Groundwater—the water that is stored beneath the Earth’s surface in soil and rock—makes up more than 95 percent of the Earth’s liquid fresh water. The subsurface aquatic realm is also the world’s largest freshwater habitat. Although organisms that live only in subterranean aquifers make up a relatively small fraction of the total number of freshwater species, they are an important component of biodiversity. They are poorly understood, however, because they are to a large extent inaccessible and are difficult or impossible to study directly.
Because groundwater overdraft is an intensifying global problem, endangering some groundwater-dependent species, research on stygobionts (fauna that live in aquifers) is urgently needed. The little that we know about these creatures hints at how much more there is to learn.
The Austrian zoologist Josephus Nicolaus Laurenti was the first person to publish a description of a vertebrate stygobiont. In his 1768 book on reptiles and amphibians, he described the blind European cave salamander Proteus anguinus, also known as the human fish or olm. These eyeless, white, eel-like creatures with small limbs were known to locals in the Alps of the western Balkan Peninsula long before they first appeared in scientific writings. Occasionally flushed from caves and springs following floods, the organisms were widely believed by those locals to be the offspring of cave-dwelling dragons. P. anguinus is thought to have the longest life span of all amphibians: Captive individuals have lived more than 70 years, and maximum life expectancy is more than 100 years. Females lay eggs only about once every 12 years, but they lay up to 70 eggs at a time, which they guard until hatching, a process that may take up to six months.
The eyelessness and lack of pigmentation of P. anguinus are features found in many stygobionts, most known species of which are highly specialized evolutionarily, having adapted to a very narrow range of stable environmental conditions. The loss or reduction of eyes and skin pigment are examples of so-called regressive evolution, in which species traits inherited from a common ancestor are lost or undergo degeneration. Other changes may compensate for this regression; for example, loss of vision is often accompanied by the enhancement of other senses that play a role in finding prey, navigating, and communicating with other individuals of their species. Collectively, this suite of morphological changes in response to the challenges of life underground is called troglomorphy. This condition has evolved repeatedly across many different unrelated or distantly related lineages of stygobionts, a process known as convergence.
Most of the known stygobionts are invertebrates, and most of the ones that are invertebrates are crustaceans, such as isopods, amphipods, and decapods, although flatworms and snails contribute to the diversity of some aquifers. Among vertebrates, only fish and salamanders have successfully colonized subterranean aquatic habitats; they are found typically in highly porous and permeable karstic aquifers (those formed from the dissolution of carbonate rocks such as limestone).
The area of the western Balkan Peninsula where olms were first found is known as the Dinaric karst aquifer system. Halfway around the world from there, the karstic Edwards-Trinity aquifer system of central Texas is home to dozens of species of stygobionts, many of which are found nowhere else on Earth, including blind catfish that live up to 600 meters belowground. A group of groundwater salamanders in the genus Eurycea has evolved there that are only distantly related to the olm. This ancient lineage of salamanders has colonized groundwater habitats both above and below ground. In west central Texas, 15 species have evolved, which inhabit different watersheds and regional aquifer segments.
Some of the Edwards-Trinity Eurycea species appear to live primarily in surface springs, only temporarily occupying the water-filled caves and conduits of the limestone aquifer. Others are strictly subterranean, rising to the surface only accidentally during periods of high spring flow. An example is the Texas blind salamander (E. rathbuni; see top photo, above), which was discovered in 1895, when a dozen of them were expelled from an artesian well in San Marcos. In 1896, Smithsonian Institution zoologist Leonhard Stejneger described the creatures as follows:
These animals, by their want of external eyes and their white color, at once proclaimed themselves as cave-dwellers, but their extraordinary proportions, absolutely unique in the order to which they belong, suggest unusual conditions of life, which alone can have produced such profound differences.
