Biogeography of Anchialine Cave Fauna
Biogeography, or the distribution of organisms over time, is a field of study shrouded in mystery. Its concern is not so much to question the who or the what of nature, but rather the when and how. If a given species is only found in one location on Earth, how did it come to be there? What historic geological and ecological events led to its current distribution pattern? Answering these questions will allow scientists to better understand the ranges of species and provide invaluable information to develop conservation strategies for the future.
While terrestrial biogeography is relatively well understood, very little is known about the marine world, particularly in the case of anchialine cave systems. Anchialine caves are underwater caves found in tropical regions throughout the world that have a subterranean connection to the ocean 1. These caves typically exhibit a freshwater layer at their surface and a marine layer at depth; both separated by a distinctive mixing layer called the halocline. This environment is considered extreme for humans because there is no light, low dissolved oxygen, and low levels of nutrient availability. However, cave-dwelling organisms (stygobionts) are known to not only survive but thrive in this harsh habitat. The dominating macrofauna within these systems are crustaceans, representing up to 60% of species diversity in caves from regions such as the Yucatán Peninsula 2. Animals such as ostracods, amphipods, isopods, and decapods are common within these systems 3. In addition, arguably one of the most charismatic and enigmatic crustacean groups also inhabits this extreme environment: Remipedia.
What are remipedes?
Remipedes are a class within crustacea that are found in anchialine caves throughout the world. Despite being only a few centimeters in length, these creatures are considered quite formidable-so much so that scientists have named species within the class after mythological monsters such as Godzilla, Mothra, and Kumonga. Remipedes have a strikingly alien appearance; their bodies are long and segmented, and the head region contains mouthparts likely used to grasp and consume prey. Their first feeding appendage even contains venom; making it the only known venomous crustacean 4. While Remipedia is an intriguing study group, little is truly known about their general ecology or distribution patterns because of their inaccessibility to researchers. Remipedes are found in the marine layer of anchialine caves, often just below the halocline, making them unobservable without the use of cave diving techniques.
Finding answers through cave diving methods
The introduction of cave diving technology to science has revolutionized the field of anchialine cave research. What was once inaccessible and unknown is now being discovered and described by researchers on a global scale. The class Remipedia was discovered in 1980’s via cave diving and has grown from one species to 29, ranging from the limestone caves in the Caribbean and Australia to the volcanic lava tubes in the Canary Islands. Using cave diving techniques, scientists can now examine these cryptic and striking creatures as they interact within their natural environment. Remipedes are known to swim upside down in an almost melodic movement through the water column. Even the hunting patterns of these organisms have been observed and photographed by scientists in the field! Specimens are collected via vials and plankton tows in order to be examined at the surface and in laboratory settings. Anchialine cave fauna are notoriously difficult to identify at the species level due to their size and the presence of pseudocryptic species (species that look similar morphologically but are distinct genetically). Remipedes are morphologically examined via light microscopy and scanning electron microscopy (SEM). High magnification can be used to examine extremely small features; with particular emphasis on their distinctive feeding appendages. Once we understand what species are found in which caves, we can begin to examine via genetic analyses how they have come to inhabit these systems around the world.
How did these organisms get to where they are today?
Many theories have tried to explain how such small anchialine crustaceans can have an incredibly disjunct distribution pattern. One hypothesis suggests that they are ancient crustaceans, dating back to the time when all the continents were merged together as the supercontinent, Pangea5. As Pangea broke apart, a large sea formed (the Tethys) and fauna such as remipedes inhabited its coastline. As the continents drifted further apart, these invertebrates remained on the shifting coastlines for millions of years until the landmasses were separated by oceans and have the geographic distribution we see today. However, there are many places where remipedes are located that do not necessarily support this theory. The lava tubes in the Canary Islands off the coast of continental Africa have two species of remipedes found within them; Morlockia atlantida and M. ondinae 6,7. What truly makes this extraordinary is the fact that scientists date the formation of these lava tubes to only 21,000 years in age, suggesting a very recent colonization took place 8. Because of these peculiar distribution patterns, it is important to not only look into geologic history but investigate the general ecology of Remipedia as well. Perhaps they are able to disperse on a local scale over time, reaching areas previously believed impossible to access? If this is the case, much more information is needed to discern the distribution pattern of these creatures such as its early developmental stages, feeding ecology, and life history strategies.
Looking to the future
Developing a better understanding of the biogeographic distribution of remipedes is critical for developing conservation strategies in the future. Anchialine cave ecosystems are extremely susceptible to pollution. As coastlines become more developed and tourism expands, anchialine caves and the organisms within them become threatened. Some remipede species are only known from one cave and if that cave were to become destroyed, the entire species could potentially be lost. Understanding the range of these species and their dispersal capabilities will allow us to find ways to protect these creatures and give future generations the opportunity to witness one of the most unique and awe-inspiring ecosystems on our planet.
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. (M1703014). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation."
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