The results of the study of local populations of the resident form of the European river lamprey Lampetra fluviatilis in medium and small watercourses (tributaries of the Pola and Msta rivers), located at a distance of 700 km from the sea, in the native part of the species range, are presented. Such populations are still poorly studied, despite the fact that they ensure the preservation of the range under conditions of fragmentation of river system. The captured adults are typical representatives of the resident form, with a slight variation in the dentition. The population density of lamprey larvae in most of the studied habitats is low and average in a small part of them. The habitat conditions of ammocoetes are determined to be optimal, however, due to the peculiarities of small watercourses, their habitats are vulnerable to the negative effects of environmental factors.

This paper describes typical habitats of European river lamprey (Lampetra fluviatilis (L., 1758)) larvae in the rivers of Leningrad Oblast. The density of ammocoetes and abiotic components of biotopes, as well as the structure of benthic and planktonic algae communities, zooplankton, and macrozoobenthos, are estimated. It is shown that the preferred grounds for larvae are sands <0.25 mm. Oligochaeta and Chironomidae larvae reach high levels of quantitative development together with ammocoetes. Algae and zooplankton communities are not decisive for lamprey larvae and have expressed seasonal patterns with peaks of abundance in the spring. The part of lamprey larvae in the abundance of benthic coenoses can reach significant values—the part in the total biomass of macrozoobenthos is 30–80

Small freshwater (resident) populations of the European river lamprey Lampetra fluviatilis from an isolated from the sea upper part of a river system with numerous lakes (the upper reaches of tributaries of the Daugava River and Lake Peipus, Pskov Oblast, Russia) have been studied. Adult individuals of the lamprey have been attributed to common and large size groups; adults are similar to other resident lamprey from the Baltic Sea basin. The presence of larvae of different size and age groups indicates regular spawning of the European river lamprey in the studied watersheds. Analysis of the modern and historical (before the isolation) distribution of lamprey revealed a reduction in habitats and a decrease in the number of individuals. Dam-associated disruption of migration routes led to the disappearance of anadromous form, and consequently, disappearance of resident lamprey from most of the studied waterbodies. We attribute the decrease in the number of rivers inhabited by lamprey and abundance of the lamprey both to anthropogenic factors (direct blocking of an access of anadromous form of the lamprey to spawning grounds by hydraulic construction) and to the characteristics of the studied area. The richness of the river system with lentic waterbodies makes it suitable for lamprey inhabitation with an influx of large anadromous individuals capable of crossing such reservoirs in search of spawning sites.

The resident nonparasitic lamprey Eudontomyzon sp., which differs from the only species of this genus known in the Baltic basin (Ukrainian lamprey E. mariae (Berg, 1931)), inhabits the Ilzhitsa River (drainage basin of the Western Dvina River). Eudontomyzon sp. larvae have been collected and the conditions of their habitat in the summer low water are identified. Abiotic and biotic characteristics of habitats are studied. The results of a morphological analysis of the river channel makes it possible to distinguish five types of biotopes: sediment in holes of the river bends (I), the sandy bottom on the straight regions of the river (II), the sediment in the pipes under the bridge (III), the sand and gravel spawning ground (IV), and the sediment at the river mouth (V). A study of the population density of lamprey larvae has made it possible to distinguish the most (I) and least (I, II, V) populated types of biotopes. An analysis of data on benthic communities has demonstrated that chironomids are the most typical representatives in the biotopes. This information complements the knowledge about the habitats of lamprey larvae with information about benthic communities in them and expand the existing classifications of habitats.

In 2019, during spring flood, we collected unusual Lampetra fluviatilis smolts migrating from a small lake, as part of a multi-year survey in the Chernaya River (Gulf of Finland, Baltic Sea). They differed from the ordinary Lampetra fluviatilis individuals in some of the features, mainly size. The emigration occurred at the darkest period of night and during extreme flooding, which significantly worsened the conditions in the Lake Gladyshevskoe and Chernaya River. In these individuals, lacus smolts, emigration occurred in the spring season with the migration of ordinary flumen specimens. Several hypotheses are discussed to explain the observation of large lacus (148–165 mm) individuals: they are seaward migrants, within expected body sizes of European river lamprey species but unusual for Gulf of Finland tributaries; the lake is a transitional habitat, and the postmetamorphic lacus juveniles migrating through it from a tributary; postmetamorphic lacus juveniles spend some time feeding in a lake but emigrated due to environmental conditions; larvae of European river lamprey, developing in the eutrophic lake, reach greater sizes than riverine larvae before transformation and subsequently produce larger juveniles.

