Groundwater animals

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Groundwater Animals (also Stygo- or groundwater fauna ) is called the animal species that the groundwater colonize and the more or less perfect in this particular habitat adapted are, this also includes the cave aquatic animals . The totality of life forms in the groundwater is also called Stygobionta .

Groundwater as a habitat

As groundwater is of hydrogeologists all the water treated under the earth's surface and cavities in columns of fixed or loose rock , in this context as aquifer , referred circulated. Groundwater as a habitat is shaped by the properties of the aquifer, especially its pore size and pore volume, the chemical properties of the water, for example calcium content and pH value, and influences from the surface, for example inflow of seeping surface or rainwater nutrients contained therein. Primary production does not play a role in the lightless medium of the groundwater ; all groundwater ecosystems with their fauna are therefore dependent on the supply of nutrients from the surface.

Solid, unweathered rock has no cavities in which water could circulate, it is a groundwater non-conductor or aquiclude. Like the rock types known as aquitars with extremely small pore volumes, they play no role as a habitat for groundwater animals. The actual aquifers ( called aquifers after their use in English-speaking countries) are divided into several types, which differ fundamentally in their habitats :

  • Pore ​​aquifers are loose or solid rock in which water can circulate in the cavities between the individual grains (depending on the grain size, sand or gravel). The cavities formed are small (in the micrometer to millimeter range), but take up a considerable proportion of the volume of the rock. They form a coherent cavity system.
  • in crevice aquifers, water circulates in the crevices and cracks of sheared or broken solid rock.
  • Karst aquifers are crevice aquifers in water-soluble rocks, especially limestone and gypsum , in which the water circulating in the crevices partially dissolves the rocks and thus enlarges the cavities. The resulting cavities can reach enormous dimensions and form caves that are also accessible to humans . Vertebrates such as fish or amphibians can only be found in the groundwater in karst aquifers with their large cavities .

Because of these conditions, groundwater as a habitat has a number of special features: Due to the lack of primary production, it is poor in nutrients ( oligotrophic ). The content of dissolved oxygen is usually also reduced, since the removal of oxygen from the surface prevents it from diffusing from the air . The temperature is very even over the year, and there are hardly any differences between the seasons. Because of this combination of factors, and in addition to the mostly small pore volume with extremely cramped space to live and the complete lack of light, the groundwater has a very independent fauna, which has hardly any species in common with that of surface waters. These are referred to with the technical term stygobionta, or stygobite.

The groundwater fauna is differentiated from that of neighboring, related or in spatial contact habitats according to the presence of these specialized species:

  • The system of gaps below surface water has many of its properties in common with groundwater, but differs in that it has a stronger water exchange with a constant supply of nutrients. It also has a number of species that migrate vertically between surface and subterranean waters. It is therefore referred to as hyporheic interstitial with its own technical term . However, some limnologists regard it as a special case of groundwater and summarize its fauna with that of the "real" groundwater.
  • In karst areas, surface waters such as rivers in ponors can disappear underground and continue to flow underground as cave rivers . According to popular belief, their fauna is not part of the “actual” groundwater.
  • The near-surface pore system in soils and rocks can be filled with water or air, depending on the amount of precipitation. In karst areas it is differentiated as a vadose zone from the constantly water-bearing phreatic zone , for which the technical term epicarst has recently been introduced. Numerous stygobiont animal species still live here, but these mix with species with other specializations. Although many limnologists and hydrogeologists do not count this area as part of the actual groundwater according to the definition they use, it is often considered together with it because of the similarity of the community.
  • A particular problem is the delimitation of the fauna of the karst groundwater from the fauna of the caves . This is referred to with its own technical term as " Troglobios ", its species as " Troglobiont ". An independent fauna of the caves, which differs from the crevice systems only decimeter to centimeter wide in the carbonate rock (which is therefore no longer directly accessible to humans), is particularly pronounced in groups of species that breathe air. Although some scientists want to exclude the karst fauna of the larger cavities from the actual groundwater fauna, most consider the two as a single habitat. Often those aquatic species are called troglobiont that are found by chance in caves or were found first.
  • Sources as natural groundwater outlets on the surface have their own fauna. This is called krenal in the limnological literature . Although groundwater animals are occasionally washed out here, many of them cannot live here permanently.

