NEXT STOP: UNDERGROUND. VARIABLE DEGREES AND VARIETY OF REASONS FOR CAVE PENETRATION IN TERRESTRIAL GASTROPODS NASLEDNJA POSTAJA: PODZEMLJE. RAZLIČNE STOPNJE IN RAZLIČNI RAZLOGI PRODIRANJA KOPENSKIH POLŽEV V JAME Alexander M. WEIGAND1,2 Abstract UDC 594.3:551.44 Alexander M. Weigand: Next Stop: Underground. Variable degrees and variety of reasons for cave penetration in terrestrial gastropods Cave-dwelling animals can be classified based on their occurrence in and relationship to the subterranean environment. Subsurface distribution data and studies addressing the initial causes for animals to enter underground habitats are sparse. By retrieving occurrence data from two voluntary biospeleological collections in Central Germany, the degree of cave penetration in terrestrial gastropods was investigated, thus to infer potential evolutionary drivers. In total, 66 identified gastropod species entered the subterranean environment with 23 of the species also recorded from the dark zone. Gastropods possessed variable degrees of cave penetration and no obligate cave forms were observed. A decline of occurrence records from the entrance to the dark zone was the most prominent pattern. Nevertheless, several species were collected from all three light zones (i.e. entrance, transition and dark). A variety of potential reasons can be inferred to explain their subsurface appearance: exploitation of alternative food sources, aphotic above-ground movement, mating sites, temporal, seasonal and long-term ref-ugia, and chance. Moreover, the results imply a frequent migration of species between the interconnected light zones and the surface. Consequently, terrestrial gastropods should be considered when investigating the origin and importation of alloch-thonous resources in caves. Keywords: Gastropoda, subterranean environment, ecology, distribution data, cave colonization, speciation. Izvleček UDK 594.3:551.44 Alexander M. Weigand: Naslednja postaja: podzemlje. Različne stopnje in različni razlogi prodiranja kopenskih polžev v jame Podzemeljske živali lahko opredelimo glede na njihovo pojavljanje v podzemeljskem okolju in odnos do tega okolja. Podatki o razširjenosti živali v podzemlju in študije, ki obravnavajo vzroke za kolonizacijo podzemlja so redki. Stopnja prodiranja kopenskih polžev v jame in morebitni evolucijski vzroki so bili proučevani na podlagi dveh biospeleoloških zbirk v osrednji Nemčiji. Skupno je bilo določenih 66 vrst polžev, ki zaidejo v podzemlje, od tega 23 vrst iz temnih predelov podzemlja. Čeprav polži kažejo različne stopnje prodiranja v jame, podzemeljska oblika polžev ni bila ugotovljena. Najpogostejši vzorec razširjenosti je bil upadanje njihovega pojavljanja od vhoda proti notranjosti, čeprav je bilo po nekaj vrst vedno ugotovljenih v vseh treh conah: na vhodu, na prehodu med svetlo in temno cono ter v temni coni. Možnih je več razlogov, zaradi katerih se pojavijo v podzemlju: izkoriščanje alternativnih virov hrane, afotično gibanje na površju, paritvena mesta, začasno, sezonsko ali dolgotrajno zatočišče ter priložnost. Rezultati kažejo na pogosto migracijo vrst med posameznimi conami in površjem. Posledično bi bilo potrebno kopenske polže upoštevati pri raziskavah izvora in vnosa alohtonih virov v jame. Ključne besede: Gastropoda (polži), podzemeljsko okolje, ekologija, podatki o razširjenosti, kolonizacija jam, speciacija. 