Assessment of Top-Down and Bottom-Up Controls on the Collapse of Alewives (<Emphasis Type="Italic">Alosa pseudoharengus</Emphasis>) in Lake Huron

Food web models are powerful tools to inform management of lake ecosystems, where top-down (predation) and bottom-up (resource) controls likely propagate through multiple trophic levels because of strong predator–prey links. We used the Ecopath with Ecosim modeling approach to assess these controls on the Lake Huron main basin food web and the 2003 collapse of an invasive pelagic prey fish, alewife (Alosa pseudoharengus). We parameterized two Ecopath models to characterize food web changes occurring between two study periods of 1981–1985 and 1998–2002. We also built an Ecosim model and simulated food web time-dynamics under scenarios representing different levels of top-down control by Chinook salmon (Oncorhynchus tshawytscha) and of bottom-up control by quagga mussels (Dreissena rostriformis bugensis) and nutrients. Ecopath results showed an increase in the relative importance of bottom-up controls between the two periods, as production decreased across all trophic levels. The production of non-dreissenid benthos decreased most, which could cause decreases in production of pelagic prey fishes feeding on them. Ecosim simulation results indicated that the alewife collapse was caused by a combination of top-down and bottom-up controls. Results showed that while controls by Chinook salmon were relatively constant before alewife collapse, controls by quagga mussels and nutrients increased jointly to unsustainable levels. Under current conditions of low nutrients and high quagga mussel biomass, simulation results showed that recovery of alewives is unlikely regardless of Chinook salmon biomass in Lake Huron, which implies that the shrinking prey base cannot support the same level of salmonine predators as that prevailed during the 1980s.     

Water quality effects following establishment of the invasive <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas) in a shallow eutrophic lake: implications for pollution mitigation measures

This study investigates whether ecosystem alteration occurred in a shallow, lowland lake following establishment of the alien zebra mussel, Dreissena polymorpha (Pallas). Measurements of total phosphorus (TP) loads, lake TP concentrations, phytoplankton (chlorophyll a) and transparency levels for the period 1990–2008 were examined to determine the water quality effects of D. polymorpha. The period of time also included the implementation of catchment measures aimed at reducing phosphorus (P) loading to the lake. A range of loading-response models was tested to explore changes in the net sedimentation rate for P. Results show that while high TP loads from the catchment were reduced, TP concentrations in the lake remained high after D. polymorpha invasion. Decoupling of the previous chlorophyll a–TP relationship also occurred. Results from a TP loading-response model that closely simulated observed concentrations in the lake prior to establishment of D. polymorpha indicated that measured TP post establishment was statistically higher than predicted for the same conditions but without the presence of D. polymorpha. Data presented in the paper highlight the need to consider the potential impacts of invasive species in evaluations of the effectiveness of measures aimed at mitigating aquatic pollution.     

The relationship between the freshwater pearl mussel (<Emphasis Type="Italic">Margaritifera margaritifera</Emphasis>) and its hosts

Endangered freshwater pearl mussels Margaritifera margaritifera reveal a complex life cycle with an obligate host-dependent phase. Only two species, Atlantic salmon Salmo salar and brown trout S. trutta, are important hosts in Europe, indicating a high degree of specialization. Whilst freshwater pearl mussels with their filtering activity can provide important ecosystem services and indirectly improve the habitat quality for their salmonid hosts, their direct effects on physiological stress, reduced swimming performance, and increased mortality at high rates of infestation all support a parasitic character of the mussel during its hostdependent phase. From an evolutionary perspective, both the much greater generation time of the parasite compared to the host, as well as the great distribution ranges of M. margaritifera and its hosts should favour local adaptation patterns. The variable suitability of different salmonid strains and species as hosts for M. margaritifera and the resulting differences in the performance of larvae during the host-dependent phase indicate that host-management strategies should focus on maintaining high quality hosts at a regional scale to avoid selection or genetic drift effects which could erode the genetic and evolutionary potential for adaptation to changing environmental conditions.     

