The biology and functional morphology of Arca noae (Bivalvia: Arcidae) from the Adriatic Sea, Croatia, with a discussion on the evolution of the bivalve mantle margin

In the Croatian Adriatic, Arca noae occurs from the low intertidal to a depth of 60 m; it can live for > 15 years and is either solitary or forms byssally attached clumps with Modiolus barbatus. The shell is anteriorly foreshortened and posteriorly elongate. The major inhalant flow is from the posterior although a remnant anterior stream is retained. There are no anterior but huge posterior byssal retractor muscles and both anterior and posterior pedal retractors. The ctenidia are of Type B(1a) and the ctenidial–labial palp junction is Category 3. The ctenidia collect, filter and undertake the primary sorting of potential food in the inhalant water. The labial palps are small with simple re‐sorting tracks on the ridges of their inner surfaces. The ciliary currents of the mantle cavity appear largely concerned with the rejection of particulate material. The mantle margin comprises an outer and an (either) inner or middle fold. The outer fold is divided into outer and inner components that secrete the shell and are photo‐sensory, respectively. The latter bears a large number of pallial eyes, especially posteriorly. The inner/middle mantle fold of A. noae, possibly representative of simpler, more primitive conditions, may have differentiated into distinct folds in other recent representatives of the Bivalvia.     

Regulation of particle transport within the ventral groove of the mussel (Mytilus edulis) gill in response to environmental conditions

To thoroughly understand the feeding processes of the blue mussel, Mytilus edulis, under the variable environmental conditions it experiences in nature, it is important to examine individually the different components of its feeding system. The ciliated ventral food groove represents one of these components, within which the majority of food particles trapped by the gill are transported to the labial palps and mouth. Any ability of the mussel to adjust food transport rates within this groove could serve as an important feeding regulatory mechanism in response to variations in the environment.

Mucous strand velocities in the ventral groove of the mussel, M. edulis, were determined by video endoscopy over different time periods and during short- and long-term manipulations of ambient particle concentration and temperature. Mucous strand velocity decreased with increasing ambient particle concentration at 14°C, but a similar relationship was not observed at 5°C. The data support the hypothesis that M. edulis possesses compensatory mechanisms to control particle transport at the level of the ventral groove cilia in response to changes in the environment. Furthermore, mucous strand velocity in the ventral groove increased when the ambient temperature of mussels acclimated to 5°C was increased to 15°C during acute and long-term acclimation temperature experiments. This response is consistent with standard physiological responses of ciliary systems to changes in temperature.     

Influence of diet on pre-ingestive particle processing in bivalves: II. Residence time in the pallial cavity and handling time on the labial palps

Previous studies have demonstrated that bivalve molluscs respond to changing food conditions through processes such as preferential selection and ingestion of particulate matter. Little is known, however, about the underlying mechanisms accountable for these responses. To further explain feeding processes at the level of the pallial organs, we determined pallial cavity residence times, or the amount of time it took particles to travel from the inhalant aperture to the stomach, in two species of bivalves, Crassostrea virginica and Mytilus edulis, under conditions of differing particle quality, particle concentration, and temperature. From these residence times, particle-handling times on the labial palps were determined. Diets of three different qualities were tested, including Rhodomonas lens cells, particles prepared from ground Spartina sp. detritus, and a 50/50 mixture of both. Bivalves were delivered one of the three diets along with 10-μm fluorescent polystyrene beads (tracer), removed from feeding chambers at intervals from 30 s up to 20 min, and placed in liquid nitrogen to halt particle transport. Digestive systems of bivalves were then dissected and examined for the presence of tracer beads. Particle-residence times in the pallial cavity and handling times on the labial palps of C. virginica were significantly affected by changes in diet type. Particle-handling times on the palps decreased with increasing diet quality and ranged from 2.2 min (100% R. lens) to 22.8 min (100% ground Spartina sp.), accounting for 88% and 99%, respectively, of the total time particles spent in the pallial cavity. In contrast, diet quality had little effect on particle-residence times in the pallial cavity of M. edulis. However, residence times were affected by temperature and diet concentration. Temperature significantly affected residence times at particle concentrations of both 20 and 100 particles μl⁻¹, whereas particle concentration affected residence times at 20 °C, but not at 5 °C. Particle-handling times on the labial palps ranged from less than 1 to 5.5 min, depending on temperature and concentration, accounting for 50% to 82%, respectively, of the total time particles spent in the pallial cavity. We suggest that (1) observed interspecific differences in particle handling on the labial palps may be due to differences in palp morphology and function, and (2) particle sorting and selection on the labial palps is a rate-limiting step of pre-ingestive feeding processes in by bivalves.     