1 Goethe-University, Institute for Ecology, Evolution and Diversity, Department of Phylogeny & Systematics, Max-von-Laue Str. 13, 60438 Frankfurt am Main 2 Croatian Biospeleological Society, Demetrova 1, 10000 Zagreb, Croatia, e-mail: weiganda@gmx.net Received/Prejeto: 08.03.2013 INTRODUCTION Since the first nominal description of a cave-dwelling animal, the blind cave salamander Proteus anguinus (Lau-renti 1768), subterranean organisms have attracted people's attention. Possessing a life in permanent darkness, this curiosity immediately can be imagined. But even more so, the perception is frequently underscored by a strongly modified, sometimes bizarre morphology. Cave-dwelling adaptive features include the (often complete) reduction of eyes and body pigmentation (= reductive troglomorphies) resulting in 'blind albinos' or the elongation of body appendages and an improved extra-optical sensory system (= constructive troglomorphies; Protas & Jeffery 2012). However, not all cave species exhibit a troglomorphic appearance (Bichuette & Trajano 2003). Subterranean organisms can be ecologically classified depending on their occurrence in and relationship to the subterranean habitat (Sket 2008). Obligate cave forms (= eutroglobionts) are restricted and highly adapted to a life in darkness, whereas in some occasions, epigean animals may accidentally occur in the subterranean environment (= eutrogloxenes). Both extremes of this ecological continuum are connected by all levels of intermediate forms. Based on the formation of stable or temporal subterranean populations, they are commonly referred to as eu-troglophiles or subtroglophiles, respectively. To explain the initial phases of cave colonization and speciation between surface and subsurface popula- tions, unsuitable environmental conditions on the surface (Climate Relict Hypothesis) and the exploitation of alternative resources (Adaptive Shift Hypothesis) have been discussed as evolutionary drivers (for a review see Juan et al. 2010). Subsequently, speciation of underground lineages may have been triggered by subsurface dispersal or vicariance events leading to the formation of allopatric populations. More recently, a combination of both non-exclusive processes has been considered (Culver et al. 2007; Weigand et al. 2013). Besides studies addressing speciation processes within the underground realm or to a surface population, causes for the initial phase of cave colonizations have been studied less intensively (Camp & Jensen 2007) although frequently reconstructed from phylogenetic patterns (Leys et al. 2003; Howarth & Hoch 2005; Cooper et al. 2007). In this survey, the affinity of gastropod species to the subterranean environment is investigated with the aim to infer potential evolutionary drivers for their occurrence in underground habitats. Since gastropods are chemically oriented organisms, they do not have to overcome the disadvantage imposed by visual orientation in caves (Culver & Pipan 2009). As a consequence, a certain number of surface litter species are expected to occur in caves, thus representing multiple case studies. VARIABLE DEGREES OF SUBTERRANEAN PENETRATION Observation records of cave-dwelling terrestrial gastropods were retrieved from two voluntary collections: from the Biospeleological Register of the Hesse Federation for Cave and Karst Research (HES; Zaenker 2008; Reiss et al. 2009a) and for the region of Rhineland-Palatinate + Saarland (RP + SAR; Weber 2012) (Fig. 1). The geology of the study area is very heterogeneous but primarily consists of limestone, argillaceous shale, slate and red sandstone. In addition to natural objects (i.e. caves, deep fissures), species were recorded from artificial cavities (i.e. bunkers, tunnels, mines) totaling 3352 investigated terrestrial objects (i.e. without springs, groundwater samples). During each visit (N), terrestrial gastropods were recorded and classified based on their occurrence in the entrance (E), transition (T) or dark zone (D) of the subterranean environment. Thereby, species observations are qualitative records for a given zone rather than representing count data of specimens. In some occasions, specimens of a single species were collected in multiple zones during a visit (i.e. N = 1 but more than one light zone with an occurrence record). Taxonomic identification is based on conchological and/or anatomical characteristics and was primarily performed by S. Zaenker, D. Weber, K. Bogon and H. Kappes. Taxonomic nomenclature is according to Fauna Europaea (2012). For species with N > 15 and occurrence records from all three light zones, a Chi2-test was conducted in SPSS 12.0 (IBM) to test for equal frequency distributions. In total, 66 gastropod species were found within the subterranean environment (Tab. 1, Fig. 2), i.e. at least penetrating the entrance zone. An amount of 58% (38/66) of the identified species was shared by both geographical regions (i.e. HES vs. RP + SAR). Notable exceptions refer to Arion ater, Arion silvaticus, Ena montana and Pomatias elegans only recorded from HES and Arion rufus and Phenacolimax major, which were only present in RP + SAR. Daudebardia rufa was almost en- Fig. 1: Overview of the study area. A: The map indicates the geographical location of the two voluntary biospeleological collections for Hesse (HES) and Rhineland-palatinate + Saarland (Rp + SAR) in Germany (colored in red). B: Detailed geographical overview of the study area. tirely restricted to HES (73 observations) with a single constituted 13% of the total records (706/5589), boett-specimen found in SAR + RP. The dark zone was pen- gerilla pallens and Oxychilus spp. accounted for 56% of etrated by 35% (23/66) of all species. Although they only all dark zone observations. fig. 2: terrestrial gastropod species penetrating the subterranean environment. A: Discus rotundatus; B: Monachoides incarnatus; C: pomatias elegans; D: Leh-mannia marginata; E: Limax cinereoniger; F: boettgerilla pallens; G: Oxychilus drapar-naudi; H: Oxychilus cellarius. Copyright of the individual pictures: Klaus bogon. tab 1: Subsurface distribution data of terrestrial gastropod species. Distribution records for the different light zones are separated for two voluntary collections: the biospeleological state register of Hesse (HES) and a private collection for the region of Rhineland-palat-inate + Saarland (Rp + SAR). N: number of visits; S: surface record; E: entrance zone record; T: transition zone record; D: dark zone record. Species with records in the E + T + D zone (N > 10) were tested by a Chi2-test. # taxon morphospecies N S E T D N E T D p (Chi2) HES RP + SAR 1 Agriolimacidae Derocerasreticulatum (O. F. Müller, 1774) 2 0 2 0 0 1 1 0 0 2 Arionidae Arion sp. 513 376 119 66 13 41 25 20 1 < 0.001 3 Arion ater (Linnaeus, 1758) 18 2 12 5 1 0.006 4 Arion circumscriptus Johnston, 1828 6 0 1 0 4 3 1 0 5 Arion distinctus J. Mabille, 1868 3 0 3 0 0 6 Arion fasciatus (Nilsson, 1823) 1 0 1 0 0 1 0 1 0 7 Arion fuscus (O.F. Müller, 1774) / A. subfuscus (Draparnaud, 1805) 10 3 7 2 0 1 0 1 0 8 Arion cf. intermedius Normand, 1852 4 0 3 2 0 9 Arion lusitanicus J. Mabille, 1868 16 0 14 3 1 2 0 2 0 0.001 10 Arion rufus (Linnaeus, 1758) 38 13 25 2 < 0.001 11 Arion silvaticus Lohmander, 1937 16 1 11 4 1 0.007 12 Boettgerillidae Boettgerilla pallens Simroth, 1912 92 24 27 32 34 14 5 6 4 0.717 13 Bradybaenidae Fruticicola fruticum (O. F. Müller, 1774) 6 3 2 1 0 14 Carychiidae Carychium tridentatum (Risso, 1826) / C. minimum O. F. Müller, 1774 850 834 16 0 0 4 1 0 2 15 Chondrinidae Chondrina avenacea (Bruguiere, 1792) 1 0 1 0 0 16 Clausiliidae Alinda biplicata (Montagu, 1803) 13 6 5 3 0 7 7 0 0 17 Baleaperversa (Linnaeus, 1758) 4 1 3 0 0 18 Clausilia bidentata (Str0m, 1765) 19 18 1 0 19 Clausilia rugosa (Draparnaud, 1801) 10 0 10 0 0 5 4 2 0 20 Cochlodina laminata (Montagu, 1803) 8 2 6 0 0 10 8 1 1 21 Macrogastra attenuata (Rossmässler, 1835) 2 2 0 0 22 Macrogastra ventricosa (Draparnaud, 1801) 1 0 1 0 23 Cochlicopidae Cochlicopa sp. 84 73 10 9 0 4 2 2 0 24 Cochlicopa lubrica (O. F. Müller, 