Several species of Eurycea have both surface-dwelling and subterranean populations, with the latter exhibiting varying degrees of troglomorphy. At a few sites, such as Honey Creek Cave, surface forms and troglomorphic forms can be found meters apart, separated only by the transition between the surface and subsurface environments. Even though there is no physical barrier, natural selection maintains the differences between cave and surface forms in their respective habitats.
Although Texas’s groundwater salamanders have been known to science for more than a century, deciphering just how many distinct species there are is a work in progress. For several reasons, species in this group have been especially difficult for systematists to discover, describe, and classify. First, collecting and studying these animals is inherently challenging because of the difficulty of accessing their habitats. Thus there may be species of which we remain unaware, despite our best efforts. The Blanco blind salamander (E. robusta) is known to us only from a single preserved specimen that was collected in 1951 in Hays County, Texas. (Four specimens were originally collected, but two were consumed by a heron in the field and a third was subsequently lost.) In addition, even experienced researchers have difficulty distinguishing among many of the surface species based on external appearance alone. Finally, the striking convergence among subterranean populations has further complicated efforts to delimit species using traditional morphological features.
The advent of molecular genetic tools (especially DNA sequencing) has been transformative in uncovering the true species diversity and evolutionary history of this group. Beginning in the early 1980s, evolutionary biologist David Hillis, then an assistant professor at the University of Texas at Austin, began studying Eurycea species in collaboration with herpetologist Andy Price, of the Texas Parks and Wildlife Department, and Paul Chippindale, then a graduate student in Hillis’s lab. (Price passed away in 2012 and Chippindale is now at the University of Texas at Arlington.) Along with other graduate students and postdoctoral researchers from Hillis’s lab, they visited hundreds of caves, wells, and springs throughout the rugged limestone canyons of the Balcones Escarpment in search of new populations and species of salamanders. (The Balcones Escarpment is the steep southern and eastern edge of the Edwards Plateau; it is a geologic fault zone several miles wide that extends in a curved line across Texas from Del Rio to Dallas.)
Although most of the field sites they explored were idyllic—fern-lined, bubbling springs flowing into crystal-clear creeks—the fieldwork was not without its hazards. The names of some of the caves (Rattlesnake Cave, Deadman’s Cave) hint at the potential dangers inherent in subterranean exploration. And in a tragic accident in 1990, during an expedition in search of salamanders at a cave on a U.S. military training reservation near San Antonio, one of two soldiers accompanying Price and Chippindale fell to his death while descending the 30-meter vertical drop into the cave.
Decades of fieldwork carried out by Hillis and his team resulted in thousands of salamander tissue samples being frozen for future DNA analysis. My colleagues and I recently conducted a study (published earlier this year in the Proceedings of the National Academy of Sciences of the U.S.A.) in which we used hundreds of these salamander samples and millions of short stretches of DNA sequences to fill nagging gaps in scientists’ knowledge about the species diversity and evolutionary history of Eurycea salamanders in the Edwards-Trinity aquifer system.
Our results show that in the past, complex geologic faults, surface drainage patterns, and subsurface groundwater divides caused populations of salamanders to become isolated in different regions of the aquifer, and in some cases those populations eventually diverged into distinct species. In the process of analyzing the samples, we refined the boundaries of named species and discovered three species new to science, descriptions of which we are preparing for publication.
The devotion of decades of research to the discovery, description, and classification of Eurycea salamanders hasn’t been simply an academic exercise. It has had and will continue to have important implications for regional conservation efforts in central Texas, because about half of these salamander species—along with several other groundwater-dependent invertebrate, fish, and plant species—are currently listed as threatened or endangered with extinction under the Endangered Species Act of 1973. The Act provides protection to rare and vulnerable species and their habitat and has prevented the extinction of hundreds of at-risk species, despite being chronically underfunded by the federal government.
The protections afforded by the Act have been used as a tool to provide some regulation of groundwater withdrawals in Texas, a state that considers groundwater to be the private property of landowners. Enforcement of the Act has provoked the ire of some, who mistakenly believe it to be a threat to the economy. But cities and counties in central Texas are some of the fastest-growing places in the nation. And protecting endangered species and the regional aquifers they inhabit directly benefits Texans, more than 2 million of whom rely on the very same high-quality groundwater as the threatened species.
