The underground world of karst

There are about a hundred caves and pits along the Krka River, with 67 within the boundaries of Krka National Park. A total of 129 taxa have been recorded in the speleological structures in Krka National Park, and pursuant to the Nature Conservation Act, all subterranean animals are protected by law. Except Oziđana pećina cave, caves and pits in Krka NP are not open to visitors.

The underground world of karst: the Dinarides

The Dinaric karst is one of the largest karst landscapes in Europe and the world. It encompasses the area of the Dinarides, a mountain chain named after Mt. Dinara, situated on the border between the Dalmatian part of Croatia and Bosnia and Herzegovina.

The main orientation of the relief and geological structures of the Dinarides range is northwest to southeast, which is often called the Dinaric orientation. Most of the Dinaric karst, which includes all karst features (both on the surface and underground) is found within Croatian borders. Due to the unique natural features, geological characteristics, cave fauna and impressive natural beauty, the Dinaric karst certain belongs to the global natural heritage.

The Dinaric karst is known worldwide as a classic type of karst (locus typicus). The word karst comes from the Indo-European root kar, meaning rock or rocky area. In Croatia, the word karst was first recorded in 1230 in the Charter of Juraj Pariježić who received the gift of "kras", a locality near the settlement of Dobrinj on the Island of Krk. The concept of karst has been accepted at the global level, and numerous names of karst phenomena from the Dinarides area have been accepted into the international terminology in their original form (uvala, kamenica, ponor, dolina, hum and polje). The first true scientific research of karst was conducted in the Dinaric karst.

In general, karst is a specific form of relief that develops on a substrate made of soluble rock. It is characterised by special morphological, hydrological and hydrogeological properties. The karst formation processes are jointly called karstification, and the dominant process is rock corrosion. Karstification is a constant process, and under its influence, the relief is constantly being shaped and altered. The Dinaric karst is currently in the phase of developed and mature karst with a strongly karstified underground, numerous depressions, dolines, poljes and a strongly interconnected underground hydrographic network.

The settling of carbonate deposits from which the karst area of the Dinarides was formed took place on the Adriatic-Dinaric carbonate platform, an environment of purely carbonate sedimentation in which the carbonate deposits of today’s outer Dinarides were formed. These conditions lasted for 150 million years, during the Jurassic and Cretaceous eras. The collision of the structure of the former Adriatic-Dinaric carbonate platform with Eurasian structures took place in the Late Eocene, about 35 million years ago. This caused a lifting of the then deposited carbonate rock, forming the Dinarides mountain range. During this lifting process, which lasted about 35 million years, the rocks were subjected to powerful tectonic forces. Intensive wrinkling, the formation of thrust faults and other faults created mechanical stress, causing rocks to crack and creating countless fissures. Under a favourable, moderate climate with sufficient precipitation, the conditions were ideal for the development of karst.

It has been estimated that almost 50% of Croatia is covered by karst. In the Croatian sections of the Dinaric karst, the most intensive periods of karstification were recorded during the Cretaceous era, and from the Eocene to the Palaeocene. The large surface area and thickness of karst has resulted in this wealth of relief forms (both on the surface and underground), along with numerous other phenomena in the Croatian karst.

Photograph: Lucija and Rade Jug

The underground world of karst: Speleogenesis

Speleogenesis is the complex process of the creation and development of speleological structures, and part of the overall process of karstification.

Speleogenesis results in the creation of speleological structures of different shape and internal morphology, which is encompassed by the term speleomorphology. Both speleogenesis and speleomorphology are part of geospeleology, a branch of speleology that studies the form and creation of speleological structures in karst and sediments within them, and the climatology, hydrology and geophysics of the underground.

The process of speleogenesis, and with it the formation and evolution of karst, requires three fundamental conditions: soluble rock, secondary porosity, and the presence of water. The main geomorphological process in which water has a destructive effect on rock is corrosion.

The course of speleogenesis can be tracked through several phases. Speleogenesis begins with the exposure of soluble rock to atmospheric influences. In addition to surface corrosion, run off water from the atmosphere trickles in through the cracks, corroding the rocks. This process is very slow. When the water penetrates a path from the entrance to an exit from the karst system (spring), the fissures take on the function of a karst conduit. Water begins to flow more quickly through these fissures, expanding it, though this still does not create a cave. Further expansion of the fissure, due to the increased denudation of the rock walls, turns the fissures into a channel. Expanding the channel increases water flow, and the corrosion process is further amplified by fluvial erosion, as the process of mechanical crushing and carrying rock particles by the water flow. Changes in the position of the erosion base and water levels have a strong effect on the process of speleogenesis. A lowering of the water level results in the cutting of a canyon, while a rise in water levels increases denudation of the channel ceiling, forming different types of grooves. This process is called paragenesis. Long-lasting reductions in the level of the erosion base creates multi-layer channels, and results in the cave in and formation of speleothems in channels above the level of water flow. The final phase in the development of speleological structures begins when they lose their hydrogeological function. Cave areas begin to fill in more with speleothems. Following the surface karst denudation, the level of the surface also lowers and may cause unstable cave ceilings to cave in in places, and the entire cave channel can cave in, creating a ceilingless cave. It is important to stress that the tectonic, hydrogeological and lithological properties of the rock, the geomorphological processes, climate change, changing sea levels and more can have significant modification impacts on this process.

In Krka National Park, the speleological structures were formed in various types of geological substrate, with the exception of flysch and dolomite deposits. Most structures were formed within the Promina deposits, conglomerate-marl-limestone systems, and in rudist limestones.

In the Quaternary structures, the formation of speleological structures is tied to the travertine barriers. Though usually of small dimensions, these caves are particularly interesting since they were created in fossil travertine and contain travertine cave ornaments.

To date, 67 speleological structures have been researched in the Krka National Park area.

The underground world of karst: Speleology

Speleology is a group of activities with the primary aim of researching the subterranean phenomena of karst: caves, pits, abysses, caverns and more.

The word speleology comes from the Ancient Greek word spelaion, meaning natural underground cavity, and logos, meaning science. Accordingly, the literal translation of the word speleology is the science of underground cavities. spelaion, što znači prirodna podzemna šupljina, i logos, što znači znanost. Prema tome, doslovni prijevod riječi speleologija je znanost o podzemnim šupljinama.

Primarni je cilj speleološkog istraživanja izrada speleološkog nacrta na temelju mjerenja dimenzija špiljskih kanala i njihova pružanja i dokumentiranje opaženih geoloških, morfoloških, hidroloških i drugih svojstava. Složenost špilja i jama zahtijeva posebne tehnike i obučene timove speleologa, tako da se aktivnost speleologa u dubokim jamama i složenim špiljama može opisati kao vrsta podzemnog alpinizma. U tom smislu dio speleoloških aktivnosti može se, poput alpinizma, smatrati sportskom djelatnošću a speleologiju promatrati kao sportsko-znanstvenu djelatnost. Da je speleologija sinteza sporta i znanosti prvi je u Hrvatskoj 1912. godine ustvrdio Umberto Girometta.