Population genetic structure and ancestry of steelhead/rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) at the extreme southern edge of their range in North America

Steelhead (Oncorhynchus mykiss) populations have declined dramatically in many parts of their range in North America, most critically in Southern California, where these anadromous trout are now classified as ‘Endangered’ under the United States Endangered Species Act. The widespread introduction of hatchery rainbow trout, the domesticated freshwater resident form of the species O. mykiss, is one factor threatening the long-term persistence of native steelhead and other trout populations. To identify where native fish of coastal steelhead lineage remained, we performed a population genetic analysis of microsatellite and SNP genotypes from O. mykiss populations at the extreme southern end of their range in Southern California, USA and Baja California, Mexico. In the northern part of this region, nearly all populations appeared to be primarily descendants of native coastal steelhead. However, in the southern, more urbanized part of this region, the majority of the sampled populations were derived primarily from hatchery trout, indicating either complete replacement of native fish or a strong signal of introgression overlaying native ancestry. Nevertheless, these genetically introgressed populations represent potentially critical genetic resources for the continued persistence of viable networks of O. mykiss populations, given the limited native ancestry uncovered in this region and the importance of genetic variation in adaptation. This study elucidates the geographic distribution of native trout populations in this region, and serves as a baseline for evaluating the impacts of hatchery trout on native O. mykiss populations and the success of steelhead conservation and recovery efforts.     

Quantitative trends of zebra mussels in Lake Balaton (Hungary) in 2003–2005 at different water levels

During the extremely dry period between 2000 and 2003, the water level of Lake Balaton decreased by 82 cm and 80% of the stony littoral, an important habitat for the zebra mussel (Dreissena polymorpha), became dry. A recovery period started in 2004 due to intense precipitation, which increased water levels in the lake. Seasonal and spatial variations of the relative abundance, population density, population structure and biomass of the zebra mussel and the relative abundance of the amphipod Chelicorophium curvispinum were monitored in the period of 2003–2005 at four different shoreline sections and in two different portions (on the bottom and near the surface portion of the rip-rap) of Lake Balaton. Along with these studies, a quantitative survey of mussel larvae found in the plankton and of the abundance of mussel feeding diving ducks were made. As a consequence of the water level fall, on the dried part of the stony littoral, numerous zebra mussel druses perished. Following the dry period in early 2004, the relative abundance of the mussel on the bottom stones was smaller than in 2003 and the bottom community was dominated by C. curvispinum. By the end of 2004 and during 2005, the water level returned to normal and the surfaces of the reinundated stones were conducive to the successful colonization of zebra mussels. Hence, they returned as the dominant fauna in 2005. The stones near the surface might provide a new substrate for the recruitment of zebra mussels, probably offering more suitable substrata for the settlement in 2005 than in 2003. Therefore, the new substrata available in 2005 may have encouraged better and more rapid zebra mussel colonization than before. Zebra mussels may be better competitors for new space than C. curvispinum. A minor change of water-level fluctuation in 2005 and the reduction in population size of the mussel feeding waterfowl could have contributed to the intensive spread of zebra mussel by 2005.     

Influence of aperiodic summer droughts on leaf litter breakdown and macroinvertebrate assemblages: testing the <Emphasis Type="Italic">drying memory</Emphasis> in a Central Apennines River (Aterno River,Italy)

Management of multiple exploited stocks of anadromous salmonids in large catchments requires understanding of movement and catchment use by the migrating fish and of their harvesting. The spawning migration of sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) was studied in the River Tweed, UK, using acoustic telemetry to complement exploitation rate data and to quantify catchment penetration. Salmon (n = 79) and sea trout (n = 65) were tagged in the tidal-influenced Tweed in summer–autumn. No tagged salmon left the river before spawning, but 3% (2010) and 8% (2011) of pre-spawning sea trout dropped out. Combined tag regurgitation/fish mortality in salmon was 12.5%, while trout mortality was 6% (2010) and 0% (2011). The estimated spawning positions of salmon and sea trout differed; tagged salmon were mostly in the main channel while trout occurred mostly in the upper Tweed and tributaries. Early fish migrated upstream slower than later fish, but sea trout moved through the lower-middle river more quickly than salmon, partly supporting the hypothesis that the lower exploitation rate in autumn of trout (1 vs 3.3% for salmon) there is generated by differences in migration behaviour.     

