Bermuda Cave Biology
The subterranean aquatic environment consists of small (cracks in bedrock or gaps between sand and gravel particles) and large (cave) water-filled spaces. Underwater caves exist either in limestone (formed by dissolving of the rock by weakly acidic groundwater) or in lava (created as lava tubes during volcanic eruptions).
Since Bermuda is a relatively small island with no place very far from the sea, most of the island’s limestone caves contain clear, tidal pools in their interior (Fig. 1). The water in such underground environments can vary from completely fresh to fully marine salinity. These pools are the gateways to complex networks of underwater passages primarily at 18 m (60 ft.) depths and were probably formed by at relatively prolonged still stand of Ice Age sea level at this depth. The Bermuda cave habitat is characterized by the absence of light, a salinity and temperature stratified water column, very limited food resources, low levels of dissolved oxygen and stable environmental conditions. Similar to Bermuda’s water filled caves are cenotes in Mexico's Yucatan Peninsula and blue holes in the Bahamas and Belize. In addition, other such cave habitats, including limestone and volcanic caves, occur on islands or peninsulas around the Caribbean, Mediterranean and Indo-Pacific.
Ecologically, animals permanently living in underwater caves can be subdivided into stygobites (cave-adapted species restricted to subterranean waters) and stygophiles (species inhabiting caves and completing their entire life cycle there, but which also occur in similar open water habitats). The prefix "stygo" refers to the subterranean River Styx which from Greek mythology circles through Hades or the underworld. Thus, stygobites are literally the "aquatic cave life", while stygophiles are "aquatic cave lovers".
Although cave-adapted animals have long been known and studied from freshwater caves, the discovery of similar animals from marine caves is a recent event brought to light by cave diving explorations. Anchialine (underground voids near the coast filled with tidal, brackish or marine water) caves typically possess surface layers of fresh or brackish water separated by a halocline from deeper, fully marine waters. Cave diving has been an essential tool to explore and study the deeper waters in such systems where most stygobitic animals live. Anchialine and freshwater stygobites are mostly crustaceans, and include several higher groups of crustaceans found only or primarily in subterranean habitats.
An extraordinarily rich and diverse group of cave adapted organisms inhabits the inland caves of Bermuda. Seventy-five cave limited species have been identified so far from Bermudian caves, including 64 crustaceans, 5 mites, 2 ciliates, 2 gastropod molluscs, and 2 segmented worms. In order of abundance, the crustaceans include 18 species of copepods, 18 ostracods, 7 amphipods, 6 shrimps, 6 cumaceans and 3 isopods. Notable in their absence are remipedes and thermosbaenaceans which are characteristic inhabitants of similar caves in the Bahamas and elsewhere in the Caribbean.
Stygobitic animals from Bermuda caves include:
- Mictaceans - small (3-3.5 mm), eyeless and depigmented, non-predatory crustaceans represented by a single species in anchialine caves in Bermuda (Fig. 2). They belong to a newly described order of crustaceans.
- Copepods - a large and diverse group, comprising the most common animals in marine plankton. They are small (typically 1-2 mm long) and have a short, cylindrical body with head and thorax (body region where the legs attached) fused into a cephalothorax. Bermuda cave copepods have closely related species from island caves on opposite sides of both the Atlantic and Pacific Oceans (Fig. 3).
- Ostracods - small (approximately 0.5-2 mm) crustaceans with a bivalve shell. A cave ostracod in Bermuda is related to cave species from the Bahamas, Cuba, Jamaica and Yucatan (Fig. 4). Another species is found only in Bermuda and Galapagos caves.
- Mysids - small (approximately 3-20 mm), shrimp-like crustaceans. The two cave mysid species from Bermuda both represent new genera (Fig. 5).
- Isopods - crustaceans that are flattened from top to bottom; they occur in terrestrial, freshwater and marine habitats. Stygobitic isopods range from several millimeters to several centimeters in length. Bermuda cave isopods include a new family (Fig. 6), as well as species with close relatives in Aruba, Jamaica, Bahamas, Canary Islands, and Curacao.
- Amphipods - crustaceans that are flattened from side to side; they occur in freshwater and marine habitats. Three of the Bermuda species are the sole members of new genera; while the others belong to genera with multiple additional species. Two of these species belong to a genus that is widely known from inland groundwater around the Mediterranean, on the Atlantic coast of Portugal and Spain, as well as in the Azores, Canary Islands, Balearic Islands, Morocco and France (Fig. 7). However, aside from Bermuda, they are not present anywhere in North America or the Caribbean.
- Decapods - crustaceans with five pairs of legs (hence the name Decapoda or “10 legs”). One Bermuda cave shrimp also occurs at sites around the Caribbean, in the Galapagos and Ascension Island, and along the coast of the Mediterranean. Another shrimp has a related species occurring in the ocean off Western Australia and Viet Nam, as well as caves in Mallorca (Fig. 8). A further species is also found in caves in the Bahama, Cayman Islands, Cuba, and Jamaica. The genus Procaris includes species from Bermuda, Ascension Island in the South Atlantic, Cozumel Island in the Caribbean, and Galapagos and Hawaii in the Pacific.
Despite being limited to caves, many groups of anchialine organisms, such as many of those found in Bermuda, are widely separated, e.g., on opposite sides of oceans or even opposite sides of the Earth, and seemingly isolated from one another on a global scale. One theory attempts to explain such distributions by suggesting that cave organisms originated more that 200 million years ago when all continents were combined into one supercontinent and subsequently were dispersed by plate tectonic rafting as the continents separated and moved to their present positions. A number of other anchialine animals show close relationships with present deep-sea species implying a possible deep-sea origin for cave fauna. Finally, some cave animals are thought to have been 'stranded' in their present locations by receding waters of the Tethys Sea – the global sea at the time the continents were combined.
Unfortunately, many of these unique and fascinating animals are threatened with extinction due to the actions of man. In Bermuda, 25 species of cave animals are internationally recognized as "critically endangered." This is the highest level of threat and roughly equates to a 50% chance of the species going extinct if nothing is done. All too frequently, anchialine cave animals can be considered endangered since they have very limited distributions, commonly being known only from a single cave, and environmental conditions in these caves are often deteriorating through the effects of water pollution or cave destruction.
Threats to caves include sewage and waste disposal, deep well injection, quarrying and construction activities and diver and other human disturbances. As an example, Bermuda is one of the ten most densely populated countries in the world and has the largest number of private cesspits per capita. Disposal of sewage and other waste water into cesspits or by pumping down boreholes is contaminating the ground and cave water with nitrates, detergents, toxic metals and pharmaceuticals, depleting the very limited amounts of dissolved oxygen in cave water, and generating toxic levels of hydrogen sulfide. Far too many caves and sinkholes are viewed as preferred locations for the dumping of garbage and other waste products (Fig. 9).
Another serious environmental problem concerns the destruction of caves by limestone quarries or construction activities. At least half a dozen or more caves have been destroyed by Bermuda limestone quarries which produce crushed aggregate for construction (Fig. 10). Many caves have been filled and built over by golf courses, hotels and housing developments in Bermuda. Recently, a series of luxury town homes were built directly on top of the largest cave lake in Bermuda.
At least seven anchialine caves in Bermuda have been used commercial tourist attractions. Unfortunately, many of the tourists visiting these sites have viewed the deep clear water cave pools as natural wishing wells in which to throw a coin or two (Fig. 11). Copper coins tend to rapidly deteriorate and dissolve in salt water, producing high levels of toxic copper ions in the cave waters. It is time that visitors to caves begin to recognize these unique biological resources and take steps to make sure such rare ecosystems and organisms are protected for future generations.
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