The primary objective of speleological research is to develop speleological maps based on measurements of cave channels and their orientation, and documenting observed geological, morphological, hydrological and other characteristics of these structures. The complexity of caves and pits demands special techniques and training for speleology teams, and the activities of speleologists in deep pits and complex caves can be best described as a type of underground alpinism. In that sense, part of the speleological activities can be considered a sport, like alpinism, and speleology can be observed as a combined sports and scientific activity. Proof that speleology is a synthesis of sport and science was first confirmed in Croatia in 1912 by Umberto Girometta.

Specially trained speleologists participate in speleological research, together with experts and scientists in different branches concerning the processes of speleogenesis, speleomorphology, underground karst hydrology, underground climatology and geophysics (geospeleology), and especially biospeleology, the branch that studies the underground fauna, consisting of a wealth of endemic taxa that are exceptionally interesting due to their isolated conditions. The first description of speleological structures, prior to the development of modern speleology, was given in 1680 by John Beaumont, who gave a detailed description of the Mendip caves in England.

Though the phrase la spéléologie was first coined by French explorer Emile Riviér, it is his countryman Édouard-Alfred Martel who is considered the father of modern speleology. At the geological congress in Besançon in 1893, he first used this phrase to describe the research of the natural underground environment of subterranean cavities, caves and pits. He described speleology as all activities involved in the research of caves and pits, penetration into these cavities, their interpretation, with the use of all available scientific and technical accomplishments. This phrase was accepted at the congress, and quickly became used worldwide.

In 1905, Dragutin Hirc was the first in Croatia to coin the phrase špiljarstvo (meaning the study of caves), which has been regularly in use since then. The phrase speleologija and its derivations was first used in Croatia by geologist Dragutin Gorjanović-Kramberger in 1912 in the journal Vjesniku geološkog povjerenstva za Hrvatsku i Sloveniju [Journal of the Geological Commission for Croatia and Slovenia], in which as its chairman he published the “Izvještaj speleološkog odbora za 1911. godinu” [Report of the speleology commission for 1911]. Both terms are used interchangeably in Croatia, as they refer to the same activities.

The oldest written record about a cave in Croatia is given in the church list from 1096, referring to the cave in Željina Bay on the island of Ugljan. The first described cave was the Šipun cave in Cavtat, described by Dubrovnik landowner Jakov Sorkočević in 1580 as the Cavtat cave.

Until the end of the 19th century, caves and pits in Croatia were only explored by individual enthusiasts. The first systematic research of caves and the first speleological expedition was organised by the Liburnia Hiking and Tourism Society from Zadar, established in 1899. The first society in Croatia that could be considered speleological was the Committee for Samograd cave in Perušić, established in 1886. Today, there are some 40 speleological societies in Croatia. Some are part of the Croatian Speleological Association, while others are part of the Speleological Commission of the Croatian Hiking Association.

The start of speleological research in the Krka River area were the surveys conducted by Mirko Malez in 1956. The first systematic speleological research in the broader area of Krka National Park was conducted by members of the Croatian National History Museum in Zagreb, and members of the Speleological Section of the Željezničar Hiking Society from Zagreb in 1989 and 1990. This successful cooperation with the Public Institute of Krka National Park continued, and in 1998, the five caves of the Miljacka spring area were surveyed. The Speleological Section of the Sveti Mihovil Hiking Society from Šibenik researched the speleological structures in the broader area of Krka National Park in 2005, 2007, 2009 and 2011. More than 100 speleological structures were surveyed, of which 65 were within the park boundaries. In order to bring the beauty of the underground of the Krka National Park area to the public, and to raise awareness of the need for its protection, the Public Institute of Krka National Park published the Speleological Guide to Krka National Park and the monograph Secrets of the Underground.

The underground world of karst: Speleothems – cave ornaments

The formation of speleothems is part of the karstification process that creates caves and pits.

The word speleothem, or cave deposit, comes from the Greek words sphlaion or spelaion meaning cave or pit, and thema meaning deposit.

Speleothems are secondary mineral deposits that form underground, in caves and pits. They are created through the action of water, and may contain one or a combination of more than 300 minerals. The most common cave minerals in Croatian caves are calcite, aragonite and ice. Calcite and aragonite account for about 95% of all cave minerals. The concept of speleothem does not in itself represent the minerals, instead it relates to the specific manner in which these minerals appear in caves and pits. For example, the mineral calcite is not a speleothem, though a calcite stalactite in a cave is. Therefore, speleothems are built of minerals, and a wide variety of minerals can make up a certain type of speleothem.

Calcite speleothems are most common and the most significant deposits in most caves. As they grow, they can also incorporate many other minerals, dust from the air and organic materials, from spores to pollen grains, even bones.

Speleothems are classified above all by their shape, and by the way they are formed (dripping water, water pouring over walls, splashing, crystal growth on the surface of a lake, deposition in underwater areas, etc.). Certain types of speleothems have received names based on their morphology, such as cave pearls or curtains, or based on the environment in which they were formed, e.g. calcite rafts or underwater speleothems.

To date, 38 different types of speleothems have been defined worldwide, some of which are exceptionally rare. The most common and certain rare types of speleothems that can be found in the Croatian karst are stalactites, stalagmites, columns, curtains, helictites, moonmilk, cascades, crystals and flowstones.

Stalaktiti su najpoznatiji tip siga. Rastu („vise“) od stropa prema podu špilje. Veličina, oblik, tekstura i sastav površine stalaktita rezultat su mnogih čimbenika: brzine kapanja, strujanja zraka, isparavanja, vlažnosti, temperature, koncentracije otopine, hidrostatskog tlaka, parcijalnog tlaka CO2 u otopini i u atmosferi špilje. Voda koja se procjeđuje kroz pukotine u stijenama skuplja se na stropu špilje u obliku tankoga filma ili kapljica. Kada kapljica postane preteška, padne, a zbog izlaženja ugljikova dioksida, na njenim rubovima zaostane sitni kalcitni prsten, ispočetka vezan za strop površinskom silom. To je inicijalni prsten, sam početak rasta buduće cjevčice. Cjevčica raste sve dok postoji kontinuirani dotok vode. Dijametar cjevčice određen je dijametrom kapljice. Najstariji dio stalaktita je njegov središnji dio, a najmlađi zadnji vanjski sloj, uključujući i sam vrh. Na poprečnom presjeku može se razlikovati, kao na godovima drveta, stariji od mlađeg sloja stalaktita. Medutim, pojedini slojevi najčešće ne odgovaraju sezonskim klimatskim promjenama (ljeto – zima), kao kod drveta, nego nastaju u većim i nepravilnijim razmacima.

