Invasive zebra mussel (Dreissena polymorpha) threatens an exceptionally large population of the depressed river mussel (Pseudanodonta complanata) in a postglacial lake

1. Freshwater mussels are in decline worldwide, with the depressed river mussel Pseudanodonta complanata being one of the rarest and most endangered species in Europe. Invasive mussels are suspected to be an important factor of decline, but there is little information on their interaction with native species.
2. This study analyzed densities, depth distribution, and individual sizes and weights in one of the largest known populations of P. complanata in Europe in relation to the co‐occurring invasive zebra mussel Dreissena polymorpha and other mussel species, using a systematic transect analysis.
3. Pseudanodonta complanata was the dominant unionid species in Lake Siecino reaching densities of up to 26 ind/m², with half of the specimens found at a water depth of 2.0–4.0 m. Densities were highest on sandy substrates in areas of underwater currents. In contrast, 67% of native Unio tumidus were found at depths < 1 m, indicating different habitat preference.
4. In the study area, 91% of P. complanata, 92% of U. tumidus, and all Anodonta individuals were fouled by D. polymorpha. The dreissenid:unionid mass ratio (mean ± SD; maximum) was 0.43 ± 0.56; 4.22 and 0.86 ± 1.87; 8.76 in P. complanata and U. tumidus, respectively. Pseudanodonta complanata fouled with D. polymorpha were impaired in their anchoring capability and had shell deformations potentially affecting shell closing and filtration activity. Fouling intensity was negatively correlated with unionid density, potentially leading to accelerated population declines.
5. The observed adverse effects of invasive zebra mussels on the depressed river mussel and the difficulties in eradicating established populations of invasive mussels suggest that D. polymorpha should be considered a serious threat to P. complanata. Therefore, the further spread of zebra mussels into habitats with native unionids needs to be avoided by all means.

     

Effects of fine-scale environmental heterogeneity on local genetic structure in Macoma balthica from the Gulf of Gdañsk (southern Baltic Sea)

Surveys of unionid populations have been incorporated into the Swedish national environmental monitoring program. Nevertheless there is still lack of knowledge of important aspects of the biology of many unionid species. There may also be differences between species. This work compares the spatial distribution of two unionid species, Anodonta anatina and Unio tumidus, at four sites in Sweden. Samples were taken at transects along the shoreline at different water depths on the surface bed and in the sediments. Individual mussel ages were determined. Our results indicated that younger mussel individuals do not show differences in horizontal spatial distribution compared to older mussel individuals. However, they showed a preference to be burrowed in the sediment compared to older individuals that were predominantly found at the sediment surface. We also found a large difference in burrowing frequency between species with a higher frequency of burrowed adult individuals of A. anatina than U. tumidus. This result may be due to the timing of our field survey that coincided with the period of glochidia release for U. tumidus. We can conclude that a monitoring program for freshwater mussel populations needs to be carefully planned in time.     

Mechanisms and impact of differential fouling of the zebra mussel Dreissena polymorpha on different unionid bivalves

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Valve movement of the freshwater mussel Anodonta anatina: a reciprocal transplant experiment between lake and river

The effects of source and caging on the valve movements of the freshwater unionid mussel (Anodonta anatina) were studied in a reciprocal transplant experiment between a lake and its outflow. Caged mussels were moved and compared with those remaining in their natural environment on the lake or river bottom. At both sites, the mussels from the study site and the transplanted mussels from the opposite site were monitored simultaneously. In river the averaged weighted valve openness was higher and the number of valve movements was lower than in the lake. The mussels monitored in the lake exhibited a diurnal rhythm of valve movements which differed between the lake-bottom and the caged animals. Caging was found to increase valve openness. On the other hand, little variation appeared in valve openness between caged and bottom animals in the river, where diurnal rhythms were almost nonexistent. In the river the valve movements were more variable in respect to time than in the lake.     