The lower limit of the groundwater fauna has hardly been researched, but there are random observations of groundwater animals from depths of several hundred meters. The absolute limit must be assumed at around 2000 meters, when the rock pressure also closes water-filled cavities. Extreme conditions can also be assumed in the zones above, among other things because the water temperature rises sharply at such great depths, while the supply of nutrients and the oxygen content decrease more and more.

Adjustments

Groundwater animals are mostly pigmentless , translucent or white, and eyeless, as they live underground without light. If they come into exposed habitats, they can be damaged there by the ultraviolet radiation of sunlight. Many are not completely blind, they have a sense of light in their skin. When exposed to light, they react with escape reactions ( photophobic ). The sense of touch and chemical senses are usually particularly well developed. Due to the narrow pore volume, especially in pore aquifers, most of them are very small, mostly significantly smaller than their relatives living in surface waters. The length of body appendages such as antennae or row legs is usually reduced (but they can also be longer in animals of the actual cave fauna, which live in less cramped caves). Many have a worm-like elongated body shape with which they can swim or slide through the narrow cavity system or push themselves through narrow crevices.

Many groundwater animals can tolerate reduced oxygen levels in the water, but they cannot live in completely oxygen-free, anoxic water. Many species are characterized by constant breathing movements to create a stream of water that brings oxygen. Otherwise they move less and usually only cover short distances. Due to the lack of nutrients in the groundwater, the metabolic rate and the rate of reproduction are usually lower than in surface water species. However, the animals are often more durable. Here they benefit from the lower predation and competition and the extremely uniform living conditions. Taken together, they are considered to be particularly well adapted to adverse environmental conditions that are actually hostile to life, with only minor adaptation due to biotic relationships; this was summarized as the "A strategy" (from adversity selection). However, this makes many species particularly sensitive to water pollution and other negative influences from the surface.

As a rule, groundwater is freshwater . However, a number of typical groundwater animals can also live in salinized groundwater and brackish water . Some groups even occur in the gap system of marine sediments, i.e. in sea water. With some animal groups it is assumed that they colonized the groundwater directly in this way, i.e. never occurred in limnic surface waters. These include, for example, the " primeval ringworm " Troglochaetus beranecki .

A number of relic forms are striking in the groundwater fauna. These are descendants of groups living in surface waters in earlier geological ages, which are now extinct there and could only survive in the groundwater. This includes many crustaceans such as the water crabs or the Thermosbaenacea .

biodiversity

Globally, around 7,000 known species of groundwater animals were given for the reference year 1986. In Europe there are around 1,220 (including the aquatic cave fauna). This means that their number of species is lower than that of the air-breathing cave fauna with its numerous specialized insect species. If one compares the groundwater fauna with that of the limnic surface water ( flowing water and standing water ), striking differences become apparent. There are almost no insect species living in the groundwater (even globally only 24 species are given), while crustaceans are remarkably well represented, so that their number of species can even exceed that of the surface waters of the same region. About 40 percent of European crustaceans belong to the groundwater fauna. Six orders of crustaceans, Mystacocarida , Gelyelloida (the copepods belonging) Syncarida (with the fountain crabs ), Mictacea , thermosbaenacea (both peracarida ) and Remipedia are almost exclusively stygobiont where thermosbaenacea and Remipedia but overwhelmingly (so-called in saltwater-bearing anchialinen ) waters Life. 92 fish species and 14 amphibian species are reported from cave waters, in Europe only one ( Delminichthys adspersus and the Grottenolm ), both of which only live in the Dinaric Karst of the Balkan Peninsula.

The distribution pattern of the species differs between underground and surface waters. Most of the groundwater and cave water is locally rather poor in species (low alpha diversity ) with often only one species per genus at each investigated location, even particularly species-rich waters such as some Dinaric karst waters rarely exceed about 40 species. However, since most of the groundwater species, at least outside of the regions glaciated in the last ice ages, are local endemites with a small distribution area, they (due to the higher beta diversity ) reach the same number of species in larger regions as that in surface waters. In addition, it was shown that the few widespread groundwater animals mostly represent complexes of cryptospecies that can be distinguished genetically, but not morphologically. For Spain it is estimated that no, or only extremely few, macroscopic species of the groundwater have distribution areas whose diameter would exceed about 200 kilometers.

Most groundwater animals are unspecialized in their diet, true herbivores are completely absent, and specialized predators are rare.

