Ostracoda are a class of small crustaceans enclosed within a bivalved calcified carapace, a feature that has contributed to their exceptional fossil record and long evolutionary history (Horne 2003; Smith et al. 2015). Although most of the ostracod diversity occurs in marine and surface freshwater environments, several lineages have independently colonized subterranean waters, constituting a distinctive and ecologically important component of groundwater biodiversity (Danielopol and Hartmann 1986).

Subterranean ostracods thrive in a wide range of groundwater habitats, including alluvial aquifers, stream hyporrheos, karstic fissures, and caves (Mori et al. 2025), and often exhibit elevated rates of speciation and endemism driven by spatial and hydrological isolation as well as persistent selective pressures (Rossetti et al. 2022). The perpetual darkness, limited and patchily distributed resources, and environmental stability of subterranean systems favour a suite of potential morphological, physiological, and behavioural adaptations, such as reduction or loss of eyes, depigmentation, and elongation of sensory setae that enhance chemo- and mechanoreception (Karanovic 2012; Pipan and Culver 2012). Stygobitic species often exhibit reduced metabolic rates, enhanced tolerance to hypoxia, and life history traits such as slow growth, delayed reproduction, and low fecundity (Rouch and Danielopol 1997; Moldovan 2018). These traits reflect the energetic constraints of most of the groundwater ecosystems, where food resources are scarce and often derived from allochthonous organic matter or microbial production (Gibert et al. 1994; Gibert and Deharveng 2002; Porter et al. 2009). Within these systems, ostracods contribute to nutrient cycling through grazing on biofilms, detritus, and microbial assemblages, and they may serve as indicators of groundwater quality and hydrological connectivity (Danielopol et al. 2003).

Among the diverse subterranean habitats colonized by ostracods, sulfidic caves represent one of the most chemically extreme and biologically specialized environments. These caves are characterized by the presence of hydrogen sulfide which may originate from geothermal sources, oxidation of sulfide-bearing minerals, or microbial sulfate reduction (Hose et al. 2000). When hydrogen sulfide–rich waters mix with oxygenated waters or cave air, steep redox gradients arise, fostering the development of abundant microbial biofilms dominated by sulfur-oxidizing bacteria (Sarbu et al. 1996; Tobler et al. 2016). These chemoautotrophy-based communities constitute the primary trophic resource of the ecosystem, sustaining life in conditions where photosynthetically derived organic matter is nearly absent.

Species inhabiting sulfidic caves have often evolved physiological tolerance to sulfide, which is toxic to most metazoans at high concentrations, as well as to hypoxia (Sarbu et al. 1996). Their presence suggests either long-term evolutionary adaptation or repeated colonization events followed by rapid selection for sulfide tolerance (Hose et al. 2000). Only a few non-marine ostracod species have so far been documented from these environments (Peterson et al. 2013; Iepure et al. 2023; Rossetti et al. 2025), yet they offer valuable insights into the limits of crustacean adaptation.

This paper describes a new species of ostracod in the genus Pseudocandona Kaufmann, 1900, discovered in a sulfidic cave in Albania, using an integrated taxonomic approach that combines detailed morphological descriptions of valves and soft parts with molecular sequence data.

The long-standing taxonomic rearrangements within the subfamily Candoninae, together with the establishment of the genus Pseudocandona, have been thoroughly described by Higuti and Martens (2014) and Namiotko et al. (2014).

The genus Pseudocandona currently comprises more than 80 extant, formally described species (Meisch et al. 2024). According to Meisch (1996, 2000), these species are assigned to five groups, based primarily on carapace morphology and the number of setae in the setal group of the second segment of the mandibular palp: caribbeana, compressa, eremita, rostrata, and zschokkei. An additional group, the prespica one, was introduced by Karanovic (1999). Subsequent revisions have transferred species formerly placed in the eremita group to the genus Typhlocypris Vejdovský, 1882, while species of the zschokkei group have been reassigned to the genus MarmocandonaDanielopol et al. 2012 (Karanovic 2005; Danielopol et al. 2012; Namiotko et al. 2014; Meisch et al. 2024). Furthermore, Higuti and Martens (2014) highlighted that the Pseudocandona species endemic to Lake Baikal, which constitute a substantial fraction of the genus diversity, will likely require transfer to a new genus pending further revision.

Pseudocandona sulphurella sp. nov. is placed in the rostrata group. This species group also includes P. atmeta Smith & Kamiya, 2015; P. becca Smith & Kamiya, 2015; P. falcula Smith & Kamiya, 2015; P. hartwigi (G. W. Müller, 1900); P. lobipes (Hartwig, 1900); P. marchica (Hartwig, 1899); P. movilaensis Iepure et al., 2023; P. rostrata (Brady & Norman, 1889); P. sarsi (Hartwig, 1899); P. semicognita (Schäfer, 1934); P. stagnalis (Sars, 1890); P. tenuirostris Hiruta & Mawatari, 2013.

