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Freshwater mussels (Bivalvia: Unionidae), which are keystone species of freshwater ecosystems, are in global decline. In addition to ecological/genetic studies, morphological examinations are needed to help provide information for the development of additional freshwater mussel studies and eventually conservation efforts for freshwater ecosystems.The microscopic structure, which can be obtained by scanning electron microscopy (SEM) and elemental composition, which can be obtained with energy dispersive X-ray spectrometry (EDS), of mollusc shells are of interest to malacologists. However, information about freshwater mussels is still limited.Kyushu Island is the southernmost island of the four major islands of Japan. Kyushu Island is a hotspot of bitterling fishes in Japan, which simultaneously means that the island is a hotspot of freshwater mussels. The Ryukyu Islands stretch southwest from Kyushu Island to Taiwan; a freshwater mussel of unknown origin was reported from the Ryukyu Islands.Digital archiving for biology and ecology is a continuing challenge for open science. This data paper describes online published photo images, 3D/CT and mtDNA data and SEM/EDS analyses of the shell of freshwater mussels that inhabit the Kyushu and Ryukyu Islands, Japan. Our data will provide basic information regarding freshwater biology and be of public interest as open science.Photo images, 3D/CT data, mtDNA data, SEM images and EDS elemental analysis of freshwater mussels that inhabit the Kyushu and Ryukyu Islands (61 individuals, nine species/subspecies) were published online in a local database (http://ffish.asia/Unionidae3D), GBIF (http://ipt.pensoft.net/resource?r=unionidae3d) and DDBJ/EMBL/Genbank (LC431810–LC431840).

Fauna Europaea provides a public web-service with an index of scientific names (including important synonyms) of all living European land and freshwater animals, their geographical distribution at country level (up to the Urals, excluding the Caucasus region), and some additional information. The Fauna Europaea project covers about 230,000 taxonomic names, including 130,000 accepted species and 14,000 accepted subspecies, which is much more than the originally projected number of 100,000 species. This represents a huge effort by more than 400 contributing specialists throughout Europe and is a unique (standard) reference suitable for many users in science, government, industry, nature conservation and education. For the Mollusca-Bivalvia, data from 5 families (Margaritiferidae, Unionidae, Sphaeriidae, Cyrenidae, Dreissenidae) containing 55 species are included in this paper. European freshwater bivalves belong to the Orders Unionoida and Cardiida. All the European unionoids are included in the superfamily Unionoidea, the freshwater mussels or naiads. The European cardiids belong to the following three superfamilies: Cardioidea, Cyrenoidea and Dreissenoidea. Among the Unionoidea there are the most imperilled animal groups on the planet while the Cardioidea includes the cosmopolitan genus Pisidium, the Cyrenoidea the Asiatic clam (Corbicula fluminea) and the Dreissenoidea the famous invasive zebra mussel (Dreissena polymorpha). Basic information is summarized on their taxonomy and biology. Tabulations include a complete list of the current estimated families, genera and species.

A new species of freshwater mussel belonging to the genus Pseudocuneopsis, namely Pseudocuneopsis yangshuoensis sp. nov., is diagnosed and described from Guangxi Province, China. This paper provides a detailed morphological description, photograph of the type specimen, and anatomical characteristics along with partial sequences of mitochondrial COI as DNA barcode data for this novel species. The new species can be distinguished from its congeners (Pseudocuneopsis sichuanensis and Pseudocuneopsis capitata) by shell shape, beak position and surface sculpture. The interspecies genetic distance based on the COI barcode between P. yangshuoensis sp. nov. and P. sichuanensis is 8%, while it reaches 9% with P. capitata. Therefore, we provide robust morphological and molecular evidence to support the validity of this new species.

The naiads, large freshwater mussels (Unionida), have very long life spans, are large-bodied, and produce thousands to millions of larvae (glochidia) which typically must attach to host fish tissues to metamorphose into a juvenile mussel. Glochidia develop within a female's marsupial gill demibranch, thus their number is restricted by female size. However, larger mussels acquire more energy, which could be invested in either larger-sized glochidia, in a more glochidia, or a combination of both. The high level of host specialization seen in many naiads may constrain glochidial size and shape around a narrow optimum, while naiads that use a wide range of host fishes may be predicted to possess greater plasticity in glochidial morphology. In this paper, we investigated the relationship between maternal body size and progeny body size and shape, aided by modern digital microscopy. We analyzed the between- and within- species variation of glochidia size and shape relative to female size in four widespread species of European naiads: Anodonta anatina, Anodonta cygnea, Unio crassus and Unio tumidus. Whereas the total reproductive output is collinear with female body size, substantial differences between species in glochidia size were found within genus Anodonta, but not genus Unio where glochidial size is remarkably consistent. The glochidial shape, however, differed within both Unio and Anodonta. We interpret this constant within-species glochidial size in Unio as reflecting a constraint imposed by the likelihood of successful transmission onto and off from a narrow range of hosts, whereas their shape seems to be less constrained. The Anodonta species, inhabiting a wide spectrum of habitats and using more than twice the number of fish hosts than Unio spp., have larger glochidia with greater variation in size and shape. Our results suggest that measures of glochidial variability may also serve as an indicator of host specificity in other naiads.

An overview of the fauna of the Perlivnitidae of Zakarpattia Region is presented in the paper. This article provides a species list of Perlivoridae with the first notes for each species in the area, some comments on their identification and modern generally accepted systematic names. The species list includes 7 species from 4 genera. Six of them (Anodonta cygnea, Anodonta anatina, Unio pictorum, Unio tumidus, Batavusiana crassa, and Pseudanodonta complanata) are native, and the seventh (Sinanodonta woodiana) is an adventive species. The article also provides some information on their general distribution, distribution and localities in Ukraine and Zakarpattia Region in particular., В роботі представлений огляд фауни Перлівницевих Закарпатської області. У цій статті наводиться перелік видів перлівницевих із зазначенням перших згадок про реєстрацію кожного з них, на даній території, окремими зауваженнями щодо їх визначення та сучасними загальновизнаними систематичними назвами. Перелік включає в себе 7 видів з 4 родів. Шість з них (Anodonta cygnea, Anodonta anatinа, Unio pictorum, Unio tumidus, Batavusiana crassa та Pseudanodonta complanata) є аборигенними, і ще один вид (Sinanodonta woodiana) є адвентивним. Також представлені деякі відомості, що стосуються загального поширення даних видів, а також знахідки з території України та Закарпатської області.

