The Hawaiian Islands are home to an abundance of natural resources that are globally rare and unique to the archipelago. The islands’ natural resources consist of a wide array of ecosystems, terrestrial and marine life, and Hawaiian cultural sites. Anchialine ponds are one of the many endemic ecosystems to the Hawaiian Islands associated with the region’s cyclical volcanic processes. 

Anchialine ponds are brackish water ponds located in coastal zones that do not have any visible connection to the sea. The word Anchialine comes from the Greek term ankhialos, meaning ‘near the sea’. The ponds are located within young lava flows on many islands, with large groupings of ponds on the islands of Hawai`i and Maui. The porous lava rocks allow fresh groundwater moving down from the mountain to mix with seawater filtering up through subterranean tunnels in the lava, forming the brackish pools.  

Anchialine habitats can form as ponds, in lava flow caves or cracks, that range in depths from half a meter to more than five meters deep, salinity from 8 to 22%, and temperatures from 22 to 28°C. No two pools are formed under the same conditions nor are they alike in shape, distance from the ocean, or supplied from the same freshwater sources. The physical characteristics of each pool are unique; each pool being its own contained ecosystem with life adapted to thrive in its specialized habitats. The organisms that inhabit anchialine ponds reflect the variability in physical characteristics among the ponds, with each pool having its organismal assemblage. 

Endemic vegetation on the Islands can be found growing around anchialine ponds with some pools supporting algae, green seagrass, and sometimes marine flowering plants. A characteristic feature of anchialine pools is photosynthetic cyanobacteria (Schizothrix) growing below the surface, growing in distinct yellow-orange mats by laying down calcium carbonate pulled from the sea-water. Anchialine ponds are known for their invertebrate communities including various species of marine snails, and one pool on Maui is the only known location of a semi-freshwater crab (Pele ramsey). The most well-known pool invertebrate is the anchialine shrimp and the best known among these shrimps being the red shrimp, Halocaridina rubra or ‘ōpae‘ula. These dominant grazers help maintain the orange-yellow color of anchialine pools by feeding across the bacterial mats, removing debris, and keeping them clean. The ‘ōpae‘ula keep anchialine pools from being overrun by algae and reduce the risk of them becoming eutrophic. ‘ōpae‘ula are less than half an inch in length with a lifespan of 10-15 years. They are able to reproduce one to two times per year. 

Anchialine systems are associated with neotropical coastal regions with the ideal conditions of limestone bedrock or young volcanic lava flows. Hawai`i contains more than half of the world’s known anchialine pools, with Mexico’s Yucatan Peninsula being another notable region with anchialine systems. The West side of the island of Hawai`i contains the largest concentration of anchialine pools with over 200 of them within Kaloko-Honokōhau National Historical Park. ‘Āhihi-Kīna‘u Natural Area Reserve on the Island of Maui contains 12 groupings of pools that are considered the most intact and the largest anchialine pool in Hawai’i. 

Anchialine pools are sensitive habitats that face a variety of human-induced threats. Physical disturbances from irresponsible hiking and land use changes cause disturbances to the bacterial mats that last for decades. The chemistry of anchialine pools is fragile and can be easily altered by upslope pollutant runoff and harmful sunscreen contaminants. The groundwater supplies to anchialine pools are threatened by human development that uses up groundwater sources while leaving behind pollutants and nutrient runoff. Sea-level rise is a future concern for anchialine pools–they could soon be covered by the tides.

The biggest threat to anchialine pools across the islands is the introduction of invasive species, those already introduced, and the potential threat of future introductions. Introduced species include tilapia, mosquitofish, and guppies that pose a significant risk to ‘ōpae‘ula through predation and competition. Species introduction to anchialine pools was primarily done intentionally by humans for aquaculture, bait, and mosquito control. Invasive species are a significant threat to island biodiversity overall, with many endemic species being vulnerable to predation and competition from non-endemic species.

Anchialine pools are prime examples of the many incredibly unique ecosystems found in Hawai`i, that support vulnerable endemic species. The protection of the unique ecosystems and species of Hawai`i is an ongoing effort by local communities. In 1970 the Hawai`i legislature created the Natural Area Reserves System (NARS), to protect and preserve, in perpetuity, the scope of unique ecosystems in Hawai`i along with the rare, threatened, and endangered species within. The first Natural Area Reserve designation was Āhihi-Kīna‘u and it is the only marine resource area with this recognition. The reserve contains the youngest ‘A‘ā lava flow on Maui, ‘A‘ā is a type of lava formation that is rough, spiny, and irregular. These flows form a geologically complex landscape supporting many endemic terrestrial species and groupings of anchialine pools. Additionally, the complex lava coastline of the reserve supports a robust rich reef system with diverse marine life. Āhihi-Kīna‘u is a culturally rich landscape with historical Native Hawaiian physical features and a living connection of the land to the Hawaiian people. The local community has a long history of stewarding and protecting the land and marine resources of this area through community organization to keep the area clean. The anchialine pools of South Maui are within a restricted area of the Āhihi-Kīna‘u preserve and are not open to the public. Anchialine ponds on the island of Hawai`i can be found in the Kaloko-Honokōhau National Historical Park and the Kīholo State Park Reserve, the latter is a joint management project between the Division of State Parks and the curatorship group Hui Aloha Kīholo. 

Anchialine pools are one of the many rich habitats found on the Islands of Hawaii. These diverse places are deeply connected to the Hawaiian people who hold many generations of ecological knowledge and stewardship. Anchialine pools are historically and culturally significant to the people of Hawai`i, being early sources of fresh water and ‘ōpae‘ula providing use as fish pond bait. The Āhihi-Kīna‘u Natural Area Researve contains numerous cultural landscapes that reflect the use of the land by Hawaiian people through structural features and oral histories. The cultural significance of the area along with the uniqueness of the ecosystems illustrates the importance of collaborative reserve systems.