Final answer:
Coral polyps create hard calcium carbonate structures that accumulate over time to form coral reefs. These structures, made by corals in the phylum Cnidaria, support marine ecosystems and protect shorelines, but are at risk due to environmental changes. The correct option is b. Calcium carbonate
Step-by-step explanation:
Coral polyps make hard calcium carbonate structures, which build up over thousands of years to form reefs. These structures are vital for the development of reefs, as they provide support and protection for various marine life forms. Corals are marine invertebrates in the phylum Cnidaria, and they exist only as polyps. Corals capture plankton and other small organisms with their tentacles for nourishment. Many corals secrete a calcium carbonate exoskeleton that, over time, contributes to the formation of large, complex coral reef systems. These reefs provide necessary food and shelter to an array of ocean organisms and help protect shorelines from erosion.
Reefs are formed in shallow tropical waters where corals have a mutualistic relationship with algae that require sunlight for photosynthesis. The exoskeletons of both living and dead corals comprise most of the reef structure, offering protection to many species from waves and ocean currents. Unfortunately, coral reefs are globally declining due to various factors, including changes in ocean water temperatures that exceed the limits that the coral and their symbiotic partners have evolved to withstand. The correct option is b. Calcium carbonate