All species of the rostrata group have a Palearctic distribution; only P. rostrata, P. semicognita and P. stagnalis have also been recorded in the Nearctic (Meisch 2000; Smith and Kamiya 2015). Most group members are associated with surface-water habitats, with only occasional occurrences in subterranean and groundwater-dependent ecosystems. Two notable exceptions are the strict stygobionts Pseudocandona sulphurella sp. nov. and P. movilaensis, both restricted to sulfidic cave environments (Iepure et al. 2023). Other species of the rostrata group tolerate acidic conditions, such as P. stagnalis, P. semicognita and P. tenuirostris (Meisch 2000; Smith and Kamiya 2015). It is important to emphasize that information on distribution and ecology remains limited for many of the above-mentioned species, and should therefore be regarded as provisional.

The external surfaces of the valves of Pseudocandona sulphurella sp. nov. are extensively covered by undetermined small bacilli and by filamentous, Thiothrix-like bacteria (Fig. 1E), which also occur on the body of the animal. These sulfur-oxidizing bacteria convert hydrogen sulfide into various sulfur compounds to obtain metabolic energy and are frequently reported as epibionts or ectosymbionts on other crustaceans, such as amphipods (Flot et al. 2014). Their association with ostracods has previously been documented in sulfidic groundwaters at Mangalia (Romania) (Boancă et al. 2022) and at methane seeps in Lake Baikal (Khalzov et al. 2021). Such bacteria-ostracod interactions may be advantageous to the bacteria, to the host, or to both: epibiotic bacteria gain dispersal opportunities and access to host-derived secretions as nutrient sources, while microbial epibiotic consortia can constitute a food resource for the host (Khalzov et al. 2021).

The study of living ostracods in Albania has been primarily advanced through comprehensive research on Lake Prespa (Macedonian, Albanian, and Greek border) and Lake Ohrid (Albanian-Macedonian border) and its adjacent waters, revealing an exceptionally rich and highly endemic ostracod fauna (Namiotko et al. 2011; Lorenschat and Schwalb 2013; Lorenschat et al. 2014). Among the species reported in these studies are Pseudocandona compressa (Koch, 1838), P. slavei Pekovski, 1969, and P. elongata Holmes, 1937, the latter having been proposed by Meisch (2000) as a synonym of Fabeformiscandona levanderi (Hirschmann, 1912). Consequently, the discovery of Pseudocandona sulphurella sp. nov. represents the first record of the rostrata group in Albania and, more importantly, the first documented occurrence of a stygobiont ostracod in the country.

Concordant with the morphological affinities, the COI phylogeny places Pseudocandona sulphurella sp. nov. in the rostrata group, together with the Western Palearctic species P. marchica (from Oslo, Norway), P. movilaensis (from the sulfidic waters in Movile Cave, Romania) and P. hartwigi (from Otomin, Poland), on a moderately-supported clade. While the rostrata species group appears to be monophyletic, members of the compressa group (P. compressa and P. albicans) cluster on separate branches suggesting a possible polyphyly. This evidence of polyphyly of the genus is also supported by two undetermined species of Pseudocandona from the Nearctic (GenBank accession numbers MG318134 and MG936030) which cluster on a separate branch. Species of the genus Typhlocypris are the closest to the rostrata group and Wysocka et al. (2019) provide evidence for the monophyletic status of the rostrata group, and also note the intricate taxonomic history of the morphogenera Pseudocandona and Typhlocypris. The 28S phylogeny places P. sulphurella sp. nov. on a clade with an undetermined species of Pseudocandona sampled in Japan (Hiruta et al. 2016; GenBank accession number AB674975). We were unable to test the potential polyphyly of Pseudocandona for 28S due to the short overlap in the alignment caused by different amplification strategies; however, Karanovic and Sitnikova (2017) also demonstrate that Pseudocandona is polyphyletic for 28S. Both mitochondrial and nuclear phylogenies support Pseudocandona sulphurella sp. nov. as a distinct species and the molecular data generated in this study might prove useful in future taxonomic studies of the family Candonidae.

In conclusion, the occurrence of ostracods in sulfidic caves broadens current knowledge of groundwater biodiversity and provides a valuable system for investigating evolutionary processes under extreme environmental constraints. Their distinctive morphological and physiological specializations, together with narrow and often localized distributions, render these taxa particularly suitable as models for studies of adaptation and speciation, as well as for elucidating the ecological dynamics of chemoautotrophic subterranean ecosystems.

This research was partially funded by Biodiversa+, the European Biodiversity Partnership under the 2021–2022 BiodivProtect joint call for research proposals, co-funded by the European Commission (GA N°101052342) and with the funding organizations Ministry of Universities and Research (Italy), Agencia Estatal de Investigación – Fundación Biodiversidad (Spain), Fundo Regional para a Ciência e Tecnologia (Portugal), Suomen Akatemia – Ministry of the Environment (Finland), Belgian Science Policy Office (Belgium), Agence Nationale de la Recherche (France), Deutsche Forschungsgemeinschaft e.V. (Germany), Schweizerischer Nationalfonds (Grant N° 31BD30_209583, Switzerland), Fonds zur Förderung der Wissenschaftlichen Forschung (Austria), Ministry of Higher Education, Science and Innovation (Slovenia), and the Executive Agency for Higher Education, Research, Development and Innovation Funding (Romania). Numerous enthusiastic amateur cave explorers provided invaluable logistical support during the field work. The authors would also like to thank PhD student Maxim-Jean Bâlcu for his unconditional help in preparing the samples for SEM imaging.