The coloration of adults from the studied populations was diverse. Lampreys from the Saragozha and the Kamenka were, in general, colored similarly to adults of the European river lamprey from various rivers of Europe [3,7,30]. The coloration of adults from the Vysochinsky Stream deserves a special discussion. Their silvery coloration is characteristic of post-metamorphic juveniles (transformers, macrophtalmia) during their downstream migration [31–33]. Sperone et al.  found what they took to be the southernmost resident population of the European river lamprey (they call it the European brook lamprey L. planeri) in the Lao River (Calabria, Italy). They caught two such specimens on October, 26 2018 with an electrofisher (total length 170 and 175 mm). It is clear from the photo of the specimen with the silvery coloration (P. 133, and the figure in ) that it is not an adult but a post-metamorphic immature specimen (secondary sexual characteristics are absent). Most probably, it was a smolt of the European river lamprey. Of the other resident species of Lampetra, silvery coloration was also noted for Lampetra aepyptera . It is unclear if the silvery coloration of lampreys is associated with their migratory activity and why lampreys from the Vysochinsky Stream do not lose this coloration upon maturation. These questions might open avenues for further research and the need for genetic analysis of archived specimens.
According to Renaud , the resident form of the European river lamprey (L. planeri sensu Renaud 2011) has a maximum total length 170 mm. Kucheryavyi et al.  grouped them according to length: “dwarf” (78–83 mm), “small” (90–104 mm) and “common” (107–140 mm). Following these definitions, the lampreys from the Kamenka River were classified as “small” (n = 1) and “common” (n = 4). All adults from the Saragozha River were “common” (n = 7). In the Vysochinsky Stream, 63% (n=19) of the lampreys were “common”, while 37% (n=11) were attributed to a new subsample, “large” (>141 mm). Lampreys like these have been recorded from northwestern Scotland (in the Endrick River, a tributary of the Loch Lomond – ) and in isolated populations in Spain (the Deva-Cares system) and Portugal (the rivers Esmoris and Vouga) [37,38].
One of the hypotheses explaining the presence of the landlocked lampreys (e.g. resident European river lamprey in the Upper Volga or lake form of sea lamprey Petromyzon marinus in the Lake Ontario) assumes the impact of global climate changes on the ichthyofauna 70–10 thousand years ago. Lawrie  and Smith  believe that the sea lamprey inhabiting the lake and its drainages is a relict Pleistocene population in North America. Dorofeev et al.  and Slynko and Tereshchenko  suggest that the retreat of the Valdai glaciation in Europe under the influence of the global warming (12–10 thousand years ago) resulted in the formation of numerous periglacial lakes and other water bodies of glacial origin. Owing to this, the entire Ponto-Caspian basin was populated by fish species of the Arctic freshwater and boreal-submontane faunistic complexes. The European river lamprey belongs to the latter. This is how the presence of the European river lamprey in the tributaries of the Upper Volga, the Ivankovo, the Uglich and the Rybinsk Reservoir is explained by Slynko and Tereshchenko .
If the European river lamprey had inhabited the Upper Volga for > 11000 years, its local populations would have mixed resulting in homogeneous phenotypes. In contrast, lampreys from the Vysochinsky Stream, the Saragozha River, and the Kamenka River show diverse phenotypes (Fig. 4). These water courses are located close to each other, are not separated by physical barriers, and have similar hydrological characteristics (all of them are lowland rivers). The observed differences indicate a recent invasion of the European river lamprey into this region. Therefore, we forward the invasion hypothesis (entry of the European river lamprey into the system of the Volga River via manmade shipways). Extremely low numbers of the European river lamprey in the entire Upper Volga are likely a consequence of the construction of dams for hydroelectric, which caused limnification of the river and a decrease in the abundance of rheophilic species, including lampreys.
In the early 18th century the Caspian, Baltic and White Sea basins were not yet connected (Fig. 5). Shipways connecting the Upper Volga with the Baltic basin started with construction of the Vyshnii Volochek water system (1708, here and below we give the year of the completion of earthworks and construction activities on the artificial canals connecting the rivers. Practically, the basins of different seas became connected after that. Officially, the water systems in question were opened later (max. 6 years later), one of the reasons was the need to build sluices for the passage of ships). This was followed by the Tikhvin water system (1805), and by the Mariinskaya water system (1808), which is now called the Volga-Baltic water system [26,28,29]. The North Dvina water system in 1828 connected the Caspian and the White sea basins . The development of the system of shipways opened new water courses, along which various hydrobionts including fish and cyclostomes have been dispersing for more than 300 years. Their dispersal along the Volga was unimpeded until the construction of the first dam, the Upper Volga Beishlot (1843), which has separated the lakes of the Upper Volga from the rest of the river. The dispersal pathways along the Upper Volga were blocked altogether after the construction of hydroelectric stations near Ivankovo (1937), Uglich (1940), Rybinsk (1941) and Nizhny Novgorod (1955).
