Scleractinian corals

Corals are marine animals of the Cnidaria phylum (Verrill, 1865), characterized by the presence of stinging cells called cnidocysts, and of the Anthozoa class (Ehrenberg, 1834), characterized by a secondary symmetry superimposed on the radial symmetry of Cnidaria. The “hard” corals of the order Scleractiniaria (Bourne, 1900), which belong to the subclass Hexacoralliaria, are unique in that they have 6th-order symmetry (or a multiple of 6), in addition to bilateral symmetry.

Simplified phylogeny of scleractinians (from World of Register Marine Species). Source : Marine Canesi – PhD.

(A) Photograph of a coral colony of the Porites genus, (B) top view of a polyp, and (C) transverse view of a polyp and (D ) enlarged view of the unicellular microalgae (zooxanthellae) located in the polyp tissue. Source : Marine Canesi – PhD.

These corals are made up of individuals called polyperites, each composed of two distinct parts, one living (tissue), the polyp, and one mineral (exoskeleton), the polypier. The living part, called “tissue”, is made up of one or more individuals, the polyps, whose size varies from a millimeter to several centimeters, depending on the species. These organisms, like most animals, breathe, absorbing oxygen and releasing CO2, and mostly live in colonies, where each individual performs the same function. The animals or polyps continuously secrete calcium carbonate (CaCO3) to build a calcareous calcareous aragonite skeleton (mineral part). In fossil corals, only the polyps are preserved and can form calcareous concretions dating back several hundred million years (Upper Jurassic). In some species, notably those studied in this study, polyps, live in symbiosis with unicellular microalgae, dinoflagellates (commonly known as zooxanthellae), which live in their tissue. This close association provides zooxanthellae a “habitat”, which in turn provides the coral with additional energy supply to the coral. Whether or not they live with zooxanthellae, all corals corals feed on organic constituents (plankton, for example), thanks to the polyps. In most cases, corals are either strict heterotrophs or mixotrophs, i.e. able to benefit from symbiosis with algae while capturing prey.
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