As a result of the study, it has been identified that the water temperature in the Kashkadarya River is 18–24 °C, the water clarity is 0.51–0.62 m and the water flow rate is 0.72–0.84 m/sec. The species of bivalve mollusks Sinanodonta gibba, S. orbicularis, S. puerorum, Colletopterum cyreum sogdianum, Corbicula cor, C. fluminalis, C. purpurea, Corbiculina tibetensis, C. ferghanensis are widespread in the region. The abiotic factors discussed in the paper play an important role in the distribution of mollusks. There are 13 species of bivalve mollusks in the mountainous region, which is the head part of the river, 9 species in the middle part of the hill region of the river and 6 species in the lower desert region.

We diagnose and describe a new freshwater mussel species of the genus Inversidens, I. rentianensissp. nov. from Jiangxi Province, China based on morphological characters and molecular data. This paper includes a morphological description and photograph of the holotype, and partial sequences of mitochondrial COI as DNA barcode data.

This paper presents results of a malacological survey of the middle and upper section of the Jihlava River, a tributary of the Dyje River (Czech Republic). Twenty-three species of aquatic molluscs (13 gastropods, 10 bivalves) were found at 35 sites during research realised mostly in 2017 and 2020. In comparison with the lower section of the Jihlava River, a significantly lower proportion of bivalves of the family Unionidae was found in the middle and upper sections.

The knowledge on the floral and faunal composition of protected areas (PAs) is crucial for formulating suitable conservation plan. In this paper, inventory and species richness of non-marine molluscs of Chintamoni Kar Bird Sanctuary has been made and is for the first time from any PA of West Bengal. A total of 276 specimens belonging to 22 species (10 species of land snails and 12 species of freshwater) of non-marine molluscs (land and freshwater) were collected and examined from this sanctuary. The malacofaunal inventory comprises of nine genera under seven families among land snails and 12 genera & seven families from both gastropods & bivalves under the freshwater forms. As far as species richness is concerned, the family Ariophantidae was found to be dominant among land forms whereas species of the families Thiaridae and Unionidae were dominant among freshwater forms.

I this paper Sinosolenaia gen. nov. a new genus of ultra-elongate freshwater mussels (Bivalvia: Unionidae) is described from Vietnam and eastern China.

In this paper, we describe Najadicola loeiensis sp. nov. (Pionidae: Najadicolinae), a new water mite species from the Loei River, a tributary of the Mekong River in Thailand. The only other species in this small genus, N. ingens (Koenike, 1895), inhabits North America. Our novel tropical Asian species can be distinguished from N. ingens by the shape of coxal plates and body size. N. loeiensis sp. nov. is a parasite of the inner cavity of freshwater mussels (Bivalvia: Unionidae) as its Nearctic congener.

The objective of this paper is to draw attention to a previously unrecognised habitat for Unio crassus Philipsson, 1788 (Bivalvia: Unionidae), namely beaver dam-and-pond complexes. The construction...

Located at the junction between Europe, Africa, and Asia, with distinct evolutionary origins and varied ecological and geographical settings, together with a marked history of changes in orogeny and configuration of the main river basins, turned the Eastern Mediterranean into a region of high diversity and endemism of freshwater taxa. Freshwater mussels (Bivalvia, Unionidae) from the Western Palearctic have been widely studied in their European range, but little attention has been dedicated to these taxa in the Eastern Mediterranean region and their diversity and phylogeography are still poorly understood. The present study aims to resolve the diversity, biogeography, and evolutionary relationships of the Eastern Mediterranean freshwater mussels. To that end, we performed multiple field surveys, phylogenetic analyses, and a thorough taxonomic revaluation. We reassessed the systematics of all Unionidae species in the region, including newly collected specimens across Turkey, Israel, and Iran, combining COI + 16S + 28S and COI phylogenies with molecular species delineation methods. Phylogeographical patterns were characterized based on published molecular data, newly sequenced specimens, and species distribution data, as well as ancestral range estimations. We reveal that Unionidae species richness in the Eastern Mediterranean is over 70% higher than previously assumed, counting 19 species within two subfamilies, the Unioninae (14) and Gonideinae (5). We propose two new species, Anodonta seddoni sp. nov. and Leguminaia anatolica sp. nov. Six additional taxa, Unio delicatus stat. rev., Unio eucirrus stat. rev., Unio hueti stat. rev., Unio sesirmensis stat. rev., Unio terminalis stat. rev. removed from the synonymy of Unio tigridis, as well as Unio damascensis stat. rev. removed from the synonymy of Unio crassus, are re-described. The nominal taxa Unio rothi var. komarowi O. Boettger, 1880 and Unio armeniacus Kobelt, 1911 are proposed as new synonyms of Unio bruguierianus, and Anodonta cyrea Drouët, 1881 and Anodonta cilicica Kobelt and Rolle, 1895 as new synonyms of Anodonta anatina. Also, the presence of Unio tumidus in the Maritza River is confirmed. The phylogeographic patterns described here are interpreted concerning major past geological events. Conservation needs and implications are presented, together with populations and species conservation priorities. We thank John Pfeiffer and David Zanatta for their helpful comments and suggestions to improve the quality of the manuscript, Jamie Smith for the help with the shell images and Cindy Bogan for reviewing and formatting several versions of the manuscript and supplementary appendix. We also like to thank the following people for their assistance with the identification and/or access to specimens or images of museumtype lots. Adam Baldinger, Museum of Comparative Zoology, Harvard University, Paul Callomon, Academy of Natural Sciences of Drexel University, Philadelphia, John Pfeiffer Smithsonian Institution, Washington D.C., Karl-Otto Nagel, Senckenberg Museum, Frankfurt, Maxim Vinarski, Saint Petersburg State University, Russia, and Harriot Wood, National Museum of Wales, Cardiff. This work was supported by a Conchological Society of Great Britain & Ireland Grant Award to MLL entitled “Conservation and Phylogeography of the Eastern Mediterranean Naiads (Bivalvia: Unionoida), the European Union’s Horizon 2020 research and innovation programme - BIOPOLIS, grant agreement no. 857251, and the project ConBiomics: the missing approach for the Conservation of Freshwater Bivalves Project No. NORTE-01-0145- FEDER-030286, co-financed by COMPETE 2020, Portugal 2020 and the European Union through the ERDF, and by FCT - Fundação para a Ciência e a Tecnologia, through national funds, the strategic funding UIDB/04423/2020 and UIDP/04423/2020 provided by FCT. FCT also supported MLL (2020.03608.CEECIND), EF (CEECIND/00627/2017), and AGS (SFRH/BD/137935/2018). The Ministry of Science and Higher Education of the Russian Federation supported INB (project 0793-2020- 0005) and AAT (project AAAA-A18-118012390161-9). The Russian Science Foundation funded INB (project No. 21-17-00126), IVV (project 19-14-00066). The Russian Foundation for Basic Research supported AVK (project 18-44-292001). Interactions of authors and writing of the paper were promoted and facilitated by the COST Action CA18239: CONFREMU - Conservation of freshwater mussels: a pan-European approach. info:eu-repo/semantics/publishedVersion

