Decline of unique Pontocaspian biodiversity in the Black Sea Basin: A review

Abstract The unique aquatic Pontocaspian (PC) biota of the Black Sea Basin (BSB) is in decline. The lack of detailed knowledge on the status and trends of species, populations, and communities hampers a thorough risk assessment and precludes effective conservation. This paper reviews PC biodiversity trends in the BSB (Bulgaria, Romania, Moldova, Ukraine, and Russia) using endemic mollusks as a model group. We aim to assess changes in PC habitats, community structure, and species distribution over the past century and to identify direct anthropogenic threats. The presence/absence data of target mollusk species were assembled from literature, reports, and personal observations. Pontocaspian biodiversity trends in the northwestern BSB coastal regions were established by comparing 20th‐ and 21st‐century occurrences. The direct drivers of habitat and biodiversity change were identified and documented. We found that a pronounced decline of PC species and communities is driven by (a) damming of rivers, (b) habitat modifications that disturbed previous natural salinity gradients and settings in the studied area, (c) pollution and eutrophication, (d) invasive alien species, and (e) climate change. Four out of the 10 studied regions, namely, the Danube Delta–Razim Lake system, Dniester Liman, Dnieper–Bug estuary, and Taganrog Bay–Don Delta, contain favorable ecological conditions for PC communities and still host threatened endemic PC mollusk species. Distribution data are incomplete, but the scale of deterioration of PC species and communities is evident from the assembled data, as are major direct threats. Pontocaspian biodiversity in the BSB is profoundly affected by human activities. Standardized observation and collection data as well as precise definition of PC biota and habitats are necessary for targeted conservation actions. This study will help to set the research and policy agenda required to improve data collection to accommodate effective conservation of the unique PC biota.

aim to identify the direct anthropogenic threats to their existence and survival (sensu Díaz et al., 2015), viz., processes and settings resulting from human decisions and actions that have direct implications for turnover/decline of PC biota, such as uncontrolled influx of sewage, invasion of alien species, and establishment of large dammed reservoirs in river basins (Shiganova, 2011, Semenchenko et al., 2015, e.g., Lattuada et al., 2020.
Pontocaspian biodiversity is also affected by indirect anthropogenic drivers such as the organization and interaction within and between societies, stakeholders, and people and their interactions with nature. For the BSB, these are treated elsewhere . Based on this review, we outline follow-up approaches to develop a conservation strategy that applies to the entire PC benthic biota in the BSB.

| Pontocaspian mollusk species in the Black Sea Basin
Most of the PC species evolved from ancestral species that radiated in the Late Miocene and Pliocene Paratethyan Basins (Krijgsman et al., 2019). The common historical origin of PC species and related ecological adaptations distinguishes this group from other groups such as Palearctic freshwater species groups and several opportunistic marine species occurring in the PC region today (Anistratenko, 2007b;Sowinsky, 1904;Starobogatov, 1970;Wesselingh et al., 2019;Zhadin, 1952).

| Habitats of Pontocaspian species and communities in the Black Sea Basin
Pontocaspian communities occur(ed) in coastal plains in areas influenced by the Black Sea and Azov Sea, such as lower stretches of rivers, lagoons, delta areas, estuaries/limans, and bays (Figures 3 and 4).
We defined optimum PC habitats as waterbodies (e.g., lakes, estuaries, bays, and river stretches) where at least one endemic PC species of two different families co-occur (Table 1). Our definition will need expansion when other groups in addition to mollusks are included. Optimum PC habitats contain(ed) communities dominated by PC species within the coastal zone, mostly in oligohaline settings F I G U R E 4 Simplified model of coastal landscapes depicting habitats of selected PC (green underlined) and other abundant mollusk species in the northwestern Black Sea coastal zone for the 20th-21st century. The optimum PC habitats are shaded (above) and indicated in green (below). FW-freshwater, U-Upper, L-Lower, Olig-Oligohaline, Mes-Mesohaline. Our model summarized personal observations as well as published accounts. In each sub-basin in the BSB, the salinity gradients and habitat successions are complex. In some areas, local salinity maxima occur that are the result of excessive evaporation rather than a simple freshwater to marine gradient