Phyllodistomum folium (Trematoda: Gorgoderidae) infecting zebra mussels (Dreissena polymorpha) in the Ebro River, Spain

Zebra mussels (Dreissena polymorpha) were first found in the Ebro River (Spain) in Ribaroja reservoir, in the summer of 2001. This paper reports a study to detect parasites in this bivalve species. From September 2003 to August 2004, a total of 1380 zebra mussels were collected and dissected or sectioned in paraffin and haematoxylin and eosin staining. We observed the presence of Phyllodistomum folium (Olfers, 1816) in two hosts (prevalence 0.14%). Sporocysts containing metacercariae were located within the gill lamellae. One of the mussels was collected in January and the other one in July. In both cases the shell length was >2 cm. P. folium had not been previously reported in Spain and D. polymorpha is its only known intermediate host. It represents a new invasive species in this river basin, presumably introduced together with the zebra mussels.     

Characterization of intracytoplasmic prokaryote infections in Dreissena sp. (Bivalvia: Dreissenidae)

This study characterizes intracytoplasmic infections with prokaryote microorganisms in Dreissena sp. (near Dreissena polymorpha) from northeastern Greece and represents the first report of such infections in freshwater bivalves. Light microscope observations of stained tissues revealed basophilic, cytoplasmic inclusion bodies in 87.5% (28/32) of the mussels sectioned. Inclusions in epithelial cells and connective tissues were noted, respectively, in 34.4 and 71.9% of the sample, with 5 mussels (15.6%) having both tissue types infected. Epithelial cell infections were observed in histological sections only in digestive gland tubules and ducts; within tubules, inclusions were present more often in secretory than digestive cells. Connective tissue infections, however, were systemic; among the 32 mussels sectioned, inclusions were found in the gills (65.6%), foot (12.5%), mantle (9.4%), labial palps (6.3%), digestive gland (6.3%), stomach (6.3%), and gonads (3.1%). Cytoplasmic inclusions (maximum dimension, 138 microm) were prominent enough in the gills to be visible in 17.0% of the 247 mussels dissected. Ultrastructurally, prokaryote cells in gill connective tissues were clearly characteristic of Chlamydiales-like organisms, with each intracytoplasmic inclusion containing a loosely packed mixture of elementary, reticulate, intermediate bodies, and blebs. Prokaryote colonies in digestive gland epithelial cells exclusively contained 1 of 4 morphological cell types and were considered Rickettsiales-like. Hexagonal, virus-like particles were present in the cytoplasm of the largest of these Rickettsiales-like prokaryotes. Although host stress was evident from localized cell necrosis and dense hemocyte infiltration, overall infection was fairly benign, with no major, adverse impact on body condition evident among sectioned or dissected mussels. A possible negative effect was partial constriction of gill water tubes, but at the infection intensity observed (typical range 1 to 7 inclusion bodies per section), significant interference with respiration and other metabolic functions of the gills was highly unlikely.     

The sorting of food particles by Abra sp. (bivalvia: tellinacea)

The selection of food particles by Abra tenuis (Montagu), A. alba (Wood) and A. nitida (Müller) has been investigated.Material for ingestion smaller than 30μm is not selected according to size by A. tenius. Particles smaller than 0.5 μm are retained by the palliai organs and ingested but only particles larger than 1 μm appear to be retained with an efficiency approaching 100 %. The mesh size of the gill filter is found to be ≈ 3.0 × 0.5 μm.A. tenuis does not appear able to discriminate between particles smaller than 20 μm by their food value; however, relatively large silica particles which are devoid of food are partially rejected by the labial palps in favour of particles of similar size but having a bacterial coating. To a lesser extent the physical nature of particles seems to influence their selection by A. tenuis; clean angular particles are rejected in favour of clean rounded ones.Small, light particles appear to be transported on the gills directly to the mouth without coming into contact with the palps. Larger, heavier particles tend to drop from the gill to be caught by the palps which extend posteriorly to cup the entire ventral margin of the inner demibranch when the animal is feeding.The material ingested by A. alba is significantly finer than that taken into the mantle cavity indicating that the pallial organs actively select food by size. Selection of material for ingestion by size in A. nitida appears to be optional since only some of the animals examined had stomach contents significantly finer than material from the mantle cavity.     