Before this study, researchers had not systematically surveyed the Rio Conchos, a tributary of the Rio Grande located in northern Mexico, for freshwater mussels. The Texas hornshell, Popenaias popeii, an endangered species under the United States Endangered Species Act, occurs in the main stem of the Rio Grande near its confluence with the Rio Conchos. The Texas hornshell's proximity to the Rio Conchos prompted us to survey to determine whether P. popeii also occurs within the Conchos basin. Additionally, previous researchers described the Conchos disk, Disconaias conchos, from the Rio Conchos but, to date, no one has reported live individuals of this species. The goal of this study was to determine the status of mussels within the Rio Conchos basin and to provide a baseline for future monitoring efforts. We qualitatively surveyed 11 sites within the Rio Conchos basin in May 2018. In total, we found two live individuals of the paper pondshell, Utterbackia imbecillis, at one site in the Rio San Pedro, a tributary of the Conchos River, and as shells of recently live individuals at two sites in the Rio Conchos. We found D. conchos as relict shells only at three sites, two in the Rio Conchos and one in a tributary of the river at Coyame. The lack of live or fresh-dead material suggests that D. conchos is extinct. We also found live individuals of nonnative Asian clams (Corbicula spp.) at 10 sites. Sand and gravel mining, high organic pollution, and inadequate instream flows were evident throughout the basin and might explain the general absence of mussels.

The Cylindrical Papershell (Anodontoides ferussacianus), considered a “Species in Need of Conservation” in Kansas, historically occurred across much of the state; however, recent studies suggest that the species is currently restricted to the upper Smoky Hill-Saline River Basin, and a survey emphasizing the status of the Cylindrical Papershell conducted in 2011 suggested its conservation status be elevated to endangered. Continuing drought since the completion of the 2011 survey raised concerns regarding the status of the Cylindrical Papershell. The objectives of this study were to evaluate possible drought-related changes in Cylindrical Papershell populations and to evaluate the status of this species in Kansas. Timed, tactile searches were conducted at 19 sites on the Saline River and 21 sites on the Smoky Hill River between July and August 2015. Thirty of these sites were revisited from the 2011 survey. In 2011, 24 live Cylindrical Papershell were observed among 11 sites. Declines in Cylindrical Papers...

The proteins of Smad family are critical components of the TGF-β superfamily signal pathway. In this paper, we cloned two intracellular mediators of TGF-β signaling, Smad3 and Smad5, from the pearl mussel Hyriopsis cumingii. The full length cDNA of HcSmad3 and HcSmad5 were 2052 bp and 1908 bp and encoded two polypeptides of 418 and 461amino acid residues, respectively. The deduced amino acid of HcSmad3 and HcSmad5 possessed two putative conserved domains, MH1 and MH2, a conserved phosphorylation motif SSXS at the carboxyl-terminal. The two Smad genes were detected muscle, mantle, hepatopancreas and gill, but with a very low level in heamocytes. The transcripts of Smad3 and Smad5 were up-regulated in hemocytes and hepatopancreas after A. hydrophila and PGN stimulation. However, the expression of Smad3 and Smad5 were only up-regulated in hepatopancreas after A. hydrophila stimulation. The transcripts of Smad3 and Smad5 had a slight change in hepatopancreas after PGN stimulation. The transcripts of HcSmad3 showed very little increase and HcSmad5 mRNA significantly up-regulated after wounding.

Polyacrylamide has become an effective tool for reducing construction-related suspended sediment and turbidity, which are considered to have significant adverse impacts on aquatic ecosystems and are a leading cause of the degradation of North American streams and rivers. However, little is known about the effects of polyacrylamide on many freshwater organisms, and prior to the present study, no information existed on the toxicity of polyacrylamide compounds to native freshwater mussels (family Unionidae), one of the most imperiled faunal groups globally. Following standard test guidelines, we exposed juvenile mussels (test duration 96 h) and glochidia larvae (test duration 24 h) to 5 different anionic polyacrylamide compounds and 1 non-ionic compound. Species tested included the yellow lampmussel (Lampsilis cariosa), an Atlantic Slope species that is listed as endangered in North Carolina; the Appalachian elktoe (Alasmidonta raveneliana), a federally endangered Interior Basin species; and the washboard (Megalonaias nervosa), a common Interior Basin species. We found that median lethal concentrations (LC50s) of polyacrylamide ranged from 411.7 to >1000 mg/L for glochidia and from 126.8 to >1000 mg/L for juveniles. All LC50s were orders of magnitude greater (2-3) than concentrations typically recommended for turbidity control (1-5 mg/L), regardless of their molecular weight or charge density. The results demonstrate that the polyacrylamide compounds tested were not acutely toxic to the mussel species and life stages tested, indicating minimal risk of short-term exposure from polyacrylamide applications in the environment. However, other potential uses of polyacrylamide in the environment (e.g., wastewater treatment, paper processing, mining, algae removal) and their chronic or sublethal effects remain uncertain and warrant additional investigation. Environ Toxicol Chem 2017;36:2715-2721. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Although eutrophication of freshwaters is a natural process, the human impact often leads to inland waters becoming overloaded with nutrients, impoverishing many valuable and vanishing habitats, such as floodplain lakes. These changes need to be reversed if the occurrence of endangered aquatic species is to be restored. In this paper we analyse the impact of a change in the water regime of a naturally eutrophic floodplain lake, which harbours a large diversity of Unionidae (large freshwater mussels), a globally threatened taxonomic group that provides important ecosystem functions and services. We found that a slight increase in the discharge from this waterbody, following the construction of an additional outflow pipe, positively influenced recruitment in three of the five mussel species inhabiting the lake. We also found that, after the construction of this additional outflow, the niches of juveniles of Anodonta cygnea and Unio spp. changed, revealing differences in their hydrological requirements. Our results suggest that, as in lotic habitats, complex hydraulic parameters are highly significant to unionid mussels in lentic conditions.

Historically, the rationale for sampling freshwater mussels was almost entirely for purposes of taxonomy, natural history surveys, and conservation and propagation efforts following a decline of the pearl button industry in the early 1900s. The concept of quantitative sampling of freshwater mussels is almost unknown in historical literature. Except for some proprietary or unpublished quantitative studies, the first study designed primarily to quantitatively sample freshwater mussels was by Scruggs [I] followed by Isom [2], Bates [3], Isom [4], and Dennis and Bates [5]. An earlier concept paper on "Quantitative sampling of mollusks and crustaceans" can be found in Wurtz [6]. This paper describes quantitative sampling methods for freshwater mussels within the context of varied study objectives, for example water pollution surveys. An example of a quantitative site-specific mussel study on the Cumberland River, Tennessee, is included. Difficulties with this study's design and application are discussed in some detail, along with site-specific studies on Kentucky Reservoir, Tennessee River, and Clinch River, Tennessee.

