerous studies that have been undertaken (especially HARMELIN) we can risk giving a vertical growth rate of 6-7 mm per year for young colonies and 3-5 for colonies with secondary ramifications.
Why does coral ramify? In my modest opinion, in order to collect more food by having multiple mouths in different positions.
Why are there branches that are nice and smooth and others that are, well, rather contorted?
That depends on where the colonies are located and on currents. A unidirectional current will give a nice smooth coral (like the ceiling of grottoes); a variable current provides a rougher coral, rather like a bush.
In order to anchor itself to the bank, coral needs a rigid substratum. Naturally it prefers zones that have a coral substratum, formed of calcium concretions but it can also attach itself to mud and sand.
We should also keep in mind that coral grows where there is little or no light. So that in shallow areas it grows wherever there are grottoes or ravines while at greater depths it can be found in open areas.
Another important fact is that coral prefers waters that are not necessarily calm but where the marine currents do not cause violent eddies or whirls.
Inversely, a total absence of marine currents does not allow coral to live as there is no “passage” of food. As for pollution, if there is little contamination coral tends to grow while it cannot live at all if the waters are contaminated.

to be Continued...
[text taken with license from the author Giuseppe Rajola from the book Sciacca Mystery]

Enjoy!