Introduction

The native range of Paramysis lacustris (P. lacustris) spans the Ponto-Caspian region, encompassing the Black Sea, the Caspian Sea, the Sea of Azov, and the rivers and streams flowing into these basins1. Within this range, the species inhabits coastal areas, river estuaries, and lakes. Over the past five million years, major geoclimatic events have repeatedly altered the connections between Ponto-Caspian water bodies with varying salinities. These changes fostered the development of today’s unique brackish-water biodiversity, characterized by numerous adaptable and euryhaline species2,3.

Assuming that P. lacustris feeds on phytoplankton and detritus and can form large populations in mesotrophic lakes4, the species was deliberately introduced into Lithuanian waters in the 1960s. The goal was to enhance the local food chain by efficiently converting primary production into energy-rich biomass, thereby bolstering fish feed stocks5. Initially, it was released into the Kaunas Reservoir and the Curonian Lagoon in the Baltic Sea, followed by the Balkhash Lake in Kazakhstan5,6.

From these locations, P. lacustris spread further westward, being recorded in Germany for the first time in 2013 in the Oder Lagoon near Kamminke7. It is believed that the species reached the Lower Havel near Berlin-Spandau via the Oder and the Oder-Havel Canal, constructed in 1914. It was documented there in 20178 and two years later in a side channel of the Havel in northeastern Saxony-Anhalt9. Following the flow of the Havel, the species likely reached the Elbe at Gnevsdorf in Brandenburg. Fig. 1 illustrates potential dispersal pathways from the Ponto-Caspian region to the Elbe.

Fig. 1
figure 1

Dispersal paths of the mysid shrimp Paramysis lacustris (P. lacustris). Red triangle: Type locality10. More recent records overlay the older ones. The overview map is shown in the European Terrestrial Reference System 1989 (ETRS89) Universal Transverse Mercator (UTM) Zone 32N coordinate system (EPSG: 28532). The gray symbols display all records from the work of Wittmann11 and references therein. Black dots – Intentional introduction into Lithuanian waters in the 1960s12. Orange dots - records by Borza et al. 201913. Dashed red line - Potential migration corridor into northern German waters via the Baltic Sea or the Vistula-Oder route. Red dots without year - Records in Germany by Zettler 20157, Müller & Martens 20198 and Hohmann & Zettler 20209. Red dot with year - First record for the tidal Elbe in this study.The map was generated using QGIS version 3.32 using the river network shapefile by the United Nations (UN)’s Food and Agricultural Organization (FAO) global information system on water and agriculture AQUASTAT (Creative Commons BY-NC-SA 3.0 IGO; CC BY-NC-SA 3.0 IGO license) as well as Natural Earth’s lakes shapefile (available under Creative Commons Public Domain: CC PD List license).

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Research site

The study area is located in the freshwater section of the tidal Elbe between river kilometers 595.5 and 596.7 (Fig. 2). The sampled 1,200-meter-long shoreline section lies adjacent to the district of Ost-Krauel and is part of the Zollenspieker Nature Reserve, protected under the Zollenspieker Nature Conservation Ordinance (Verordnung über das Naturschutzgebiet Zollenspieker, ZollenNatSchGebV HA) and the Flora-Fauna-Habitat Directive (Fauna-Flora-Habitat-Richtlinie, FFH).

Fig. 2
figure 2

Study area in the tidal Elbe in the area of the Zollenspieker nature reserve, here at low tide. The overview map is shown in the World Geodetic System 84 (WGS84) coordinate system (EPSG: 4326). The map was generated using QGIS version 3.32 using a Google Satellite background map accessed (Maps Data ©2025) with data from Airbus Imagery of 6/9/2023 and Maxar Technologies, available for research without needing permission, according to the Google geo guidelines (as of March 2025).

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At Zollenspieker, the shoreline features stone revetments and groynes extending into the river. A narrow strip of herbaceous riparian vegetation grows atop and behind the stone walls, transitioning into tall herbaceous fringes and floodplain forests. The groyne fields are classified as FFH habitat type “unvegetated river mudflats” (Flusswatt ohne Vegetation, FWO) and protected under §28(1) of the Hamburg Nature Conservation Act (Hamburgisches Gesetz über Naturschutz und Landschaftspflege, HmbNatSchG).

The groynes, spaced 200 meters apart, increase flow velocity in the river’s center while reducing it near the banks. This reduced flow allowed fine-grained and silty sediments, transported by floods and erosion in the navigational channel, to accumulate in the groyne fields. On one hand, this process has created flow-reduced areas near the groynes covered with anoxic mud, which often contains large amounts of undecomposed detritus. On the other hand, significant portions of the intertidal zone have silted up over time, resulting in the loss of biologically valuable shallow water zones.