Population density and age structure of the freshwater pearl mussel, Margaritifera margaritifera, in two Iberian rivers

1. The European freshwater pearl mussel, Margaritifera margaritifera (Bivalvia: Unionoida), is one of the most threatened mussels. The Iberian populations of this species are considered peripheral because their distinct characters such as growth rate and longevity, and require development of effective conservation strategies. 2. We assessed population density and age structure of pearl mussels in two Galician rivers (Eo and Masma in north‐west Spain). Four sampling sites were selected in each river to cover stretches of 100 m. The mean density of mussels in each of these sampling transects was estimated using the adaptive sampling technique, given that mussels occur at low densities and are highly aggregated in these rivers. 3. Age structure was inferred for each population using length–age keys. The empty shells encountered during sampling were used to determine the length of the specimens at different ages (years), together with length‐at‐age data from shells previously analysed for computing growth rates from the same rivers. Water samples from both rivers were analysed for typical physicochemical parameters. 4. Mean densities were very variable, even within the same river (from 0.27 to 6.55 m^?2 in the River Eo and from 0.98 to 2.38 m^?2 in the River Masma). Individuals in the 0‐ to 5‐year age class were scarce in both rivers. 5. Margaritifera margaritifera showed a preference for the strip of river bed within 1.5 m from the river bank and avoided sites at greater distances. The species also showed a preference for sites with more than 80% tree cover and avoided sites with <50% cover. 6. Iberian populations exhibit the highest growth rate, together with the lowest maximum age and maximum length known for M. margaritifera. Detailed knowledge about these peripheral Iberian populations will contribute to developing strategies for conservation and management of this endangered species.     

Identification of ecophenotypic trends within three European freshwater mussel species (Bivalvia: Unionoida) using traditional and modern morphometric techniques

Most species of freshwater mussels (Unionoida) show a wide variability in shell form and size but an understanding of which factors determine unionoid morphology is poor. We identified ecophenotypic trends in shell and internal characters within three unionoid species from two habitat types (marinas and river) of the River Thames, UK, using traditional and modern morphometric techniques. In marinas, all species grew to larger maximum sizes than in the river, which might be a result of higher temperatures and phytoplankton densities in marinas. Unio pictorum in marinas was more elongated than in the river and Fourier shape analysis revealed a trend from dorsally arched river specimens to straight dorsal and pointed posterior margins in marina individuals. The degree of shell elongation and shape of dorso‐posterior margin were not associated with sediment composition, but were associated with the different hydrological characters of the two habitat types. Relative shell width was a poor indicator of collection site and influenced by allometric growth. Unlike U. pictorum, a difference in shell elongation of marina and river mussels could not be detected in Unio tumidus and Anodonta anatina. However, all three species showed the same trends regarding the shape of the dorso‐posterior shell margin. This shell character may thus have broad ecological significance and could have considerable utility to palaeontologists, taxonomists, and conservation biologists. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 814–825.     

Seasonal settlement and succession of fouling communities in Kalpakkam, east coast of India

Seasonal distribution and community succession of macrofoulants were studied using concrete panels in the coastal waters of Kalpakkam, east coast of India, for a period of two years. The panels were suspended at 1 m, 4 m and 7 m depths and categorised into short-term and long-term exposures. A high total of 105 fouling taxa were recorded. The major fouling organisms observed were hydroids, barnacles, mussels, anthozoans and ascidians. Considerable faunistic and biomass variations were noticed both with respect to season and depth. The month of panel exposure had a significant influence on the succession of fouling communities. On the short-term panels, the maximum fouling biomass was 64 kg m^–2 in 30 days at 4 m depth, whereas on the long-term panels, it was 250 kg m^–2 after 216 days at 4 m depth. A comparison with the biomass values reported from elsewhere shows that biomass build-up in Kalpakkam coastal waters is one of the highest ever reported. Such a very high biomass accumulation is due to the extremely dense settlement of mussels, especially the green mussel,Perna viridis (L).     