1774) 9 9 0 0 0 9 8 1 0 25 Cochlicopa lubricella (Rossmässler, 1834) 4 1 3 2 0 26 Azeca goodalli (A. Ferussac, 1821) 24 20 4 0 0 27 Daudebardiidae Daudebardia rufa (Draparnaud, 1805) 73 63 10 6 0 1 0 1 0 28 Enidae Ena montana (Draparnaud, 1801) 35 22 11 5 1 0.011 29 Merdigera obscura (O. F. Müller, 1774) 2 0 2 0 0 2 2 0 0 30 Zebrina detrita (O. F. Müller, 1774) 1 1 0 0 31 Euconulidae Euconulus sp. 39 39 0 0 0 32 Euconulus fulvus (O. F. Müller, 1774) 88 82 6 2 0 6 3 3 0 33 Ferussaciidae Cecilioides acicula (O. F. Müller, 1774) 10 9 1 0 0 34 Helicidae Cepaea sp. 49 17 28 12 2 12 10 2 0 < 0.001 35 Cepaea hortensis (O. F. Müller, 1774) 48 17 21 14 4 9 6 3 0 < 0.001 36 Cepaea nemoralis (Linnaeus, 1758) 48 11 28 13 5 34 18 13 4 < 0.001 # taxon morphospecies N S E T D N E T D p (Chi2) 37 Ariantaarbustorum (Linnaeus, 1758) 21 15 6 3 0 2 2 0 0 38 Helicigona lapicida (Linnaeus, 1758) 95 3 85 23 3 25 17 8 0 < 0.001 39 Helix pomatia Linnaeus, 1758 23 4 12 7 3 23 15 7 2 < 0.001 40 Isognomostoma isognomostomos (Schröter, 1784) 22 7 14 4 0 41 Helicodontidae Helicodonta obvoluta (O. F. Müller, 1774) 107 33 60 36 2 34 21 15 1 < 0.001 42 Hygromiidae Helicella sp. 3 3 0 0 43 Trochulus sp. 125 106 16 9 1 2 2 0 0 < 0.001 44 Trochulus hispidus (Linnaeus, 1758) 6 2 3 1 0 2 1 1 0 45 Trochulus plebeius (Draparnaud, 1805) 4 4 0 0 0 46 Trochulus sericeus (Draparnaud, 1801) 1 1 0 0 47 Monachoides incarnatus (O. F. Müller, 1774) 511 399 95 50 7 49 39 10 0 < 0.001 48 Limacidae 191 148 39 18 4 26 14 15 1 < 0.001 49 Lehmannia marginata (O. F. Müller, 1774) 16 0 15 4 2 4 4 0 0 < 0.001 50 Limaxcinereoniger Wolf, 1803 31 0 25 10 4 83 40 42 5 < 0.001 51 Limax maximus Linnaeus, 1758 59 21 30 21 5 23 12 16 3 < 0.001 52 Milacidae Tandonia rustica (Millet, 1843) 6 2 3 1 1 2 1 0 1 53 Orculidae Sphyradium doliolum (Bruguiere, 1792) 1 0 1 1 0 1 1 0 0 54 Oxychilidae Oxychilus sp. 67 24 29 23 10 31 10 18 4 0.001 55 Oxychilus alliarius (J. S. Miller, 1822) 6 3 2 0 1 56 Oxychilus cellarius (O. F. Müller, 1774) 167 7 98 88 48 244 87 136 38 < 0.001 57 Oxychilus draparnaudi (H. Beck, 1837) 12 0 10 6 1 73 31 36 10 < 0.001 58 Aegopinella nitens (Michaud, 1831) 5 2 2 1 0 3 3 0 0 59 Aegopinella nitidula (Draparnaud, 1805) 15 9 6 1 0 2 2 0 0 60 Aegopinella pura (Alder, 1830) 15 11 4 1 0 1 0 1 0 61 Nesovitrea hammonis (Str0m, 1765) 5 3 2 1 0 3 3 0 0 62 Patulidae Discus rotundatus (O. F. Müller, 1774) 655 386 226 108 19 299 174 132 14 < 0.001 63 Pomatiidae Pomatias elegans (O. F. Müller, 1774) 13 2 11 3 0 64 Pristilomatidae Vitrea crystallina (O. F. Müller, 1774) 6 5 1 0 0 2 1 1 0 65 Pupillidae Pupillamuscorum (Linnaeus, 1758) 2 1 1 0 0 66 Succineidae Oxyloma elegans (Risso, 1826) 7 6 0 1 0 67 Succinella oblonga (Draparnaud, 1801) 6 5 1 0 0 68 Valloniidae Vallonia sp. 1 1 0 0 69 Vallonia pulchella (O. F. Müller, 1774) 13 12 1 0 0 70 Acanthinula aculeata (O. F. Müller, 1774) 15 13 2 1 0 71 Vitrinidae Eucobresia diaphana (Draparnaud, 1805) 4 2 1 2 0 72 Phenacolimax major (A. Ferussac, 1807) 77 52 26 0 73 Vitrina pellucida (O. F. Müller, 1774) 14 5 8 2 0 12 7 3 2 0.002 74 Vitrinobrachium breve (A. Ferussac, 1821) 2 0 2 0 I 4331 2855 1181 608 173 1258 681 555 95 VARIETY OF REASONS FOR SUBTERRANEAN PENETRATION Knowledge about the occurrence of a species among different light environments (i.e. entrance, transition and dark zone) is important to understand the selective pressure of the subterranean environment imposed on the gene pool of the population. Numerous gastropod species were (sporadically) found within the subterranean environment. No true obligate cave forms were observed and only one third of all species possessed records in the dark zone. Those species were generally collected from other light zones as well. An unequal distribution of observation records with a peak in the entrance zone and decline to the dark zone