Stalactites are the best know type of speleothem. They grow (hang) from the ceiling down towards the floor. The size, shape, texture and composition of the stalactite surface is the result of numerous factors: drip rate, air circulation, evaporation, humidity, temperature, solution concentration, hydrostatic pressure, partial CO2 pressure in solution and in the cave atmosphere. The water penetrating through fissures in the rocks collects on the cave ceiling in the form of a thin film or drops. When a drop becomes too heavy, it falls, and the carbon dioxide inside is released, leaving a fine calcite ring along its edges, which is initially bound to the ceiling through surface tension. This is the initial ring, and the start of the future tube. The tube continues to grow as long as there is a continuous flow of water, and its diameter is determined by the diameter of the water droplet. The oldest part of the stalactite is the central part, while the outer layers are youngest, including the tip. In cross section, it is possible to differentiate the older from younger layers of the stalactite, similar to reading tree rings. However, individual layers most often do not correspond to seasonal climatic changes (summer/winter) as in trees, but are instead formed in much larger, irregular intervals.

Stalagmites are speleothems that grow upwards from the cave floor. They vary widely in shape, and are formed from the dripping of water off the ceiling or off a stalactite directly above it. They typically have a rounded tip, and a larger diameter than the accompanying stalactite above. Unlike stalactites, stalagmites do not have a central canal. When a stalagmite grows so much that it becomes connected to the stalactite, a column is formed, and such columns may grow to enormous size.

Flowstones are very common speleothems. They may vary in shape, depending on the surface on which they are formed. Flowstones are formed from the slow flow of a thin layer of water over a broad surface, where CO2 is released from the solution into the air, leaving the calcite to crystallise. These crystals continue to grow, with a general orientation perpendicular to the surface of the flowstone. The flowstone grows in thin layers, which can be many different colours, depending on the changes to the composition of the solution from which the crystals are deposited.

Ice speleothems are created by the freezing of dripping, flowing and percolating water, and water vapour from the cave air. The freezing dripping and flowing water creates forms like ordinary calcite speleothems: ice stalactites, stalagmites, columns, curtains and flowstones. Ice speleothems typically have a smooth surface, and rarely have visible ice crystal surfaces. Thin ice speleothems are transparent, while thicker ones take on a white, dark blue or blue-green hue.

Speleothems are used in paleoclimatic research. Paleoclimatology is the scientific discipline that studies climate change throughout the past. Information of these changes are recorded in the physical, chemical and organic properties of the speleothems. The cross-section of speleothems enables reconstruction of former climate conditions, and can provide answers to the questions of the spatial and temporal variability of climate, and of the speed of and causes behind natural climate change.

The underground world of karst: Caves near the Miljacka waterfall

Along the canyon section of the Krka River, the following caves are found on the right bank of the Krka River: Miljacka I-V cave system, Miljacka II, Miljacka III (cave under the falls), Miljacka IV (cave by the Miljacki mill), Miljacka VI and VII.

The Miljacka I-V cave system is the only cave system in Krka National Park. It is situated on the right bank, upstream of the Miljacki water mills, at an elevation of 102 metres. The total length of the investigated part of the system is 2456 metres, with a depth of 33 metres and elevation difference of 44 metres. The system includes the unique area of two former caves, Miljacka I and Miljacka V, which were connected during research conducted in 2001 by a dry channel and in 2017 by a submerged channel. The cave system has two entrances, just 20 metres apart, and 10 metres above the water level of the Krka River. During periods of high waters, the cave entrances are submerged, and water springs from them. The cave system was formed within Promina deposits on a tectonic fault extending in the direction NW-SE. The highest part of the grand cavern, situated 50 metres from the first entrance, is above the level of the main torrential flow, and its stalactites and stalagmites are well preserved. In the lower areas, the stalagmites are eroded due to occasional flooding of the structure.

The cave Miljacka IV (cave near the Miljacki mill) is situated within the plant building of the old water main system on the right bank, upstream of the Miljacka Hydroelectric plant, at an elevation of 78 metres. The length of the investigated part of the cave is 302.9 metres, with a depth of 8 metres and elevation difference of 10 metres. The cave has two entrances: a terrestrial and aquatic entrance. The aquatic entrance can only be accessed by diving. A concrete wall with an access shaft is found at the terrestrial entrance. In the cavern at the entrance of the cave, there is a drystone wall. A layer of coal is visible at several sites in the small chamber.

Due to the strong influence on the cave and water flow within it, the temperature in the cave varies substantially through the year. The difference between the highest and lowest measured temperature is 10.9°C, while the average temperature is about 14.5°C.

The Miljacka IV cave is habitat for 29 taxa, including snails (Gastropoda), spiders (Araneae), pseudoscorpions (Pseudoscorpiones), harvestmen (Opiliones), mites (Acari), isopods (Isopoda), amphipods (Amphipoda), decapods (Decapoda), centipedes (Chilopoda), millipedes (Diplopoda), springtails (Collembola), bristletails (Diplura), beetles (Coleoptera), booklice (Psocoptera), grasshoppers, crickets (Orthoptera), and butterflies and moths (Lepidoptera). Among them are several species that are endemic to the Dinarides mountain range and Croatia.

The cave Miljacka VI is found on the left bank, along the stream that drains into the Krka River 50 metres downstream from the waterfall. It is completely formed in a fossil travertine barrier. The cave is a small and simple speleological structure, without any opportunities for further research. It is just 5.5 metres long and 2 metres deep. It has no active water flow, though occasionally water percolates from the layers above the cave to form speleothems. In the geological past, it likely belonged to a larger cave system, but the erosion action of the river, deepening of the riverbed and loss of material denuded the once larger structure. The stalactites and stalagmites inside indicate a longer period with a dry atmosphere, and the fossil stage of development of the structure suitable for the growth of speleothems. It can be assumed that this cave was once a much larger structure.

The cave Miljacka VII is just 15 metres from the cave Miljacka VI. This is a simple speleological structure formed in fossil travertine. It is 9 metres long and 2 metres deep. The structure was formed in the horizontal and subhorizontal layers, which determined its general morphology. The cave has no active water flow. Deeper sections of the cave contain numerous speleothems (stalactites, stalagmites, columns, other forms). Like in the cave Miljacka VI, these indicate a long period of the fossil phase suitable for their growth, and it is likely that in the geological past, this was a larger speleological system, that was possibly connected with other speleological structures in the canyon.