Fouling of European freshwater bivalves (Unionidae) by the invasive zebra mussel (Dreissena polymorpha)

1. The zebra mussel (Dreissena polymorpha) is well known for its invasive success and its ecological and economic impacts. Of particular concern has been the regional extinction of North American freshwater mussels (Order Unionoida) on whose exposed shells the zebra mussels settle. Surprisingly, relatively little attention has been given to the fouling of European unionoids. 2. We investigated interspecific patterns in fouling at six United Kingdom localities between 1998 and 2008. To quantify the effect on two pan‐European unionoids (Anodonta anatina and Unio pictorum), we used two measures of physiological status: tissue mass : shell mass and tissue glycogen content. 3. The proportion of fouled mussels increased between 1998 and 2008, reflecting the recent, rapid increase in zebra mussels in the U.K. Anodonta anatina was consistently more heavily fouled than U. pictorum and had a greater surface area of shell exposed in the water column. 4. Fouled mussels had a lower physiological condition than unfouled mussels. Unlike tissue mass : shell mass ratio, tissue glycogen content was independent of mussel size, making it a particularly useful measure of condition. Unio pictorum showed a stronger decline in glycogen with increasing zebra mussel load, but had a broadly higher condition than A. anatina at the time of study (July). 5. Given the high conservation status and important ecological roles of unionoids, the increased spatial distribution and fouling rates by D. polymorpha in Europe should receive more attention.     

Faunal structures associated with patches of mussels on East Asian coasts

Aggregations of mussels harbor a variety of associated animals and make it possible for diverse species to coexist at the shore. Species composition and diversity of the associated fauna are controlled by the position of mussel beds or patches, e.g. tidal level, age structure of mussels, quality of ambient water and by mussel species. When patches of mussels were surrounded by algal growth, a difference in the species composition of the associated fauna was recognized between the patches and algal mats. Mechanisms promoting coexistence are discussed. Biodeposit production by mussels may affect the environment both within the bed and the ambient waters. Reducing sediments showing low Eh values caused by the accumulation of biodeposits was observed in calm waters where the polychaete Capitella capitata, an indicator for organic enrichment, occurred both in the intertidal mussel bed and the subtidal sandy bottom communities. In a shallow subtidal sandy bottom of the Gulf of Thailand, where heavy bioturbation by the spatangoid urchin Brissus latecarinatus was occurring, small patches of the mussel Modiolus metcalfi increased species diversity and equitability in this habitat. Species composition was different between mussel patches and pure sandy bottoms. Electronic Publication     

Water clearance efficiency indicates potential filter-feeding interactions between invasive <Emphasis Type="Italic">Sinanodonta woodiana</Emphasis> and native freshwater mussels

The Chinese pond mussel (Sinanodonta woodiana Lea, 1834) is a benthic filter-feeder that prefers soft-bottomed freshwater habitats and has successfully spread into both tropical and temperate water bodies outside its natural Southeast Asian range. Due to its preference for nutrient-rich waters with high levels of suspended food particles, the capacity of S. woodiana to influence natural seston concentrations is thought to be relatively low in comparison to that of other invasive bivalves. The experimental quantification of seston removal efficiency reported here demonstrates that S. woodiana is able to reduce seston loads to levels comparable to those by the control native freshwater mussel species Unio tumidus Philipsson, 1788. Moreover, increasing food depletion did not cause detectable changes in the filtration regime of S. woodiana, although the activity of native U. tumidus was significantly reduced. The seston clearance rate (volume of water cleared of particles per unit time) of S. woodiana averaged 9.3 ± 4.0 mL g^?1 wet mass h^?1, which corresponds to the total daily volume of water filtered being up to several hundreds to thousands L m^?2 at the maximal S. woodiana population densities reported in the literature. The observed filtration capacity of S. woodiana and its current invasional spread into areas inhabited by endangered freshwater mussels call for more careful consideration of filter-feeding interactions with native mussels. The potential impacts of S. woodiana should be studied in more detail with respect to available food resources and long-term nutritional needs of native species and reflected in management strategies in the invaded range.     

Guild structure in water mites (Unionicola spp.) inhabiting freshwater mussels: choice,competitive exclusion and sex

Summary Unionicolid water mites inhabit freshwater unionid mussels during the nymphal and adult stages of their life-cycle. Regular sampling of mussels from two sites in St. Mark's River, Fl. established that each of four species of water mite (Unionicola abnormipes, U. fossulata, U. serrata and U. formosa) occurred mainly in one or two of the mussel species available at each site.The role of preference for particular mussel species during host location was assessed for the first three mite species by choice experiments, in which mites were offered different mussel species simultaneously. In five out of six experiments, mites entered normally unused mussels as often as they did normally used ones. Additionally, a sexual difference in choice was found for U. fossulata, with males preferring one mussel species and females showing no preference. One mussel species, (Anodonta imbecilis), normally unused but chosen by mite species during the lab. experiments, is inhabited exclusively by the fourth mite species, U. formosa, in the field. An experiment showed that U. formosa excludes other mite species aggressively from Anodonta imbecilis.The results illustrate the sometimes misleading nature of simple sampling data as an indication of host specificity or host preference in parasites. They suggest also that the population dynamics of some parasites might be more fruitfully compared to unrelated, free-living species than to other parasites.     