| Bulgarian coastal lagoons and limans
The Bulgarian Black Sea coast contains 31 wetland areas such as lakes, marshes, and lower river floodplain areas (Varbanov, 2002), from where living PC species and shells have been reported (Georgiev & Hubenov, 2013;Hubenov, 2007Hubenov, , 2015Sands et al., 2019;Appendix S2). Theodoxus fluviatilis has been reported from more than 15 wetlands (Hubenov, 2015), while Dreissena polymorpha occurred in about ten wetlands in the past, and currently is confirmed from five of these native habitats (Hubenov, 2015;Vidinova et al., 2016). Theodoxus danubialis (reported as T. pallasi) occurred in Lake Varna before salinization in the first half of the 20th century (Drensky, 1947;Kaneva-Abadjieva, 1957;Sands et al., 2020) and is now considered extinct in Bulgaria (Hubenov, 2015). Living specimens of L. lincta (reported as Micromelania lincta) were recorded in Lake Mandra (June 1944) and Lake Beloslav (August 1945) by Drensky (1947). The species was considered rare for Bulgaria (Drensky, 1947), and since then, no further occurrences have been recorded (Hubenov, 2015 Valkanov (1957), Marinov (1990), and Hubenov (2015) and shells of C. variabilis reported by Genov and Peychev (2001) and Hubenov (2015). It is unclear whether these littoral shells represent possible 20th-century occurrences, as older Holocene and even Late Pleistocene occurrences are well known from shallow deposits in the Black Sea coastal and shelf areas .
The Bulgarian Black Sea coastal wetlands have been exposed to a variety of strong anthropogenic pressures owing to agricultural, recreational, urban, and industrial development over the past two centuries (Hubenov, 2015;Trichkova, 2007). Increased eutrophication and substantial variation in physico-chemical parameters such as salinity, oxygen content, mineral content, and temperature in the wetlands have caused pronounced changes in benthic invertebrate communities (Trichkova, 2007). Some of the past habitats sustaining PC species have completely changed. For example, Lake Varna was connected to the sea through a navigation canal in 1909 and to Lake Beloslav in 1923. Later, in 1975, a bigger canal and a sea port were built, increasing salinity within both lakes, driving the loss of their natural fauna, including PC species (Trichkova, 2007;Varbanov, 2002). Benthic invertebrate biota in other wetlands (e.g., Durankulak, Shabla-Ezerets, Burgas, Mandra, and Dyavolsko Blato Marsh) declined or vanished due to restriction or complete disconnection from the Black Sea because of damming, and/or due to intensive fish-farming activities, overfishing, and household and industrial pollution (and Trichkova, 2007, summarized in Hubenov, 2015.

| Lower Danube River
Theodoxus and Dreissena are and have always been common in the Danube River (Angelov, 2000;Russev, 1966;Sands et al., 2019;Trichkova et al., 2019). In the Bulgarian sector, PC hydrobiid shells were reported in the 20th century. In June 1958, empty shells of L. lincta (reported as M. lincta) were recorded at Oryahovo (678 rkm) by Russev (1966). Shells of C. variabilis were found upstream of Lom (474 rkm) in September 1957, at Ruse (493 rkm) in October 1959, and upstream of Silistra (381 rkm) in June 1963 (Russev, 1966). No 21st-century records exist of these PC hydrobiids from the Bulgarian Danube River stretch. However, recently a Clathrocaspia sp. has been described as Caspia milae in Boeters et al. (2015) from Vardim Island in the Bulgarian sector of the Danube, whose identity is subject to further study (see Appendix S1).
The main threats to the aquatic mollusks in general and the PC fauna in the Lower Danube River in particular are the loss and degradation of habitats, pollution, and introduction of invasive alien species (Trichkova et al., 2019). Throughout the years, the Danube River has been contaminated by urban, industrial, and agricultural waste and has experienced increasing economic activities, such as ship traffic (Russev & Naidenow, 1978). A major threat that has become a problem in the 21st century is the introduction, establishment, and spread of invasive alien species (Paunović & Csányi, 2018). In recent years, owing to the increase in abundance and biomass of the newly introduced invasive alien mussels Corbicula fluminea, Sinanodonta woodiana, and Dreissena bugensis, benthic habitats in the Bulgarian sector of the Danube River completely changed (Hubenov, 2001(Hubenov, , 2006Hubenov & Trichkova, 2007;Hubenov et al., 2012Hubenov et al., , 2013, which may have potential adverse impacts on several PC species.
Additionally, the invasive mussels may directly impact PC species through competition and fouling.