The quality of groundwater discharging from karst aquifers such as the Edwards-Trinity depends on the health of the aquifer’s ecosystem. Stygobionts living in the aquifer underpin vital ecosystem services (the contributions of natural ecosystems to human well-being), including water purification and the biodegradation of contaminants and pathogens. So the health of these aquifer-dwelling species is a measure of the quality of the water supply. Unfortunately, the same properties of karst aquifers that make them so valuable for water storage, namely rapid recharge and transport, make them especially vulnerable to contamination. Of particular concern is the direct discharge of treated sewage into the creeks and rivers that recharge the aquifers. This discharge increases the levels of nutrients—in particular, nitrogen and phosphorus—in the stream ecosystem, resulting in explosive algal growth. As algae die and are decomposed by bacteria, the dissolved oxygen needed by aquatic organisms is depleted and water quality deteriorates, posing a threat to the creeks and streams that draw people to the region. In addition, the oil and gas pipelines that cross the Edwards-Trinity aquifer present a serious contamination risk.
Groundwater is vital to human health and the environment, especially in arid and semiarid regions, where droughts are frequent and permanent surface water is scarce. Conservation of groundwater-dependent species and their habitats should be a low-cost by-product of water conservation programs, yet groundwater depletion is intensifying, threatening the stability and resilience of these ecosystems. If declines in groundwater quality and quantity continue unchecked, they will result in declines in the abundance of stygobionts, extirpation of populations, and eventually, global extinction of these creatures.
Why should we care? Conservationist Aldo Leopold addressed this question in 1949 in A Sand County Almanac:
The last word in ignorance is the man who says of an animal or plant, “What good is it?” If the land mechanism as a whole is good, then every part is good, whether we understand it or not. If the biota, in the course of aeons, has built something we like but do not understand, then who but a fool would discard seemingly useless parts? To keep every cog and wheel is the first precaution of intelligent tinkering.
The decline or extinction of one species in a community may have cascading negative effects on other species through complex interactions, leading to ecosystem decay. In the worst case, ecosystem function could be impaired to the point that water quality declines.
Conserving sufficient clean fresh water will require a sea change in the way water is currently valued and governed.
Life on Earth is at greater risk than most people realize. The United Nations–backed 2019 Global Assessment Report on Biodiversity and Ecosystem Services, prepared by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), is the most comprehensive assessment yet of the state of our environment. It concludes that the loss of species and habitats poses as great a threat to life on Earth as that posed by climate change. The pace of this loss in biodiversity is many times greater than the “background” (normal) rate over geologic time. Some groups of organisms are going extinct more quickly than others. Terrestrial vertebrates are suffering from serious decreases in population size and range, amounting to “biological annihilation.” More amphibian species are in decline and at risk of extinction than any other group of species.
Threats to human and environmental health will intensify as the human population continues to grow. Scientific evidence such as that included in the IPBES report makes it clear that, collectively, we’re not doing enough to stave off environmental destruction.
Yet pathways to a more sustainable world do exist. One major focus identified by the IPBES report is the maintenance of sufficient clean fresh water for nature and humanity. Conserving this precious resource will require a sea change in the way water is currently valued and governed. Improvements that will facilitate this change include integrated management of surface water and groundwater, decentralized (including home-based) collection of rainwater, preservation of natural stormwater infrastructure, strengthening of environmental standards, and water pricing and incentive programs.
Actions that we can take as individuals to reduce our water footprint include installing efficient plumbing and native landscaping, and choosing foods that require less water to produce. And, of course, we must hold our elected officials accountable. We can lead happy, healthy lives while consuming less and conserving the environment, and we owe it to ourselves, our children, and the planet—including the stygobionts—to do just that.