Final answer:
Hard corals, or hermatypic corals, create coral reefs by depositing calcium carbonate skeletons over thousands of years. Reefs are currently threatened by bleaching due to warmer temperatures and ocean acidification from rising CO₂ levels, impacting coral health and the marine life dependent on these ecosystems.
Step-by-step explanation:
Calcium builds up over the years from coral colonies to form coral reefs. Hard corals are also known as hermatypic or reef-building corals. These marine invertebrates belong to the phylum Cnidaria and are crucial to reef creation due to their ability to deposit calcium carbonate skeletons. Coral reefs are essential ecosystems providing food and shelter to a diverse array of marine life.
Global Decline of Coral Reefs: Unfortunately, these vibrant ecosystems are under threat. The hard calcium carbonate skeleton of coral reefs is damaged by a process known as bleaching due to rising ocean temperatures. Additionally, increased CO₂ levels lead to a change in the pH of water, which interferes with the process of calcification necessary for reef building. This has a cascading effect on the various marine animals that rely on coral reefs for food and shelter, as their numbers also decline.
Corals live in large colonies in shallow tropical waters because they have a mutualistic relationship with photosynthetic algae that require sunlight. This mutualism is vital for their survival, as the algae provide essential nutrients to the corals. However, coral reefs can only thrive within certain temperature ranges, at the upper limit of ocean water temperatures, beyond which corals and their symbiotic algal partners are at risk.