Selected groups of species

Copepods

Copepods , scientific name Copepoda, are the most species-rich group of crustaceans. Of the more than 2,800 species living in freshwater (as of 2008), around 1,000 are found in groundwater. This makes them the most species-rich group of animals in the groundwater. Since the copepods are generally small animals, they were able to colonize the groundwater more easily, but there is still a further decrease in body size in many kin groups. Numerous species have a simplified body structure with numerous morphological regressions (reductions), which often go back to pedomorphosis (shortened development in which the sexually mature animals retain characteristics of the larval stages). The advantage for the animals could be better mobility in the narrow cavity systems. Most of the groundwater copepods belong to the orders Cyclopoida and Harpacticoida . The small order of the Gelyelloida, which includes only three species, is only known from there (they live in the Karst of Southeast Europe and North America). In general, karst aquifers and porous aquifers are populated by copepods with about the same species-rich population.

Cave shrimp

Cave shrimp of the genus Niphargus belong to the species-rich species of shrimp (order Amphipoda) found in the groundwater . Among the more than 330 species of the genus, almost 50 live in the Dinaric Karst, of which only 10 occasionally also live in surface waters. Many of the species are extremely similar morphologically and very difficult to distinguish, some only according to genetic markers (cryptic species). The genus is widespread in Central and Southern Europe, the Black Sea region, Anatolia, to the east as far as the mountains of Iran. It is species-rich in the Mediterranean, poor in species north of the Alps and absent in Scandinavia. This is attributed to the influence of the ice ages. But you could z. B. as far north as in Ireland and southern England survive the last Ice Age, as shown by the occurrence of numerous endemic species here. The northern limit of the distribution coincides almost perfectly with the southern limit of the glaciation in the Vistula Ice Age. Compared to surface water gammarids, cave shrimp are slimmer, more delicate and eyeless. In contrast to surface species, some investigated species can survive a complete lack of oxygen (anoxia) for several days and very low oxygen levels even for months.

Groundwater and spring snails

Mollusks are widespread in the groundwater and are sometimes very diverse. With very few exceptions, such as the endangered mussel Congeria kusceri in the Dinaric Karst, these are snail species. Almost all species are very small, with a thin, partially transparent housing and colored white. Almost all groundwater snails belong to the sea ​​snails and are included in the Hydrobiidae family in the broader sense (superfamily Rissooidea ). Worldwide around 350 species were specified in 2005 (groundwater and cave waters summarized here), with many species within the group since then, e.g. Sometimes even genera have been newly described. Although widespread worldwide, two regions of the world are particularly rich in species, the Dinaric Karst in Europe and Southeast Asia. More recently, after intensive research, the Iberian Peninsula was identified as another region with a comparable biodiversity. Almost nothing is known about the ecology of most species, not a few have been described only after empty shells that had been washed to the surface from karst waters. Within the genus of spring snails ( Bythinella ), which is mainly distributed in springs, it could be shown that the species of stygofauna have become more similar in their characteristics through convergent evolution .

The most common groundwater snails in Central Europe are the species of the genus Bythiospeum . Numerous species from Central Europe have been described here, mostly based on details of the shell morphology. According to genetic data, however, the genus actually appears to be significantly poorer in species. In Germany, instead of up to 25, as previously assumed, there are probably only three species.

Annelid worms

A number of leech species that belong to the Erpobdellidae family have become known in southern Europe exclusively from karst and cave waters . This also includes the species Croatobranchus mestrovi, newly discovered in 2001 in the Dinaric Karst, with a very unusual morphology, including lateral appendages . More typical for groundwater habitats, however, are the little bristles (Oligochaeta). These are represented in all examined groundwater ecosystems. A good 100 specialized stygobionte species have been described so far, although the non-European fauna has hardly been studied. In this group too, species with a very small distribution area predominate (local endemic species), but widespread species from surface waters are also found in the groundwater. Little bristles of the groundwater are morphologically indistinguishable from other limnic species and from many species of the ground fauna; one can say that they are pre- adapted to this habitat . The most species-rich genera, Trichodrilus (family Lumbriculidae ) and Rhyacodrilus (family Tubificidae ), also have numerous species in surface waters. Although it is assumed that annelworms in the groundwater are probably of great ecological importance, little research has been done on them in this regard.