Field and laboratory studies of Ophryoglena sp. (Ciliata: Ophryoglenidae) infection in zebra mussels,Dreissena polymorpha (Bivalvia: Dreissenidae)

This study, conducted in the Dnieper-Bug Canal in Belarus, is the first to monitor the seasonal (June-November) dynamics of infection with the parasitic ciliate Ophryoglena sp. in a zebra mussel (Dreissena polymorpha) population. Mean population prevalence and intensity of infection varied, respectively, from 11 to 62% and from 0.9 to 24.1 ciliates/mussel. Mean prevalence was highly correlated with mussel length in mussels <20 mm (R(2)=0.97) and was lower in larger mussels. Mean infection intensity in mussels 1-25 mm long was similarly correlated with their size (R=0.98), reached a maximum in the 20-25 mm size-class, and then sharply decreased, thus providing evidence, albeit limited, that high intensity of infection might be lethal. Transinfection of zebra mussels by Ophryoglena sp. was achieved in the laboratory-a first for a protozoan parasite of D. polymorpha; from an initial complete lack of infection, mean prevalence and intensity rose, respectively, to 86.7% and 8.3 ciliates/mussel.     

The functional morphology of Sinonovacula constricta with a discussion on the taxonomic status of the Novaculininae (Bivalvia).

Sinonovacula constricta is representative of two genera of deep burrowing bivalves hitherto placed in the Novaculininae of the Tellinacea. This species, widely cultured in China, has not previously been critically examined. Aspects of its anatomy show that it should more properly be placed in the Solenacea. The presently accepted classification of this superfamily suggests that it should be placed close to Pharella in the Cultellidae but it is clear that the group requires more detailed re-examination.
Sinonovacula is somewhat posteriorly elongate with long separate siphons. The anterior pedal gape acts as a valve to the piston-like foot enabling rapid reburrowing to occur. Sensory tentacles around the pedal gape respond to mechanical disturbance and water can be squirted from the gape to, presumably, flush away the source of irritation. Modifications of this kind are rare in the Bivalvia.
The anatomy of Sinonovacula is described and related to its life style as a suspension feeder in soft intertidal muds. The ciliary currents of the mantle cavity are powerful, the labial palps in particular being large as compared with the relatively small ctenidia. The stomach is adapted for dealing with large amounts of food, though the sorting areas are not extensive; principal sorting is clearly the function of the labial palps. The style sac and mid-gut are separate (unlike the conjoined style sac and mid gut of the Tellinacea), and ensure separation of the style from the sediment loaded mid gut.
In many ways Sinonovacula foreshadows the highly modified true razor shells and conditions in these bivalves can be accounted for by further posterior elongation of a bivalve essentially similar to Sinonovacula.     

Worth a second look: gill structure in Hemipecten forbesianus (Adams & Reeve, 1849) and taxonomic implications for the Pectinidae

The gill of the reef-dwelling Hemipecten forbesianus was examinedusing histology and scanning electron microscopy, in order toverify its reported homorhabdic state, and to provide the firstdetailed information on tropical pectinid gill structure. Dueto the difficulty of obtaining live individuals, museum referencespecimens were used for this study. The gill is heterorhabdic,and presents several notable differences compared to the larger,temperate species studied to date: (1) the small number of ordinaryfilaments per plica (probably associated with the very smallsize of these specimens), (2) the outer demibranch is approximately15% shorter than the inner demibranch, (3) several characteristicswhich confer gill cohesion in other pectinid gills are absentand (4) the principal filaments are devoid of mucocytes, suchthat particle transport of positively-selected particles mustbe entirely effected in water. The apparent universality ofthe heterorhabdic condition within the Pectinidae leads us torecommend its use as a reliable soft-body taxonomic characterat the family level. The anatomical differences between thegill of H. forbesianus and that of the larger, temperate speciescall for further comparison with both large and small, temperateand tropical species, in order to elucidate the effects of sizeand habitat on gill structure in this family. (Received 26 June 2007; accepted 14 January 2008)     

Influence of diet on pre-ingestive particle processing in bivalves: I: Transport velocities on the ctenidium