Phosphorus recycling by zebra mussels in relation to density and food resource availability

Using flow-through microcosms, we examined phosphorus (P) recycling by zebra mussels under conditions of nearly constant food resource supply and varying zebra mussel population densities (600–5200 ind./m²). At all density levels, zebra mussels filtered substantial algae, measured as chlorophyll biomass. Because chlorophyll biomass inputs were low throughout the study, zebra mussel biomass-specific rates of chlorophyll filtration declined with increasing density, suggesting food resource limitation at the higher densities. We observed net total P export and high zebra mussel biomass-specific rates of P recycling over time in microcosms at high zebra mussel densities. In systems with a low zebra mussel density, net total P export did not occur over time. Our results suggest the occurrence of P remineralization by zebra mussels and net loss associated with emaciation during periods of temporary starvation. These findings have implications for P dynamics since zebra mussels can be subjected to periods of starvation over seasonal and annual time scales.     

Predation by the invasive American signal crayfish, Pacifastacus leniusculus Dana, on the invasive zebra mussel, Dreissena polymorpha Pallas: the potential for control and facilitation

Non-indigenous crayfish often have major ecological impacts on invaded water bodies, and have contributed to the decline of native crayfish species throughout Europe. The American signal crayfish, Pacifastacus leniusculus, is the most widespread invasive crayfish in Great Britain, where the zebra mussel, Dreissena polymorpha, is similarly an invasive pest species. The potential for the American signal crayfish to regulate zebra mussel populations was investigated through a series of laboratory experiments. Crayfish were found to be highly size selective, consuming significantly more of the smallest size class of zebra mussels offered (7–12 mm), over medium (16–21 mm) and large (25–30 mm). Crayfish feeding rate on zebra mussels was not altered when mussels were presented clumped together in natural druses compared with mussels in a disassembled druse. Crayfish spent significantly more time foraging when mussels were unattached, and a greater proportion of attacks were on medium and large than on small mussels (83% of attacks were on medium and large mussels when unattached as opposed to 47% when on druses). Individual crayfish feeding rate decreased significantly at densities of > ~5 crayfish m⁻². Signal crayfish are, therefore, unlikely to be able to significantly impact established populations of zebra mussels in the wild, although zebra mussels have the potential to provide crayfish with a substantial food source.     

The status of the freshwater pearl mussel <Emphasis Type="Italic">Margaritifera margaritifera</Emphasis> in Scotland: extent of change since 1990s,threats and management implications

All known rivers in Scotland with recent records of freshwater pearl mussels Margaritifera margaritifera were surveyed in 2013–2015 using a standard methodology. Freshwater pearl mussel populations were classed as: (i) apparently extinct in 11 rivers, (ii) not successfully recruiting in 44 rivers, and (iii) evidence of recent successful recruitment in 71 rivers. On a regional basis, a high proportion of extant populations were located in North and West Scotland. In all regions extant populations were characterised by low pearl mussel densities, with 97 of 115 extant Scottish populations defined as ‘rare’ (0.1–0.9 mussels per 1 m ²) or ‘scarce’ (1.0–9.9 mussels per 1 m ²). Only 18 Scottish rivers now hold pearl mussel populations in densities that are considered to be ‘common’ (10–19.9 mussels per 1 m ²) or ‘abundant’ (>20 mussels per 1 m ²). Based on survey evidence, the number of apparently extinct pearl mussel populations in Scottish rivers is now 73. The decline is particularly pronounced in the West Highlands and Western Isles strongholds. The key threats are: (i) pearl fishing, (ii) low host fish densities, (iii) pollution/water quality, (iv) climate change and habitat loss, (v) hydrological management/river engineering and (vi) ‘other factors’, such as non-native invasive species. Over the last 100 years this endangered species has been lost from much of its former Holarctic range. Scotland’s extant M. margaritifera populations continue to be of international importance, but their continued decline since the first national survey in 1998 is of great concern.     

