We explore the population of the troglophilic harvestman Ischyropsalis ravasinii inhabiting the Buso del Valon ice cave located in the Italian Prealps. Spatial and temporal distributions of the specimens are investigated in relation to the variation of environmental abiotic conditions in the cave, such as the seasonal temperature and substrate surface typology. Our results show that I. ravasinii is distributed unevenly in the cave, most of individuals being present in the scree-covered section of the cave with superficial activities limited to the warm seasons only. In addition, our data suggests that the presence of a thick layer of rocky debris, together with high humidity and cold temperatures, are important limiting factors for the species. Seven additional species of harvestman are recorded in the cave, including the congeneric troglophilic species Ischyropsalis strandi. This is the first known record of these two troglophilic Ischyropsalis species coexisting within the same cave. An updated map of the distribution of I. ravasinii and I. strandi in the Italian Prealps is provided.

Age classes, global warming, Lessinia Mountains, Northern Italy, seasonality, subterranean environment

Harvestmen (Arachnida: Opiliones) are one of the largest orders within the class Arachnida, numbering 65 families and 6637 species (Blick and Harvey 2011; Kury et al. 2020). Harvestmen show highly diverse morphology and biology, allowing them to successfully colonize a large number of habitats including terrestrial and subterranean habitats. The genus Ischyropsalis C.L. Koch, 1839 (Family Ischyropsalididae Simon, 1879) contains some of most iconic European harvestmen, which are easily recognizable by their relatively large body size, massive, prominent chelicerae and dark coloration. Numbering 22 species, all geographically limited to Europe, Ischyropsalis species are characterized by a high level of endemism. They are often restricted to a single mountain chain (Schönhofer 2013; Schönhofer et al. 2015). They frequently show frigophilic and hygrophilic habits, having marked preferences for microhabitats with low temperature and constantly high humidity (Martens 1969; Schönhofer et al. 2015). Recent studies also suggest a direct relationship between the current distribution of Alpine Ischyropsalis species and the Pleistocene glaciations (Mammola et al. 2019). Although in high mountains these arachnids can be found in open or shallow humid habitats, such as scree and mossy landscapes, at lower altitudes they inhabit caves and other subterranean habitats. Thus, several Ischyropsalis species display an affinity for hypogean habitats and different degrees of adaptations to the subterranean life, including numerous obligate cave-dwellers (Marcellino 1982; Schönhofer et al. 2015). Despite their relevance as part of the European cave-dwelling fauna, the ecology of this genus has been scarcely explored (Martens 1969). To date, only six species have been partially analyzed in terms of ecology and life cycle, and most of the data rely on old studies carried out over 50 years ago (e.g. I. luteipes Simon, 1872 and I. pyrenaea Simon, 1872 see Juberthie 1961; I. strandi Kratochvil, 1936 see Juberthie 1963; I. kollari C.L. Koch, 1839 see Martens 1969; I. dentipalpis Canestrini, 1872 and I. lithoclasica Schönhofer & Martens, 2010 see Schönhofer and Martens 2010). For most known cave-dwelling Ischyropsalis no updated information concerning their life cycle, seasonality, micro-habitat preference or even area of distribution are available.

In caves, where the environmental conditions remain rather constant along the year (Badino 2010), abiotic factors represent a critical element to define the structure and composition of the subterranean communities (Howarth 1980). Local variations may deeply affect the spatial and temporal distribution of the cave-dwelling arthropods (Latella et al. 2008). In particular, frigophilic cave species have adapted to living in cold subterranean habitats characterized by low temperature and the presence of an ice or snow layer throughout the year, albeit with some seasonal variation (Iepure 2018).

Located in the Italian Prealps (Fig. 1B) at relatively low altitude, the Buso del Valon ice cave (cadaster number: 438 V/VR) has a permanent internal ice body and superficial snow (Zorzin et al. 2015). The cave hosts a well-defined assemblage of arthropods adapted to cold environments including three species of the wingless limoniid crane flies of the genus Chionea Dalman, 1816 (Avesani and Latella 2016; Latella et al. 2019). It also shelters a large population of the harvestman I. ravasinii Hadži, 1942 (Fig. 1D), a troglophilic species endemic to the Venetian Prealps in North-East Italy. Due to its unique features, the Buso del Valon ice cave represents an ideal natural laboratory to explore the phenology and microhabitat preference of frigophilic subterranean species (Avesani and Latella 2016). With this study, we aim to complete our knowledge of the life cycle and spatio-temporal distribution of I. ravasinii living in cold subterranean environments in relation to the cave substratus and seasonal changes. We further plan to use data herein collected as a starting point for long-lasting studies aiming to monitor the effects of climate change on the frigophilic cave fauna and its resilience to changes in micro-habitat conditions (see e.g. Howarth, 2021).

Among the harvestman fauna inhabiting the Buso del Valon ice cave, two coexisting species belonging to the genus Ischyropsalis were collected: I. ravasinii and I. strandi. Both the species belong to the Alpine clade sensu Schönhofer et al. 2015, and show a reduced distribution along the Venetian Prealps. Ischyropsalis strandi is endemic to the Lessini and Baldo mountains, while I. ravasinii extends its distribution to the East toward the Cansiglio plateau. Thus, the Buso del Valon ice cave represents the southernmost record for I. ravasinii (Fig. 1B). The distributions of both species overlap in the western part of the Venetian Prealps (Schönhofer et al. 2015). Our data are in agreement with this finding (Fig. 1B). Despite being sympatric, to our knowledge this is the first known case of coexistence of these two troglophilic Ischyropsalis species within the same cave.