| Danube Delta-Razim Lake system
The Danube Delta (up to its apex near Galati), the neighboring drowned valley lakes both on the Romanian side (e.g., Brates, Crapina, and Jijila) and on the Ukrainian side (Yalpuh, Katlabukh, Kagul, and Kitai), and the coastal Razim-Sinoe Lake complex to the south of the delta and Sasyk Lake to the north make up a large (c 6,000 km 2 ) and varied area that hosts many PC species ( Figure 6).
Lake Sasyk was historically separated from the Danube Delta, but was included when, in 1978, a feeder channel from the Danube was constructed. Most of the Danube-Razim region consists of freshwater habitats (e.g., river channels, floodplain delta lakes, drowned river valleys, and swamps) but, importantly, salinity gradients toward mesohaline settings occur in the outer delta and in the coastal la- annual fieldwork in the Razim complex has shown that their abundance has strongly declined in the past 15 years ).
In the 20th century, H. plicata was common in the Razim-Sinoe Lake complex (Teodorescu-Leonte, 1966). The last time this species was found alive in Razim-Sinoe Lake complex was in 2004 (Tatiana Begun, PO). Within the lakes and lagoons very close to the Black Sea coast, A. fragilis has been a common occurrence in the 20th century (Borcea, 1926b;Grossu, 1962;Markovsky, 1955), but the species has declined recently . Velde et al. (2019) showed that the Razim communities have almost entirely been replaced by freshwater communities in the past decades. In Romania, PC hydrobiid species were reported mostly from the Razim-Sinoe complex and low salinity habitats near the mouth of the Danube distributaries (Grossu, 1956). In most cases, these records are represented by empty shells and their historical distribution (e.g., 20th-century occurrences) is not well known. In the past decade, no living specimens were encountered apart from a 2003 record of L. lincta (Wilke et al., 2007).
In the Ukrainian part of the Danube Delta, in the Kitai Lake, PC communities have recently disappeared completely and PC species abundances in this lake and in other lakes are decreasing (MOS and VVA, PO). The distribution ranges of L. lincta and A. fragilis have decreased compared with occurrences reported over a century ago (Markovsky, 1953(Markovsky, , 1954a(Markovsky, , 1954b(Markovsky, , 1955Milaschewitsch, 1916;Ostroumov, 1898 (Pavel et al., 2017) and potential interactions of this successful invasive (Crespo et al., 2015) with PC species are a reason for concern.

| Dniester Liman
The lower Dniester, comprising the Dniester Delta and Liman, the Kuchurgan Liman (Figure 7), and the lower Dniester River up to Dubăsari Dam (Moldova) historically host a rich array of PC fauna that includes 10 mollusk species (Grinbart, 1953a;Markovsky, 1953;Son, 2007b). The Dniester Liman is about 45 km long, with a surface area of about 400 km 2 , and a maximum depth is 2.7 m. In the 20th century, the Liman was subdivided into an inner freshwater-oligohaline zone (up to 0.5 psu), a middle oligohaline zone (up to 4 psu), and an outer mesohaline zone (salinities typically between 4 and 9 psu with episodic lowering during peak floods; Markovsky, 1953). Salinity regimes changed due to human interference. A deep-water sea canal has enabled seawater intrusions during storm surges. In the upper Dniester basin, a system of fish ladders decimated natural flow regimes (Zhulidov et al., 2015). In general, the lower Dniester basin is characterized by problems of seasonal runoff deficiency and associated degradation of floodplain ecosystems, common to all large PC rivers with cascades of dams (Shevtsova, 2000). The episodic release of large amounts of freshwater from reservoirs in the feeding rivers causes strong episodic freshening of the inner and middle parts of the Dniester system. This freshening sharply steepens the salinity gradient and minimizes optimum salinity areas of F I G U R E 6 Pontocaspian habitats in the Danube Delta region. (a) Regional overview and major trends, (b) 20th-century occurrences, (c) 21st-century occurrences. See data in Appendix S2, Table A2.1, outline of subareas in Figure A2 (Grinbart, 1953a;Markovsky, 1953). Dam construction has been a major driver for Dniester floodplain ecosystem demise (Shevtsova, 2000), which has been further affected by an increase in water extraction, climate change, and organic pollution. Increased episodic intrusions of seawater and variability of freshwater inflow from the catchments have severely impacted the salinity gradients. Salinity increase in estuaries under the conditions of climate change and artificial flood-changing constructions is a global trend (Rahel & Olden, 2008). In freshwater and oligohaline zones, among numerous alien species, two species of mollusks (a) D. bugensis, a PC species from the Dnieper-Bug Estuary, and (b) Potamopyrgus antipodarum, a species from New Zealand, have affected the original PC communities (Son, 2007a(Son, , 2008. In the lower zone of the Dniester Liman, alien species (especially Mytilopsis leucophaeta) occupy the vacant niches of PC species, which are not adapted to rapid salinity changes (Zhulidov et al., 2015). M. colorata) and marine cardiids (Grinbart, 1953b). However, D. polymorpha, M. colorata, and the Theodoxus spp. that lived in the liman have disappeared as a result of a human-driven salinity increase (Moroz et al., 1986;Son, 2007b).