A possible evolutionary lag in the relationship between freshwater mussels and European bitterling

The coevolutionary dynamics between European bitterling Rhodeus amarus and freshwater unionid mussels, which the former parasitize by laying eggs on their gills, were tested. In a series of experiments fish preferences and mussel responses were compared in parasites and hosts of recent (Europe) and ancient (Asia) sympatry. Rhodeus amarus readily oviposited on the gills of all mussel species tested. Fish that laid their eggs on the gills of Asian Anodonta woodiana, however, suffered a dramatic reduction in reproductive success compared to fish that oviposited on the gills of European mussels: Unio pictorum , Unio tumidus , Anodonta anatina and Anodonta cygnea . This difference was the result of egg ejection behaviour by mussels rather than the unsuitability of the internal gill environment for European bitterling embryo development. The ejection response of mussels with a long sympatry with European bitterling was considerably more pronounced than that of mussels with a substantially shorter sympatry. The data support a coevolutionary arms race between bitterling and mussels and point to an evolutionary lag in the relationship between R. amarus and its European mussel hosts.     

Population-specific responses to an invasive species

Predicting the impacts of non-native species remains a challenge. As populations of a species are genetically and phenotypically variable, the impact of non-native species on local taxa could crucially depend on population-specific traits and adaptations of both native and non-native species. Bitterling fishes are brood parasites of unionid mussels and unionid mussels produce larvae that parasitize fishes. We used common garden experiments to measure three key elements in the bitterling–mussel association among two populations of an invasive mussel (Anodonta woodiana) and four populations of European bitterling (Rhodeus amarus). The impact of the invasive mussel varied between geographically distinct R. amarus lineages and between local populations within lineages. The capacity of parasitic larvae of the invasive mussel to exploit R. amarus was higher in a Danubian than in a Baltic R. amarus lineage and in allopatric than in sympatric R. amarus populations. Maladaptive oviposition by R. amarus into A. woodiana varied among populations, with significant population-specific consequences for R. amarus recruitment. We suggest that variation in coevolutionary states may predispose different populations to divergent responses. Given that coevolutionary relationships are ubiquitous, population-specific attributes of invasive and native populations may play a critical role in the outcome of invasion. We argue for a shift from a species-centred to population-centred perspective of the impacts of invasions.     

Behavioral aspects of host recognition by the symbiotic water mite Unionicola formosa (Acarina,Unionicolidae)

Summary The occurrence and specificity of host recognition behavior of adult and nymphal Unionicola formosa and the capability of adult mites to recolonize various mussel species were examined. Adult U. formosa aggregated on excised tissue from their host mussel, Anodonta imbecilis, in preference to that of two other species of mussels. Nymphs also exhibited an aggregation response to host tissue. A radioisotope (⁵¹Cr) technique was used to monitor the recolonization behavior of U. formosa. Adult female mites preferentially re-entered A. imbecilis rather than the sympatric mussel A. cataracta. The specificity of this behavior parallels the distribution of this water mite among potential bivalve hosts in the southeastern U.S. Host recognition by U. formosa may contribute to re-establishing contact with a host after accidental separation and probably helps to maintain mite-mussel symbioses. Whether or not larval U. formosa employ similar host recognition behavior while selecting a potential host has not as yet been determined.     