The Volga River system is now one of the major invasion corridors of Europe. According to Konovalov et al. , hydrobionts mainly disperse from the Caspian basin northwards, to the Baltic basin (i.e., Volga pikepech Sander volgensis, blue bream Ballerus ballerus, wels catfish Silurus glanis) and the White Sea basin (i.e. sterlet Acipencer ruthenus, spined loach Cobitis taenia). This, probably, was made possible not only by the shipways but also because global warming causing northward displacement of a number of the species . Dispersal in the opposite direction has been noted for fewer fish species, mainly due to their small adult sizes and short life cycle . It may also be associated with the regulation of the run-off of the Volga River, which results in an increase in the water temperature, low oxygen conditions, a change in the water mineralization, demands on spawning substrate, and growing eutrophication .
The Tikhvin water system is closer than the other two to the system of the Saragozha River and to the Kamenka River. The Vyshnii Volochek water system is situated in direct proximity of the Vyazma River, the Malaya Dubenka River, the Malaya and the Bolshaya Kosha rivers, the Shutinka Stream and the sites of collection of the European river lamprey in the studies by Viktorov , Viktorov et al.  and Nezdolii & Kirillov . We consider these two systems of shipways as the most probable invasion pathways of the European river lamprey from the Baltic to the Caspian basin in the corresponding parts of the distribution. However, distance cannot be the only criterion in this matter. To ascertain the pathways used by the European river lamprey for the invasion in the Caspian basin, phenogeographic studies are necessary.
The foregoing reasoning about the colonization pathways opened due to the human activity closely resembles the process underway in the North American Laurentian Great Lakes. While the situation with the Lake Ontario remains a topic of debate, the remaining invasion pathways are clear to most researchers. Sea lamprey (Petromyzon marinus) has spread throughout the system of the lakes due to the construction of channels, which has made it possible to bypass Niagara Falls. It took only 25 years for sea lamprey to get to the farthest Lake Superior . Of note is that as both sea and European river lamprey penetrate more deeply into these freshwater systems, they reduce their adult sizes and fecundity, producing parasitic lake forms. The European river lamprey goes further to produce a nonparasitic resident form , which after metamorphosis has no need of feeding and can disperse wider and inhabit more types of habitats.
Anadromous lampreys are quite capable of covering the distances mentioned above during one upstream migration . For instance, the presence of an anadromous lamprey from the Gdovka River in the collection of the Museum of the Zoological Institute of the Russian Academy of Sciences (ZISP 25430–25433) indicates that the European river lamprey could migrate upstream the Narva River and cross Lake Peipus, covering in total more than 3000 km. In water-abundant years, lampreys in the Luga River overcome the Kingisepp and the Sabsk rapids and entered the tributaries of the Luga, e.g. the Krupa River, ascending 150 km upstream (ZISP 26437; 26438).
A possible scenario of the dispersal of the European river lamprey into the Upper Volga, based on the evidence from the Tikhvin water system, is as follows. Anadromous lamprey adults were noted in the Syas River . Berg (ZISP 39080; 42976) and Ivanova-Berg (ZISP 42977) ascertained the presence of adults of the lake form in that river. This means that anadromous lampreys could have migrated upstream from the Baltic Sea along the Syas River up to the upper reaches of the Tikhvinka River, i.e. up to the water-parting line (457.3 km). For the lake form of lampreys from Lake Ladoga this way is 271.3 km long.
Earthworks on the Tikhvin water system were completed in 1805 , heralding the connection between the Baltic and the Caspian basin. After that, anadromous, lake adults (or resident adults from the Syas and the Tikhvinka) could cross the water-parting line and start migrating along the Volga slope. Further downstream dispersal along the rivers of the Tikhvin water system and the Volga River was achieved by the larval stages. Primary dispersal in the form of downstream migration is shown for lampreys aged 0+ [51,52], while downstream migration of older ammocoetes has been repeatedly recorded in various rivers throughout the year. During downstream migrations lampreys can cover considerable distances (tens of kilometers) over a short period of time. This means that it could take as little as several decades for the species to disperse across the Upper Volga. Upstream dispersal into rivers such as the Saragozha, the Kamenka and the Tunoshonka could be performed both by the resident adults and by the larvae of the European river lamprey .
Thus, the mechanism employed by the European river lamprey for colonization of the Caspian basin was a combination of upstream and downstream migrations. At the first stage, the lampreys migrated upstream along the rivers of the Baltic basin until they reached the water-parting line. Reaching and crossing the water-parting line became possible owing to the anthropogenic interference: the construction of sluices on the rivers (allowing lampreys to navigate up the rapids) and on the water-parting line, and the opening of shipways. The second stage was represented by mass, mostly downstream migrations along the rivers of the Caspian basin. Dispersal along the system of the Volga River was also a combination of upstream and downstream migrations in accordance with the migration cycle of the European river lamprey.
The European river lamprey – a species capable of long-term and long-distance migrations both upstream and downstream – could disperse across the Caspian basin along corridors of anthropogenic origin. Considerable morphological diversity of its local populations reported in our study provides evidence for this hypothesis. The diversity of lampreys from the Vysochinsky Stream, the Saragozha River and the Kamenka River is probably associated with the fact these young (not more than 60 generations) local populations formed from a handful of pioneering adults.