The cave Miljacka II is the longest speleological structure in Krka National Park, and features a wide diversity of cave fauna, while Miljacka III is interesting hydrologically, as in this part, the water flows at two levels – both through the cave and on the surface, in the Krka riverbed.

The cave springs in the area of the Miljacka waterfall are habitat to the only true stygobiont vertebrate species (amphibian) in Europe, the olm (Proteus anguinus), an endemic of the Dinaric karst.

Photo: Vedran Jalžić

The underground world of karst: the cave Miljacka II

The cave is a valuable part of Croatia’s geoheritage

The entrance into the Miljacka II cave is in the canyon on the right bank of the Krka River, downstream of the Miljacka waterfall, at an elevation of 115 m. It was formed in Promina deposits. This is the longest topographically explored speleological structure in Krka National Park, currently researched to a depth of 3365 metres. From the geological and geomorphological characteristics of the terrain, it can be concluded that the main cave canal continues further in the same direction, and it is possible that this cave is over 5 kilometres long.

The Miljacka II cave is a great challenge for speleological and speleodiving research. This is a highly obstructed speleological structure, with a maximum cave depth of 29 metres and maximum vertical difference of 50 metres. The cave has a typical large entrance chamber with divisions made of stone blocks, and it branches near the entrance (which in the past was used to keep sheep). The main channel, which is also the longest, extends in the direction NW-SE. In addition to the dry channels, the cave is also characterised by the presence of a temporary siphon, large lake and six (currently known) lake siphons. The third siphon is the first active siphon in the cave. During periods of high water, the first and second siphons also become active, and at that time, diving behind them is nearly impossible. The sixth siphon has been investigated and topographically mapped to a distance of 445 metres, though it continues further.

though it continues further. On the basis of the topographical sketches and collected speleomorphological data, based on its morphological type, the cave Miljacka II can be categorised as a layered and branching speleological structure. In terms of its genesis, the cave is classified as a tectonic-erosional (polygenetic) speleological structure, as both tectonic movements and the erosion and corrosion effects of water played the main roles in its formation. In terms of its hydrology, the Miljacka II structure has a constant water course, where the quantity of water depends on the general hydrological situation in the broader area. The cave is a valuable part of Croatia’s geoheritage. 

The entrance part of the Miljacka II cave is dominated by conglomerates and marly limestones, while the final channel is completely formed in marly limestones. The cave is periodically a strong source. During periods of high water, an underground river flows through the cave. The cave is dominated by erosion-corrosion rock relief with clastic sediments deposited from sporadically strong water flow or the tectonic splitting of blocks caved in from the channel ceiling. The channels sides and ceiling throughout the cave show strongly developed erosional forms, eddy pools and current flows, that indicate the strength of the water flowing through it. The cave is ornamented by calcite stalagmites and stalactites of exceptional beauty. Due to the frequent flooding, the stalagmites are caked in mud, so most are brown in colour. At the entrance to the cave, loose, black accumulations of guano (bat excrement) are visible, while the ceiling above these accumulations is covered by a shiny, black crust, which is likely formed from phosphate minerals created from the reaction of bat urine with the limestone.

The air temperature in the entrance part of the cave is under the influence of external weather conditions, and can significantly vary, with an average value of 13.9°C. Further into the cave, the climate is more stable, with an average value of 13.1°C.

To date, 54 taxa of invertebrates have been confirmed to inhabit the Miljacka II cave, of which 18 fall within higher systematic categories: mites (Acari), spiders (Araneae), centipedes (Chilopoda), beetles (Coleoptera), springtails (Collembola), millipedes (Diplopoda), true flies (Diptera), molluscs (Gastropoda), isopods (Isopoda), amphipods (Amphipoda), decapods (Decapoda), oligochaetes (Oligochaeta), harvestmen (Opiliones), bees, wasps and other flying insects (Hymenoptera), grasshoppers and crickets (Orthoptera), pseudoscorpions (Pseudoscorpiones), booklice (Psocoptera) and bristletails (Diplura).

Among these taxa are numerous species that are endemic to the Dinarides mountain range and Croatia. For example, the cave centipede species Eupolybothrus cavernicolus is stenoendemic, and Krka National Park is the type locality. The cave is also inhabited by the olm, Proteus anguinus, an amphibian endemic to the Dinaric karst.

Research conducted in the Miljacka II cave to date has confirmed the presence of nine bat species, and the cave has been included on the list of Internationally important underground bat shelters (UNEP/EUROBATS).

Photo: Vedran Jalžić

The underground world of karst: the cave Miljacka III

The cave stands out due to its length of 113 metres, which is highly unusual for a travertine cave

The Miljacka III cave (also called the Cave under the waterfall) was accidentally discovered in 1988 when examining the banks of the Krka River under the travertine barrier at the Miljacka waterfall. It is positioned within the course of the Krka River. The entrance to the cave is at the base of a 7.2 metre tall travertine cliff of polygenetic origin, formed from the alternation of accumulation and denudation phases. The cave stands out due to its length of 113 metres, which is highly unusual for a travertine cave. It is 6.6 metres deep. It was formed by the expansion of what were initially fissures in the travertine after it solidified as rock. It can only be accessed by diving. During periods of high water, entrance is not possible. The hydrological conditions are also interesting in the cave, for in this section, the water flows both through the cave and along the surface in the Krka River, so it flows at two different levels.

In terms of its morphology, the Miljacka III cave is a simple cave with only one main channel and no branches. The formation of the cave was based on the fracture systems caused by the breakage of travertine, and shares the same orientation as the bed of the Krka River. The cave is a constant source, with water appearing in it both as percolating water and as dripping water. In the channel expansions, there are abundant depositions of speleothems in the form of stalactites, flowstone and speleothem crusts on the channel walls.

The cave was formed in travertine deposits (an elongated travertine body), and is likely of Holocene age, and more precisely lies in the zone of contact between the travertine and base rock upon which the travertine was deposited. The rock base is visible in parts of the entrance, sporadically throughout the cave and at the end of its channel. In the rock, micro-relief forms are visible, such as troughs and eddy pots, created by erosion. The main part of the cave is formed in travertine, as it is softer than the base rock and therefore more strongly affected by erosion. This initiates paragenetic processes and results in mechanical wear and a widening of the channels, as seen by the predominantly travertine system of accumulated fluvial materials covering the floor of the cave. In addition to erosion, the channel has also been shaped by cave-ins, with karst rock and large blocks of travertine also found in the sediments on the floor. Research of this sediment confirmed that large particle sediments are deposited on the cave floor, mostly sands with varying ratios of gravel and silt fractions. The mineral composition of the sediments has shown that the primary mineral is calcite, followed by quartz, with traces of dolomite and mica.