The contribution of marine snow to the particle food supply of the benthic suspension feeder, Mytilus edulis

In order to examine the importance of the settling of large particles to the food supply and feeding behavior of a benthic culture of the blue mussel, Mytilus edulis, we investigated the tidal dynamics of large (>0.5 mm diameter) marine aggregates, commonly known as marine snow, during three tidal cycles in July 1998 at a shallow, subtidal, low current flow regime site along the coast of Maine (Shorey Cove, Roque Island, Englishman's Bay, Maine).In situ, optically measured marine snow showed a distinct tidal signal displaying an increase in size and abundance through high tide with a peak on the early ebb tide as it settled to the bottom. Marine snow volume ranged an order of magnitude through the tidal cycle, from under 8 to over 80 mm³ l⁻¹. An increase in the in situ marine snow volume corresponded with an increase in benthic mussel feeding activity (from 20% to 60% of maximum exhalant siphon area, which is an estimate of pumping rate) and maximum rates of pseudofeces production by the mussels during periods of low tidal current speeds. In contrast, mussels from the same population feeding on surface waters in shipboard chambers produced no pseudofeces and had high pumping rates (80-100% maximum exhalant siphon area) over the whole tidal period. A second peak in benthic mussel pumping rates also occurred during flood tide.Food quality was lower in the bottom waters due to significantly higher particulate inorganic matter (PIM, >2 mg l⁻¹) when compared with the surface waters. PIM accounted for 95% of the total settled mass flux of 3.4 g m⁻² day⁻¹ measured in sediment traps deployed 1 m off the bottom, with organic carbon representing only 2.5% of the mass flux during the mid-summer conditions. At low-current sites such as Shorey Cove, Roque Island, Maine, the settling of marine snow provides an important additional source of food, albeit of low quality, to benthic populations of blue mussels.     

Variability in annual recruitment success as a determinant of long-term and large-scale variation in annual production of intertidal Wadden Sea mussels (<Emphasis Type="Italic">Mytilus edulis</Emphasis>)

To understand the background of the strong variation and recent decline of stocks and production of mussels (Mytilus edulis) on tidal flats of the Wadden Sea, we analysed long-term (twice-annual for 26 years) and multi-station (15 sites) estimates of numbers, mean individual weights, biomass, and annual production on Balgzand, a 50-km² tidal-flat area in the westernmost part of the Wadden Sea (The Netherlands). Somatic production was estimated from summed growth increments of soft tissues per half-year period and expressed in ash-free dry mass (AFDM). In adults, positive values in spring/summer regularly alternated with negative values in autumn/winter, when up to ∼25% (mean: 14%) of individual weight gains in the preceding season were lost. No weight losses were observed during the first winter of the life of mussels. The 26-year mean of net somatic tissue production P amounted to 5.5 g AFDM m⁻² a⁻¹ at a mean biomass B of 3.2 g AFDM m⁻²; the ratio P/B varied strongly with age composition of the mussel population and ranged between 0.5 and 3.0 a⁻¹ (mean: 1.7). Within the restricted areas of mussel beds, mean biomass and annual production values were two orders of magnitude higher. In the Wadden Sea, mussel beds cover a typical 1% of extensive tidal flat areas. Numerical densities of recruits showed straight-line relationships with subsequent life-time year-class production. Once recruits had reached an age of ∼10 months, their numbers predicted subsequent production within narrow limits. Production per recruit averaged 0.21 g AFDM for 10-mo recruits and was not related to recruit density. Local variation in annual production varied strongly, with maximal values between mid-tide and low-tide level, where recruitment was also maximal. Production per recruit was higher at low than at high intertidal levels. Frequently failing recruitment is indicated as the main cause of declining mussel stocks in the Wadden Sea. As in other bivalve species, a declining frequency of the occurrence of cold winters appears to govern declining recruitment success and consequently declining production.     

Impact of Seasonal Ice on the Structure of the <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas, 1771) Beds in the Swash-Ice Zone of the Curonian Lagoon and the Peculiarities of Formation of the Zebra-Mussel Bed

The swash-ice zone of the Curonian Lagoon comprises the depths from a lower margin of the supralittoral to a lower margin of the swash subzone (0.7 m). This zone extends from a few meters to 250 m. This zone is the area of expatriation of Dreissena polymorpha because of its complete freezing in winter. However, this species prevails in the swash-ice zone in terms of the frequency of occurrence and the ratio in the total abundance and biomass of macrozoobenthos. Zebra mussels form large beds; their abundance and biomass reach 1420 ind./m² and 74.334 g/m², respectively. Population replenishment in this zone occurs sporadically due to adult individuals brought here from the consortium communities, where zebra mussel + Unionidae and zebra mussel + Viviparidae are the determining centers, as well as due to the settling of the mollusks of the veliconch stage.