Recent Advances in the Diversity and Taxonomy of Subterranean Arthropods

The subterranean fauna of arthropods is one of the richest on the planet [1]; however, it remains poorly understood. The characteristics of the subterranean environment have led hypogean arthropods to develop both morphological and physiological adaptations, such as depigmentation, complete loss of eyes, limb elongation, a slow metabolism, reduced energy expenditure, and many others [2,3]. These animals are excellent models for studying the general principles of evolution and the mechanisms of adaptation to a new environment [4,5]. Since the diversity of subterranean arthropods worldwide is still insufficiently explored and most previous studies have focused on faunal diversity [6], it is desirable to analyse further aspects of the diversity of this specific group of animals, including morphological, molecular, ecological, chemoecological, and behavioural diversity. Since the editors’ expertise covers both troglobitic (associated with terrestrial subterranean habitats) and stygobitic (associated with all types of subterranean waters) arthropods [7], this Special Issue is dedicated to exploring the diversity of both groups. We hope that it will contribute to a better understanding of the diversity of subterranean arthropods across various biological disciplines, highlighting their great importance within their habitats and for humans in general [8,9].

This Special Issue provides an overview of current research on subterranean arthropods and the interactions between these specific animals and their habitats. A total of 11 papers were accepted for inclusion in this Special Issue, comprising 10 original research articles and 1 hypothesis. Four contributions deal with the diversity, phylogeny, and phylogeography of stygobitic arthropods. Another four are dedicated to the diversity and taxonomy of troglobitic representatives, while the topics of diversity of both categories of subterranean arthropods, diversity of chemicals of subterranean ground beetles, and a hypothesis on land colonisation by arthropods are each represented by one paper.

Eme et al. (contribution 1) investigated the hitherto inconclusive phylogeographic history and showed the mito-nuclear discordance of an endemic groundwater amphipod from Iceland, Crangonyx islandicus, which is the only known metazoan to have survived the Pleistocene under the glaciers. Double digest restriction-site associated DNA sequencing (ddRADseq) was used to clarify this mito-nuclear discordance. Various statistical methods were applied to assess sensitivity to different analytical approaches (data type, differentiation indices, and base call uncertainty). The majority of nuclear markers and methods supported ITS divergence. Nevertheless, a more complex scenario emerges, possibly involving introgression caused by male dispersal between northern sites or mitochondrial capture exacerbated by natural selection.

Balázs et al. (contribution 2) analysed 14 localities of Niphargus in Hungary, covering the 8 valid species of the country and including 9 previously unstudied populations. Based on the sequences of three gene fragments, they reconstructed the phylogeny of the Hungarian species using maximum likelihood and Bayesian approaches. They discovered that all Hungarian Niphargus species are closely related to each other and that even species sampled from the same localities may belong to different clades. Some species form monophyletic clades, while others are nested in different non-Hungarian lineages. The new populations are all genetically distinct from the known species. The results suggest that the Hungarian Niphargus fauna has originated from seven unrelated clades, and its diversity is underestimated due to unknown populations and cryptic species.

In the study by Weber and Weigand (contribution 3), groundwater amphipods were sampled in hyporheic interstitials throughout Luxembourg and the Greater Region, and the samples were analysed using DNA barcoding. Sites characterised by gravel or coarse sand and high flow velocities of the inflowing water were the richest in species and individuals. A total of 11 species were detected, of which the Niphargus aquilex lineage EF of the N. aquilex cryptic species complex and juveniles of N. schellenbergi dominated, but an unknown lineage of the N. aquilex complex was also found. In some regions, groundwater amphipods appeared to be absent. The authors suggest that underlying sandstone formations leading to low sediment porosity may prevent physical colonisation, but also that historical water pollution may have a long-lasting effect, either through the persistence of contaminants in the sediment or through low recolonisation rates of affected populations.

Jaime et al. (contribution 4) presented an overview of the stygobitic crustaceans of the Yucatan Peninsula. They represent potential markers for water reserves, which is why the authors wanted to use them to delineate aquifers on the peninsula. As part of a literature review, 75 crustacean species from 132 subterranean systems on the peninsula were recorded together with geomorphological, hydrological, hydrogeochemical, and historical precipitation data. Fourteen UPGMA clusters were informative for mapping species composition, with the Ring of Cenotes, Caribbean Cave, and Cozumel Island delineated as consolidated aquifers. These aquifers also differ in terms of abiotic factors: the Ring of Cenotes is dominated by freshwater species, while the Caribbean Cave and Cozumel Island are characterised by marine species. The use of stygobitic crustaceans for the integrative delineation of aquifers may represent an opportunity to improve the monitoring networks of regional aquifers.

Vargovitsh (contribution 5) described two new troglomorphic springtail species from the Arrhopalitidae family that live in the Snezhnaya (=Snow) Cave System in the western Caucasus. Both are morphologically highly specialised, but have evolved into different troglobitic life forms. Arrhopalites profundus, with extremely long claws, a lamellar mucro, and an enlarged sensory organ on the third antennal segment, lives exclusively in hygropetric and epineustonic habitats. In contrast, Pygmarrhopalites rystsovi, with strongly elongated and multi-subsegmented antennae and long legs, favours open terrestrial spaces. The author discussed the troglomorphy and ecological life forms of the obligate cave-dwelling Arrhopalitidae, in general, and classified them as neustonic-hygropetric, atmobiontic, intrasubstrate, and intermediate troglomorphs.