Morphological and chromosomal characteristics of a number of unionid freshwater mussels were studied from northeastern Thailand. Karyotypes of eight species from seven genera (Chamberlainia, Ensidens, Hyriopsis, Physunio, Pseudodon, Scabies and Trapezoideus) were examined. Six species possess 2n = 38 karyotypes, whereas Scabies crispata and an unidentified Scabies sp. lack three small chromosome pairs, giving a diploid number of 32. Moreover, the karyotypes of the unidentified Scabies differ from Scabies crispata as it exhibits a telocentric chromosome pair (6m + 7sm + 2st + 1t). Most of the conchological characters also differ between the two species – adult size, colour pattern, muscle scars, pseudocardinal and lateral teeth. The name Scabies songkramensis sp. n. is proposed for the unidentified species, and its description is included in this paper. Interestingly, seven species contain mostly bi-armed chromosomes, but only the mud-dweller in stagnant water, Ensidens ingallsianus, contains predominantly five telocentric pairs. In addition, the marker chromosome characteristics of an unbalanced long arm, twisted centromere, a wider angle 180° arrangement, a twisted arm and telomeric end union reported in this study are described for the first time for unionid mussels.

The freshwater mussel Anodonta (Sinanodonta) woodiana (Lea, 1834) (Chinese Huge Mussel or Swan Mussel) (Bivalvia: Unionidae) is the largest unionid species present in the European fauna. Its native range is in East Asia (South-Eastern Russia to Malaysia), but it has spread rapidly across Europe over the last few decades and the species is invasive also in other parts of the world ( Bogan et al., 2011 and Demayo et al., 2012). Studies of the population genetics of A. woodiana in Europe used slowly evolving markers such as allozymes and mitochondrial COI DNA sequences for the analysis of several isolated populations ( Nagel et al., 1996, Soroka, 2005 and Soroka et al., 2014). However, for the recent and rapid spread of the species across Europe, fast evolving markers, such as DNA microsatellites, are needed to understand important aspects of the population genetics of this invasive species: the route(s) of invasion, the time and number of colonization events, and other details. The first eight microsatellite markers for the species have been described in 2011 (Popa et al., 2011). This number is rather low to infer aspects of the evolutionary history of populations and additional microsatellite loci are needed to increase to power of future genetic studies of this species (Koskinen et al., 2004). In this paper we describe the development of nine new polymorphic microsatellite loci for A. woodiana. We also combined new and previously described loci into three multiplex sets allowing reducing the time and money costs of genotyping, as well as decreasing the risk for samples mishandling.

A total of 6 species of freshwater mussels under the family Unionidae, order- Unionoida were collected, which are new records for Bangladesh. The information on the distribution and ecology, population density and seasonal variation of Lamellidens phenchooganjensis, L. indawgyiensis, L. generosus, Parreysia (Parreysia) favidens var. assamensis, P. (P.) var. marcens and P. (P.) var. viridula were provided in this paper. Population density and morphometric parameters were measured. Moreover, economic importance and economic role were stated.Bangladesh J. Zool. 41(1): 29-42, 2013

Freshwater bivalves in the family Unionidae are one of the most endangered groups of animals in North America. In Texas, where over 60% of unionids are rare or very rare, 15 species have been added to the state's list of threatened species, and 10 are under consideration for federal listing. Due to insufficient survey efforts in the past decades, however, primary data on current distribution and habitat requirement for most of these rare species are lacking, thus challenging their protection and management. Although the species are listed as valid, there was no genetic confirmation to test for the possibility of ecophenotypes, which complicates conservation efforts. In this paper, we present genetic and distributional data for two rare Texas species, Truncilla macrodon and Truncilla cognata, and we suggest appropriate conservation measures. Tests of genetic affinities of these species using three gene regions, cox1, nad1 and ITS1, supported recognition of T. cognata and T. macrodon as full species. Analysis of historic and current species distribution showed that both these regional endemics are particularly vulnerable, and their distribution range has been reduced in the last 80 years.

The presence of thin organic layers embedded in the nacreous layer in the shell of several species of Unionoida has been widely considered to be largely a reaction to prevent further shell dissolution once part of the shell has been corroded. Their microstructural characteristics have also been used as taxonomic characters to separate the families Margaritiferidae and Unionidae. This paper focuses on the description and interpretation of these organic layers in samples of four extant species of European Unionidae coming from acidic and alkaline waters, six extant species of Margaritiferidae, one Cretaceous Unionidae and two Cretaceous Margaritiferidae species. Our results demonstrate that organic layers have characterized species of Unionoidea at least since the Cretaceous and occur in specimens coming from both acidic and alkaline waters. Moreover, the presence of these layers in noncorroded shells can be interpreted as a preventative or ‘prophylactic’ measure against corrosion. Their thickness is the only useful character verified to separate the two families. In the Margaritiferidae, organic layers were found to be present in Margaritifera margaritifera, M. valdensis, M. idubedae, M. laosensis, M. laevis and M. monodonta, and absent in M. auricularia and M. marocana. In Unionidae, they appear in Protopleurobema numantina, Potomida littoralis and in some individuals of Unio mancus, U. delphinus and U. tumidiformis. We argue that organic layers played a fundamental role in shell protection during the invasion of freshwater by the earliest Unionoida, while today they function to improve fitness in acidic water in some species.

Freshwater mussels are amongst the most threatened invertebrate species worldwide. For some species, current distribution has contracted to a point whereby original extent and ecological requirements are difficult to assess. As a consequence, extirpation rate is poorly evaluated, surveys are not targeted towards suitable areas and species are not taken into account in development impact studies. In this paper, we developed species distribution models (SDM) and produced maps of suitable habitat for conservation purposes. We focused on Margaritifera and Unio species for which enough reliable data could be collated for whole continental France. For the first time at this scale, SDM were based on a river stretch framework which proved to be very efficient. Models performances were highly discriminative, allowing drawing predictive maps of habitat suitability. As no human-induced variables were included, our models approximate species’ ideal original range. Range contraction could then be quantified by comparing the extent of occurrence (EOO) predicted by the models to the currently known EOO of the species. For most species, known distribution matches the predicted suitable habitat suggesting that geographic barriers between main drainages do not impact naiads’ distribution.