was the most common pattern. One striking exception refers to the slug Boettgerilla pal-lens, for which an equal frequency distribution cannot be rejected (Chi2-test, p = 0.717). The majority of observations for this species originated from the transition and dark zone. Typically known from soil samples, B. pal-lens demonstrates a rather limited mobility by ground-dwelling or moving in earthworm burrows (Gunn 1992). In the study area, however, it was found moving above-ground and climbing on walls in the subterranean environment. Supposedly, the species has recently spread from the Caucasus region over large parts of Central Europe. Even sightings from North- (Canada) and South-America (Columbia) exist (Reise et al. 2000; Hausdorf 2002). Although human-mediated dispersal is considered to explain the rapid pan-European colonization and transatlantic dispersal, this study points to a local above-ground movement within the more spacious, aphotic subterranean environment. This behavior may enable a faster establishment and spread after arrival. Specimens of the carnivorous and omnivorous taxa Oxychilus and Boettgerilla constituted the majority of dark zone observations (56%). The penetration of non-surface habitats in those groups can be related to the exploitation of alternative food sources. Species of both taxa are known to prey on small gastropods and egg clutches. In particular, Boettgerilla pallens, Oxychilus cel-larius and Oxychilus draparnaudi were recorded to feed on arionid eggs (Daxl 1967; Gunn 1992; von Proschwitz 1994). Their prey, juvenile slugs of the Arionidae and Limacidae, were frequently encountered in the subterranean environments of the studied region. The presence of juvenile slugs either suggests an underground colonization during infancy or a subsurface egg deposition with in situ hatching. Similar observations were recorded from caves in Luxembourg, including juvenile Boettgeril-la for which egg-laying is known to occur below ground (Gunn 1992; Renker et al. 2012; Renker et al. 2013). Heller and Dolev (1993) hypothesized that "crevices might be a suitable habitat for land snails in which the hatching period, and period of juvenile growth, are very long". By implication, the prolonged development could be seen as a result of the less nutrient rich subsurface conditions. Avoidance of unsuitable surface conditions must be seen as the most likely factor explaining the colonization of subterranean habitats by the majority of gastropod species. Local surface habitats are constantly changing and gastropod populations continuously feature the risk of extinction (Regnier et al. 2009). Because temperature and humidity levels in caves demonstrate rather low fluctuations and are generally buffered compared to the surface (Pipan et al. 2011), dwelling in the entrance zone may allow survival in patchy microhabitats (Poulson & White 1969). Hence, the risk of temporal or seasonal suffering from unsuitable environmental conditions can be locally decreased. By studying the Mediterranean helicid snail Iberus gualtieranus gualtieranus, Moreno-Rueda (2007) has found evidence for summer survival of this species in karstic crevices. Since slugs are particularly prone to desiccation, increased humidity levels in the subterranean milieu may ensure survival during droughts. Gunn (1992) showed that soil-dwelling is a common response of boettgerilla pallens to escape direct light exposure and potentially heat. On the other site, winter freezing especially affects slugs and larger gastropods (Getz 1959; Biannic & Daguzan 1993; Ansart et al. 2001; Cook 2004). The frequent penetration of underground habitats by slugs (e.g. Arion, Boettgerilla, Limax) and helicids (Cepaea and Helix) in the study area may be best explained as a response to winter frost using the subterranean