| Berezan Liman
The Berezan Liman is 26 km long, with a surface area of c 60 km 2 , a maximum depth of 26 m and is connected to the Black Sea by a canal (Figure 8) (Son, 2007b). Salinities within the Berezan Liman historically ranged between about 3-6 psu but were depressed by an influx of low saline waters during peak discharges from the adjacent Dnieper-Bug estuary through a channel connecting the liman to the Black Sea (Grinbart, 1955).
In the earlier part of the 20th century, Berezan Liman was dominated by M. colorata, as well as Theodoxus spp. (Grinbart, 1953b) and further contained D. polymorpha. In recent times, M. colorata has disappeared in several sites it previously occurred, but some areas within the estuary have not been explored (MOS, PO); other PC species still occur in this liman (Son, 2007b).
Some PC species, including C. variabilis, were recorded in the Yagorlyk Bay on the south side of the Dnieper-Bug Estuary (Anistratenko, 1996)  pollutants occur (Bespalov, 2005). The upper sediment layer in the bay is commonly disturbed by storm waves. The wind is a major factor determining water circulation and therefore salinity distribution in the bay . Strong western storms can push mesohaline waters to the eastern end of the bay and even occasionally flood the adjacent Don Delta with 4-5 psu waters F I G U R E 9 Pontocaspian habitats and trends in the Taganrog Bay-Don Delta region. (a) Regional overview and major trends, (b) 20thcentury occurrences, (c) 21st-century occurrences. See data in Appendix S2, Table A2.4, outline of subareas in Figure A2  . Other drivers affecting the salinity gradients in the bay are the river flow volume and Black Sea water advections . Two large limans adjoin the bay approximately in its middle. The Mius Liman (33-40 km long and only 1 m deep: Vishnevetskiy & Popruzhniy, 2018) to the north is a drowned estuary with average salinities between 0.9 and 1.8 psu (Kreneva et al., 2013), while the Yeysk Liman to the south is an open estuary with hydrological conditions similar to the adjacent Taganrog Bay. The benthic fauna is different here due to small nature of this water body (Nabozhenko & Kovalenko, 2011 (Anistratenko, 2007b). Historically, PC species were common in the Taganrog Bay and the outer Don River Delta. In early 2000, communities were changing (Shokhin et al., 2006) but later works showed the persistence of, slightly altered but nevertheless diverse, M. colorata communities in the inner and central bay area (Nabozhenko, 2008)  Bay. However, its sharp increase has not been associated with considerable shifts in Monodacna abundance or species structure of corresponding communities thus far. Corbicula cf. fluminea, which was first found in the Don River in 2017 (Zhivoglyadova et al., 2018), is considered one of the most aggressive invasive species tending to lead to negative environmental consequences (Bespalaya et al., 2018;Crespo et al., 2015) and is therefore likely to be a hazardous exotic species for PC mollusks in the freshwater and oligohaline zones. Recently, the brackish water mussel M. leucophaeta was reported from the inner Taganrog Bay (Zhulidov et al., 2015), which, if capable to survive low winter temperatures, can disrupt PC habitats, as has been reported in the Dniester Liman.
The Taganrog Bay and the Don River are located in a densely populated area with intensive shipping, agricultural, and industrial activity. Dredging and dumping are common in the eastern parts of Taganrog Bay where artificial fairways are subject to permanent siltation. Continuous dredging also occurs in the Don River, especially in the delta. The Lower Don and the Taganrog Bay waters are strongly eutrophicated due to the sewage discharge and terrigenous nutrients from agricultural fertilizers (Matishov, 2005;Moses et al., 2012). Large industrial ports (e.g., Taganrog and Mariupol) are sources of local toxic contamination as well. A considerable threat is the Bagayevskiy waterwork facility which is planned to be put into operation in 2023 (http://bguzel.ru/). According to preliminary estimates, the waterworks will lead to wide-scale changes in the Lower Don ecosystem (Dubinina & Zhukova, 2016;Krivoshey, 2016).