Polyacrylamide has become an effective tool for reducing construction-related suspended sediment and turbidity, which are considered to have significant adverse impacts on aquatic ecosystems and are a leading cause of the degradation of North American streams and rivers. However, little is known about the effects of polyacrylamide on many freshwater organisms, and prior to the present study, no information existed on the toxicity of polyacrylamide compounds to native freshwater mussels (family Unionidae), one of the most imperiled faunal groups globally. Following standard test guidelines, we exposed juvenile mussels (test duration 96 h) and glochidia larvae (test duration 24 h) to 5 different anionic polyacrylamide compounds and 1 non-ionic compound. Species tested included the yellow lampmussel (Lampsilis cariosa), an Atlantic Slope species that is listed as endangered in North Carolina; the Appalachian elktoe (Alasmidonta raveneliana), a federally endangered Interior Basin species; and the washboard (Megalonaias nervosa), a common Interior Basin species. We found that median lethal concentrations (LC50s) of polyacrylamide ranged from 411.7 to1000 mg/L for glochidia and from 126.8 to1000 mg/L for juveniles. All LC50s were orders of magnitude greater (2-3) than concentrations typically recommended for turbidity control (1-5 mg/L), regardless of their molecular weight or charge density. The results demonstrate that the polyacrylamide compounds tested were not acutely toxic to the mussel species and life stages tested, indicating minimal risk of short-term exposure from polyacrylamide applications in the environment. However, other potential uses of polyacrylamide in the environment (e.g., wastewater treatment, paper processing, mining, algae removal) and their chronic or sublethal effects remain uncertain and warrant additional investigation. Environ Toxicol Chem 2017;36:2715-2721. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

The conservation of endangered affiliate species, which are critically dependent on the presence of another species, is often hindered by a poor understanding of the relationships between the interacting partners. The parasitic stage of endangered unionid bivalves constitutes a tight host–affiliate linkage between the mussels and their host fishes. However, the threats resulting from potential shortages of the host species are rarely sufficiently quantified and incorporated into conservation strategies. In this paper, we integrated both host quality and availability analyses to assess the potential threats to the endangered thick-shelled river mussel Unio crassus that result from the impairment of its host resources in Central Europe. The experimental determination of the compatibility of U. crassus parasitic larvae (glochidia) with its potential host fishes revealed an intermediate level of host specificity. At least, some glochidia successfully developed on 14 of the 27 potential hosts that were evaluated. Nevertheless, only three fish species (Scardinius erythrophthalmus, Phoxinus phoxinus and Cottus gobio) enabled the majority of the attached glochidia to transform successfully. Subsequently, our analysis of host availability at sites inhabited by living or extirpated populations of U. crassus showed that the local extirpations of U. crassus are likely associated with an impaired status of the fish assemblage and with the absence of the primary host fishes. These results indicate that the availability of host fish resources may have played at least an additive role in the present pan-European decline of U. crassus and that the evaluation of host limitation without precise data on the host compatibility or host abundance may be ineffective for identifying the threats to particular species. It demonstrates a strong need for more thorough incorporation of host limitation issues into conservation strategies for U. crassus and probably also for other species of freshwater mussels that were previously considered safe from host limitation.

Geochemical records of bivalve shells have been increasingly studied in the last decade to obtain information on climate conditions. In this paper we present stable isotope compositions of living and prehistoric shells of freshwater mussels (Unionidae) and their relationships with climate conditions in a shallow lake environment of Lake Balaton, West-Central Hungary. Physical conditions and stable oxygen isotope compositions of lake water samples were monitored where living bivalves were collected. Comparisons between seasonal variations in ambient temperature, water composition and within-shell isotopic variations indicate that the shells of Unio pictorum do reflect local changes at high resolution and thus can be used to study past conditions. Additionally, shells covering the last two decades were gathered at several locations along the lake in order to determine spatial and temporal variations in the shells’ isotopic compositions as a function of weather conditions. As an application, prehistoric shells collected in archaeological excavations were analysed in order to study past environmental variations. Climate variations during the Late Copper Age (5460–4870 cal. yr BP) have been assumed on the basis of geomorphological and archaeozoological observations at the site Balatonkeresztur-Reti-dűlő (south of Lake Balaton), that suggested increasing humidity as a cause of changes in settlement location and domestic livestock husbandry. Stable carbon and oxygen isotope compositions of prehistoric bivalve shells were analyzed from excavations representing five archaeological subphases (Boleraz subphase, 5460–5310 cal. yr BP; two transitional subphases around 5310 cal. yr BP; Early Classic subphase, 5310–5060 cal. yr BP; Late Classic subphase, 5040–4870 cal. yr BP). The analyses revealed significant negative C and O isotope shifts in the transitional subphases relative to the earlier and later subphases. The isotopic variations indicate that the local climate became relatively wet and possibly cold around 5310 cal. yr BP, then it returned to drier (and likely warmer) conditions during the Classic subphases. This interpretation is in agreement with previous studies on climate changes related to the “5.3 ky event” in the European continental area and the North Atlantic Region, indicating an Atlantic influence in the Carpathian Basin.

Sinanodonta woodiana (Lea, 1834) is a large Unionid species with a real invasion success. It colonized Europe, Central America, the Indonesian Islands and recently North America. The species life cycle involves a larval parasitic stage on freshwater fish species which contributes to the spread of the mussel. In this paper we describe, for the first time, eight polymorphic microsatellite loci for the species Sinanodonta woodiana. The genetic screening of individuals confirmed that all loci were highly polymorphic. The number of alleles per locus ranged from 7 to 14 and the observed heterozygosity ranged from 0.650 to 0.950. These loci should prove useful to study the species population genetics which could help to infer important aspects of the invasion process.

Aim Geographic patterns of species distributions and the factors contributing to species endangerment are necessary for the development of integrative conservation strategies. Freshwater mussels Unionidae have among the highest levels of imperilment recorded in North America. This paper describes the biogeography and diversity of Unionidae along climate and habitat gradients in Texas, evaluates human impact, and identifies the hot spots of diversity and endemism that should be targeted for conservation. Location Texas, North America. Methods Unionids were surveyed in all major Texas river basins in 2003–2009. Multivariate statistics were used to test for differences in environmental parameters and among unionid assemblages in different bioprovinces, and to determine to what extent the multivariate pattern of species distribution was affected by environmental factors. To estimate human impact, we examined the relationship between human population density and the proportion of rare species, as well with the proportion of historically present species that persist in the watershed. Results Correlation between biotic and environmental similarity matrices indicated concordance in the differences among unionid assemblages and environmental factors that could cause these differences. Lake surface evaporation rate and percentage of forest cover in the watershed were among the most important parameters explaining the differences in unionid assemblages. Human population density was negatively correlated with the proportion of rare species. The proportion of species found live relative to the total number of live and relic species found in our surveys and to the number of historically known species decreased with the increase in human population density. Main conclusions Climate, landscape, geology, and land use type were important factors influencing unionid distribution patterns among biotic provinces. Increased human population density was associated with the loss of rare species over several decades, but this loss was not recognized because of a lack of assessing the conservation status of unionids.

