Eglė Šidagytė-Copilas

The decline of salmonid stocks in the Baltic Sea region is a matter of serious concern, prompting many countries to implement widespread stocking of artificially reared individuals to restore or enhance populations. While such interventions are intended to be beneficial, their efficacy remains a subject of ongoing debate. Artificially reared fish often face challenges in adapting to natural environments and may struggle to compete with wild counterparts, potentially leading to reduced growth rates and diminished overall fitness. This study evaluated the growth and physiological condition of naturally hatched versus artificially reared Salmo trutta juveniles during their first two years of life, prior to smoltification and seaward migration. The results demonstrated that stocked juveniles exhibited significantly slower growth, a higher incidence of fin damage, and a greater abundance of cultivable gut bacteria compared to wild individuals. Conversely, no significant differences were observed in blood parameters. Such growth retardation suggests potential difficulties in adaptation and recruitment. Consequently, while the release of artificially reared S. trutta fry facilitates the restoration of extinct populations, its capacity to enhance existing stocks within Baltic Sea riverine ecosystems may be limited.

Alien Ponto-Caspian mysids Paramysis lacustris, Limnomysis benedeni and Hemimysis anomala were introduced into Lithuanian waters from Ukraine’s water reservoirs in the early 1960s. Their expansion from the primary introduction site in the Kaunas Reservoir, located on the Nemunas River, proceeded through secondary introductions and natural dispersal. The two species, P. lacustris and L. benedeni, are currently quite widespread in Lithuanian waters, whereas H. anomala has been observed exclusively in the Kaunas Reservoir until recently. Here, we present data from the most recent comprehensive survey of Ponto Caspian mysids and analyze the impact of sampling method on the likelihood of species detection. The results clearly indicate that the detection of larger-sized, more mobile species with good swimming abilities, such as P. lacustris, requires, in addition to conventional macroinvertebrate sampling, the use of devices designed to capture active nektobenthic animals. For this purpose, an epibenthic dredge or sledge is recommended. In contrast, the detection probability of the smaller-sized L. benedeni was not affected by the sampling method. The recent detection of the bloody-red mysid H. anomala near the Nemunas Delta suggests it may now be well-established in the area. However, due to its nocturnal lifestyle, the effective detection of this mysid requires dusk or nighttime sampling using equipment appropriate for capturing nektobenthic fauna.

A species' expansion beyond the native range is often assumed to be associated with an increased dietary niche breadth. However, empirical evidence remains limited due to a scarcity of studies comparing both the parental and invaded ranges. Here, we test the trophic niche expansion hypothesis by examining stable isotopes and functional morphology across native (NW Black Sea) and invaded (SE Baltic Sea) ranges of two amphipods, Dikerogammarus villosus and Pontogammarus robustoides, originating from the Ponto‐Caspian region—a major source of species invading Holarctic inland waters. Stable isotopes revealed that both species underwent a marked dietary niche contraction by becoming less carnivorous in the invaded range. This dietary shift was morphologically mirrored by an overall reduction of functional traits related to predation. The magnitude of dietary and morphological change was greater in D. villosus. Our findings indicate that previous experimental reports of aggressive predation in D. villosus reflect opportunistic foraging and align with local stable isotope studies which generally indicate a low trophic position. We conclude that Ponto‐Caspian species can undergo rapid dietary changes outside the native range, and that dietary niche expansion is not a prerequisite for successful non‐native establishment.

The spread of non-native species is one of the outcomes of global change, threatening many native communities through predation and competition. Freshwater ecosystems are particularly affected by species turnover with non-native species. One species that has been established in Central Europe for many decades – or even a few centuries – is the amphipod crustacean Gammarus roeselii. Although G. roeselii is nowadays widespread in major river systems, there have been recent reports of its spread into smaller streams that are typically inhabited by the native species Gammarus fossarum. Due to their leaf shredding ability, G. fossarum takes up a key position in headwater streams. This raises the important question, to what extent G. roeselii can equivalently take over this function. To answer this question, we collected both species from nine different sites in a mid-mountain river system (Kinzig catchment, Hesse, Germany) and investigated their functional similarity using a combination of stable isotope analysis, gut content and functional morphology. The species hardly differed in morphological characteristics, only females showed differences in some traits. Gut content analysis indicated a broad dietary overlap, while stable isotopes showed a higher trophic position of G. roeselii. The observed functional overlap could intensify interspecific competition and allow the larger and more predaceous G. roeselii to replace G. fossarum in the future as a headwater keystone species. However, the differentiation in the stable isotopes also shows that co-existence can occur by occupying different trophic niches. Moreover, the wide range of inhabited sites and exploited resources demonstrate the omnivorous lifestyle of G. roeselii, which is likely to help the species succeed in rapidly changing environments.

1. Niche conservatism posits that a species' non‐native populations establish in areas that match their native environmental conditions. Although the Ponto‐Caspian biodiversity hotspot is a major source of freshwater alien species, the extent to which their climatic niches diverge between invaded and native ranges remains poorly understood.2. Using an n‐dimensional hypervolume approach, we quantified climatic niche overlap and inferred patterns of niche differentiation (shift, contraction, or expansion) among native and invaded ranges for 12 widespread Ponto‐Caspian amphipod species (six genera in three families).3. Our results show that all species experience substantially different climatic conditions in their invaded range compared to their native range. The invasive niche either contracted (five species), shifted (four species), expanded and shifted (two species), or shifted and contracted (one species) relative to the native niche.4. We conclude that although the focal taxa share a common geographic origin and evolutionary history, they exhibit disparate patterns of climatic niche change outside their native range. The niche conservatism hypothesis receives mixed support given that half of the studied species underwent niche shifts/expansion. Furthermore, congeners exhibited both identical and contrasting patterns of niche differentiation, suggesting a limited phylogenetic effect.5. The uncovered diversity of niche dynamics among closely related species indicates that each has a unique potential for invasiveness and long‐term persistence. This has important implications for predicting invasion risk and refining management strategies. Our study also underlines the importance of biodiversity hotspots as reservoirs of highly adaptable alien species.

