Corals

Corals typically form colonies of genetically identical polyps. Each polyp is a living organism that cannot survive independently and must live alongside others to build the massive and complex structures known as coral reefs. These structures are formed from calcium carbonate secreted by the polyps, which over time accumulate into coral reefs.

Hard corals (Scleractinia) capture suspended particles from the water through filter feeding. Using short tentacles and hair-like appendages, they direct plankton, bacteria, and fine organic particles toward their mouths. This process reduces turbidity and suspended matter around coral reefs, thereby enhancing water clarity.

Although corals cover less than 0.1% of the ocean’s surface, they provide habitat for more than 25% of all marine species. For this reason, these ecosystems are often referred to as the “rainforests of the sea.” They play a crucial role in maintaining biodiversity, protecting coastlines, and supplying food resources for millions of people.

The oldest coral fossils date back more than 500 million years. Early forms of corals, different from those living today, went extinct about 225 million years ago. Modern corals are abundant in tropical regions. Since many corals live in warm, shallow marine waters, their fossils serve as excellent indicators of past environmental conditions. Additionally, some corals are used today in the production of ornamental gemstones.

Coral bleaching is one of the direct consequences of global warming. This phenomenon occurs when corals lose their symbiotic algae, known as zooxanthellae. These algae are responsible for coral coloration and provide a significant portion of their energy through photosynthesis. When expelled, corals lose their color and appear white. In recent years, large-scale bleaching events have been reported worldwide. For instance, in 2024 and 2025, an unprecedented marine heatwave along the western coast of Australia caused severe coral bleaching and resulted in the death of over 90% of corals.

Moreover, by extracting calcium carbonate from seawater to build their skeletons, corals reduce dissolved calcium levels and thus indirectly contribute to stabilizing seawater chemistry.