In the Miljacka III cave, erosional remnants of sediments are found along the entire profile of the channel (on the floor, walls and ceiling). These sediments were likely deposited during phreatic (submerged) conditions in the geological past of the cave. Since they are somewhat eroded, it can be assumed that they were deposited in conditions different to those today. The phreatic sediments of the Miljacka III cave were also deposited onto the stalactites that formed under dry air conditions, so it can be concluded that this cave has undergone numerous stages of alternating dry air and submerged conditions.

Research of the laminate cave sediments and stalactites in the submerged part of the cave channel in Miljacka III indicated the recrystallisation of speleothems following the circulation of pore water, presenting a problem in dating these samples, as the U-Th dating method would not be able to successfully determine the age of the stalactites due to this recrystallisation.

Unusual organic, white to yellowish formations have been discovered in the Miljacka III cave, attached to the submerged cave walls. Analysis of these formations indicated that these are anthozoan type of colonies likely containing more than 30 bacterial species.

Photograph: Nenad Buzjak

The underground world of karst: the Stara jametina cave

The Stara jametina cave is the deepest speleological structure in Krka National Park

It is situated at an elevation of 200 metres on the left bank of the Čikola River over Punčka Draga, near the settlement Gornji Krnići by Konjevrate. The cave is overgrown with dense vegetation. It has two entrances that resemble depressions, one is a cave entrance the other a pit entrance. Both lead into the entrance hall.

Though the name (meaning Old Large Pit) implies it is a pit, based on the average slope of the channel and the speleological classification, this structure is actually a cave. It is nearly completely passable, without the need for any speleological techniques, with the exception of a single 4-metre vertical drop.

Stara jametina is a branching speleological structure, explored to a length of 188 metres and a depth of 85 metres. Further exploration is not possible due to the narrow fissures. The cave is characterised by large spaces divided by narrow channels, and it can be naturally divided into four main halls.

Stara jametina was formed within transitional deposits of conglomerates, marly limestones and marl. A hall in eroded marl is found at a depth of 40 metres. This cave has no hydrogeological function, though there is some percolating water. It is ornamented with lovely stalagmites.

The average air temperature throughout the structure is 11.9°C. However, the entrance hall is highly influenced by external weather conditions, and when this hall is excluded, the mean air temperature in the rest of the cave is 12.4°C. The relative air humidity is 100%, with the exception of the entrance hall where the humidity is also influenced by the external conditions.

The cave fauna inhabiting the Stara jametina cave is very diverse. According to the ecological classification of cave taxa, it is inhabited by troglobionts, troglophiles, subtroglophiles and trogloxenes.

More than 50 taxa have been recorded in the cave, including snails (Gastropoda), spiders (Araneae), pseudoscorpions (Pseudoscorpiones), harvestmen (Opiliones), mites (Acari), isopods (Isopoda), centipedes (Chilopoda), millipedes (Diplopoda), springtails (Collembola), bristletails (Diplura), silverfish (Thysanura), beetles (Coleoptera), booklice (Psocoptera), true flies (Diptera), grasshoppers and crickets (Orthoptera), butterflies and moths (Lepidoptera), earthworms (Lumbricidae), bees, wasps and ants (Hymenoptera) and bats (Chiroptera).

Among these, many taxa are endemic to the Dinarides and to Croatia. The cave is also home to Verhoeffiella margusi, a troglobiont springtail species that is stenoendemic to Krka National Park, meaning it is found nowhere else on Earth.

Since 2018, research on the environmental parameters have been ongoing in the Stara jametina cave. Verhoeffiella margusi, troglobiontna vrsta skokuna koji je stenoendem NP „Krka“.

This is the only research of its kind in Croatia, aiming to determine the rate of rock wearing and denudation, and the climatic and palaeoclimatic properties of the karst in Krka National Park.

Photo: Marijana Cukrov

The underground world of karst: Jazinka and Oziđana pećina cave

On the left bank of the Krka River, there are two exceptional caves thanks to the archaeological discoveries found within them

Oziđana pećina cave (meaning Walled cave) is found at the top of the cliff of the Krka River Canyon above Roški slap waterfall. The cave has a simple morphology, and it was formed in the ceiling structure of horizontal layers shaped in conglomerate deposits. The cave is 59 metres long, and composed of one channel 5 to 10 metres wide, with two nearly vertical channels (chimneys) that do not reach the surface. The cave was named after the drystone wall at its entrance.

Given its morphology, external weather conditions have a strong influence on the microclimate inside the cave, seen by the drop in humidity and increasing air temperature in the cave during the summer months. The average air temperature in the cave at a site off the marked trail, i.e., not exposed to artificial light, is 16.1°C, while the mean relative humidity is about 95.5%. The cave is mostly dry even in the wettest conditions, with visible percolation visible only at a few sports where small tubelike stalactites and flowstones have formed, with spreading stalagmites on the floor underneath.

Numerous ceramic fragments have been found in the cave, clearly indicating the presence of all Neolithic cultures of the Adriatic area (Impresso, Danilo and Hvar cultures), and of the Eneolithic period and Early and Middle Bronze Age. This proves the continuous human presence in the cave from the Early Neolithic period, from about 5000 until about 1500 B.C. Important discoveries have been made within the cave such as flint artefacts (small knife), stone tools (hand-held millstone), bones of various animals (sheep, deer, wildcat) and fragments of mussel shells. Two children’s skeletons were unearthed, lying in the foetal position on the right side, like a child in a mother’s womb. According to their position in the vertical stratigraphy, they belong to the Neolithic period.

To date, 36 invertebrate taxa have been identified in Oziđana pećina cave, belonging to 19 higher systematic categories: mites (Acari), microwhip scorpions (Palpigradi), spiders (Araneae), centipedes (Chilopoda), beetles (Coleoptera), springtails (Collembola), millipedes (Diplopoda), true flies (Diptera), snails (Gastropoda), isopods (Isopoda), butterflies and moths (Lepidoptera), annelids (Anelida), harvestmen (Opiliones), grasshoppers and crickets (Orthoptera), pseudoscorpions (Pseudoscorpiones), booklice (Psocoptera), scorpions (Scorpiones), bristletails (Diplura) and silverfish (Thysanura). The springtails are the dominant groups, followed by crickets, spiders and isopods. These four groups together account for 84% of the total cave fauna. The cave is also habitat for bats.

The cave has been equipped and is open for visitors as part of the educational walking trail Stinice – Roški slap waterfall – Oziđana pećina cave.