To counteract this, three of the five groynes in the study area were notched to enhance hydrodynamics and promote sediment removal from the groyne fields. This measure was implemented in January 2023 by the Elbe Habitat Foundation14 and has been monitored scientifically since August 2022 by members of the early-career research group “Future Urban Coastlines” at TU Braunschweig.

During the sampling month, on August 16, 2023, the River Basin Community Elbe (RBC Elbe, Flussgebietsgemeinschaft Elbe, FGG Elbe) recorded the following chemical-physical parameters for water quality15: a Potential of Hydrogen (pH) value of 8.4, a water temperature of 19.4 °C, and an oxygen saturation of 112 % (8.3 mg\(\text {L}^{-1}\)). The oxygen saturation values exceeding 100 % and the elevated pH indicate increased activity of phototrophic organisms during the sampling period.

Results

During the sampling of the groyne fields in the lower tidal Elbe on August 31, 2023, a total of 36 individuals of P. lacustris and a single specimen of Limnomysis benedeni (L. benedeni) were recorded (see Table 1).

Table 1 Site description of Paramysis lacustris (P. lacustris) in the upper tidal Elbe river with recorded Water Temperature (WT) and Salinity (PSU) During Sampling.
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The specimens were exclusively found in aquatic habitats near the groynes (Fig. 3). Thirty-one individuals were observed swimming between the reinforcement stones of a notched groyne, in small pools formed by the receding tide. No colonization of this habitat, characterized by hard substrate and coarse sand, by other species was detected.

Fig. 3
figure 3

Notched groyne in the upper tidal Elbe near Ost-Krauel – Numerous mysid shrimps Paramysis lacustris (P. lacustris) were recorded in the water column near the groyne and in tidal ponds (bottom right in the picture). Photograph was provided to the authors courtesy of the Stiftung Lebensraum Elbe 2023.

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In landing net samples taken at two different groynes, a total of five free-swimming P. lacustris (Fig. 4) and one L. benedeni were identified. Besides two chironomid larvae and three juvenile bivalves, no additional species were present in the net. Crustacean representatives were also found inhabiting sandy sediment as well as within and on deadwood (Table 2).

Fig. 4
figure 4

Paramysis lacustris (P. lacustris) and characteristic features. I: Ventral view of a female with characteristic dots on abdominal somites one to five, II: Telson with statocysts, III: Antennal scale with tooth, IV: Rostrum and eyes banded. Pictures were recorded with a Zeiss Stereo Microscope Stemi 305 trinocular EDU-Set (Carl Zeiss AG, Germany – see also methods).

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Table 2 List of Crustacea recorded in the study area with special focus on alien species and their origin. P-C : Ponto-Caspian, NW-A : Northwest Atlantic, NW-P: Northwest Pacific. The abundances of individuals in each habitat type are based on estimates.
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Among the eight crustacean species identified during the monitoring, 50% are of Ponto-Caspian origin. The presence of Dreissena polymorpha (D. polymorpha) in the study area with individuals inhabiting the sandy habitats near the low water line or colonizing on native duck mussel further highlights the influence of Ponto-Caspian species in the surveyed area.

Discussion

For the Elbe, this study presents the first record of the mysid shrimp P. lacustris. The freshwater zone of the Elbe estuary featuring flow-reduced groyne fields indicates a suitable habitat for this species. P. lacustris prefers fine sediments such as sand, mud, and silt in standing or slow to moderately flowing water1,22.

P. lacustris is euryhaline, tolerating salinity fluctuations between 0 PSU to 13.8 PSU in its native range, which besides the Elbe includes the Black and Caspian Seas as well as major rivers such as the Danube, Volga, and Dnieper11,23.

Although P. lacustris can endure water temperatures as low as 0°C, seasonal storm surges and declining winter temperatures present challenges for its survival in the study area. In lakes and ponds, individuals retreat to deeper areas when temperatures drop below the species’ natural range11. However, in a dynamic river system like the Elbe, where deeper areas are near the navigation channel with flow velocities far exceeding the species’ habitat preferences, this strategy alone is unlikely to support successful establishment: Recent studies suggest that a combination of cold-tolerant traits – such as entering diapause or hibernation – enhances the ability of Ponto-Caspian invertebrates like P. lacustris to cope with very low temperature conditions24. This physiological adaptability to fluctuating temperature and salinity is a key factor for colonisation success of Ponto-Caspian species beyond their native range1,23.

