The paper provides the first detailed account on the taxonomic richness of the subterranean freshwater triclads from Sardinia, including the description of four new species for the genera Dendrocoelum and Phagocata. New records for Dugesia benazzii, Dugesia sp., Crenobia alpina, and Phagocata sp. are also reported. The three new species of Dendrocoelum are the first reported for the island of Sardinia. These species display a bursal canal sphincter and a large adenodactyl with a characteristic anatomy with a zone of fine circular muscle fibers running through the mesenchyme of its papilla. A detailed analysis of the structure of the penial flagellum in the genus Dendrocoelum highlighted six main conditions, some of which have not been previously reported, in regard to the histology of the tip of the penis papilla and the extent of its inversion. The new species of Phagocata represents the first species recorded from Italy and the first anophtalmous species reported from Europe.

Although hypogene cave systems have been described since the beginning of the 20th century, the importance in speleogenesis of ascending fluids that acquired their aggressiveness from in-depth sources has been fully realized only in the last decades. Aggressiveness of waters can be related to carbonic and sulfuric acids and the related corrosion-dissolu­tion processes give rise to different types of caves and under­ground morphologies.

The abundance of hydrothermal springs and associated traver­tine deposits, and the widespread interaction between volcanic or sub-volcanic phenomena and karst in many sectors of the Ital­ian peninsula are a strong evidence of hypogene speleogenesis. Furthermore, researches on secondary minerals have allowed to discover hypogene caves formed by highly acidic vapors in sub­aerial environments, also showing that most of these caves have extremely rich mineral associations.

Despite this, until the late 1980s the only known important cave systems of clear hypogene origin in Italy were considered to be the ones hosted in the Frasassi Canyon and Monte Cucco, in which important gypsum deposits undoubtedly showed that sulfuric acid played an important role in the creation of voids (Galdenzi, 1990, 2001; Galdenzi & Maruoka, 2003; Menichetti et al., 2007). Afterwards many other caves were categorized as formed by the sulfuric acid speleogenesis throughout the entire Apennines. Following the broad definition of hypogene caves by Palmer in 1991, and the even more general one of Klimchouk in the last decade (Klimchouk, 2007, 2009), the number of caves considered of hypogene origin in Italy has grown rapidly. Figure 1 shows the hypogene karst systems of Italy, including, besides the well-known and published ones, also the known and less studied, and presumed hypogene cave systems (see also Table 1).

More recently, in some of these caves detailed studies have been carried out including geomorphology, mineralogy, and geochem­istry. Sulfuric acid caves are known from many regions along the Apennine chain (Tuscany, Umbria, Marche, Latium, Campa­nia, Calabria) (Forti, 1985; Forti et al., 1989; Galdenzi and Me­nichetti, 1989, 1995; Galdenzi, 1997, 2001, 2009; Galdenzi et al., 2010; Piccini, 2000; Menichetti, 2009, 2011; Mecchia, 2012; De Waele et al., 2013b), but also from Piedmont, Apulia, Sicily (Vattano et al., 2013) and Sardinia (De Waele et al., 2013a). In this last region ascending fluids have also formed a hypogene cave in quartzite rock. Oxidation of sulfides can locally create hypogene cave morphologies in dominantly epigenic caves, such as in the Venetian forealps (this cave is not shown in Figure 1, being largely epigenic in origin) (Tisato et al., 2012). Ascend­ing fluids have also created large solution voids in Messinian gypsum beds in Piedmont, and these can be defined hypogene caves according to the definition by Klimchouk (Vigna et al., 2010). Some examples of hypogene cave systems due to the rise of CO2-rich fluids are also known in Liguria and Tuscany (Pic­cini, 2000). In the Alps and Prealps (Lombardy), some ancient high mountain karst areas exhibit evidences of an early hypo­gene origin, deeply modified and re-modeled by later epigenic processes. Hypogene morphologies are thus preserved as inac­tive features, and it is often difficult to distinguish them from epigenic ones.

At almost twenty years distance from the first review paper on hypogene cave systems in Central Italy by S. Galdenzi and M. Menichetti (1995), we give a review of the state-of-the-art knowledge on hypogene caves actually known from the whole of Italy