Suspension-feeding bivalve molluscs can assume a large ecological role by linking benthic and pelagic ecosystems. Therefore, a knowledge of the factors that influence feeding rates and processes at the level of the individual is important in understanding bivalve-dominated environments. We examined the roles of diet quality and concentration on particle processing by the ctenidia of four species of bivalves: the mussels Mytilus edulis L. and Mytilus trossulus Gould, and oysters Crassostrea virginica (Gmelin) and Crassostrea gigas (Thunberg). Bivalves were delivered diets of three different qualities at three different concentrations (1-2×10³, 10⁴, 10⁵ particles ml⁻¹). The high-quality diet consisted of the cryptophyte Rhodomonas lens Pascher et Ruttner; the low-quality diet consisted of particles prepared from ground Spartina sp. detritus; the medium-quality diet consisted of a 50:50 mixture (by particle number) of both particle types. Particle transport velocities on the ventral groove and dorsal tracts were then measured by means of video endoscopy. Ventral-groove transport velocities of M. edulis were the most responsive, demonstrating a significant increase with increasing diet quality, and a significant decrease with increasing diet concentration. Transport velocities in the ventral groove and dorsal tracts of C. virginica were not significantly affected by changes in diet quality, but significantly increased with increasing diet concentration. Transport velocities of M. trossulus and C. gigas demonstrated little change with diet quality or concentration, indicating that responses are species specific. Our data suggest that differential control of particle transport on the bivalve ctenidium is one of the underlying mechanisms that contribute to compensatory feeding responses exhibited by the entire organism.     

Effects of the benthic suspension feeder Dreissena polymorpha on zooplankton in a large river

1. We conducted a series of in situ enclosure experiments to assess the impact of zebra mussels ( Dreissena polymorpha ) on the plankton of the Ohio River. Adult mussels were suspended in pelagic enclosures ('potamocorrals') at three densities (0, 1000, 2500 mussels per corral) and incubated for 6 days with daily plankton and physiochemical sampling.
2. The presence of adult zebra mussels was correlated with a shift in composition of the phytoplankton community and a severe reduction in some rotifers. The effects of zebra mussels on the larger zooplankton were taxon-dependent, but bacterial density showed no trend among treatments.
3. Zebra mussels may have significant negative impacts on zooplankton, which may in turn alter riverine food webs.     

The effect of seasonal variation in selective feeding by zebra mussels (Dreissena polymorpha) on phytoplankton community composition

1. To investigate the impact of zebra mussels ( Dreissena polymorpha ) on phytoplankton community composition, temporal variability in selective feeding by the mussels was determined from April to November 2005 in a natural lake using Delayed Fluorescence (DF) excitation spectroscopy.
2. Selective grazing by zebra mussels varied in relation to seasonal phytoplankton dynamics; mussels showed a consistent preference for cryptophytes and avoidance of chlorophytes and cyanobacteria. Diatoms, chrysophytes and dinoflagellates responded differentially to zebra mussel grazing depending on their size. Analysis of excreted products of the zebra mussels revealed that in addition to chlorophytes and cyanobacteria, phytoplankton >50  μ m and very small phytoplankton (≤7  μ m) were largely expelled in pseudofaeces.
3. The zebra mussel is a selective filter-feeder that alters its feeding behaviour in relation to phytoplankton composition to capture and ingest high quality phytoplankton, especially when phytoplankton occur in preferred size ranges. Flexibility of zebra mussel feeding behaviour and variation in susceptibility among phytoplankton groups to mussel ingestion indicate that invading zebra mussels could alter phytoplankton community composition of lakes and have important ecosystem consequences.     

Tissue distribution of a coliphage and Escherichia coli in mussels after contamination and depuration

Experiments were undertaken to determine the tissue distribution of Escherichia coli and a coliphage after contamination of the common mussel (Mytilus edulis). Mussels were contaminated with high levels of feces-associated E. coli and a 22-nm icosahedral coliphage over a 2-day period in a flowing-seawater facility. After contamination, individual tissues were carefully dissected and assayed for E. coli and the coliphage. Contaminated mussels were also analyzed to determine the tissue distribution of the contaminants after 24- and 48-h depuration periods. The majority of each contaminant was located in the digestive tract (94 and 89% of E. coli and coliphage, respectively). Decreasing concentrations were found in the gills and labial palps, foot and muscles, mantle lobes, and hemolymph. Our results indicate that contamination above levels in water occurred only in the digestive tract. Contaminated mussels were depurated in a commercial-scale recirculating UV depuration system over a 48-h period. The percent reductions of E. coli occurred in the following order: digestive tract, hemolymph, foot and muscles, mantle lobes, and gills and labial palps. The percent reductions of the coliphage were different, occurring in the following order: hemolymph, foot and muscles, gills and labial palps, mantle lobes, and digestive tract. Our results clearly demonstrate that E. coli and the coliphage are differentially eliminated from the digestive tract. The two microorganisms are eliminated at similar rates from the remaining tissues. Our results also clearly show that the most significant coliphage retention after depuration for 48 h is in the digestive tract. Thus, conventional depuration practices are inappropriate for efficient virus elimination from mussels.     

