The currently accepted taxonomy recognizes two subspecies: the whitish, depigmented and blind Proteus anguinus anguinus (“White Proteus”), and the dark-colored, eyed P. a. parkelj (“Black Proteus”). The subspecies P. a. parkelj was considered to be of doubtful validity (Goricki and Trontelj 2006).

The White Proteus is distributed throughout the entire range of the species, the Black Proteus is only known from a few sites in the region of Bela Krajina in southeastern Slovenia.

Recent molecular studies revealed that the species is deeply subdivided into nine lineages supported by both mitochondrial DNA and microsatellites (Gorički and Trontelj 2006, Trontelj et al. 2009, Trontelj and Zakšek 2016, Vörös et al. 2019). The Black Proteus constitutes one of those lineages. Their ranges are mostly allopatric, although they are touching or even overlapping in some areas (Gorički et al. 2017).

The assessment of the exact taxonomic and conservation status of the Proteus lineages is the subject of ongoing research (Bizjak Mali and Sket 2019), but they should be treated as evolutionary significant units, to ensure the survival of diversity in this threatened species.

Global and European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

This species is listed as Vulnerable because its area of occupancy (AOO) is 1,132 km², its population is considered to be severely fragmented, and there is continuing decline in its AOO, the extent and quality of its habitat, and the number of mature individuals.

In the EU27 member states the species is also assessed as VU B2ab(ii,iii,v), as its AOO is less than 1,132 km², but larger than 500 km², its population is considered to be severely fragmented, and there is continuing decline in its AOO, the extent and quality of its habitat, and the number of mature individuals.

This species is endemic to the European region, where it is restricted to subterranean aquatic habitats within the Dinaric Karst region of the Balkans, ranging from southern Slovenia and just across the border into adjoining northeastern Italy, through disjunct areas of western Croatia, and western Bosnia and Herzegovina, reaching the Trebišnjica River Basin in the south (e.g. Sket 1997, Jablonski et al. 2012, Koller et al. 2019, Kostanjšek et al. 2022). Environmental DNA of the species was also detected in western Montenegro (not mapped) (Aljančič et al. 2014a, Gorički et al. 2017), while the closest confirmed localities lie just across the border in Bosnia and Herzegovina. Its global AOO is 1,132 km² and the global EOO is 55,470 km².

Outside its natural range, the species has been introduced and naturalised in the subterranean basin of the Valstagna Caves near Oliero in northeastern Italy, which was populated from the Postojna-Planina Cave system in Slovenia in 1822 (Bressi and Dolce 2006). Captive subpopulations are maintained at the Subterranean Laboratory CNRS Moulis (France) and the Tular Cave Laboratory (Slovenia), and in the show caves, Postojna Caves (Slovenia), Hermannshöhle (Germany), Grotte de Choranche (France) and Baredine (Croatia).

Due to its inaccessible habitat, few data are available on the abundance of this species (Briegleb 1962, Trontelj and Zakšek 2016), but according to the number of documented sites (Sket 1997, Koller et al. 2019), it is apparently most common in Slovenia and Croatia. A study by Vörös et al. (2019) of four cave systems in Croatia showed the highest population density of 7–11.45 individuals/10 m² of cave, and the lowest of 0.45–1.08 individuals/10 m² of cave.

A decline in the number of sites and individuals has been reported for the subpopulations of Goriza (Italy), Potojna (Slovenia) (Gasc et al. 1997), Dolenjska and Bela Krajina (Slovenia) (Hudoklin 2011) and Croatia (Jelić et al. 2012). In Herzegovina, the subpopulation from the subterranean Trebišnjica River has severely declined in the past (Milanović 1990). Local subpopulations have gone extinct in the region of Kočevje (Slovenia) (Sket 1997). The number of individuals of the subspecies P. a. parkelj is considered very low due to its extremely narrow range, documented at no more than 3 km² (Gorički et al. 2017).

More than half (> 50%) of the individuals (or more than half of the occupied habitat area) are in small and isolated patches between which there is little to no dispersal. The population is therefore considered to be severely fragmented. Due to ongoing decline in its AOO and the extent and quality of its habitat, the population is inferred to be decreasing.

The species occurs in karstic subterranean aquatic systems formed in limestone and dolomite rocks. It inhabits subterranean karstic habitats up to 1500 m vertical depth (dry cave systems with water streams at the bottom) and up to -113 depth of water column, withstanding water pressure of up to 12 atmospheres (Kovač-Konrad and Koller 2017). Many of the caves that the species occurs in are connected to rivers that run above ground for the first 50–100 km and then disappear into the ground. Subpopulations may be found close to the surface or as much as 300 m underground depending on the thickness of the karstic formation. It may also be found at cave entrances or washed-out of caves after episodes of high rainfall and flooding (Aljančič et al. 2014b).

The water temperature of the habitat ranges from about 5°C to 15°C, which is also the approximate extent of annual fluctuations. Proteus anguinus parkelj is found in warmer surface waters and some subpopulations of the nominal subspecies were found in such conditions as well. Animals feed on detritus and endemic cave invertebrates and hide in crevices or bottom sediment when disturbed, and are well adapted to cave life (e.g. Briegleb 1962, Aljančič et al. 1993, Bulog 2006, Balázs et al. 2015, Zakšek et al. 2017, Gorički et al. 2019). As it is very difficult to study the species in its natural subterranean habitat, most ecological data comes from cave laboratories (Briegleb 1962, Parzefall et al. 1980, Hervant et al. 2000). In captivity, they become sexually mature at 15 years of age and reproduce on average only every 12 years, while their lifespan can exceed 100 years (Voituron et al. 2011). Females lay up to 70 eggs on the undersides of stones (Briegleb 1962, Juberthie et al. 1996), and in some cases, the eggs are retained within the body and two fully formed young are produced.