Ischyropsalis ravasinii appears to prosper inside the Buso del Valon ice cave, forming a large population and being the most abundant representative of the local harvestman fauna. Additionally, the lack of specimens collected outside the cave corroborates the strong affinity of I. ravasinii for subterranean habitats. However, the population of I. ravasinii is not uniformly distributed inside the Buso del Valon ice cave. This species shows a marked preference for the micro-habitat formed by the thick scree in the central part of the cave, being most abundant near the border of the permanent ice during both the warm and cold seasons. Most of the specimens were sampled from the central part of the cave, including juveniles belonging to all six instars and all the adults. A similar distribution pattern seems to be followed by I. strandi although on a smaller scale. Ischyropsalis ravasinii is a troglophilic-hygrophilic species strictly bound to high humidity to deposit eggs (Juberthie 1964, 1965; and observations with specimens in captivity). It needs a cool environment of about 4–6 °C to remain active as observed with captive specimens. The interstitial spaces within the rocky debris may contribute to retain the suitable humidity and temperature for the harvestmen survival throughout the year. The scree likely represents the most stable micro-habitat inside the cave in both the seasons, consequently serving as an ideal habitat for egg deposition and juvenile development. Other areas of the cave pose colder or drier conditions, at least for a part of the year (Fig. 2A), or lack a coverage of debris that can be used as a refuge, thus being a less suitable habitat for this species.

Our data suggests a conspicuous difference in the seasonal distribution of I. ravasinii in the scree-covered area of the Buso del Valon ice cave. During the warm season, when the slightly warmer temperatures and the reduced extension of the ice coverage allows surface activity, I. ravasinii seems to be similarly present in both superficial and deep layers within the scree, with a preference for being close below the surface. In contrast, very few specimens were sampled on the surface during the cold season, most collections occurred in the DPS trap only. Lower surface temperatures and the presence of a larger and thicker layer of ice and snow in comparison to the warm season, most likely hinder the surface activity of I. ravasinii during the cold season. Such conditions may force I. ravasinii to move deeper into the scree where the micro-climatic conditions remain more suitable.

Martens (1969) reports that the reproductive season in Ischyropsalis spp. extends from spring to early summer, with egg hatching in late summer/autumn. Similar results were observed by us with I. ravasinii eggs hatched in captivity. Accordingly, the presence of the highest number of early instars (IN 1–3) collected in the cave during the cold season, together with the low numbers of adults, supports this hypothesis. The high percentage of the subadult IN 5 found during the warm season also suggests that the final maturation of this species occurs mainly during the warmer period. Therefore, it is likely that juvenile I. ravasinii need at least nine months to reach adulthood and probably even longer. Such hypotheses are in line with our experimental observations with specimens raised in captivity. After reaching their maturation, adults of some Ischyropsalis species may live for several months (e.g. I. kollari, see Martens 1969). The records of adults collected in the Buso del Valon ice cave during both the warm and cold periods also support this hypothesis. Dead adults of this species have occasionally been found in caves during the early summer months (July and August, Petri I. personal observation 2020) and captive adults have survived several months before dying. Such findings imply that the life cycle of I. ravasinii extends for more than one year, possibly around two or more years, as reported for other Ischyropsalis species (see Juberthie 1968) and experimentally tested by us hatching and raising specimens in captivity.

The present study offers new data on the spatio-temporal distribution of the troglophilic harvestmen I. ravasinii adapted to living in cold subterranean environments. We investigated for the first time the ecological and seasonal preferences for microhabitats in I. ravasinii and we report additional data on its life cycle. Despite being the dominant harvestman in the cave, this species is absent in nearby cavities, including artificial tunnels, which are instead occupied by I. strandi. Since I. ravasinii seems to strongly rely on the presence of stable, humid and cool habitat, the Buso del Valon ice cave may provide refuge for this species similarly to other local frigophilic arthropods. In addition, the typology of substrate seems to play an important role in its survivability, the wide majority of individuals being collected in the scree-covered area of the cave.

Due to its strict bond with specific environmental conditions I. ravasinii may be strongly affected by even limited changes occurring to its habitat. Following the rise in temperatures related to climate change, and the consequent progressive reduction of the internal ice body, the conditions of the microhabitats inside the Buso del Valon ice cave are changing at a fast pace (Latella et al. 2019). Such changes may threat the species survivability similarly to what is occurring to other arthropods living in ice caves (Mammola et al. 2019; Howarth 2021). Additional studies on the ecology of I. ravasinii in the Buso del Valon ice cave may help us to explore how this or other frigophilic subterranean arthropods face the long-term effects of climate change.

The authors would like to thank the speleologists of the Commissione Speleologia Veronese (CSV) who took part in the field research and the Parco Naturale Regionale della Lessinia for the authorization to carry out the field research. Special thanks to Giorgio Annichini for helping with the cave surveys and to Roberta Salmaso for the research in the Museum collections. We are also thankful to Victoria Smith (New Zealand) to revise the English text of an early draft of this manuscript. Special thanks to Prof. Dr. Jochen Martens for confirming the identifications of the populations collected by the first author reported in the map (Fig. 1B). We are particularly grateful to Stefano Mammola and an anonymous referee for their detailed suggestions and advices which helped to substantially improve the work. This research was granted by the Federazione Speleologica Veneta and the Natural History Museum of Verona.