Cave fish

The cave fish Phreatichthys andruzzii from Somali cave waters

Cave fish occur exclusively in caves and wide crevices in karst aquifers and are therefore not counted as part of the actual groundwater fauna by numerous researchers. More than 80 species, subspecies or local populations of cave fish have been described worldwide, which are divided into eight orders, and new ones are still being discovered all the time. In 2016, for example, the discovery of the loach species Eidinemacheilus proudlovei in Iraq attracted some attention. Almost all cave fish are descendants of genera and families widespread in limnic surface waters; in some cases they live in direct contact with closely related aboveground populations. Most of these related species are pre-adapted to low lighting conditions, for example nocturnal. Cave fish are also characterized by regression of the eyes, which have progressed to different degrees depending on the species, and a white (sometimes pink or yellowish) color, which is caused by the lower content of the pigment melanin in the skin. It is hereditary, but in some species a darkening can be determined with artificial light. Often other senses are enhanced to compensate for the loss of optical sense. The Typhliasina pearsei (family Dinematichthyidae ) living in Yucatán has a lateral line organ that is greatly improved by a cavity system in the head area , which acts as a remote sense of touch. With others like the American Amblyopsidae ( blind fish ), however, this is rather weak, but they have improved chemical senses. Many cave fish have a broadened head. The reason is controversial; it may make sensory orientation easier.

More detailed genetic studies, which have been carried out mainly on North and Central American species, have shown that the apparently more widespread species of cave fish are in fact complex local endemic cryptic species or local populations. This applies, for example, to the cave population of the Atlantic penguins living in the Mexican Cueva del Azufre or the most common cave fish in North America, Typhlichthys subterraneus.Normally only one cave fish species occurs in each cave system, as a major exception sometimes two. The only real cave fish in Europe exist in the Aachtopf cave system . They belong to the creek loaches , their discovery was announced in April 2017. In the Dinaric Karst there are a number of species that temporarily live on the surface, but when the water level drops, they follow this into the underground karst cavities. The carp fish species Delminichthys adspersus is best adapted to underground conditions .

Groundwater fauna of Germany

In Germany, almost 500 animal species have so far been found in the groundwater, at least occasionally, of which 178 are specialized, i.e. stygobiont, animal species. The German fauna shows a clear species difference with a decrease in the number of species from south to north, which also continues in the neighboring countries. In almost all the places examined, the crustaceans are both the most individual and the most species-rich group. However, some groups such as the roundworms (Nematoda) and the water mites can only be identified by a few specialists worldwide and therefore hardly researched. The more widespread stygobiont fauna of Germany includes, in addition to the well crabs and cave shrimp, also the cave isopods of the genus Proasellus and the snails of the genus Bythiospeum . Most of the species-rich groups represented in the Mediterranean region are absent in Germany, as in all of Central and Northern Europe.

A little more than 100 species were found in investigations of numerous groundwater measuring points in the state of Baden-Württemberg over several years. The local species density per measuring point was low with an average of hardly more than two species per sample. More than half of the species were found in only one percent of the sample sites or less. More than a fifth of the measuring points showed no fauna at all. However, a sample site in the karst at a depth of over 200 meters was still populated with a variety of species. Among the 20 or so species with a regional focus of distribution, a noticeable focus was found in the catchment areas of the large rivers Rhine, Danube and Neckar. A peculiarity arose here: the fauna in the area of ​​the High Rhine was more similar to that of the Danube area. This is probably due to the fact that this region still belonged to the Danube region in the Pliocene (see article Urdonau ).

Drinking water networks

Groundwater animals can also migrate from the groundwater that is promoted for use as drinking water into drinking water networks, where they are undesirable primarily for aesthetic reasons. In particular, technical systems such as sand filters in drinking water treatment systems can be densely populated, whereby the animals keep the pore space free by breaking down particulate organic matter and are therefore more useful. While real groundwater species are usually harmless, species with a distribution focus in surface waters can definitely become a problem. For example, stygobionte cave isopods of the genus Proasellus such as Proasellus slavus are usually not a particular problem. If, on the other hand, the water louse ( Asellus aquaticus ) occurs in pipe networks, it can develop real mass occurrences. The species can also multiply here by grazing the biofilm in pipelines and thus keep indefinitely.

Individual evidence

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  30. ↑ Groundwater animals - studies of the groundwater fauna that are unique in the world have been successfully completed. ( Memento of the original from October 19, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.lubw.baden-wuerttemberg.de
  31. ^ LUBW State Institute for the Environment, Measurements and Nature Conservation Baden-Württemberg (ed.), Andreas Fuchs, Hans Jürgen Hahn, Klaus-Peter Barufke: Groundwater monitoring program. Survey and description of the groundwater fauna in Baden-Württemberg - short report. PDF file
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