The MSS is one of the least explored ecosystems in terms of biodiversity. Unlike caves, which have increasingly attracted scientific and public attention for their unique fauna, the MSS remains poorly studied despite its global distribution across varied geomorphological settings. The habitat is formed when rock fragments accumulate on steep slopes, creating a network of voids that connects the surface with deeper subterranean realms. These voids offer relatively stable microclimatic conditions, with reduced thermal fluctuations compared with the surface, providing both a refuge from climatic extremes and a permanent habitat for a diverse range of invertebrates, including both subterranean-adapted and surface-dwelling species.

Terrestrial isopods — the crustaceans commonly known as woodlice or pill bugs — are frequent inhabitants of subterranean ecosystems. They have limited dispersal abilities and a strong preference for high moisture, making them ideal indicators of hidden habitat connectivity. Very few studies have dealt with terrestrial isopods from the MSS, and comprehensive data on their diversity, distribution and seasonal dynamics in Portugal were still lacking. The new research set out to fill that gap, analysing new spatial-temporal distribution patterns and describing a new species of the genus Eluma Budde-Lund, 1885.

Sampling was conducted in five MSS areas across Portugal: Unhais da Serra in Serra da Estrela at 1,330 metres above sea level, Vale dos Poios in Sicó at 150 metres, Alcobertas in the Estremenho Karst Massif at 400 metres, Cabanas de Torres in Montejunto at 420 metres, and São Lourenço in Arrábida at 300 metres. The team used pitfall traps modified to sample MSS habitats, with a single collecting cup placed at a depth of 50 centimetres. Five traps were installed at each location, spaced 10 metres apart and positioned one metre from the shrub edge, baited with pork liver and preserved with propylene glycol. Samples were collected every three months for one year, from March 2022 to March 2023.

Specimens were identified to species level based on morphological characters under Wild M5 and M20 microscopes. They were dissected and mounted in microscopic preparations using Hoyer’s liquid, drawn with a camera lucida, and photographed with a Leica DMC4500 camera and Leica Z6 APO microscope, with images stacked using Leica LAS X software. Data analysis was performed in R software version 4.4.2, with species abundance visualised using the ggplot function. Non-metric multidimensional scaling (NMDS) was used to visualise spatial and seasonal community patterns based on Bray-Curtis dissimilarity, considering only stress values below 0.2, which indicate a good two-dimensional representation of the data.

The results revealed 12 species of terrestrial isopods across the five families Trichoniscidae, Philosciidae, Porcellionidae, Armadillidiidae and Armadillidae. Porcellionidae was the most well represented family with five species, followed by Armadillidiidae with four, and the remaining families were each represented by a single species. Among the most abundant were Porcellionides cingendus, found in four locations during all seasons, and Eluma caelata, captured in every sampled site. Widespread species such as Ctenoscia minima and Porcellionides cingendus occurred in multiple locations throughout the year, while others, including the new Eluma cristata and Porcellio novus, were restricted to single MSS sites.

The star of the study is undoubtedly Eluma cristata sp. nov., a new species found exclusively in the MSS of Alcobertas, in the Estremenho Karst Massif. The species name derives from the Latin cristatus, meaning crested, and refers to the high protruding crests on the posterior margins of its body segments — the cephalon, the seven pereonites and, to a lesser extent, the pleonites. Males reach a maximum length of 7.5 millimetres and females 9.5 millimetres. The body is strongly convex and able to roll up into a perfect ball, a euspheric type typical of the family Armadillidiidae. The dorsal surface bears ornamentation consisting of protruding crests ending in lobes of irregular shape and number, with the telson carrying two small paramedian tubercles. The dorsum also carries scattered pointed scale-setae and a line of noduli laterales per side located near the top of the crests on pereonites 1 to 7.