Effects of zebra mussels on cladoceran communities under eutrophic conditions

The purpose of this study was to determine how zebra mussels affected cladoceran community structure under eutrophic conditions. We conducted a mesocosm study where we manipulated the presence of zebra mussels and the presence of large-bodied Daphnia (Daphnia magna and Daphnia pulicaria). We also conducted a complimentary life-table experiment to determine how water from the zebra mussel treatment affected the life history characteristics of the cladoceran species. We anticipated that small- and large-bodied cladoceran species would respond differently to changes in algal quality and quantity under the effects of zebra mussels. Large-bodied Daphnia successfully established in the zebra mussel treatment but failed to grow in the control. We did not observe positive relationships between food concentrations and cladoceran abundances. However, the phosphorus content in the seston indicated that food quality was below the threshold level for large-bodied cladocerans at the beginning of the experiment. We believe that zebra mussels quickly enhanced the phosphorus content in the seston due to the excretion of inorganic phosphorus, thus facilitating the development of large-bodied Daphnia. In conclusion, our results suggest that zebra mussels can alter the phosphorus content of seston in lakes and this can affect the dynamics of crustacean zooplankton.     

Long-term nutrient trends and harmful cyanobacterial bloom potential in hypertrophic Lake Taihu,China

Rapid economic development in China’s Lake Taihu basin during the past four decades has accelerated nitrogen (N) and phosphorus (P) loadings to the lake. This has caused a shift from mesotrophic to hypertrophic conditions, symptomized by harmful cyanobacterial blooms (CyanoHABs). The relationships between phytoplankton biomass as chlorophyll a (Chla) and nutrients as total nitrogen (TN) and total phosphorus (TP) were analyzed using historical data from 1992 to 2012 to link the response of CyanoHAB potential to long-term nutrient changes. Over the twenty year study period, annual mean Chla showed significantly positive correlations with both annual mean TN and TP (P < 0.001), reflecting a strong phytoplankton biomass response to changes in nutrient inputs to the lake. However, phytoplankton biomass responded slowly to annual changes in TN after 2002. There was not a well-defined or significant relationship between spring TN and summertime Chla. The loss of a significant fraction of spring N loading due to denitrification likely weakened this relationship. Bioavailability of both N and P during the summer plays a key role in sustaining cyanobacterial blooms. The frequency of occurrence of bloom level Chla (>20 μg L^?1) was compared to TN and TP to determine nutrient-bloom thresholds. A decline in bloom risk is expected if TN remains below 1.0 mg L^?1 and TP below 0.08 mg L^?1.     

Special traits of biology and of the parasite fauna of juvenile salmonid fish of the genus <Emphasis Type="Italic">Salmo</Emphasis> of the Tornio River system (The Baltic Sea basin)

Juveniles of Atlantic salmon Salmo salar and trout S. trutta populating the system of the Tornio River (the Baltic Sea basin) are investigated. The obtained parasitological data indicate the absence of rigid spatial and food competition between juvenile salmon and trout in the case of their cohabitation.     

Combined evidence indicates that <Emphasis Type="Italic">Perna indica</Emphasis> Kuriakose and Nair 1976 is <Emphasis Type="Italic">Perna perna</Emphasis> (Linnaeus, 1758) from the Oman region introduced into southern India more than 100 years ago

The specific status of the brown mussel, Perna indica, in southern India and northern Sri Lanka has long been questioned. Its limited geographical distribution within the extensive range of the Asian green mussel, P. viridis, and its morphometric similarity to the African brown mussel, P. perna, have led several authorities to suggest that P. indica is not a distinct species but is in fact introduced P. perna. Analysis of DNA sequences for nuclear ITS and mitochondrial COI sequence data from newly collected mussels from southern India examined in the context of data from GenBank for mussels of the genus Perna reveals that P. indica is indeed P. perna from the Oman region. A literature review indicates that P. indica (now P. perna) has been established in southern India for at least 100 years. Reports of ecological interactions between the native P. viridis and the introduced P. perna reveal that P. perna grows to a larger size than P. viridis and outcompetes the native mussel. P. perna forms high density populations that have long been fished by local coastal communities and recently the fishery for P. perna has expanded to markets beyond the coastal area. Eradication of the introduced P. perna is not feasible and is not desirable given its important role as a source of protein and revenue for the local community. Because no monitoring of the range of P. perna in southern India occurs it is not possible to know if this species is increasing its area of distribution, but a qualitative assessment of ongoing range expansion is made based on limited evidence from published reports. The possibility exists for inter-specific hybridisation between P. viridis and P. perna and we recommend that testing commence to check for this. This research highlights the value of a detailed understanding of the genetic structure (four different clades) of the genus Perna, without which the status of P. indica as P. perna from Oman in southern India could not have been made.     

The brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in the lake, Øvre Heimdalsvatn: long-term changes in population dynamics due to exploitation and the invasive species,European minnow (<Emphasis Type="Italic">Phoxinus phoxinus</Emphasis>)

Studies of the brown trout (Salmo trutta) population in the Norwegian subalpine lake, Øvre Heimdalsvatn, over a 50-year period have revealed major changes in population dynamics. In 1958, the population density was high, with individuals stagnating in growth at lengths below 30 cm. After heavy exploitation during the years 1958–1969, the number of older fish declined substantially, and growth rates increased significantly. Since 1969, the European minnow (Phoxinus phoxinus) have been observed in the lake, with increasing densities from 1977–1978 to 1999–2000. The age structure of the brown trout population has changed markedly from the 1960s to the period 1993–2006. Annual recruitment significantly declined, from an average number of 3746 individuals in age-class 4 during the period 1958–1966 to an average of 1668 during the period 1993–2006. However, due to lower exploitation rates, the number of old fish was significantly higher in the latter period. The summer diet of brown trout has changed substantially from a dominance of the large crustaceans Lepidurus arcticus and Gammarus lacustris to a high occurrence of European minnows, while L. arcticus has become practically absent from the diet. There was a negative relationship between brown trout biomass and annual length increment. However, despite a brown trout biomass at the same level during the years 1993–2006 as in the 1960s, annual individual growth rates have significantly declined. The reduced recruitment and reduced annual growth rates of the brown trout, as well as changes in the diet, are most likely related to the introduction and establishment of the invasive species, the European minnow.     

Gamete-associated flavobacteria of the oviparous Chinook salmon (<Emphasis Type="Italic">Oncorhynchus tshawytscha</Emphasis>) in lakes Michigan and Huron,North America

Flavobacterial diseases, caused by multiple members of the Family Flavobacteriaceae, elicit serious losses in wild and farmed fish around the world. Flavobacteria are known to be transmitted horizontally; however, vertical transmission has been suspected but proven only for one fish-pathogenic flavobacterial species (e.g., Flavobacterium psychrophilum). Herein, we report on the isolation and molecular identification of multiple Flavobacterium and Chryseobacterium taxa from the ovarian fluid and eggs of feral Great Lakes Chinook salmon (Oncorhynchus tshawytscha). Identified egg- and ovarian fluid-associated flavobacteria were either well-known flavobacterial fish pathogens (e.g., F. psychrophilum and F. columnare), most similar to emerging fish-associated flavobacteria (e.g., F. spartansii, F. tructae, F. piscis, C. piscium, C. scophthalmum), or were distinct from all other described Chryseobacterium and Flavobacterium spp., as determined by phylogenetic analyses using neighbor-joining, Bayesian, and Maximum Likelihood methodologies. The gamete-associated flavobacteria fell into three groups (e.g., those that were recovered from the ovarian fluid but not eggs; those that were recovered from the ovarian fluid and eggs; and those that were recovered from eggs but not ovarian fluid), a portion of which were recovered from eggs that were surface disinfected with iodophor at the commonly used dose and duration for egg disinfection. Some gamete-associated flavobacteria were also found in renal, splenic, and neurological tissues. Systemic polymicrobial infections comprised of F. psychrophilum and F. columnare were also detected at nearly an 11% prevalence. This study highlights the potential role that sexual products of female Great Lakes Chinook salmon may play in the transmission of fish-associated flavobacteria.     

Comparative Analysis of Subcellular Distribution of Heavy Metals in Organs of the Bivalve Mollusks <Emphasis Type="Italic">Crenomytilus grayanus</Emphasis> and <Emphasis Type="Italic">Modiolus modiolus</Emphasis> in a Continuously Polluted Environment

The distribution of Zn, Fe, Ni, Cu, Mn, Cd, and Pb in subcellular fractions, and of Cd, Zn, and Cu in cytoplasm proteins of the kidney and digestive gland of the mussels Crenomytilus grayanus and Modiolus modiolus, sampled from contaminated and conditionally clean areas, was studied. It was found that, in a contaminated environment, the organs of mussels were more highly enriched with metals. It was shown that essential trace elements were accumulated mostly in the cytosol of organs of both molluscan species from contaminated areas, whereas in the background areas the trace elements were associated mostly with membrane structures in Gray's mussel, C. grayanus, and with the cytosol in M. modiolus, the northern horse mussel. The lead was bound mostly to membrane structures in organs of both mussel species at all stations. The method of gel chromatography enabled us to isolate metallothionein-like proteins from the kidney of the northern horse mussel sampled in contaminated areas, whereas their concentration in the kidney of Gray's mussels was lower than the limiting error of the method. It is supposed that in the kidney of Gray's mussel the synthesis of metallothionein-like proteins was quenched by the integrated effect of the accumulated metals.     