The Jazinka cave is found in the canyon downstream of the Nečven fortress, at an elevation of 216 metres. The cave morphology is simple. It is 42 metres long, 4 metres deep, with a 12-metre vertical difference. After the low entrance, it opens onto a hall that continues in a 20-metre channel with many speleothems. The front part is dry, while the back end of the cave is very wet due to seeping and percolating water. Clayey sediment is present on the floor of most of the cave, and partly also on the cave walls.

The seasonal changes most affect the entrance part of the cave, as expected due to its morphology. The cave areas furthest from the entrance feature true cave conditions, with constant air temperature with a mean value of about 12.5°C.

Ceramic fragments dating to the Bronze and Late Iron Ages, animal bones, bronze arrows, a bronze fibula, twisted bronze torc necklace with chain have been found in the front part of the cave. Deeper in, human bones, glass paste beads, bronze spiral pendant, hooks and bronze needles have been found. The front part of the cave was likely used for housing, while the back part of the cave was a burial site.

To date, 29 taxa have been identified in the Jazinka cave, from the groups snails (Gastropoda), spiders (Araneae), pseudoscorpions (Pseudoscorpiones), harvestmen (Opiliones), mites (Acari), isopods (Isopoda), centipedes (Chilopoda), millipedes (Diplopoda), springtails (Collembola), bristletails (Diplura), beetles (Coleoptera), booklice (Psocoptera), true flies (Diptera), grasshoppers and crickets (Orthoptera) and butterflies and moths (Lepidoptera).

Among them are numerous taxa that are endemic to the Dinarides and Croatia. The troglobiont springtail species, Verhoeffiella margusi, is stenoendemic to Krka National Park, and the Jazinka cave is the type locality.

The troglobiont springtail species, Verhoeffiella margusi, is stenoendemic to Krka National Park, and the Jazinka cave is the type locality.

U špilji Jazinki ukupno je evidentirano dvadeset devet svojta iz skupina puževa (Gastropoda), pauka (Araneae), lažištipavaca (Pseudoscorpiones), lažipauka (Opiliones), grinja (Acari), jednakonožnih rakova (Isopoda), striga (Chilopoda), dvojenoga (Diplopoda), skokuna (Collembola), dvorepaca (Diplura), kornjaša (Coleoptera), grizlica (Psocoptera), dvokrilaca (Diptera), ravnokrilaca (Orthoptera) i leptira (Lepidoptera).

Među njima je i velik broj endemskih svojti Dinarida i Hrvatske. Troglobiontnoj vrsti skokuna Verhoeffiella margusi, stenoendemu NP „Krka“, špilja Jazinka tipski je lokalitet.

Photo: Mario Romulić & Dražen Stojčić

The underground world of karst: biospeleology

More than 500 subterranean taxa are known in Croatia and 50% are endemic. Many are relicts of fauna from deep in the geological past.

Biospeleology is a scientific branch that studies subterranean animals, their habitats and their mutual relationships. The name comes from three words of Greek origin: bios, meaning life, speleos, meaning cavity, and logos, meaning science. This is a synthesis of sciences that bring together two fundamental scientific disciplines: biology and speleology.

Biospeleologists are scientists and researchers that enter into caves and pits in search of subterranean animals, who collect and photograph these animals, and then in the laboratory determine which species they have found and study them in great detail. When a previously unknown species is found, the animal is described in detail, given a scientific name, and this is published in a scientific journal to make this information about the new find available to all.

The word biospeleology was first used by Armand Vire of France in his publication La Biospeologie published in Paris in 1904. In it, he described the science which researches the living creatures that inhabit the underground.

Speleological structures are inhabited by animals that are partly or completely adapted to the demanding conditions of life underground, such as a lack of light, scarcity of food, and very high air humidity. These animals live, or only visit, these habitats for various reasons, from taking shelter from unfavourable conditions on the surface, hibernation, or reproduction.

Until 1832, when the narrow-necked blind cave beetle Leptodirus hochenwartii was discovered in the Postojna Cave in Slovenia, it was believed that cave habitats did not contain life. This discovery stimulated new research of underground habitats, especially caves. To date, more than 7500 subterranean taxa (species and subspecies) have been described in the world, with some 1200 of these found in the Dinarides Mountain range. More than 500 subterranean taxa are known in Croatia, nearly 7% of the total number of global species. More than 50% of subterranean taxa are endemic. Many are relicts of fauna from deep in the geological past. The cave habitats of numerous species are dominated by the beetles, followed by crustaceans, molluscs, pseudoscorpions, spiders and centipedes. These groups account for nearly 90% of all known cave taxa.

In addition to the many species found in Croatia, the country also has certain unique representatives of the subterranean fauna, such as the only known freshwater cave sponge, the Ogulin cave sponge Eunapius subterraneus, the only known freshwater subterranean bivalve mollusc, the Dinaric cave bivalve Congeria kusceri, the only European freshwater subterranean vertebrate (amphibian), the olm Proteus anguinus, the only subterranean freshwater cnidarian Velkovrhia aenigmatica, the only subterranean freshwater arthropod, the Dinaric cave-dwelling tube worm Marifugia cavatica, and the only flying troglobiont in the world, the orthoclad Troglocladius hajdi.

The lack of study of speleological structures in the area of Krka National Park is the main reason for the relatively late start of biospeleological research here. It was not until the 1960s that Slovenian biospeleologists made the first important discovery of subterranean fauna in the present day area of Krka National Park: they described the subspecies of the cave-bug Monolistra pretneri spinulosa (Cave near the mill at Miljacki – Miljacka IV), the order of the stygobiont snail Dalmatella with the type species D. sketi, and the species of stygobiont snail Lanzaia skradinensis (karst spring near Skradinski Buk). The first comprehensive biospeleological research in the broader area of Krka National Park was conducted in 1989 and 1998 by the staff of the Croatian Natural History Museum. The Croatian Biospeleological Society, in cooperation with the Public Institute of Krka National Park, conducted an inventory of the cave fauna in 2005 and 2010, and in 2013 conducted research of rare and new cave taxa. Throughout the park area, speleological and biospeleological research is ongoing, in addition to monitoring of bat nurseries and olm populations.

Photo: Branko Jalžić

The underground world of karst: cave fauna

The caves of the Dinaric karst are the richest in the world in the number of cave species of which 70% are endemic

Animals that inhabit underground areas developed from aboveground ancestors, developing adaptations suited for cave habitats over long time periods. In developing these new traits, through regressive evolution, they simultaneously lost others (loss of individual acquired traits and development of simpler morphological traits).

Some of the main adaptations to underground life are the reduction of sight organs, loss of pigment, thinning of the integument (skin), elongation of limbs, slowed metabolism, longer life, low level of reproduction, accumulation of fat reserves, reduced aggressiveness, loss of the circadian rhythm of activity, loss of seasonal changes and activities, and changes to brain function (increased avoidance of barriers and better spatial memory).