Many speleothems can be assigned to one of two morphological groups: massive speleothems, which consist of compact bulks of material, and coralloids, which are domal to digitate in form. Faster growth on protrusions of the substrate occurs in the typical growth layers of coralloids (where those layers are termed “coralloid accretions”), but it is not observed in the typical layers of massive speleothems, which in contrast tend to smoothen the speleothem surface (and can therefore be defined as "smoothing accretions"). The different growth rates on different areas of the substrate are explainable by various mechanisms of CaCO3 deposition (e.g., differential aerosol deposition, differential CO2 and/or H2O loss from a capillary film of solution, deposition in subaqueous environments). To identify the causes of formation of coralloids rather than massive speleothems, this article provides data about d13C and d18O at coeval points of both smoothing and coralloid accretions, examining the relationship between isotopic composition and the substrate morphology. In subaerial speleothems, data show an enrichment in heavy isotopes both along the direction of water flow and toward the protrusions. The first effect is due to H2O evaporation and CO2 degassing during a gravity-driven flow of water (gravity stage) and is observed in smoothing accretions; the second effect is due to evaporation and degassing during water movement by capillary action from recesses to prominences (capillary stage) and is observed in subaerial coralloids. Both effects coexist in smoothing accretions interspersed among coralloid ones (intermediate stage). Thus this study supports the origin of subaerial coralloids from dominantly capillary water and disproves their origin by deposition of aerosol from the cave air. On the other hand, subaqueous coralloids seem to form by a differential mass-transfer from a still bulk of water towards different zones of the substrate along diffusion flux vectors of nutrients perpendicular to the isodepleted surfaces. Finally, this isotopic method has proved useful to investigate the controls on speleothem morphology and to obtain additional insights on the evolution of aqueous solutions inside caves.

Many speleothems can be assigned to one of two morphological groups: massive speleothems, which consist of compact bulks of material, and coralloids, which are domal to digitate in form. Faster growth on protrusions of the substrate occurs in the typical growth layers of coralloids (where those layers are termed “coralloid accretions”), but it is not observed in the typical layers of massive speleothems, which in contrast tend to smoothen the speleothem surface (and can therefore be defined as "smoothing accretions"). The different growth rates on different areas of the substrate are explainable by various mechanisms of CaCO3 deposition (e.g., differential aerosol deposition, differential CO2 and/or H2O loss from a capillary film of solution, deposition in subaqueous environments). To identify the causes of formation of coralloids rather than massive speleothems, this article provides data about d13C and d18O at coeval points of both smoothing and coralloid accretions, examining the relationship between isotopic composition and the substrate morphology. In subaerial speleothems, data show an enrichment in heavy isotopes both along the direction of water flow and toward the protrusions. The first effect is due to H2O evaporation and CO2 degassing during a gravity-driven flow of water (gravity stage) and is observed in smoothing accretions; the second effect is due to evaporation and degassing during water movement by capillary action from recesses to prominences (capillary stage) and is observed in subaerial coralloids. Both effects coexist in smoothing accretions interspersed among coralloid ones (intermediate stage). Thus this study supports the origin of subaerial coralloids from dominantly capillary water and disproves their origin by deposition of aerosol from the cave air. On the other hand, subaqueous coralloids seem to form by a differential mass-transfer from a still bulk of water towards different zones of the substrate along diffusion flux vectors of nutrients perpendicular to the isodepleted surfaces. Finally, this isotopic method has proved useful to investigate the controls on speleothem morphology and to obtain additional insights on the evolution of aqueous solutions inside caves.

Thiswork reports the results of geomorphological observations carried out in the coastal Fico Cave and surrounding areas (Baunei, Central East Sardinia) in the Gulf of Orosei. A tidal notch, generally believed to be of Eemian (MIS 5e) age, is barely visible at 8.5 above present sea level (asl), some metres below the main entrance of the cave. Old cave passages, now partially opened by cliff retreat and parallel to the coastline, are clearly visible at around 14 m asl and correspond to the main level of Fico Cave. Two more notches are located higher, at 22 and 50 m asl. Fico Cave itself is composed of at least 6 clearly distinguished more or less horizontal levels (−10 m below present sea level (bsl), and +14, +22, +40, +50, and +63 m asl), independent of the stratal dip, arguing for a sea-level, and hence, fresh-water lens control. Cave passages develop along main fractures more or less parallel to the coastline and never extend landward for more than 150 m, mostly ending blindly, or diminishing in their dimensions progressively landward. Most passages only contain clay deposits, lacking fluvial or marine sediments or typical fluvial erosion morphologies (i.e. scallops).

It is suggested from this body of evidence that Fico Cave was formed in the coastal mixing zone along major discontinuities during several Quaternary interglacial periods, when sea level was high and relatively stable for enough time to develop large dissolutional voids. The geomorphological observations indicate the main +14 m asl level of the cave to have formed during MIS 9, and was heavily reworked during MIS 5, while the higher levels are relative to older interglacial highstands that occurred between 1 Ma and 325 ka. The small active branch developed below present sea level has formed during MIS 7 (225 ka). These observations shed new light on the position of the MIS 5e highstand markers in this area of the coast, much higher than previously thought.