Freshwater bivalves are declining globally (Lydeard et al., Reference Lydeard, Cowie, Ponder, Bogan, Bouchet and Clark2004), with numerous species threatened. One such species is the thick-shelled river mussel Unio crassus. It is categorized as Endangered on the IUCN Red List (Lopes-Lima et al., Reference Lopes-Lima, Prié, Österling and Zając2024b), although it appears to be a complex of cryptic species (Lopes-Lima et al., Reference Lopes-Lima, Geist, Egg, Beran, Bikashvili and Van Bocxlaer2024a). Unio crassus is considered common in the eastern part of its range and is not included in the Red Data Book of Russia (Popov et al., Reference Popov, ., Fadeeva, Shamilishvily, Gorin, Burdo and Melchakova2017), although it is included in regional Red Books for Volgogradskaya Oblast, Kostromskaya Oblast, Mari El, Crimea (Mikhailov, Reference Mikhailov2018) and Saint-Petersburg city (Kiashko, Reference Kiashko and Geltman2018). Empty shells of U. crassus had been previously reported from two rivers in Kaliningradskaya Oblast (Manakov, Reference Manakov and Ovcharenko2018, Reference Manakov2020), a Russian territory lying between Lithuania and Poland, and we therefore surveyed this area using methods initially developed for other species.

Recent publications suggest declines of the species in Lithuania (Skujienė, Reference Skujienė2018) and in several Russian territories (Mikhailov, Reference Mikhailov2018), and we expected a similar situation in Kaliningradskaya Oblast. This area, formerly East Prussia, has undergone extensive development, resulting in significant alterations of its river systems. The north-east of the oblast is a polder resembling those found in the Netherlands, characterized by wet, flat areas at or below sea level, intersected by a network of canals (Adamov, Reference Adamov2011). The remaining terrain is predominantly flat, with minor variations in elevation. The rivers, having been modified by hydraulic engineering, have weak currents and are mostly surrounded by farmland, resulting in eutrophication and siltation. Although many farms in the region were abandoned in the 1990s, since 2014 farmland acreage has been increasing (Fedorov, Reference Fedorov2022).

With 125 rivers listed in the State Register of Water Bodies of Russia (2024) and numerous streams, a comprehensive survey of Kaliningradskaya Oblast is infeasible. However, this was also the case for the freshwater pearl mussels Margaritifera margaritifera and Margaritifera laevis in part of their range, yet hidden populations were successfully located (Popov, Reference Popov2021a,Reference Popovb). The methods used in these studies could also be effective for surveying U. crassus as these species share common characteristics: inhabiting small, pure rivers, sensitivity to eutrophication, and susceptibility to water pollution, river canalization and siltation. To identify previously unknown populations of pearl mussels, we assessed potential habitats using indirect evidence. The principal criterion was the proportion of forest in a river basin. Pearl mussels are absent in rivers surrounded by agricultural lands, but are potentially present if the proportion of deforested banks is < 22% (Popov, Reference Popov2021a). The presence of forest in the upper reaches of rivers is also critical. Even if the lower reaches are forested, deforestation in the upper reaches degrades mussel habitats. In northern Europe, rivers with pearl mussel populations typically have lakes at their sources. Lakes absorb heat during summer, warming the rivers that flow from them and providing optimal temperatures for pearl mussel reproduction. Lakes also reduce pollution and maintain the water supply. In addition, freshwater pearl mussels rarely coexist with other large bivalves, and this is probably also the case with U. crassus: at the Gulf of Finland several U. crassus habitats have been identified that do not have any other bivalve species (Popov, Reference Popov2021a).

We first made a remote assessment of Kaliningradskaya Oblast using land-cover maps (Buchhorn et al., Reference Buchhorn, Smets, Bertels, Lesiv, Tsendbazar and Masiliunas2020; Federal Service of State Register, Cadastre and Cartography, 2024). Diva GIS 7.5.0 (DIVA-GIS, 2012) was used for data processing. We plotted the forests and the rivers flowing through them, outlined the drainage areas, mapped the sources of inflows and estimated the configurations of forest and lakes with reference to previous studies on pearl mussels. From this, we compiled a list of watercourses where U. crassus could potentially be present, and then surveyed these locations during 5–15 June 2023. Surveys for pearl mussels indicated they are most common in certain microhabitats. We identified 1–2 m wide streams, and river bends (especially the inner side of the bend), as favourable locations, with the largest pearl mussel aggregations near sandbars where a stream enters a river and in the lower sections of riffles. We prioritized these microhabitats, surveying with an aquascope whilst wading, or by snorkelling.

