The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two. It does not include the temperate and polar regions of the Indian and Pacific oceans, nor the Tropical Eastern Pacific, along the Pacific coast of the Americas, which is also a distinct marine realm. The term is especially useful in marine biology, ichthyology, and similar fields, since many marine habitats are continuously connected from Madagascar to Japan and Oceania, and a number of species occur over that range, but are not found in the Atlantic Ocean. The region has an exceptionally high species richness, with the world's highest species richness being found in at its heart in the Coral Triangle, and a remarkable gradient of decreasing species richness radiating outward in all directions. The region includes over 3,000 species of fish, compared with around 1,200 in the next richest marine region, the Western Atlantic, and around 500 species of reef building corals, compared with about 50 species in the Western Atlantic. The term first appeared in academic use in oceanography and geopolitics. Scholarship has shown that the "Indo-Pacific" concept circulated in Weimar Germany, and spread to interwar Japan. German political oceanographers envisioned an "Indo-Pacific" comprising anticolonial India and republican China, as German allies, against "Euro-America". Since the late 2010s, the term "Indo-Pacific" has been increasingly used in geopolitical discourse. It also has a "symbiotic link" with the Quadrilateral Security Dialogue, or "Quad", an informal grouping between Australia, Japan, India, and the United States. It has been argued that the concept may lead to a change in popular "mental maps" of how the world is understood in strategic terms. In its widest sense, the term geopolitically covers all nations and islands surrounding either the Indian Ocean or the Pacific Ocean, encompassing mainland African and Asian nations who border these oceans, such as India and South Africa, Indian Ocean territories such as the Kerguelen Islands and Seychelles, the Malay Archipelago (which is within the bounds of both the Indian Ocean and the Pacific), Japan, Russia and other Far East nations bordering the Pacific, Australia and all the Pacific Islands east of them, as well as Pacific nations of the Americas such as Canada or Mexico. ASEAN countries (defined as