The animals found underground can be divided into:

– troglobionts on land and stygobiont in water – inhabitants of cave habitats whose entire life cycle takes place underground

– eutroglophiles on land and eustygophiles in water – inhabitants of cave habitats that can have both underground and aboveground populations, i.e. they can spend their entire life underground or above ground

– subtroglophiles on land and substygophiles in water – occasional residents of cave habitats that use caves for specific parts of their life cycle, such as hibernation, reproduction, raising young, taking shelter from inclement weather

– trogloxenes on land and stygoxenes in water – incidental residents of cave habitats.

The caves of the Dinaric karst are the richest in the world in terms of the number of cave species living within. There is virtually no island or mountain that does not have at least a few of its own endemic taxa of cave fauna. Most cave species have very limited distribution ranges, and of the known species in Croatia, nearly 70% are endemic.

Greater attention should be paid to raising awareness of this high abundance of the unique endemic cave species of the Dinaric fauna and Croatian fauna, and the need to preserve this heritage as a priceless treasure.

According to the Red Book of Cave Fauna of Croatia, 186 cave taxa are threatened in accordance with the threat criteria of the International Union for the Conservation of Nature (IUCN). One of the greatest threats is the pollution of underground habitats.

With the proclamation of the Natura 2000 ecological network, areas important for the conservation of European threatened species and habitats were defined. As such, the subterranean habitats of Croatia have been recognised as natural habitats of interest for the EU. About 400 speleological structures are listed in the category ‘Caves and pits closed to the public’ (code 8310) and 220 as ‘Submerged or partially submerged sea caves’ (code 8330). Three troglobionts are also included on the Natura 2000 list of target species: narrow-necked blind cave beetle Leptodirus hochenwarti, southern Dinaric cave bivalve Congeria kusceri and the olm Proteus anguinus, while 12 species of bats are also listed.

To date, 170 cave taxa have been recorded in the speleological structures in the area of Krka National Park, of which many are endemic to the Dinarides and to Croatia, and four are endemic to Krka National Park (stenoendemic): the snails Dalmatella sketi and Lanzaia skradinensis, the centipede Eupolybothrus cavernicolus and the springtail Verhoeffiella margusi. Recent research has collected several taxa that, upon preliminary examination, appear to be new species to science, but they have not yet been scientifically described.

Photo: Branko Jalžić

The underground world of karst: the olm

The olm, the only stygobiont species that inhabits the karst region of Croatia, is likely the best known subterranean resident

It is the only true subterranean vertebrate, a paleoendemic species (living fossil) of the Dinarides mountain range, and is completely adapted to life in cave conditions. It inhabits the karst underground areas in northern Italy, Slovenia, Croatia and Bosnia and Herzegovina. In Croatia, it was first recorded in 1840 at the Goručica spring near Sinj.

Due to the cold and scarcity of food in the cave habitats it is found in, this amphibian species reaches sexual maturity still in the larval phase, and remains in this phase its entire life (neoteny). Behind the head, there are three pairs of pinkish to bright red external gills on the neck, a characteristic of the larval stage of tailed amphibians. Since it never completes metamorphosis and enters the adult stage, like most other amphibians, the olm lives its entire life as an aquatic organism.

The base colour of the body is white, with a pinkish tinge (due to the capillaries right under the surface), and due to its similarity with human skin, in Croatia this species is called the “man fish”. Due to living in conditions of constant darkness, the eyes are only visible in young individuals, while adults have tiny eyes covered in a thin layer of skin and are not visible. Only one olm population, in Bela Krajina in Slovenia, have a dark grey skin colouration and visible eyes. The body is slender and elongated, from 25 – 35 cm (max 40 cm) from the tip of the snout to the tip of the tail. Since it very rarely leaves the water, the legs are small, with three toes on the forelegs and two toes on the hind legs, that serve only for minimal movements in seeking food. The tail is laterally flattened, exceptionally strong, and serves for movement through the water. In oxygen rich waters, it breathes through the gills and through the skin, while in hypoxic conditions it can also breathe with its lungs. It feeds on aquatic cave invertebrates, such as crustaceans, snails and insect larvae, and can live up to eight years without food. The olm is a small tailed amphibian, and weighs 15–20 grams. Its development is very slow, and it reaches adulthood after 14–18 years. Since it can live for over 100 years (68.5 years in captivity), the olm is the longest-living amphibian. It prefers clean, oxygen rich waters and relatively low temperatures, from 5 to 15°C. It is usually found in the deep part of caves, and may occasionally be found in shallow underwater lakes in search of food. During high spring rains, individuals may be forced out of the underground with the rushing water through karst springs and estavelles.

It usually lives in small groups. During mating, the males become exceptionally territorial. When a female enters into a male’s territory, the courtship begins. During courting, the male attracts the female by releasing hormones into the water, waving its tail to direct them towards her head. The courting ritual reaches its climax when the male lays a packet of sperm (spermatophore) on the surface, which the female then picks up with her cloaca, resulting in internal fertilisation of eggs. Courting can be repeated multiple times over several hours. When she leaves the male’s territory, the female seeks out an adequate place to lay her eggs. After 2–3 days, she begins to lay the eggs under rocks and continues to guard them until the young larvae hatch. She repeats this for 25 days, and can lay more than 70 eggs, giving rise to the next generation of larvae. The eggs are round, white, about 5–6 mm long. The incubation period varies and depends largely on temperature (at 15°C lasts about 85 days).

In Croatia, the olm inhabits caves and sinkholes of karst fields, from Istria to Dubrovnik. There are three separate populations that inhabit the regions of Istria, Gorski Kotar and the Gacko Polje field (northern Lika), and Dalmatia (from the Krka River to Dubrovnik). It is interesting that in the area of the Lika Polja field, Mt. Velebit and Zrmanja River, it has never been recorded (this is assumed to be due to a hydrogeological barrier). The olm population in Istria has been confirmed through genetic research to be a separate species – the Istrian olm. In Croatia, the olm is a strictly protected species, and at the European level it is considered a priority species and listed in the Annexes of the EU Habitats Directive. It has also been included on the list of the EDGE Conservation programme, aimed at protecting and conserving threatened species that are unique in their evolution and that are irreplaceable parts of the world natural heritage.

Despite protection at the national and European levels, this species was assessed to fall within the category of endangered species in the most recent assessment of threats to amphibians and reptiles in Croatia. The basic cause of its threat is the degradation of karst habitats due to the pollution of ground waters, and works that alter the hydrological region of ground waters.

The olm was discovered in Krka National Park in 1989. Monitoring of the species in the park area has shown that this population is stable.