This paper compiles the pre-Linnean iconography of freshwater mussels or naiads and provices an understanding of the historical framework in which they were created. The first book providing a figure of a freshwater mussel would seem to be Hortus sanitatis, an anonymous encyclopaedia first published in 1491. This and other early images of these animals related to the activity of obtaining pearls from Margaritifera margaritifera. The first traditional image of a freshwater mussel, drawn by Rondelet in 1555, was thereafter used by such other early naturalists as Boussuet, Gessner, Aldrovandi, and Jonston. The first microscopists Heide and Leeuwenhoek and pioneer malacologists Lister, D'Argenville and Ginanni were the first to depict the anatomy of naiads. We also have discovered a pre-Linnean image of a living naiad by Poupart showing its typical furrow in the sediments, and a pencil sketch by Linnaeus showing a pearl fishery of M. margaritifera.

This paper examines if shell oxygen isotope ratios (delta O-18(ar)) of Unio sp. can be used as a proxy of past discharge of the river Meuse. The proxy was developed from a modern dataset for the reference time interval 1997-2007, which showed a logarithmic relationship between discharge and measured water oxygen isotope ratios (delta O-18(w)). To test this relationship for past time intervals, delta O-18(ar) values were measured in the aragonite of the growth increments of four Unio sp. shells; two from a relatively wet period and two from a very dry time interval (1910-1918 and 1969-1977, respectively). Shell delta O-18(ar) records were converted into delta O-18(w) values using existing water temperature records. Summer delta O-18(w) values, reconstructed from delta O-18(ar) of 1910-1918, showed a similar range as the summer delta O-18(w) values for the reference time interval 1997-2007, whilst summer reconstructed delta O-18(w) values for the time interval 1969-1977 were anomalously high. These high delta O-18(w) values suggest that the river Meuse experienced severe summer droughts during the latter time interval. delta O-18(w) values were then applied to calculate discharge values. It was attempted to estimate discharge from the reconstructed delta O-18(w) values using the logarithmic relationship between delta O-18(w) and discharge. A comparison of the calculated summer discharge results with observed discharge data showed that Meuse low-discharge events below a threshold value of 6 m(3)/s can be detected in the reconstructed delta O-18(w) records, but true quantification remains problematic.

Expectations for survival of vertebrate remains have been well developed and intensely studied in the zooarchaeological taphonomic literature. Taphonomic studies of shellfish remains focus on marine species and on variables relevant to remains from paleontological contexts (e.g., fossil marine beds). In this paper we develop a conceptual framework from which to derive expectations concerning the preservation of freshwater mussel remains focusing on two parameters, shell microstructure and shell shape. Shell size does not influence survivorship. Our model is validated through application to late Holocene zooarchaeological mussel assemblages from north Texas. Taphonomically robust species are important regarding zooarchaeological and biogeographic interpretations based on mussel paleofaunas, and fragile species are important indicators of whether or not an assemblage is well preserved.

SUMMARY 1. There may be bias associated with mark–recapture experiments used to estimate age and growth of freshwater mussels. Using subsets of a mark–recapture dataset for Quadrula pustulosa, I examined how age and growth parameter estimates are affected by (i) the range and skew of the data and (ii) growth reduction due to handling. I compared predictions from von Bertalanffy growth models based on mark–recapture data with direct observation of mussel age and growth inferred from validated shell rings. 2. Growth models based on a dataset that included observations from a wide range of length classes (spanning ‡ the upper 50% of the population length range) produced only slightly biased age estimates for small and medium-sized individuals (overestimated by 1–2 years relative to estimates from validated shell rings) but estimates became increasingly biased for larger individuals. Growth models using data that included only observations of larger animals (< the upper 50% of length range) overestimated age for all length classes, and estimated maximum age was two to six times greater than the maximum age observed in the population (47 years). Similarly, growth models using a left-skewed dataset overestimated age. 3. Reductions of growth due to repeated handling also resulted in overestimates of age. The estimated age of mussels that were handled in two consecutive years was as much as twice that of mussels that were handled only once over the same period. Assuming a constant reduction in the annual rate of growth, handling an individual for five consecutive years could result in an estimated age that is five times too high. 4. These findings show that mark–recapture methods have serious limitations for estimating mussel age and growth. A previous paper (Freshwater Biology, 46, 2001, 1349) presented longevity estimates for three mussel species that were an order of magnitude higher than estimates inferred from shell rings. Because those estimates of extreme longevity were based on mark–recapture methods and subject to multiple, additive sources of bias, they cannot be considered accurate representations of life span and cannot be used to conclude that traditional methods of bivalve ageing by interpretation of shell rings are flawed.

In this paper we have compiled all information on Chinese pond mussel Sinanodonta woodiana (Lea, 1834) in the Czech Republic. This species was found in the Czech Republic for the first time in 1996 and altogether 20 records about its occurrence had been obtained till the beginning of 2008. The species was found in rivers, oxbow lakes and ponds. The expansion is the most intensive in Southern and Central Moravia, which is likely due to the fact that it is the warmest part of the country.

1.This paper documents a diverse, reproducing freshwater mussel community (20 species) in Lower Lake — an impounded, regulated portion of the Little Tallahatchie River below Sardis Dam in Panola Co., Mississippi, USA. 2.Despite being regulated and impounded, the lake has a heterogeneous array of habitats that differ markedly in mussel community attributes. Four distinct habitat types were identified based on current velocity and substrate characteristics, representing a gradient from habitats having lotic characteristics to lentic habitats. All four habitat types supported mussels, but habitats most resembling unimpounded, lotic situations (relatively higher current velocity and coarser substrate) had the highest mussel abundance and species density (10.1 mussels m−2, 1.8 species m−2, respectively). Lentic habitats (no flow, fine substrate) were characterized by lower abundance and species density (2.0 mussels m−2, 0.8 species m−2, respectively), but supported mussel assemblages distinctive from lotic habitats. 3.Evidence of strong recent recruitment was observed for most species in the lake and was observed in all four habitat types. 4.Although impounded and regulated, Lower Lake represents one of the few areas of stable large-stream habitat in the region. The presence of a diverse, healthy mussel community in this highly modified habitat suggests that a large component of the regional mussel fauna is relatively resilient and adaptable and is limited primarily by the absence of stable river reaches. Management actions that increase stream stability are likely to result in expansion of the mussel fauna and restoration of a valuable component of ecosystem function in this region. Published in 2006 by John Wiley & Sons, Ltd.