environment as a hibernation site. Despite temporal or seasonal survival, subterranean habitats may act as long-term refugia at the edge of a species distribution range. The northern range limit of the Mediterranean snail pomatias elegans is connected with the 2 °C January isotherm line. Hibernating specimens can withstand frost down to -6 °C for several days (Welter-Schultes 2012). In this study, several geographically restricted populations of p. elegans were found in the region of Treffurt at the NW-margin of the Thuring-ian Basin (i.e. at the NE-boarder of the HES state register). During a single visit, up to twelve individuals were collected from the soil populating the entrance and transition zone. More general, peripheral populations surviving in patchy subterranean microhabitats at the edge of a species distribution range can serve as an origin for future range expansion (thereby, significantly increasing the speed of colonization). Additionally, those populations are particularly susceptible to adapt to local environmental conditions because of an increased genetic isolation and/or enhanced/divergent selective pressures (Sexton et al. 2009; Vergeer & Kunin 2013). Two other cave-penetrating gastropods possessed their range limits within the study area: the Scandinavian slug Arion ater (S-range limit) and Phenacolimax major (NE-range limit), the latter being rare east of the Rhine River (Renker et al. 2013). Finally, one must be cautious to link species observations with their ecological requirements. As an example, frequent observations of living specimens in the dark zone do not necessarily reflect a cave-dwelling ecology of the species as best illustrated by the situation of Discus rotundatus. This abundant species was usually found above-ground or within in the entrance zone of subterranean environments comprising 20% of all re- cords. However, and because of their sheer number, numerous specimens were also collected in the dark zone (12% of all dark zone observations). As a fungivorous species, D. rotundatus may be able to survive in the dark zone for a longer period of time (Renker et al. 2013). Nevertheless, their presence is most probably best explained by population density as some specimens of this highly abundant, cave-penetrating species (Reiss et al. 2009b) will "end up" in the dark zone. Because eco-clas-sifications of cave-dwellers are inferred from the relationship of the animal to the subterranean environment (Sket 2008) and this relationship is reconstructed from observation and species abundance data, one might face a general problem. CONCLUSIONS Subsurface distribution data is paramount to reconstruct species affinities to subterranean habitats, since behavioral and ecological changes may precede morphological change. In this study, several gastropod species were observed from different subterranean light zones possessing a variety of reasons to explain their underground appearance (e.g. exploitation of alternative food sources, aphotic above-ground movement, mating site, temporal, seasonal and long-term refugia, and chance). These individual patterns are important to consider when reconstructing the evolutionary history of subterranean animals and addressing the question of what has caused animals to enter subterranean environments. Finally, the presented data imply the frequent migration of terrestrial gastropod species between the interconnected light zones and the surface. This knowledge may shed light on the origin and importation of allochthonous resources in cave environments (Souza-Silva et al. 2012; Boch et al. 2013). Cave-penetrating gastropods as well as their progeny (incl. deposited egg clutches and the newly hatched juveniles) may end up as nutrients for the cave community. 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