The Pacific region National Park Service has the following description (https://www.nps.gov/im/pacn/anchialine_pool.htm): “Anchialine pools are unique brackish water environments that form in lava fields near the ocean. They are landlocked pools fed by subsurface groundwater (freshwater) and tides (seawater) with no visible connection to the ocean. Anchialine pools vary significantly in size and structure from large fishponds to small lava cracks. Hawai‘i is the only state with these special pools.
Anchialine pools provide the habitat for rare invertebrate species including shrimp, snails, and damselflies. Many of these species are endemic to the Hawaiian Islands, meaning they exist nowhere else in the world.” The fauna of anchialine pools are described on another page of this website.
According to the Nature Conservancy’s Ecoregion website (http://www.hawaiiecoregionplan.info/anchpoolNC.html) “Some Hawaiian anchialine pools show extremely high invertebrate and algal diversity, among the highest in the Indo-Pacific, and perhaps the world.” Also “The island of Hawai‘i clearly supports the majority of the state’s anchialine resources: a total of more than 460 pools in about 80 sites have been identified on the ground, and an additional 54 sites (about 170 pools) have been viewed or photographed from the air.”