Atoll? Pukapuka?

    Corals are animals belonging to the phylum Cnidaria. Reef-building corals are those that have algae called zooxanthellae living symbiotically inside their bodies. Reef-building corals grow their calcareous skeletons by receiving photosynthates from zooxanthellae, and because of their symbiosis with zooxanthellae, they inhabit relatively shallow waters in the tropics and subtropics. When reef-building corals create landforms with their own skeletons over a long period of time, these landforms are called coral reefs. There are three main types of coral reefs: fringing reefs bordering land, barrier reefs with shallow water (lagoon) several tens of meters deep between them, and ring-shaped atolls with no land. Atolls are formed by coral reefs piled on top of volcanic islands that submerge below the sea surface. When coral, calcareous algae, foraminifera, shellfish, and other foraminifera carcasses - in other words, calcareous gravel and sand - accumulate on top of the coral reefs of an atoll, low-lying state islands that are higher than the sea surface are formed in a scattered manner surrounding the lagoon. It is said that there are about 500 atolls in the world, 400 of which are distributed in the Pacific Ocean. Some countries, such as the Republic of the Marshall Islands in the Central Pacific, the Republic of Kiribati, Tuvalu, and the Republic of Maldives in the Indian Ocean, consist almost entirely of atoll islands. Some inhabited atolls are relatively close to other islands or continents, where the flow of people and goods is intense, while others are far from other islands or continents, where the flow of people and goods is limited. Some of the former types of atolls have high population densities and are called "urban atolls," while the latter types are more isolated and are referred to as "remote atolls" in this study.

Source of the left figure: "Coral Reef Q&A" on the website of the Japanese Society for Coral Reef Society

　Pukapuka is the name of an atoll in the Cook Islands country of Oceania. Oceania, excluding the Australian continent, is conventionally divided into Melanesia, Micronesia, and Polynesia, with the Cook Islands being located in the middle of Polynesia. The Cook Islands, like the surrounding islands, have been at the mercy of European colonizers, and in 1888 they became a British colony. In 1901, New Zealand, a former British colony that had gradually become more independent, took control of the islands, which spread from north to south, and officially named them the Cook Islands. In the 1960s, following the end of World War II, the Cook Islands gained independence, and in 1965, they gained self-government and became part of a free association with New Zealand. Although not a member of the United Nations, the Cook Islands has diplomatic relations with the United States, the European Union, China, Japan, and other countries, and on many occasions acts as a practically independent nation. The Cook Islands, with a population of about 15,000, are a typical archipelagic nation, consisting of six atolls, mainly in the Northern group, and seven volcanic islands and two atolls in the Southern group. The capital, Avarua, is located on the volcanic island of Rarotonga in the southern part of the archipelago, and more than 70% of the population lives on this island.

    Pukapuka is one of the six atolls in the Northern Islands and is known as Te Ulu o te Watu among the local people. Located more than 1,100 km from Rarotonga and 640 km from the nearest large island, Samoa, it is considered one of the "far-remote" islands in the country. The land area is approximately 1㎢ and the population is currently less than 500. There are no regular shipping or air routes connecting Pukapuka to other islands or the continent, but passenger and cargo ships operate between Rarotonga and the Northern Islands three to four times a year. The atoll includes three islets: Wale (the main island in the north), Motu Kō (in the southeast), and Motu Kotawa (in the southwest). The Pukapuka/Nassau Island Government has jurisdiction over Pukapuka Atoll and Nassau Island, a single reef island located approximately 90 km south of Pukapuka atoll.