A first report on coexistence and hybridization of <Emphasis Type="Italic">Mytilus trossulus</Emphasis> and <Emphasis Type="Italic">M. edulis</Emphasis> mussels in Greenland

Semi-sessile Mytilus mussels are used as indicators of climate changes, but their geographic distribution is not sufficiently known in the Arctic. The aim of this study was to investigate the taxonomic status and genetic differentiation of Mytilus populations in a Northwest Greenlandic fjord at Maarmorilik, impacted by contaminations from a former mine. In this study, mussels were collected at three sites differing in exposure to environmental factors. A total of 54 polymorphic SNPs found in the Mytilus EST and DNA sequences analyzed were successfully applied to 256 individuals. The results provided the first evidence for the existence of M. trossulus in Greenland. The mussel from M. trossulus and M. edulis taxa are shown to coexist and hybridize in the fjord. The three studied sites were found to differ significantly in the distribution of taxa with a higher prevalence of M. trossulus in the inner fjord. The identified M. edulis × M. trossulus hybrids mostly had a hybrid index score of about 0.5, indicating a similar number of alleles characteristic for M. trossulus and M. edulis. There was a low number of backcrosses between ‘pure’ taxa and hybrids. This newly discovered hybrid zone between the two taxa is unique in comparison with the Canadian populations. As Mytilus mussels in Greenland hitherto have been regarded as the one taxon M. edulis, the results have importance for biogeography and future monitoring and environmental studies.     

Effects of nonnative invertebrates on two life stages of the native eastern oyster <Emphasis Type="Italic">Crassostrea virginica</Emphasis>

Since their recent introductions into Florida waters, three sessile invertebrates [Perna viridis (Asian green mussel), Mytella charruana (charru mussel) and Megabalanus coccopoma (pink titan acorn barnacle)] have expanded their range along the Atlantic coast in estuarine waters. Little research has been done to understand how these nonnative species interact with the ecologically and economically important eastern oyster Crassostrea virginica. To assess the potential effects of P. viridis, M. charruana and M. coccopoma on C. virginica, the following questions were addressed in manipulative experiments. (1) Does the presence of nonnative species decrease oyster larval settlement? (2) Do oyster larvae avoid settling on nonnative species? (3) Do nonnative species decrease survival of juvenile oysters (spat)? (4) Do nonnative species hinder spat growth? We included two controls: absence of nonnative species and presence of the native mussel Geukensia demissa. The nonnative species influenced settlement, growth and survival of C. virginica in different ways. M. coccopoma and P. viridis negatively influenced larval settlement, whereas M. charruana had no influence on the total number of settled larvae. Oyster larvae avoided settling on all three nonnative species and the native G. demissa. Both nonnative mussels negatively affected survival of juvenile oysters but only M. charruana also reduced spat growth. The native mussel, G. demissa, had no negative impacts on total settlement, survival and growth of C. virginica; in fact, it increased larval settlement in some trials. These three nonnative species should be classified as invasive because all had negative effects on native C. virginica.     

Mutual effects of several fouling organisms of the White Sea (<Emphasis Type="Italic">Mytilus edulis,Styela rustica</Emphasis>, and <Emphasis Type="Italic">Hiatella arctica</Emphasis>) on their growth rate and survival

The mutual effects of several fouling species (the bivalves Mytilus edulis and Hiatella arctica and a solitary ascidian Styela rustica) on their growth rate and mortality were studied through field experiments. The interactions between S. rustica and H. arctica appeared to be the least antagonistic. In contrast, the mussel was the most “aggressive” species with regard to both competitors. It was observed that the ascidians died, because they were intensively braided and gummed up with the byssus threads of the mussels. However, in some cases the intraspecific competition was stronger than the interspecific one.     