Manmade waterways increase hydrologic connectivity thus enabling alien species to spread across European watersheds. This is epitomized by the Elbe River being a crucial component of Central Europe’s inland waterway network, linking the North Sea, Baltic Sea, Rhine, Danube, and Black Sea through an extensive system of rivers and canals. While this connectivity supports commerce, it also facilitates the dispersal of non-native aquatic invertebrates25. The saltwater-influenced zone of the Elbe estuary, in particular, now hosts an increasing number of alien macroinvertebrate species23,26. For instance, of the eight crustacean taxa identified in the study area, six were non-native with four of them originating from the Ponto-Caspian region. Of the two non-Ponto-Caspian aliens, the Chinese mitten crab Eriocheir sinensis (E. sinensis) was the only decapod found during our monitoring. Its range expansion is an example of human-mediated dispersal via maritime transport, which supposedly started in Germany in the port of Hamburg in the early decades of the last century and continued to rapidly distribute accross Europe27. As for P. lacustris in Lithuanian waters, the introduction of the North-American euryhaline G. tigrinus in German rivers was intentional: It was released into a river west of the Elbe (Werra) in the late 1950s to account for a declining population of the native gammarid populations due to salt pollution from mining activities28. From there, it expanded its range northeastward, possibly facilitated by the German Mittelland Canal, which connects major German rivers since 193829,30,31. Although our sampling results showed the species vastly outnumbering the native amphipods Gammarus zaddachi (G. zaddachi) and Gammarus duebenii (G. duebenii), this is likely due to the habitat preferences of the latter, which left ecological niches open for colonization by alien species26,30. This factor needs to be taken into account when trying to assess possible ecological impacts of the arrival of P. lacustris in the Elbe.

Former observations indicate that P. lacustris can form large populations under favorable conditions11,12. The introduction of the Ponto-Caspian mysid shrimp P. lacustris into European mesotrophic lakes and reservoirs has led to significant alterations in local food webs5. Originally assumed to feed primarily on phytoplankton and detritus, P. lacustris has demonstrated omnivorous feeding habits, preying on various invertebrates, including mesozooplankton species. Its dietary flexibility enables P. lacustris to occupy a higher trophic level, thereby extending food chain length and potentially affecting the trophic position of native planktivorous fish, such as the European perch4,5, Perca fluviatilis (P. fluviatilis). Thus the establishment of P. lacustris may lead to increased competition for food resources among native zooplanktivorous species and could entail negative effects on the overall energy flow within the ecosystem. Yet, the Elbe river is a heavily modified waterway that has undergone extensive alterations to its riverbanks and habitats, and conclusions from research in lakes and water reservoirs can probably not be drawn directly. Comparing our current observations to biological river water quality assessments based on the European Water Framework Directive32, we found that species composition and abundance is classified as only moderate in our study area, with only four Crustacean species found in 201833. Thus, in the context of the Elbe Habitat Foundation’s objective to create suitable habitats for fish and lampreys within groyne fields, the occurrence of P. lacustris as an additional energy-rich food source may be considered beneficial.

Methods

The sampling was conducted on behalf of the Elbe Habitat Foundation (Stiftung Lebensraum Elbe) to evaluate the impact of the construction measures, particularly regarding the development of suitable habitats for fish and lampreys within the groyne fields. In late summer 2023, a total of 17 samples were collected from three different habitat types: sand, deadwood, and stone revetments, to assess the communities inhabiting the groyne fields.

The sandy, partially silty soft substrate along the low-water line was sampled at ten locations, each covering an area of 25 cm x 25 cm, using a 500 \(\upmu\)m mesh sieve. At three additional locations, deadwood components were collected from an equivalent area using tweezers. The remaining four samples were taken from aquatic habitats between and adjacent to the immobile stones of the groynes. In these cases, stones were brushed, and a simple hand net with a mesh size of 500 \(\upmu\)m and a frame length of 25 cm was used to perform four net sweeps.

For all habitat types, a preliminary live sorting was conducted in the field, with only a small number of individuals preserved for further identification in the laboratory.

We used a Zeiss Stereo Microscope Stemi 305 trinocular EDU-Set (Carl Zeiss AG, Germany) equipped with an eyepiece 16\(\times\)/14 Br. foc and an Objective 2.0\(\times\) FWD 43 mm front optics system 3 for Stemi 305. Images were captured using an Axiocam 208 color microscope camera (USB3, 8 MP, 1/2.1 in). Imaging and analysis were conducted using the free version of Zen Core 3.6 software (Carl Zeiss AG).