At present, the genus Eluma comprises only five species: the four included in this study and Eluma praticola from northern Morocco. Eluma cristata is readily distinct from all other species by its protruding dorsal crests. It further differs from Eluma caelata in lacking the basal triangular lobe on the male pereopod 7 merus, and from that species and Eluma praticola in the male pleopod 1 endopod, which has a thickset distal part not bent outwards. It also differs from Eluma matae in the much shorter dorsal scale-setae of a different shape. The new species was captured during all seasons at Alcobertas, with abundance decreasing from spring to summer, reaching its highest during fall, and decreasing again in winter.

The ecological analysis showed that terrestrial isopod communities differ across locations and seasons. All species were found in either three or four of the sampled seasons. The spatial distribution plot revealed that communities are relatively different from each other among the five sampled locations. Serra da Estrela showed a distinctly different isopod community composition compared with other locations, largely driven by the strong influence of Porcellio scaber. Alcobertas also had a distinct community, characterised primarily by Eluma cristata and Porcellio novus. In contrast, the other three locations — Sicó, Arrábida and Montejunto — showed greater overlap in species composition, suggesting more similar isopod communities. The seasonal site-specific plots revealed varying degrees of variation: in Serra da Estrela, communities were composed of only two species with no clear seasonal pattern, while in Sicó, Alcobertas and Montejunto the seasonal ellipses overlapped almost completely, indicating similar communities throughout the year. In Arrábida there was some marked seasonal variation, with Armadillo officinalis and Porcellionides cingendus influencing summer and fall, and the other four species influencing spring.

The diversity found in Portuguese MSS habitats includes both surface-dwelling species, such as Ctenoscia minima, Porcellionides cingendus, Porcellio scaber, Porcellio dispar, Porcellio novus, Eluma caelata, Eluma matae and Armadillo officinalis, and species more frequently found in the surface-cave ecotone, such as Trichoniscoides machadoi, Eluma tuberculata and Porcellio dilatatus. This mix emphasises the connectivity between surface and subterranean habitats and reflects the broad ecological tolerance of terrestrial isopods and their preference for cryptic and dark environments. The discovery of Eluma cristata highlights the relevance of shallow subterranean habitats as refuges for short-range distributed species of high conservation value. Despite several species showing strong seasonal abundance patterns, seasonality does not seem to influence community composition in the majority of the studied locations.

Subterranean habitats and their specialised fauna face increasing threats from land-use change, global warming and pollution. While cave invertebrates have been increasingly included in conservation assessments such as the IUCN Red List and the Portuguese Red Book of Invertebrates, species from shallow subterranean habitats remain overlooked, despite their vulnerability and ecological importance. Given the observed high diversity and ecological specialisation, the conservation of MSS habitats and their isopod communities is critical, not only in Portugal but worldwide. Despite being legally protected and facing mounting pressures, MSS habitats and their fauna remain largely absent from biodiversity assessments. The researchers argue that their study and protection are essential to safeguard the unique invertebrate diversity these hidden rocky worlds harbour — a reminder that some of the planet’s most remarkable species live just beneath our feet, in the crevices between stones.

Subject of Research: Terrestrial isopods (Crustacea, Isopoda, Oniscidea) from Mesovoid Shallow Substratum habitats of Portugal

Article Title: Terrestrial isopods ( Crustacea , Isopoda , Oniscidea ) from Mesovoid Shallow Substratum habitats of Portugal

Article References: Eusébio, R. P., Taiti, S., & Reboleira, A. S. P. S. (2025). Terrestrial isopods (Crustacea, Isopoda, Oniscidea) from Mesovoid Shallow Substratum habitats of Portugal. Subterranean Biology, 53, 129-153. https://doi.org/10.3897/subtbiol.53.163632

Image Credits: AI Generated

DOI: 10.3897/subtbiol.53.163632

Keywords: Terrestrial, isopods, Crustacea, Isopoda, Oniscidea, Mesovoid, Shallow, Substratum, habitats, Portugal, scientific research