A species’ expansion beyond the native range is often assumed to be associated with an increased dietary niche breadth. However, empirical evidence remains limited due to a scarcity of studies comparing both the parental and invaded ranges. Here, we test the trophic niche expansion hypothesis by examining stable isotopes and functional morphology across native (NW Black Sea) and invaded (SE Baltic Sea) ranges of two amphipods, Dikerogammarus villosus and Pontogammarus robustoides , originating from the Ponto-Caspian region – a major source of species invading Holarctic inland waters. Stable isotopes revealed that both species underwent a twofold contraction of the dietary niche with a shift towards decreased carnivory in the invaded range. This dietary shift was morphologically mirrored by an overall reduction of prey grasping appendages, antennae, and mouthpart palps. The magnitude of dietary and morphological change was greater in D. villosus . Our findings indicate that previous experimental reports of aggressive predation in D. villosus reflect opportunistic foraging and align with local stable isotope studies which generally indicate a low trophic position. We conclude that Ponto-Caspian species can undergo rapid, if non-intuitive, changes in both diet and functional morphology outside the native range, likely contributing to their invasive potential.

European inland surface waters are home to a rich diversity of native amphipod crustaceans, many of which face threats from invasive Ponto-Caspian counterparts. In this study, we analyse mitochondrial genomes to deduce phylogenetic relationships and compare gene order and nucleotide composition between representative native European and invasive Ponto-Caspian taxa across five families, ten genera and 20 species (with 13 newly sequenced herein). We observe various gene rearrangement patterns in the phylogenetically diverse native species pool.Pallaseopsis quadrispinosaandSynurella ambulansexhibit notable deviations from the typical organisation, featuring extensive translocations of tRNAs and the nad1 gene, as well as a tRNA-F polarity switch in the latter. The monophyletic invasive Ponto-Caspian gammarids display a conserved gene order, primarily differing from native species by a tRNA-E and tRNA-R translocation, which reinforces previous findings. However,Chaetogammarus warpachowskyishows extensive rearrangement with translocations of six tRNAs. The invasive corophiid,Chelicorophium curvispinum, maintains a highly conserved gene order despite its distant phylogenetic position. We also discover that native species have a significantly higher GC and lower AT content compared to invasive species. The mitogenomic differences observed between native and invasive amphipods warrant further investigation and could provide insights into the mechanisms underlying invasion success.

Niche conservatism posits that a species’ non-native populations establish in areas that match the native environmental conditions. Although the Ponto-Caspian region is a major source of inland aquatic alien species, the extent to which their climatic niches diverged between the invasive and native ranges remains poorly understood. Using an n-dimensional hypervolume approach, we quantified climatic niche overlap and inferred patterns of niche differentiation (shift, contraction, or expansion) among native and invaded ranges for 12 widespread Ponto-Caspian amphipod species (six genera in three families). Our results show that all investigated species experience substantially different climatic conditions in the invaded range. The invasive niche either contracted (five species), shifted (four species), expanded and shifted (two species), or shifted and contracted (one species) relative to the native niche. We conclude that although the focal taxa share a common geographic origin and evolutionary history, they exhibit disparate patterns of climatic niche change outside the native range. The niche conservatism hypothesis receives mixed support given that half of the studied species underwent niche shifts/expansion. Furthermore, congeners exhibited both identical and contrasting patterns of niche differentiation, suggesting a limited phylogenetic effect. The uncovered diversity of niche dynamics among closely related species indicates that each has a unique potential for invasiveness and long-term persistence. This has important implications for predicting invasion risk and refining management strategies.

European inland surface waters harbor an extensive diversity of native amphipod crustaceans with many species facing threats from invasive counterparts of Ponto-Caspian origin. Herein, we examine mitochondrial genomes to infer phylogenetic relationships and compare gene order and nucleotide composition between representative native European and invasive Ponto-Caspian taxa belonging to five families, ten genera, and 20 species (13 newly sequenced herein). We observe diverse gene rearrangement patterns in the phylogenetically disparate native species pool. Pallaseopsis quadrispinosa and Synurella ambulans show significant departures from the typical organization, with extensive translocations of tRNAs and the nad1 gene, as well as a tRNA-F polarity switch in the latter. The monophyletic alien Ponto-Caspian gammarids display a conserved gene order, mainly differing from the native species by a tRNA-E and tRNA-R translocation, which strengthens previous findings. However, extensive rearrangement is observed in Chaetogammarus warpachowskyi with translocations of six tRNAs. The alien corophiid, Chelicorophium curvispinum , displays a very conserved gene order despite its distant phylogenetic position. We also find that native species have a significantly higher GC and lower AT content than invasive ones. The observed mitogenomic differences between native and invasive amphipods need further investigation and could shed light on the mechanisms underlying invasion success.