Tissue distribution of a coliphage and Escherichia coli in mussels after contamination and depuration.

Experiments were undertaken to determine the tissue distribution of Escherichia coli and a coliphage after contamination of the common mussel (Mytilus edulis). Mussels were contaminated with high levels of feces-associated E. coli and a 22-nm icosahedral coliphage over a 2-day period in a flowing-seawater facility. After contamination, individual tissues were carefully dissected and assayed for E. coli and the coliphage. Contaminated mussels were also analyzed to determine the tissue distribution of the contaminants after 24- and 48-h depuration periods. The majority of each contaminant was located in the digestive tract (94 and 89% of E. coli and coliphage, respectively). Decreasing concentrations were found in the gills and labial palps, foot and muscles, mantle lobes, and hemolymph. Our results indicate that contamination above levels in water occurred only in the digestive tract. Contaminated mussels were depurated in a commercial-scale recirculating UV depuration system over a 48-h period. The percent reductions of E. coli occurred in the following order: digestive tract, hemolymph, foot and muscles, mantle lobes, and gills and labial palps. The percent reductions of the coliphage were different, occurring in the following order: hemolymph, foot and muscles, gills and labial palps, mantle lobes, and digestive tract. Our results clearly demonstrate that E. coli and the coliphage are differentially eliminated from the digestive tract. The two microorganisms are eliminated at similar rates from the remaining tissues. Our results also clearly show that the most significant coliphage retention after depuration for 48 h is in the digestive tract. Thus, conventional depuration practices are inappropriate for efficient virus elimination from mussels.     

Zebra mussels mediate benthic–pelagic coupling by biodeposition and changing detrital stoichiometry

1. The zebra mussel ( Dreissena polymorpha ) is one of the most successful invasive species; it has colonised many aquatic systems in Europe and North America with strong impacts on various ecosystem processes. The effect of D. polymorpha filtration on pelagic seston concentrations has been quantified in several studies, but the magnitude and stoichiometry of the transfer of sestonic biomass into benthic detritus by D. polymorpha and the accompanying enrichment of the benthic habitat is still under-investigated.
2. We studied biodeposition by zebra mussels in two series of laboratory experiments with the food algae Cryptomonas erosa and Scenedesmus obliquus . We also measured the year-round biodeposition rate under natural conditions in the oligotrophic Lake Constance.
3. In all experiments, zebra mussel biodeposition was linearly related to seston concentration. In the field, the relationship changed with a seasonal shift in algal composition and lower biodeposition rates during the spring algal bloom.
4. For both algal species in laboratory experiments, biodeposited material was depleted in phosphorous at an algal concentration ≤0.6 mg ash-free dry mass L⁻¹, but not at higher concentrations. This effect was not observed in the field, probably because of high variation in C : N : P stoichiometry.
5. By mediating the transfer of pelagic resources into the benthos zebra mussels provide a sufficient amount of detritus for benthic invertebrates, especially during summer. Thus, material biodeposited by the mussels might increase benthic secondary production from pelagic resources, and zebra mussels are important mediators of this flux of organic matter from the pelagic zone into the benthos.     

Effects of zebra mussels on phytoplankton and ciliates: a field mesocosm experiment

Many observational studies in North American lakes have documenteddecreases in phytoplankton abundance after the invasion of thezebra mussel (Dreissena polymorpha). However, few field experimentshave examined in detail the effect of zebra mussels on phytoplanktonabundance and species composition over an extended period. Replicatedin situ mesocosms were used to evaluate the impact of naturaldensities of zebra mussels on phytoplankton and ciliate biovolume,and algal species composition over a 5-week period in a habitatthat lacked extant mussel populations. Mussel biomass used inthe experiment was determined using a regression model basedon a data analysis that predicts zebra mussel biomass from totalphosphorus concentration. Within 1 week, zebra mussels decreasedphytoplankton biovolume by 53% and ciliate biovolume by 71%.The effect of zebra mussels on ciliate biovolume was sustainedthroughout the study. However, the effect of zebra mussels onphytoplankton abundance gradually waned over the remaining 4weeks of the experiment, such that the declining effect of zebramussels could not be explained by a shift towards less edibleand/or faster growing algal species. The mussels’ decliningcondition could help to explain the effect observed over thecourse of the experiment.     

