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How do corals build calcium carbonate skeletons?

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A pangolin walks through grass with a blue circle around it and the word endangered written in white across the image. — Integument The pangolin is the only mammal in the world covered in scales.

Corals build their calcium carbonate skeletons through a continuous secretory process performed by the epidermis, specifically within a region known as the basal disk. As the coral polyp secretes this material, it progressively raises itself upon the growing stem of its skeleton.1

This skeletal formation is significantly enhanced by a symbiotic relationship with zooxanthellae, which are dinoflagellate algae living within the coral's tissues. These algae perform photosynthesis and aid the coral by removing metabolic byproducts, such as carbon dioxide, that would otherwise be excreted. By accelerating the removal of these metabolic products, the zooxanthellae increase the speed at which the coral can form its calcium carbonate skeleton. Through this process, colonial corals develop rigid, limestone-like structures that serve as the framework for coral reefs.23