Anchialine pools, or ponds, are land-based bodies of water with an underground connection to the sea.
Though Hawaii is where many of the anchialine pools on this planet are found, they are not exclusive to Hawaii; in Bermuda, I’ve snorkeled in one of these. Some of these anchialine systems are underground caverns with km of caves. The largest cave system is Sistema Ox Bel Ha in the Yucatan of Mexico. It has 180 km of tunnels. In cave anchialine systems, the water may be anoxic due to the lack of plants and algae doing that photosynthesis thing.
Why such an odd word? anchialine is from the Greek word ankhialos meaning near the sea. Why not just salt-water ponds or brackish ponds? Short answer, they are referred to by these names as well. They are also known as blue holes, cenotes, and grietas, in different regions.
There are at least two reasons to call them anchialine. First, it’s a neat word, and second, as is the trend in science generally, odd words that most people don’t know add to the pompous nature of science (yes, this is tongue-in-cheek). In science, we should strive to avoid this type of verbiage. Right? Vote below:
In Bermuda, the larger Anchialine ponds have plenty of ocean water flow, and salinities are from high brackish to very near the salinity of the ocean. They have been formed in several ways. Several were historically freshwater ponds within inner-island dunes. They now connect to the sea at or near the surface due to sea level rise (post-glacial rise, not the current rise). Other ponds have been formed due to the limestone rock dissolving. making room for freshwater to collect and form a connection to the ocean. Pools with a hefty connection to the ocean tend to have a large tidal range—ponds with smaller connections to the sea form stratified salinity layers, particularly during rainfall.
While Bermuda has a handful of smaller and only five large anchialine pools, Hawaii has a very large number of these pools, 700 or more. In Hawaii, pools are often formed due to lava flows. Hot lava creates a physical channel or at least a connection of porous rock, that ends at the sea: a few gaps, a little erosion, and the seawater flows in.
Among these types of ponds, biodiversity is highest when they have a large ocean connection. The organisms in the ponds of Bermuda are mostly marine varieties, common ones being the Caribbean Worm Shell (Vermicularia spirata), the flat tree Oyster (Isognomon alatus), and the Variable ceirth snail (Cerithium lutosum).
Though most organisms in these ponds are also found in the nearby marine environment, some species are restricted to these pools. Shrimp from the genus Procaris have only been found in anchialine ponds, and species are endemic to a particular region. Procaris chacei is only found in Bermuda while Procaris hawaiiana is endemic to Hawaii. Even pools on the Ascension islands have their endemic shrimp, Procaris ascensionis. As you might expect, these species are endangered due to their rarity.
Besides Procaris hawaiina, which is only found in a few pools, Hawaiian pools have other shrimp species found nowhere else; the most well know is Halocaridina rubra, or ‘ōpae ‘ula, the Hawaiian Red Shrimp. And even though red is in their name, both common and scientific, there are white morphs of this species. Several other shrimp species are observed in Hawaiian anchialine ponds, some very rare, others found throughout the Indo-Pacific region.
Some species thrive in anchialine waters. Palau’s somewhat famous Jellyfish Lake (on the island of Mecherchar) is an anchialine system inundated by seawater due to past rising sea levels. The lake houses an abundance of jellys, including moon jelly species, Aurelia sp., and golden jellys, Mastigias sp., a group with symbiotic algae (zooxanthellae).
Jellyfish Lake requires an entire blog entry for itself. Among the interesting facts about this lake is the anoxic layer with concentrated hydrogen sulfide in the lower depths of the lake
The endemic snail Neritilia hawaiiensis is found in anchialine pools and other brackish environments in Hawaii.
The anchialine cave habitats appear to house “living fossil” species. In some cases, marine species may have been trapped when the sea level fell. The mysid shrimp found in many cave systems seem to have been affected by sea level. Or perhaps these environments allowed them to escape from predators by living in these harsh environments where they while away the years hardly changing.
These interesting habitats are, of course, at risk. Climate change-driven sea level rise may erase them and the rare species living there. Pollution runoff from human activities is also a threat.
Final Note:
No, no, no, like in the rest of fascist America, you can’t really vote.
Sources and Further Reading:
Hawaiian DAR. Fact Sheet. The Eight Anchialine Pool Shrimp of Hawai‘i.
https://www.accord3.com/wp-content/uploads/2021/04/Anchialine-Pool-brochure.pdf
Iliffe, TM and Kornicker LS. 1987. Observations on the biology and geology of anchialine caves. Proceedings of the 3rd Symposium, Geol. of Bahamas: 73–79.
Iliffe, T. 2009. Worldwide diving discoveries of living fossil animals from the depths of anchialine and marine caves. Smithsonian Contributions to the Marine Sciences. 38: 269–280.
Thomas MLH, Eakins, KE, and Logan A. 1991. Physical characteristics of the anchialine ponds of Bermuda. Bulletin of Marine Science 48(1): 125–136.
Thomas MLH, Logan A, Eakins, KE, and Mathers SM. 1992. Biotic characteristics of the anchialine ponds of Bermuda. Bulletin of Marine Science 50(1): 133–157.
U.S. Fish and Wildlife Fact Sheet. Anchialine Pool Shrimp of Hawaii.
https://webhome.auburn.edu/~santosr/pdf/anchialinefactsheet041608.pdf