A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
Anchialine caves are partially or totally submerged caves in coastal areas. Anchialine (pronounced “AN-key-ah-lin”) is a Greek term meaning “near the sea,” and anchialine caves often contain freshwater and/or brackish water in addition to seawater. These caves may be formed in karst landscapes as well as in rock tubes produced by volcanic activity. Karst landscapes are areas where limestone is the major rock underlying the land surface, and often contain caves and sinkholes formed when acidic rainwater dissolves portions of the limestone rock. Water in anchialine caves tends to stratify according to salinity, with the heavier seawater below the level of fresh and brackish water. This stratification produces distinctive habitats inhabited by a variety of species that are endemic to these habitats (endemic means that these species are not found anywhere else). Some of these species are “living fossils” known as relict species, which means that they have survived while other related species have become extinct.
Animals that live only in anchialine habitats are called stygofauna or stygobites. Investigations of these species have revealed some puzzling relationships, including:
Most investigations of anchialine caves have been confined to relatively shallow depths; yet, the observations described above suggest that connections with deeper habitats may also be important to understanding the distribution of stygobite species. Bermuda is a group of mid-ocean islands composed of limestone lying on top of a volcanic seamount. Because they are karst landscapes, Bermuda has one of the highest concentrations of cave systems in the world. Typical Bermuda caves have inland entrances, interior cave pools, underwater passages, and tidal spring outlets to the ocean. Bermuda’s underwater caves contain an exceptional variety of endemic species, most of which are crustaceans. Most of these organisms are relict species with distinctive morphological, physiological, and behavioral adaptations to the cave environment that suggest these species have been living in caves for many millions of years. Yet, all known anchialine caves in Bermuda were completely dry only 18,000 years ago when sea levels were at least 100 m lower than present because of water contained in glaciers. Such observations suggest the possibility of additional caves in deeper water that would have provided habitat for anchialine species when presently-known caves were dry.
The primary goal of the Bermuda Deep Water Caves 2011: Dive of Discovery Expedition is to explore the uppermost 200 meters of the Bermuda seamount and adjacent seamounts to confirm the existence of underwater caves at depths between 60 and 200 meters. A related goal is to document underwater features that indicate sea level during the last Ice Age, which was much lower than at present.
Key activities to achieve these goals are divided into three phases. The first two phases were completed in 2009. In Phase 1, high-resolution multibeam sonar was used to produce detailed maps that assist with locating deep-water caves and sea level benchmarks. During Phase 2, a remotely operated vehicle was used to examine and photograph sites of interest identified by the multibeam survey. In particular, expedition scientists were looking for signs of water movements around possible cave entrances, such as congregations of schooling fish, plumes of brackish water, sand ripples, or unusual abundance of filter-feeding organisms such as sponges. Phase 3 of the Expedition (2011) involves exploration of caves by technical divers to collect biological specimens and place or recover instrument packages.The 2009 multibeam sonar survey produced detailed maps of the entire shelf edge around the Bermuda Platform and 75% of the shelf edge around the Challenger Bank, in water depths greater than 150 m. Data from the mapping surveys were used to identify more than 100 points of interest. The mapping surveys also revealed what appeared to be several submarine landslides around the perimeter of the Bermuda Shelf. During the final week of the 2009 expedition, 33 video survey dives were completed at selected points of interest, and 25 cumulative hours of dive time were logged on these sites. For more information about results from the 2009 Expedition, see the Mission Summary.