The migration activity of ammocoetes of the European river lamprey Lampetra fluviatilis does not decline following the primary dispersal from the nest, but continues throughout the larval stage, ensuring the redistribution of juveniles along the river channel. The mechanisms of their diurnal and seasonal dynamics are determined both by the anatomical and physiological development of individuals and by external factors (illumination, water level, availability of sediments and food particles suitable for a particular developmental stage). The generalization of our own long-term studies and literature data shows that larvae are mobile and move mainly downstream, and the migration aspect of this part of the life cycle should be divided into phases: primary dispersal of early larvae, secondary redistribution of ammocoetes, downstream migration of smolts to the sea.

The extended freshwater life of Petromyzontiformes larvae (ammocoetes) takes place mainly in lotic waters where it has been studied in detail. Information on ammocoetes in lentic habitats is extremely limited, with few data on the occurrence of specimens in ponds or large lakes. The area of the Smolenskoye Poozerye National Park has a dense network of lakes and rivers, with streams abundantly populated by lamprey larvae of the genera Lampetra and Eudontomyzon. In this paper, we evaluate the habitability of lakes belonging to such systems and the manner in which they are populated by lamprey. Larval habitat identification and documentation (bed analysis where ammocoetes exist, including ground composition and biotic and abiotic habitat characteristics) were carried out, resulting in the first documentation of larvae and metamorphic specimens of Eudontomyzon sp. inhabiting littoral areas of small lakes. These specimens were found in the fringe biotopes between the river mouth and lakes. One peculiarity that distinguishes the lentic habitats from the typical river ones is the absence of distinct currents. Freshwater flows diffusely into lakes, mixing continuously at the sites we studied. Preferred habitats for Eudontomyzon sp. consisted of swales filled with debris carried from inflowing streams and deposited as current diminished. In ammocoete beds of this type, the examined larvae had a good trophic position. Such habitats can be both permanent and ephemeral as tributary water levels changes. During flood events, elevated velocity can flush debris and ammocoetes away from these fringe habitats and further into a lake. No accentuated active decomposition was found in the areas where the ammocoetes were present. The results of the study indicate that Petromyzontiformes larvae may be more common in lentic habitats than is currently documented.

After the construction of the Volga Hydroelectric Station and other dams, migration routes of the Caspian lamprey were obstructed. The ecological niches vacated by this species attracted another lamprey of the genus Lampetra to the Upper Volga, which probably came from the Baltic Sea via the system of shipways developed in the 18 th and 19 th centuries. Based on collected samples and observations from sites in the Upper Volga basin, we provide diagnostic characters of adults, and information on spawning behavior. Silver coloration of Lampetra fluviatilis was noted for the first time and a new size-related subsample of “large” specimens was delimited, in addition to the previously described “dwarf”, “small” and “common” adult resident sizes categories. The three water systems: the Vyshnii Volochek, the Tikhvin and the Mariinskaya, are possible invasion pathways, based on the migration capabilities of the lampreys. Dispersal and colonization of the Caspian basin was likely a combination of upstream and downstreams migrations. First, the lampreys migrated upstream along the rivers of the Baltic basin until they reached the water-parting line, followed by mostly downstream dispersal into rivers of the Caspian basin. Dispersal in the Volga River was similar, in accordance with the migration cycle of this opportunistic lamprey species.

The European river lamprey came to the Upper Volga from the Baltic Sea most probably via a system of shipways developed in the 18th and 19th centuries. The Vyshnii Volochek, Tikhvin, and Mariinskaya water systems are possible invasion pathways for this species. Dispersal and colonization of the Caspian Basin was likely a combination of upstream and downstream migrations. Analysis of museum and our own samples showed that lamprey possibly migrated upstream (for spawning) along rivers of the Baltic Basin until they reached the watershed boundary from which they could disperse downstream (in the juvenile period) into rivers of the Caspian Basin. Dispersal in the Volga River could occur in accordance with the migration cycle of this opportunistic lamprey species and lead to the present distribution. Key features (dentition and number of trunk myomeres) showed that lamprey from the studied area are similar to lampreys from the Baltic basin, although specimens in each population have their own peculiarities in morphology (size and coloration). Genetic data (Cyt-b) support the idea of a relatively recent invasion of lamprey into the Upper Volga. The haplotype, found in three rivers, is one of the most widespread in Europe and is found along the supposed route of invasion.

