Name the term for the underwater ecosystems formed by coral reefs.

The term for the underwater ecosystems formed by coral reefs is indeed coral reef ecosystems. Coral reefs are remarkable marine structures formed primarily from the calcium carbonate skeletons of corals, which are small, marine invertebrates. These ecosystems are rich in biodiversity and provide habitat, food, and protection for a multitude of marine species, from fish to mollusks to crustaceans.

Coral reef ecosystems are often referred to as "the rainforests of the sea" due to their high levels of species diversity and productivity. They play a crucial role in coastal protection, supporting fisheries and tourism, and providing various ecological services.

The other options refer to different aquatic environments. Photic zones denote regions of the ocean where sunlight penetrates, allowing photosynthesis, but they do not specifically refer to coral reefs. Estuaries are areas where freshwater from rivers meets and mixes with saltwater from the ocean, leading to unique ecosystems that are different from those of coral reefs. Pelagic zones pertain to open ocean areas that are not near the bottom or the shore, distinguished from the structured and biodiverse environment of coral reefs. Thus, the correct choice highlights the specific and rich nature of these underwater ecosystems uniquely linked to coral formations.