Anchialine pools form some of the longest submerged caves structures on Earth. (Image courtesy of National Science Foundation)

The word “anchialine” (AN-key-ah-lin) comes from a Greek word meaning “near the sea." These typically small pools, which form in limestone or volcanic rock, are located throughout the world but are most common in the Hawaiian Islands and on Mexico’s Yucatán Peninsula.

Anchialine pools have their own unique ecosystems populated by tiny and often rare species of crustaceans, fish, and eels. Among these species is Hawaii’s legendary red shrimp, the ʻōpaeʻula (oo-PAY-oo-la). Water levels in the pools can fluctuate in response to ocean tides. Due to their subterranean connection to the ocean, anchialine surface waters are often brackish and become more saline (salty) with increasing depth.

Many anchialine pools are found in underground cave systems and are favorite destinations for cave divers seeking to observe unusual cave species. One such creature is the Mexican blind tetra, an albino fish that has lost not only its sight but also its eyes!

More than half of the world’s known anchialine pools are found in the Hawaiian Islands. On the Big Island of Hawaii, the NOAA Restoration Center funded a two-acre restoration project for the pools, which have fallen victim in recent years to invasive fish species that upset the pools’ natural balance. To restore the ponds, nearly 400 citizen volunteers helped remove the foreign invaders and reintroduce native vegetation. The restored sites are monitored twice a month to remove algae and maintain the revitalized vegetation. Scientists are learning that the pools’ fragile natural balance makes them good indicators of overall coastal ecosystem health.