In the remote assessment, we found a limited number of rivers with potentially suitable locations for U. crassus. Only the river Pissa appeared to provide ideal habitat, as it flows from a large lake surrounded by forest, and is also bordered by forest in its upper reaches. River Krasnaya also appeared to be potentially suitable as it flows through a forest, but the lake at its source is small and lies outside the forest. Four other rivers have small areas of forest in the upper or middle reaches and lakes at their sources, although the lakes are distant from the forested areas (Table 1).

Table 1

The six locations where we searched for the thick-shelled river mussel Unio crassus in Kaliningradskaya Oblast, Russia (Fig. 1), with details of forested banks, lakes, area surveyed and the area that had to be searched to find at least one mussel.

We found U. crassus in five of the six rivers (Fig. 1, Table 1). It was most common in the Krasnaya and Pissa rivers (Plate 1), and scarce in three other rivers. In the Sheshupe river, where we did not locate U. crassus, we found the bivalves Unio pictorum and Anodonta sp. in riffles, potentially precluding the presence of U. crassus. The empty shells that we found demonstrated the main diagnostic characteristics of U. crassus (thick shell, two teeth). The cryptic species belonging to the U. crassus complex (Lopes-Lima et al., Reference Kiashko and Geltman2024b) are indistinguishable by their external characteristics, but the species in the five rivers in Kaliningradskaya Oblast is most likely U. crassus sensu stricto, as specimens from neighbouring areas of Lithuania and Poland have been described as this species.

Fig. 1

Kaliningradskaya Oblast, a Russian territory lying between Lithuania and Poland, indicating the six rivers (Table 1) where we surveyed the thick-shelled river mussel Unio crassus.

Plate 1

(a) Pissa River, Kaliningradskaya Oblast, Russia (Fig. 1), and (b) thick-shelled river mussels Unio crassus on the bottom of the river.

All of the forests through which the six rivers pass are natural, with no monoculture tree plantations. The predominant trees are broad-leaved, with spruce Picea abies and pines Pinus sylvestris in small numbers. The forests around the Pissa and Krasnaya rivers are particularly well preserved. This area has been a designated game hunting area since the 16th century and has been under protection since then (Fedorov, Reference Fedorov1990). The rivers in which we found low densities of U. crassus show that even a small section of the upper reaches of a river flowing through forest can support a small population of this species. Further deforestation, however, could lead to extirpation of U. crassus.

Unio crassus inhabits a broader range of habitats, including deeper rivers, than pearl mussels (Vaessen et al., Reference Vaessen, Houbrechts, Peeters and Van Campenhout2021). Further refinement of the assessment and survey methods used in this study is therefore required for the survey of larger areas. Nonetheless, for Kaliningradskaya Oblast and similar regions the methods are probably adequate given that the combination of lakes and forested river banks indicates potential presence of the species, and mitigates to some degree the negative influences of farmland on river ecosystems.

Our study underscores the importance of forests for the conservation of riverine mussels. While emphasis is often focused on ground-level microhabitats (Stoeckl & Geist, Reference Stoeckl and Geist2016; Vaessen et al., Reference Vaessen, Houbrechts, Peeters and Van Campenhout2021), lack of host fishes (Taeubert et al., Reference Taeubert, Gum and Geist2012), water pollution, and habitat alteration and degradation (Downing et al., Reference Downing, Van Meter and Woolnough2010), many other negative influences result from deforestation. Forested river banks help prevent soil erosion, maintaining optimal river bed and water conditions. The presence of trees on riverbanks increases fish abundance, as both living trees and fallen trunks create diverse habitats for fish. Shading by trees is also crucial for preventing excessive macrophyte growth, which can be detrimental to mussels. Any plans for the restoration of U. crassus will require concurrent restoration of riparian arboreal vegetation.