| South East Azov Sea coast
The South East Azov Sea coast includes the coastal zone of Temryuk Bay, northwards to Primorsko-Akhtarsk and the estuaries and channels of the Kuban Delta. The marine part has typical features of the southern Azov Sea, with mesohaline conditions and faunas, sandy beaches, and silty and shelly sediments at depths over 2 m (Simonov & Altman, 1991 (Korpakova et al., 2007) and the Temryuk Bay itself (Korpakova et al., 2008).
Also, D. polymorpha communities, with relatively high biomass, were mentioned across the area as a dominant species (Korpakova et al., 2010). No recent records of PC hydrobiid species are known from the region, even though their general presence in the area was reported by Golikov and Starobogatov (1972).
As the PC species occurrences are poorly known, we have no insights into their trends, but the area is subject to severe anthropogenic modifications. These include invasive species (Syomin et al., 2020), oil/gas exploration and production in Temryuk Bay (Nagalevsky & Lobko, 2017), and the shallowing and siltation in the estuaries of the Kuban Delta area resulting from hydraulic engineering and pollution by the drainage waters from rice fields.
Some limans have been transformed in aquaculture ponds losing PC habitats.  Such deterioration also applies to other rivers of the NW Black Sea region (South Bug, Dniester), as well as the lower Don River and Taganrog Bay (Anistratenko et al., 2011;Shokhin et al., 2006).

| Threats
Siltation should be considered as an important, perhaps even a key factor triggering habitat reduction threatening PC biota. that sustain PC species and communities, but overall, the variability has strongly increased. In many of the PC areas, (episodic) influx of mesohaline Black Sea waters increased as a result of canal construction and dredging. For example, deep-water shipping canals, that require regular dredging, resulted in massive seawater intrusion into estuaries and river deltas during storm surges causing rapid salinity fluctuations. The impact may be magnified due to large-scale water withdrawal upstream from these estuaries and river deltas. In several regions, breaching of sand barriers and spits resulted in strong salinity increases and the breakdown of the pre-existing stable gradients (Mikhailov & Gorin, 2012). Other estuaries and bays have become isolated hypersaline lakes as a result of their separation from the major limans, either by natural or by man-made interventions (Vinogradov et al., 2014). These hypersaline lakes (including the entire Tiligul Liman) are hostile to PC species. The breakdown of salinity gradients in Danube coastal lake systems, due to the closing of Black Sea inlets and river diversion, has been a major factor driving the demise of PC species and communities there (Son, 2007b;Velde et al., 2019). Pontocaspian species in the nontidal BSB estuaries live across wide salinity gradients but often occur in the relatively constant salinity regimes of the bottom water layers (Khlebovich, 1974).

| The modification of marine and freshwater influx in coastal areas
Populations of PC species have local acclimatization optima and are negatively affected by rapid salinity fluctuations even when occurring within the limits of their autecological tolerance (Orlova, 1987;Orlova et al., 1998;Zhulidov et al., 2018). Increasing salinity variability is especially beneficial to generalist alien and native species (Shiganova, 2011;Zhulidov et al., 2018).
In some of the lower estuaries, increased salinity has resulted in the replacement of PC communities by marine communities, which have colonized these areas from the Black Sea (Zhulidov et al., 2018).
These marine communities are heavily affected by three invasive mollusk species, especially in the NW Black Sea: Mya arenaria, Rapana venosa, and Anadara sp. (see for taxonomy discussion of the latter Anistratenko et al., 2014;Anistratenko & Khaliman, 2006;Krapal et al., 2015). In areas with strong freshening, such as the Razim-Sinoe system, freshwater mollusk species, including non-native bivalves (i.e., S. woodiana, C. fluminea) and viviparids, expanded at the cost of PC species Velde et al., 2019). Some PC species have become invasive themselves. The Quagga mussel, D. bugensis, expanded in the second half of the 20th century from its native NW BSB range into all PC habitats, major westerncentral European inland water systems and even freshwater ecosystems in North America (Lyashenko et al., 2012;Son, 2007aSon, , 2007b. The BSB species M. colorata has recently been introduced into the Volga River and the Caspian Sea, as well as Lake Balkhash-Kazakhstan Wesselingh et al., 2019). A native Caspian subspecies, A. vitrea glabra, recently expanded into the Don River drainage and has a large impact on local benthic species and communities . Increased shipping activity between the Volga and Don River systems has increased the introduction risk of Caspian PC species in the BSB.