Embryonic development characteristics and host mussel utilization of flat bitterling <Emphasis Type="Italic">Acheilognathus rhombeus</Emphasis> (Cyprinidae) during winter in Korea

This study aimed to investigate the embryonic development characteristics and host mussel utilization of Acheilognathus rhombeus to determine its adaptations or host utilization for improving survival during winter in the wild; the study was conducted from August 2014 to May 2015. Spawning of A. rhombeus occurred between mid-September and early November, peaking between late September and early October of 2014 when the water temperature was 19–21°C. The embryos of A. rhombeus were first found in early October of 2014 and remained inside the host mussel for 7 months. Development retardation and diapause in embryonic stages (stages C and D) occurred before eye pigmentation and continued until late March of 2015. Subsequently, the embryos resumed development (stage E) when the water temperature increased to about 10 °C. The positions during the development of A. rhombeus embryos in the mussels were mainly present in the farthest suprabranchial cavity and not in the water tube. The embryos were frequently found in the two inner demibranches of the four gills of the mussels regardless of their sex. We assume that embryonic diapause and use of suprabranchial cavity and inner demibranches of the gills of the mussels by A. rhombeus might be the optimal adaptation to maintain high embryonic survival in low water temperature during winter and to avoid interspecific competition.     

Freshwater pearl mussel in Finland—current status and future prospects

Information on the present distribution and status of the threatened freshwater pearl mussel Margaritifera margaritifera populations in Finland was updated. The status of the populations was classified into seven categories which are viable, maybe viable, non-viable/partly viable, dying-out, almost extinct, probably extinct, and extinct. The main criteria for judging the viability class were the population size and proportion of juvenile mussels. According to calculated estimate 1.7% populations were viable, 8.5% maybe viable, 40.2% non-viable/partly viable, 14.5% dying-out, 30.8% almost extinct, and 4.3% probably extinct. The present number of rivers with M. margaritifera in Finland is 117.     

Same habitat,different species: otolith microchemistry relationships between migratory and resident species support interspecific natal source classification

We tested the hypothesis that otolith trace elemental signatures (microchemistries) of mottled sculpin Cottus bairdi, slimy sculpin C. cognatus, and juvenile coho salmon Oncorhynchus kisutch were predictive of those of juvenile steelhead O. mykiss across many sites within the Lake Michigan basin. Laser ablation inductively coupled plasma mass spectrometry was used to generate otolith microchemistry signatures for each individual fish. For each species pair, statistical correlations of mean otolith concentrations of Mg, Mn, Cu, Zn, Sr, Ba, and Pb for each site were estimated. Linear equations describing these relationships were used to transform juvenile steelhead otolith microchemistry data to those of each of the other species. Transformed otolith microchemistry data were subjected to random forest classifications developed for mottled sculpin, slimy sculpin, and juvenile coho salmon to assess interspecific natal source assignment accuracies. Steelhead otolith concentrations of Sr were significantly correlated with those of each of the other species, whereas otolith concentrations of Ba and Mn were significantly correlated among some species pairs, but not others. Natal source assignment accuracies of juvenile steelhead to site and watershed generally decreased when otolith microchemistry data were transformed to those of mottled sculpin, slimy sculpin, and coho salmon. Miss-assigned fish often classified into nearby watersheds within larger hydrologic units, leading to higher assignment accuracies at coarser geographical resolutions (75–97% correct assignment to hydrologic unit for each species). These findings suggest that applications of otolith microchemistry data may extend beyond the species from which they are collected.     

The endangered thick-shelled river mussel (<Emphasis Type="Italic">Unio crassus</Emphasis>): a new host species for the European bitterling (<Emphasis Type="Italic">Rhodeus amarus</Emphasis>)

Large freshwater mussels (Unionoida) are declining throughout the world. The European bitterling Rhodeus amarus (Bloch, 1782) female spawns its eggs inside the unionids’ shells, where fertilisation and further embryonic development take place; thus its reproduction depends fully on the presence of large freshwater mussels. Unio crassus, previously regarded as one of the most numerous unionids in Europe, is now listed in the IUCN Red Data List as being globally endangered. Despite its previous prevalence, it was never reported as a host for the bitterling. A large population of U. crassus was studied in small river at the ?wi?tokrzyskie Mts (Poland), where also electrofishing was conducted. In each bitterling territory located on the study plots, we found individuals of U. crassus, with the bitterling eggs or larvae developing on mussel’s gills. That proves that this species can be also used by the bitterling for reproduction. We suggest that this relationship has not been reported to date due to the mussels’ rarity and ongoing decline. However, it is also possible that the endangered mussel is a novel host for the bitterling, which is expanding its range throughout Europe.