C111 Evaluating the Diverse Functions of Coral Reefs and Proposing Sustainable Restoration Methods

By creating coral reefs, corals expand the surface area exposed to sunlight for photosynthesis and establish topographical zonation, creating a diverse environment where the open ocean meets the calm inner bay. Coral reefs are home to the greatest diversity of marine species. This biodiversity is due to the photosynthetic products of symbiotic algae within corals, which form the basis of the food chain via the corals, and to the habitats provided by the reef's topography and coral colonies. Coral reefs can be considered a symbiotic ecosystem-topography system based on coral photosynthesis and calcification.

The gravel provided by coral reefs includes coral fragments, foraminifera, and shellfish. Foraminifera, in particular, inhabit reef ridges and reef flats above low tide, where corals cannot survive, and their shells become sand, nourishing the coastline. In Pacific atoll islands, half to three-quarters of the sediment that makes up the land is foraminiferal sand. Atoll islands are formed when coral reefs build their foundations below low tide, and then coral gravel accumulates on top of them during storm surges, creating mounds. Foraminiferal sand then accumulates behind them, expanding the islands. Because these islands are made up entirely of the remains of living organisms, preserving and restoring their ecosystems is synonymous with preserving and restoring the country's land. However, due to the effects of climate change, ocean temperatures have been rising worldwide in recent years, leading to coral bleaching and death around the world. Small island nations in the Pacific have not been immune to this phenomenon, and the Republic of the Marshall Islands experienced large-scale coral bleaching and subsequent coral reef degradation in 2014, 2019, and 2024.

Coral restoration methods can be broadly divided into asexual and sexual reproduction. The former involves breaking off live coral branches, growing them, and then planting them in the restoration area. The latter involves growing fertilized eggs and larvae from live corals into small corals and then planting them in the restoration area, or scattering the larvae directly in the area and allowing them to settle and grow. To restore coral reefs in island nations, we envision the application of sexual reproduction, which preserves genetic diversity without damaging living corals and enables large-scale restoration.

Specifically, surviving corals from bleached and dead coral colonies are collected as parent corals and collected in waters suitable for growth to form a larval supply base (donor farm). During this process, it is desirable to select coral strains that can tolerate high water temperatures through water temperature tests and genetic analysis.

Larvae obtained through sexual reproduction at the larva supply base are collected and settled on settlement substrates, after which they are subjected to intermediate rearing in a relatively quiet location for approximately two years or more. Alternatively, larvae obtained through sexual reproduction at the larva supply base are collected, allowed to settle on settlement substrates for several days, and then the settlement substrates with the larvae settled are installed in the restoration area to promote growth. Alternatively, larvae obtained through sexual reproduction at the larva supply base are scattered on settlement substrates installed in the restoration area to promote settlement and subsequent growth.

Alternatively, the natural settlement of sexual eggs or larvae that have emerged from the larva supply base on the restoration area as a result of natural tidal currents and advection is planned and encouraged.