| Pollution and eutrophication
Pollution and eutrophication (IUCN threat categories 9.3.1 Nutrient loads, 9.3.3 Herbicides & pesticides, 9.6.2 Thermal pollution) are rampant throughout the region, resulting from large-scale industrial and agricultural activities in the BSB river systems (Lyashenko et al., 2012;Semenchenko et al., 2015). Organic pollution and eutrophication negatively affect PC communities and species that are sensitive to oxygen regimes (Mordukhay-Boltovskoy, 1960;. Thermal pollution is a local threat to Kuchurgan Estuary and the lower Dnieper River by simultaneously affecting the PC communities and creating preferable conditions for alien species (Protasov et al., 2013;Son, 2007a;Son et al., 2013). Eutrophication has been proposed as a driver for the demise of lymnocardiine species in many lakes in the Danube Delta area ) and also appears to negatively affect communities in Lake Sasyk at the northern end of the Danube Delta, yet pollution levels in the Razim-Sinoe system were found to be low (Catianis et al., 2018

| D ISCUSS I ON-TOWARD EFFEC TIVE CON S ERVATI ON OF P ONTO C A S PIAN B I OTA IN THE B L ACK S E A BA S IN
The combined evidence of this review paper indicates a decline of PC mollusk species and their communities throughout the BSB.
However, while the decline seems evident, its ecological consequences are not. It is largely unknown to what extent the species associated with the PC taxa (e.g., their parasites or predators) may be affected by their demise. The decline in abundance and apparent fragmentation (and isolation) of populations is a problem in itself, but may drive genetic depletion, which should also be another reason for concern. Data on genetic diversity of PC species in the BSB are scarce, and little understanding exists on patterns and processes of gene flow between populations, even though it may be an important determinant of PC biodiversity maintenance (Audzijonyte et al., 2006(Audzijonyte et al., , 2017. The first step toward effective conservation is improving (a) scientific knowledge on PC biodiversity at community, species, and genetic levels and (b) understanding population and community dynamics as well as species distributions and their ecological tolerances (Cardoso et al., 2011). Recurring and standardized collection and observation efforts are paramount as a basis for establishing trends.
These efforts shall be cross-country collaborative efforts given the Historical distribution data are often imprecise and also hampered by uncertainty in identifications (see Appendix S1). Updated taxonomy will enable targeted research into autecological tolerances and species responses to disturbances. Additionally, the extinction risk of species should be updated through IUCN assessments, as many of the taxa concerned are currently data deficient to perform such analyses (see Wesselingh et al., 2019). New data on PC populations, species, and communities will enable a more inclusive and comprehensive definition of PC habitats and their inclusion in conservation schemes.
Secondly, our proposed optimum PC habitats shall be validated using the quantitative data on up-to-date PC population sizes and standardized threat analyses shall be performed such as those conducted by Lattuada et al. (2019) (Díaz et al., 2015). Institutional alignment and responsibilities to address PC biodiversity conservation and governance have been studied by ,  who showed that this biota is not a priority for conservation plan- habitats/species management area (Dudley, 2008) can be a useful approach. The IUCN protected area management categories provide a global framework for sorting the variety of protected area management aims. Category IV aims to "maintain, conserve and restore species and habitats" (https://www.iucn.org/theme/ prote cted-areas/ about/ prote cted-areas -categ ories/ categ or y-iv-habit atspe cies-manag ement -area). Such categorization can take place in different phases of establishing a protected area, such as the initial phase: before the protected area is established and category has to be decided, or in later phase: after the protected area has already been established and category decided, but management aim is to address emerging conservation priorities (Dudley, 2008). Managing and mitigating the wholesale decline of the unique PC biota in the BSB will require long-standing commitment from various stakeholders across countries bordering the Black Sea.

| CON CLUS IONS
Pontocaspian mollusk species and communities in the BSB have suffered a severe decline over the past century. Five major drivers for the decline are identified. However, basic distribution data and integrated approaches to mitigate the decline are lacking. Some PC communities have already vanished and many species have gone extinct or are under increased risk of extinction. The identification of optimum PC habitats will enable targeted conservation actions.
Sustained, transnational collaboration is required to improve conservation of PC species, communities, and their habitats in the BSB.
Only then can the effective conservation of the unique and threatened PC biota be achieved in the region.

CO N FLI C T O F I NTE R E S T
The authors have no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data that support the findings of this study are provided in appendices. Pontocaspian habitat polygon shapefiles and the attributes describing historical (20th century) and modern (21st century) distributions of PC target taxa are available on Dryad, https://datad r yad.org/st ash/ share/ cMhMU -zTUUU LuZM1 XjtQ K ZNwN5 M-L6cwK iKP4k af6go.