In numerical dominance Mollusca are second only to Arthropoda living in habitats connected with water. Of the six classes under the phylum two, namely Gastropoda and Bivalvia are restricted to inland water bodies and wetlands. The gastropods of Punjab were reported by the present authors earlier (Battish & Sharma, 1993) and the bivalves of northern India, Bihar, Punjab, Himachal Pradesh and Chandigarh are presented in this paper. The inland bivalves contribute to the nation’s economy, as these support pearl and button industries. Culturing of Lamellidens marginalis and Lamellidens corrianus is picking up for pearl production. The shell pieces are used in preparation of poultry feed and mosaic tiles. Larvae of Unionidae are known to parasitize fishes. The literature scan reveals that inland bivalvia of India remain unattended by and large. Benson (1862) was perhaps the first to work on Unio. Blanford (1866, 1867, 1870) published a series of papers on the Malacology covering bivalve genera Unio and Anodonta from India, Sri Lanka, Burma and Yunnan.

Archaeological evidence demonstrates that molluscs, either as unworked or worked artefacts, have been deposited in burial settings as objects which speak to secular or various philosophical, religious or cosmological concerns. Those molluscs imbued with secular or cosmological significance are identified and selected with reference to the specific behaviour, characteristics or qualities of the mollusc itself within a given society’s socio-cultural belief system. This paper examines data from mainland Southeast Asia to explore the distribution and potential significance of Unionidae bivalves in human burials from a range of large, later prehistoric burial sites. This family of bivalves was important in the mortuary tradition of the region from the Neolithic through to Iron Age, especially during the Bronze Age.

Genetic and Morphological Variability and Diff erentiation of Mussels (Bivavia, Unionidae, Anodontinae) in Ukraine. Mezhzherin S. V., Yanovich L. M., Zhalay Е. I., Vasilieva L. А., Pampura М. М. - Th e study of allozymes variation and sequence analysis of two mitochondrial genes supports the concept that there are four species of subfamily Anodontinae in the Eastern European fauna, three of them; Anodonta cygnea Linnaeus, 1758; A. anatina Linnaeus, 1758; Pseudanodonta complanata (Rossmassler, 1835) are indigenous and one is invasive (Sinanodonta woodiana Lea, 1834). Th is paper analyzes morphological diagnostic features of which some can be used in identification of these species.

Effective, species-specific zebra mussel control is needed urgently for Ireland׳s freshwater bodies, which became infested with non-native zebra mussels in the 1990s. Zequanox®, a newly commercialized product for zebra and quagga mussel control, is composed of dead Pseudomonas fluorescens CL 145A cells. This paper describes ecotoxicology tests on three representative native Irish freshwater species: Anodonta (duck mussel), Chironomus plumosus (non-biting midge), and Austropotamobius pallipes (white-clawed crayfish). The species were exposed to Zequanox in a 72-h static renewal toxicity test at concentrations of 100-750mg active ingredient per liter (mga.i./L). Water quality parameters were measured every 12-24h before and after water and product renewal. After 72h, endpoints were reported as LC10, LC50, and LC100. The LC50 values derived were (1) Anodonta: ≥500mga.i./L (2) C. plumosus: 1075mga.i./L, and (3) A. pallipes: ≥750mga.i./L. These results demonstrate that Zequanox does not negatively affect these organisms at the concentration required for >80percent zebra mussel mortality (150mg a.i/L) and the maximum allowable treatment concentration in the United Sates (200mga.i./L). They also show the overall species-specificity of Zequanox, and support its use in commercial facilities and open waters.

Freshwater mussels of the family Unionidae exhibit a particular form of mitochondria inheritance called double uniparental inheritance (DUI), in which the mitochondria are inherited by both male and female parents. The (M)ale and (F)emale mitogenomes are highly divergent within species. In the present study, we determine and describe the complete M and F mitogenomes of the Endangered freshwater mussel Potomida littoralis (Cuvier, 1798). The complete M and F mitogenomes sequences are 16 451 bp and 15 787 bp in length, respectively. Both F and M have the same gene content: 13 protein-coding genes (PCGs), 22 transfer RNA (trn) and 2 ribosomal RNA (rrn) genes. Bayesian analyses based on the concatenated nucleotide sequences of 12 PCGs and 2 rrn genes of both genomes, including mitogenome sequences available from related species, were performed. Male and Female lineages are monophyletic within the family, but reveal distinct phylogenetic relationships., The authors report that they have no conflicts of interest. The authors alone are responsible for the content and writing of the paper. This work was financially supported by the Portuguese Foundation for Science and Technology (FCT) project PTDC/AAC-AMB/ 117688/2010, and partly by the Strategic Funding UID/Multi/04423/2013 through national funds provided by FCT and European Regional Development Fund (ERDF), in the framework of the programme PT2020. JC is supported by Portuguese Foundation for Science and Technology (FCT) post-doc fellowship with reference SFRH/BPD/100912/2014. HMG and YPL are supported by the Monash University Malaysia Tropical Medicine Biology Platform., info:eu-repo/semantics/publishedVersion

This paper discusses the spatial distribution patterns of the various species of the Unionid mussels as functions of their respective life-cycle characteristics. Computer simulations identify two life-cycle characteristics as major factors governing the abundance of a species, namely the movement range of their fish hosts and the success rate of the parasitic larval glochidia in finding fish hosts. Core mussels species have fish hosts with large movement range to disperse the parasitic larval glochidia to achieve high levels of abundance. Species associated with fish host of limited movement range require high success rate of finding fish host to achieve at least an intermediate level of abundance. Species with low success rate of finding fish hosts coupled with fish hosts having limited movement range exhibit satellite species characteristics, namely rare in numbers and sparse in distributions.

The southeastern United States has one of the richest and most diverse communities of freshwater mussels in the world. However, the population size and species diversity has steadily declined since early in this century, resulting in many species being on the brink of extinction. In this paper, a modelling framework using a spatially explicit and age structured computer model is used to conduct spatial-temporal dynamics analysis of the mussel population. The important parameters studied are the effects of fecundity rate, availability of fish hosts and suitability of the habitat for the mussel communities.

Colour and anatomy of the glochidial larvae of Anodonta anatina (L.), Anodonta cygnea (L.), Pseudanodonta complanata (RossM.), Unio pictorum (L.) and Unio tumidus (PHILIPSSON) were studied by using light and scanning electron microscopy. The structures which were considered were the shell, hooks and their spines, larval thread and sensory hair cells. Characteristic features helpful in the identification of the glochidia of the five species described in this paper and those of Margaritifera margaritifera (L.) are summarised in a table. The features are compared with those reported in other descriptions of unionacean glochidia.