The paper by Vesović et al. (contribution 6) gives an overview of the diversity of subterranean terrestrial arthropods in the Resava Cave in eastern Serbia, which is under protection as a natural monument and is one of the most visited caves in Serbia and its surroundings. A total of 107 arthropod species from the four main subphyla were recorded in the cave: 66 species of Hexapoda, 27 species of Chelicerata, 11 species of Myriapoda, and 3 species of Crustacea. The authors provided microhabitat descriptions and information on distribution in the cave for 4 troglobitic, 16 troglophilic, and 87 trogloxenic species. The cave was found to be relatively rich in hypogean terrestrial arthropod fauna compared to other caves in Serbia that have been biospeleologically investigated so far.

Latella and Brighenti (contribution 7) investigated the terrestrial invertebrate diversity in two ice caves (Bus delle Taccole and Caverna del Sieson) in northeastern Italy. Ice samples were taken from each explored part of the caves, which the authors used to measure the stable isotopes of oxygen and hydrogen and monitor the air temperature. The two caves had different invertebrate communities, both dominated by a combination of troglobitic and cryophilic taxa. Despite the low taxonomic richness, both caves harboured rare and endemic species, four of which are yet to be described. The isotopic signatures of the ice water differed in the different parts of the caves, suggesting that the ice formed under different climatic conditions and/or due to different frequencies of thawing/freezing.

In the article by Blasco-Aróstegui and Prendini (contribution 8), the first truly troglobitic European scorpion species, Euscorpius studentium, which was described a few years ago on the basis of two immature specimens, is redescribed on the basis of adult specimens. In addition, the ecological classification of all currently known subterranean scorpions of the family Euscorpiidae is updated, a key to the identification of cavernicolous scorpions occurring in the Dinaric Karst is provided, and an overview of the historical and geographical factors affecting the distribution and conservation of cave-dwelling scorpions in the Balkans is given.

In the work by Sarbu et al. (contribution 9), biological investigations were carried out in the Sulfur Cave in the Vromoner Canyon on the Greek–Albanian border, focusing on invertebrate, vertebrate, and microbial communities and investigating the structure of the subterranean food web. The invertebrate fauna includes 5 aquatic species, 25 terrestrial species, and 4 amphibiotic species. Two fish species from two families and six bat species from three families were documented. The microbial communities in the different biofilms are dominated by chemosynthetic, sulphur-oxidising microorganisms, especially filamentous bacteria. Stable isotope analysis revealed low carbon and nitrogen levels for the terrestrial and aquatic invertebrates, suggesting that the subterranean communities rely on food produced in situ by chemoautotrophic microorganisms.

Vranić et al. (contribution 10) investigated both the chemical composition of the pygidial gland secretion and the morphology of the glands in adults of the troglophilic ground beetle species Laemostenus cavicola. A total of seven chemical compounds were detected in the secretion mixture. Formic acid was the most dominant compound, followed by dodecyl acetate and undecane. Other chemicals were present in small amounts. The morphological structure of the pygidial glands of the studied ground beetle was compared with the structure of the glands of the related troglophilic species Laemostenus punctatus. In addition, the authors presented summarised data on the semiochemicals previously recorded in subterranean ground beetle species and discussed the differences in the chemical composition of secretions between and among troglobitic and troglophilic species. The results of this study show the great diversity of chemicals in the pygidial gland secretions of subterranean ground beetles.

The issue of land conquest by arthropods is discussed by Frumkin and Chipman in their paper (contribution 11), in which they offer an opinion on how arthropods could survive on land without established plants and an ozone shield. Their hypothesis is that chemolithoautotrophic cave ecosystems, which are independent of photosynthesis, may have served as a subterranean stepping stone, providing a possible explanation for the land invasion conundrum. These caves could provide abundant food and radiation protection, allowing ancient arthropods to develop strategies to adapt to new frontiers by gradually spreading from the sea into shielded cave waters, then into the hygropetric margins of cave waters, and finally to the surface.

In summary, this Special Issue contains 11 papers that provide a comprehensive study of the diversity and taxonomy of subterranean arthropods. These studies examine a variety of arthropods inhabiting different hypogean habitats, including temperate caves, ice caves, sulphur caves, groundwater, and interstitial habitats. The studies were conducted on three continents—Europe, Asia, and North America—and in all major climatic zones: temperate, polar, and tropical. The main focus was on terrestrial and aquatic arthropods, which were analysed at both species and population level, with some papers covering other invertebrate groups, vertebrates, and microbes from the same habitats in addition to arthropods. The authors of the papers used both traditional and modern data collection techniques and advanced statistical methods, as well as taxonomic, molecular, faunistic, morphological, ecological, and chemoecological methods for laboratory analyses. The results of the articles in this Special Issue described, characterised, and clarified presently little-researched topics related to subterranean arthropods. Overall, this Special Issue represents an important and encouraging contribution to the understanding of the diversity and taxonomy of subterranean arthropods on a global scale and contains valuable data that shed light on part of their biology and support the conservation of these specific organisms and their habitats.

In view of the enormous diversity of the animal group studied [10], there is still much to be discovered, examined, and clarified. Particular attention should be paid to the investigation of hotspots that have not yet been sufficiently researched (North America, Australia, Southeast Asia, Brazil, and China) [11,12,13,14,15,16,17]. In addition, future studies should increasingly incorporate modern techniques, such as mapping and modelling [18,19]. It is necessary to adopt an integrative, synergistic approach [20], using and combining different types of data (faunistic, morphological, molecular, environmental, etc.) to gain a more comprehensive insight into the richness and diversity of this specific group of invertebrates.