The results of studying the thermal tolerance and thermoregulatory behavior of the river lamprey larvae Lampetra fluviatilis (L.) have been reported. The upper lethal temperatures, recorded at a waterheating rate of ~8°C/h –1 and acclimation temperature of 18°C, do not differ in the specimens regarded as descendants derived from the ancestors of anadromous and resident life forms, comprising 34.3 and 34.5°C, respectively ( p > 0.05). In addition, no significant differences in the temperatures preferred by larvae from the populations of different life strategies are revealed. These individuals do not show any distinct thermal preferendum, distributing across the thermal gradient field presented to them. The mean values for the preferred temperatures on each day of a 3-day experiment comprised 18.3, 19.3, and 19.8°C.

Представлена дополненная информация о распространении и условиях обитания речной миноги (Lampetra fluviatilis L.) на территории Тверской области. Ареал на территории области представляет собой сочетание исторического (водотоки бассейна Балтийского моря) и инвазионного (водотоки бассейна Каспийского моря) компонентов. Обоснована идея вселения миноги в систему р. Волга по системам искусственных судоходных каналов. Состояние популяций в Каспийском бассейне указывает на продолжающийся процесс освоения новых местообитаний. Additional information on the distribution and habitat of the European river lamprey (Lampetra fluviatilis L.) in the Tver Region is presented. The areal of the European river lamprey in the Region consists of the historical (watercourses of the Baltic Sea Basin) and invasive (watercourses of the Caspian Sea Basin) components. The idea of the lamprey invasion in the Volga River through the system of manmade shipping canals is substantiated. The condition of the lamprey’s populations in the Caspian Sea Basin indicates the ongoing process of the species expansion.

The contranatant migration of juvenile cyprinids was studied in the Volga River delta in September–November 2016. A motivational component of rheoreaction (the ratio of rheoreaction types) of migrating and nonmigrating juveniles was evaluated in the course of experiments. It is shown that the motivational component of rheoreaction is a behavioral mechanism of contranatant migration.

Thermoadaptation characteristics of smolts of European river lamprey Lampetra fluviatilis are studied in experimental conditions for the first time. The final preferred temperature in the smolts preliminary acclimated to water temperature of 9 is 15.3°С. The value of sublethal temperature determined by the method of critical thermal maximum at a 9°С/h temperature increase rate is 29.0°С, while that of the upper lethal temperature is 30.8°С. The values of the final selected and upper lethal temperatures in river lamprey smolts are close to those in salmonids and coregonids.

Locomotor activity was investigated in smolts of the European river lamprey at alternation of light and darkness (12/12 h) and in the dark. Two groups of smolts were discerned differing in the level of locomotor activity. The presence of the circadian rhythm in smolts has been revealed, and its role in downstream migration has been shown. The locomotor activity leading smolts to channel flow increases with decreasing illumination at evening twilight. The period of high nocturnal activity is completed a short time before morning twilight, under the action of the individual circadian rhythm. This decreases the probability of impact of predators on migrating smolts.

Rheoreaction of the downstream migrating smolts of the European river lamprey was studied in the experimental conditions at illuminations of day and night intensity. It was found that at the daytime the smolts are mostly dormant and if move downstream then in active-passive form (with the head against the stream, and their speed going beyond the velocity). This data is well within the findings on the night downstream migration of the smolts during the 24 h period in natural conditions. Thus, the downstream migration of the smolts has an active form.

The effect of thiourea (exposure to 0.05% solution) on the migratory activity of the climbing perch Anabas testudineus has been studied. It is established that thiourea simultaneously affects two components of the migratory behavior of climbing perch: rheoreaction and frequency of jumps out of water. Thiourea stimulates (by 27%) movement of specimens against the current and increases (by 31%) the frequency of jumps out of water. The effect of the substance is manifested on the fifth day of exposure, while specimens almost completely stop feeding on the twelfth day. Starvation caused by thiourea may play a crucial role in the effect of thiourea on the migratory activity of climbing perch.