The main threat to this species is increasing pollution of groundwater in karst landscapes, coming from intensive agriculture (overuse of fertilisers and phyto-pharmaceuticals), urbanisation (non-functioning sewage and waste treatment, public landfills and illegal dumping in caves or permeable karst terrain), traffic and industry (Kostanjšek et al. 2022).

In Croatia, invasive fish introduction present major threat (Jelić et al. 2012)

In Slovenia, more attention is put on increased nitrates in groundwater (Năpăruş-Aljančič 2017, Kolar 2018) and emerging threats from pathogens (Kostanjšek et al. 2017). Past ecological incidents are still a considerable source of pollution; high accumulation of heavy metals and PCB were detected in tissues of selected subpopulations in Slovenia (Bulog et al. 2002, Pezdirc et al. 2011). Destruction of habitat, such as extensive hydrotechnical works for hydroelectric power stations remains a particularly severe threat in Bosnia and Herzegovina (Čučković 1983, Lewarne 1999) and in Croatia (Jelić et al. 2012). Considerable threat comes from changes to the forested and pastoral land above the subterranean cave systems, including intensive agriculture - excessive groundwater pumping for irrigation in agriculture, salinisation of karst springs and fish farming (Ozimec 2011, Aljančič et al. 2014a). These changes have a direct influence on the quality and extent of the groundwater habitat, which also represents the only reliable source of drinking water in the karst for the Western Balkans. Significant threats to this species could also arise through tourism – especially sport diving in the caves (see in Balázs and Lewarne 2017). The species is highly dependent on clean water, and local subpopulations in the wild are therefore very susceptible to pollution and fungal infections other than Bsal (e.g. Lukac et al. 2015, Bizjak-Mali et al. 2018). Additionally, climate change is expected to dramatically increase the timing, frequency and magnitude of periodical floods, and the ability of Proteus to cope with environmental changes will be apparently reduced (Aljančič et al. 2014). There is some occasional illegal collection of the species for the pet trade.
In Italy and Slovenia, introduced freshwater species may present major threat in the future.

Batrachochytrium salamandrivorans (Bsal) is an emerging fungal pathogen affecting amphibians and is closely related to B. dendrobatidis (Bd), which has had a devastating impact on amphibian populations around the world. Both pathogens cause the infectious disease chytridiomycosis, and both are believed to have originated in Asia (for Bsal: Martel et al. 2014, for Bd: O'Hanlon et al. 2018). While Bd can cause disease in frogs, salamanders and caecilians, Bsal has only been reported to cause disease in salamanders and newts, although the pathogen has been detected in some frog species (Martel et al. 2013). Bd has spread globally, but at present Bsal is only known in Asia and has been introduced to Europe, most likely through the pet trade in salamanders (Martel et al. 2014, Nguyen et al. 2017). Currently in Europe, Bsal has been detected in the wild in the Netherlands, Belgium, Germany (e.g. Thein et al. 2020) and Spain (Martel et al. 2020), and has been shown to be highly pathogenic to most urodelan taxa in Europe (Martel et al. 2014). Laboratory trials for susceptibility to Bsal demonstrate low-level infection and subsequent disease for this species (Gilbert et al. 2020, F. Pasmans and A. Martel pers. comm. June 2021). It is thought that wild subpopulations would have some resistance to Bsal and it is not considered to be a major threat. Extinction risk modeling demonstrates that this species will not be at significant risk from the natural spread of the fungus in the next 50 years.

There are some anecdotes about this species being found in the pet trade, but it is not considered a major threat.

Conservation Actions In-Place
This species is listed on Appendix II of the Bern Convention, and on Annexes II and IV of the EU Habitats Directive. It is recorded in the National Red Lists and is protected by national legislation in Italy, Slovenia, Croatia, Bosnia and Herzegovina, and Montenegro. In Slovenia, the subterranean habitat of the species is also protected by national legislation, and much of the range of this species is within Natura 2000 or other protected areas (Hudoklin 2011). In Italy and Croatia parts of its range are also included in Natura 2000 (Koller et al. 2019). The subspecies Proteus anguinus parkelj is particularly in need of protection, as its habitat is limited to only a few karst springs in connection with subterranean networks in a very small geographic area. Rehabilitation centres for veterinary treatment of injured or ill specimens are organised in Slovenia and Croatia, and at the Trebišnjica River Basin in Bosnia and Herzegovina (Aljančič et al. 2016, Lewarne 2018, Koller et al. 2019).

Conservation Needed
Once the identity of the different conservation units is confirmed, they are likely to have highly restricted ranges, are likely to be threatened and will require specific conservation actions (K. Poboljsaj pers. comm. September 2019). Urgent action is needed particularly to reduce pollution in the habitat of subspecies P. a. parkelj (G. Aljančič, P. Trontelj, V. Zakšek and K. Poboljsaj pers. comm. December 2019) and, generally, to remove aquatic invasive species. There is also an urgent need for an EU (European Union) wide coordinated approach and implementation of the Bsal Action Plan by Gilbert et al. (2020), which was commissioned by the European Commission.

Research Needed
Further taxonomic work is required to confirm the identity of the different conservation units.Ecological research on lifespan and population needed.