Zebra mussel adhesion: structure of the byssal adhesive apparatus in the freshwater mussel, Dreissena polymorpha

The freshwater zebra mussel (Dreissena polymorpha) owes a large part of its success as an invasive species to its ability to attach to a wide variety of substrates. As in marine mussels, this attachment is achieved by a proteinaceous byssus, a series of threads joined at a stem that connect the mussel to adhesive plaques secreted onto the substrate. Although the zebra mussel byssus is superficially similar to marine mussels, significant structural and compositional differences suggest that further investigation of the adhesion mechanisms in this freshwater species is warranted. Here we present an ultrastructural examination of the zebra mussel byssus, with emphasis on interfaces that are critical to its adhesive function. By examining the attached plaques, we show that adhesion is mediated by a uniform electron dense layer on the underside of the plaque. This layer is only 10-20 nm thick and makes direct and continuous contact with the substrate. The plaque itself is fibrous, and curiously can exhibit either a dense or porous morphology. In zebra mussels, a graded interface between the animal and the substrate mussels is achieved by interdigitation of uniform threads with the stem, in contrast to marine mussels, where the threads themselves are non-uniform. Our observations of several novel aspects of zebra mussel byssal ultrastructure may have important implications not only for preventing biofouling by the zebra mussel, but for the development of new bioadhesives as well.     

Northern map turtles (Graptemys geographica) derive energy from the pelagic pathway through predation on zebra mussels (Dreissena polymorpha)

1. Zebra mussels ( Dreissena polymorpha ) derive their energy from the pelagic energy pathway by filtering plankton. Because zebra mussels occur in high densities in littoral habitats, they potentially constitute an important trophic link between littoral consumers and pelagic energy sources. Northern map turtles ( Graptemys geographica ) are widespread in North America and consume zebra mussels.
2. We used stable isotopes analyses to quantify the flow of energy from the pelagic pathway to northern map turtles and to infer the contribution of zebra mussels to map turtle biomass. We then built a bioenergetic model to estimate the annual intake of zebra mussels by northern map turtles in Lake Opinicon, Ontario, Canada.
3. Stable isotopes analyses indicated that zebra mussels constitute between 0% and 14% of the diet of males and between 4% and 36% of the diet of females. Assuming that zebra mussels account for all of the pelagic contribution, we estimated that map turtles consume 3200 kg of zebra mussels annually. Because female map turtles are much larger than males and consume more zebra mussels, they are responsible for 95% of the zebra mussel biomass ingested annually.
4. The pelagic pathway supports an important part of the standing crop biomass of map turtles in Lake Opinicon. We highlight the importance of freshwater turtles in lake ecosystems. Unravelling the trophic interactions mediated by freshwater turtles will lead to a more integrated picture of lake ecosystems.     

Haplosporidium raabei n. sp. (Haplosporidia): a parasite of zebra mussels, Dreissena polymorpha (Pallas, 1771)

Extensive connective tissue lysis is a common outcome of haplosporidian infection. Although such infections in marine invertebrates are well documented, they are relatively rarely observed in freshwater invertebrates. Herein, we report a field study using a comprehensive series of methodologies (histology, dissection, electron microscopy, gene sequence analysis, and molecular phylogenetics) to investigate the morphology, taxonomy, systematics, geographical distribution, pathogenicity, and seasonal and annual prevalence of a haplosporidian observed in zebra mussels, Dreissena polymorpha. Based on its genetic sequence, morphology, and host, we describe Haplosporidium raabei n. sp. from D. polymorpha - the first haplosporidian species from a freshwater bivalve. Haplosporidium raabei is rare as we observed it in histological sections in only 0·7% of the zebra mussels collected from 43 water bodies across 11 European countries and in none that were collected from 10 water bodies in the United States. In contrast to its low prevalences, disease intensities were quite high with 79·5% of infections advanced to sporogenesis.