Photo: Branko Jalžić

The underground world of karst: bats

Bats have been recorded as present in 15 speleological structures within Krka National Park

Bats make up about one-fifth of the total number of mammals in the world. They differ from all other mammals due to their ability to fly and their use of sonic waves (echolocation) for orientation. They can live up to 40 years. The body is covered with fur consisting of short dense to downy-dense hairs, and the colour is usually darker on the back than on the stomach. The neck is short, head relatively large with a large mouth and strong jaws containing large canine teeth and sharp molars. The annual cycle of bat activity is both seasonal and weather dependent.

In winter, the cold weather limits the amount of food available for insectivorous bat species. Therefore, bats usually hibernate through the winter to save energy. In spring, their body temperature rises with the increasing external air temperature, and they again become capable of flying and hunting. In spring, the embryos begin to mature in females that mated the previous fall. In summer, pregnant females form nursing colonies, while the males are most often separated from the females and young. Females usually give birth to only one offspring each year, those some species may occasionally have two. The offspring are born in June to early July and feed on milk for several weeks. The young are capable for flight after about a month. At the end of summer, the males become more active and begin courting the females. Some shelters, particularly speleological structures, are often used as gathering spaces, with bats congregating here to mate, and later they seek out new shelters and show their young the new locations. By the end of autumn, they move to their winter habitats to begin hibernation, where they remain during the cold months, individually or in colonies that can include up to several thousand individuals.

Many species combine different types of shelters (speleological structures, attics, forests, wall cracks, cliffs and trees), depending on the season, opportunities available, and the environmental conditions. Therefore, shelters are classified based on their function, into hibernation, nursing colony, aggregation, and transitional (occasional) shelters.

Speleological structures can serve as constant shelters for many generations of bats. Cave species of bat, such as the species of the genus Rhinolophus, are often faithful to the same locations throughout their lives. Bats can used underground habitats in all phases of their life cycle, depending on the air temperature within them. They prefer speleological structures with a dynamic atmosphere, with good air circulation, which also means that the temperature within them fluctuates.

From dusk until the early morning hours, bats leave the shelter in search of food. They pass over open habitats (meadows, pastures), shrubby vegetation and forest habitats, habitats near aquatic surfaces and urban areas, based on the ecological characteristics of the species.

Most European bat species primarily feed on insects. For that reasons, bats play an important ecological role as a natural pesticide, since one bat can catch from 500 to 1000 insects per hour. Bats are also considered very good bioindicators, and their abundance is a direct indicator of the health and stability of the ecosystem, while declining bat populations may be the result of the impacts of human activities.

Of the 45 bat species recorded in the European Union, 34 species are found in the Republic of Croatia, making Croatia the country with the highest bat biodiversity in all of Europe. The bat fauna in the broader area of Krka National Park has been researched sporadically since the mid 19th century. According to research to date, at least 22 bat species inhabit the park area, including the common bent-wing bat Miniopterus schreibersii, lesser mouse-eared bat Myotis blythii, long-fingered bat M. capaccinii, Geoffrey’s bat M. emarginatus, greater mouse-eared bat M. myotis, Blasius’ horseshoe bat Rhinolophus blasii, Mediterranean horseshoe bat R. euryale, greater horseshoe bat R. ferrumequinum and lesser horseshoe bat R. hipposideros. All these species use caves and pits as shelters.

In the broader area of Krka National Park, nine speleological structures have been included on the list of internationally important subterranean shelters for bats (Miljacka II, Topla pećina, Špilja izvor Krke / HE Krčić, Škarin samograd, Tradanj, Stražbenica, Mandalina, Dobra voda and Ćulumova pećina). Given the great mobility of bats and their range of movement during feeding, and their daily and seasonal migration patterns, the park area is exceptionally important for their conservation.

Bats are susceptible to changes in habitat conditions and disturbances. Loss and degradation of bat shelters is caused by a range of human activities. The decreasing numbers of insects, due to the use of various pesticides and herbicides, and due to habitat change such as the loss of wetland and forest habitats, also negatively impacts bat populations. Furthermore, there is still much misunderstanding and prejudice about bats, due to a lack of knowledge about their lives and their ecology.

Bats have been recorded as present in 15 speleological structures within the park boundaries in surveys conducted by many speleologists.

Bats have been recorded as present in 15 speleological structures within the park boundaries in surveys conducted by many speleologists.

Photograph: Daniela Hamidović

The underground world of karst: Dinaric cave tubeworm

The Dinaric cave tubeworm inhabits the submerged speleothems in the Pit opposite Torak in Krka NP

The Dinaric cave worm, Marifugia cavatica Absolon & Hrabe, 1930, is a paleoendemic species of the Dinarides. This stygobiont species inhabits the underground freshwaters of the Dinaric karst from Italy to Albania, and is the only known representatives of the polychaetes, tubeworm family Serpulidae, to live a subterranean life. It is believed that during the Pliocene, in inhabited surface lakes.

This is a sessile organism, living attached to the rock (or other hard substrate). It builds a limestone tube around its body, about 1 mm in diameter, and up to 6 cm long. Along the tube is an irregularly toothed and interrupted ridge, with irregularly spaced rings. It extends its feathery branches outside the tube to catch and filter organic matter from the water. In unfavourable conditions, it retracts the entire body back into the tube, and closes the operculum. This adaptation has enabled it to survive low water levels underground, and even longer periods without water. It typically inhabits waters in the temperature range of 4 to 19°C. Depending on the speed of the current in the ground waters, it can live individually or in colonies. Dense colonies are built in calmer flow area, with numerous other aquatic cave organisms living between the limestone tubes of this tubeworm. There are some 70 localities of this species known in Croatia, and with the exception of the K2 cavern on the island of Brač, all are on the mainland.

It is interesting that this species inhabits all localities where other subterranean sessile species are found, such as the Dinaric cave clam Congeria kusceri Bole, 1962 and the Ogulin cave sponge Eunapius subterraneus Sket and Velikonja, 1984.

  1. In Croatia, M. cavatica is a strictly protected species. Its colonies are a unique habitat in the National Habitats Classification, H Marifugia deposits.

As a sessile species, it is directly threatened by changes to the water regime following different hydrotechnical works, and different types of pollution that can enter into the ground water.

In speleodiving research of the structure Pit opposite Torak in 1998, the first locality of this stygobiont polychaete in Krka National Park was found. In this cave, the Dinaric cave tubeworm colonizes submerged speleothems. The Pit opposite Torak was submerged with the rising water levels in Visovac Lake and the growth of the travertine barrier at Skradinski buk. The dive survey explored the structure to a depth of 17 metres, and confirmed a connection between all three lakes in the pit.

Photo: Vedran Jalžić