Paper deals with data on interrelation of mollusks of the family Unionidae and Dreissena polymorpha (Pallas, 1771) and other epibiotic invertebrates in the cooling pond of the Khmelnitsky nuclear power plant (Ukraine). The quantitative indices of unionids and Dreissena, inhabiting shells of the living Unionidae, size structure of populations, and morphometric indexes of the living Unionidae were analyzed. No significant negative impact of settlements of the zebra mussels on Unionidae was noted. Приведены результаты исследований взаимоотношений между двустворчатыми моллюсками семейсва Unionidae, Dreissena polymorpha (Pallas, 1771) и другими эпибионтными беспозвоночными в водоёме-охладителе Хмельницкой АЭС (Украина). Проанализированы количественные показатели перловиц и дрейссены, обитающей на живых перловицах, размерная структура популяций моллюсков, морфометрические характеристики живых перловиц. Явно выраженного негативного влияния поселений дрейссены на перловиц не выявлено.

The larvae of unionid freshwater mussels (i.e., glochidia) undergo a parasitic stage requiring their attachment to the external epithelia of fish hosts, where they metamorphose into free-living juveniles. We describe the physiological effects in bluegill sunfish (Lepomis macrochirus) of infection with glochidia from the paper pondshell (Utterbackia imbecillis). Glochidia accumulation on bluegill increased dramatically at concentrations of 2000 glochidia liter(-1) and above, reaching a maximum attachment density of about 30 glochidia g(-1) fish at 4000 glochidia liter(-1). Plasma cortisol was the most sensitive indicator of biological effect to glochidial exposure, increasing significantly in hosts exposed to 2000 glochidia liter(-1) or greater. Glochidia were 31% more likely to undergo successful juvenile metamorphosis when attached to bluegill with elevated plasma cortisol, largely due to the enhanced survivorship of these larvae during the first 48 h after infection. We tested the hypothesis that glochidial attachment and juvenile metamorphosis were stimulated directly by plasma cortisol in fish hosts. Bluegill were given an intraperitoneal injection of cortisol, then infected with 1000 glochidia liter(-1) at 48 h after hormone supplementation. Cortisol-injected fish had a 42% increase in the number of attached glochidia g(-1) fish and a 28% increase in larval metamorphosis compared to sham-injected and control fish. We provide evidence that cortisol enhances glochidial metamorphosis on hosts by improving the retention of attached glochidia. This study gives insights into the influence of host physiology on glochidial attachment and juvenile mussel transformation.

Based on the EST sequence of mantle cDNA library of Hyriopsis cumingii, the complete cDNA of Hyriopsis cumingii GPX gene was cloned by rapid amplification of cDNA ends (RACE) in this paper. The GPX full-length cDNA sequence was 1286 bp including a 39 bp 5'-untranslated region, a 659 bp 3'-untranslated region and an open reading frame (ORF) 588 bp in length, which encodes for 195 amino acids with the predicted molecular weight of 22.2 kDa and the isoelectric point of 8.44. Molecular structure analysis revealed that GPX was Se-GPX. The amino acid sequences have three highly conserved loop structures that have a stabilizing effect on the tertiary structure of the enzyme. The amino acid sequence did not contain obvious hydrophobic domain and signal peptide. Homologous analysis of amino acid sequences indicated that the GPX of H. cumingii shared a high similarity with GPX-2 and GPX-1 of vertebrates (73.1%-80.8%), and low similarity with other types (lt; 60%). Phylogenetic trees showed that GPX of Hyriopsis cumingii was clustered together with GPX of some fishes and far away from GPX of several other published Molluscas. These results indicated GPX gene of H. cumingii cloned in this study was different from the GPX genes from other Molluscas.

Simultaneous hermaphroditism is an infrequent mode of reproduction among bivalves of the family Unionidae: only five of the 220 North American species are simultaneous hermaphrodites. However, hermaphroditic individuals of otherwise predominantly dioecious species have been encountered in 30 of I01 species examined. These hermaphroditic individuals as well as simultaneous hermaphrodites can exhibit considerable variability in the ratio of spermatogenic: oogenic tissue within the gonad, and the purposes of this paper are to determine the underlying causes of both this variability and the occurrence of occasional hermaphroditic individuals among dioecious species. Results indicate that the ratio of male: female gonodal tissue of a simultaneous hermaphrodite is bimodally distributed, and several hypotheses to account for this observation are presented. It is proposed that populations occurring in different habitats and under conditions of different individual density are subject to fundamentally different sexual selection pressures acting on ratios of allocation to male and female gametes. Occasional hermaphroditism among otherwise predominantly dioecious species was in this study associated with infection of the gonads by digenean trematodes. A model of sexual determination among unionids presented in this paper proposes that sex is determined by genetically controlled hormone levels: occasional hermaphrodites result from alterations in these hormone levels caused by developmental errors and trematodal infections which mimic the results of such errors. Predictions of this model are consistent with observed levels of variability in male: females gonadal tissue among occasional hermaphrodites.

In 1827, von Baer described a trematode which he named Aspidogaster conchicola. The original material was from a fresh-water mussel of the genus Anodonta, from Prussia. This same species has long been known to occur in the pericardial and renal cavities of North American UNIONIDAE, but most of the studies carried on by American investigators have dealt chiefly with morphology and distribution of the species. Kelly (1899) carried on extensive studies of the host relationships of this species in the Illinois River and Stunkard (1917) made a study of the general morphology which confirms and adds somewhat to the earlier observations by Stafford (1896). There has been but fragmentary information concerning the development of this parasite of the UNIONIDAE. Stafford (1896), Faust and Tang (1936), and others have made the suggestion that the life cycle is fundamentally direct, i.e., that the young worm develops directly into an adult, either in the same host individual which shelters the mature worm, or in another individual of the same or of a different species. The systematic position of Aspidogaster has long been in doubt. Whether it belongs in MONOGENEA, DIGENEA, or an intermediate group has been a controversial question. It is the object of this paper to trace the life cycle of this worm and to review critically the evidence as to its systematic position. In the present study, the process of hatching of the eggs of Aspidogaster has been observed and, in natural infections, series of immature stages have been found representing fairly complete developmental stages all within the same host species. Aspidogaster conchicola therefore differs from the general pattern of host-parasite relationship among trematodes in that the entire life cycle occurs within an invertebrate host. UNIONIDAE from the Sangamon River near Mahomet, Illinois; from the Salt Fork, at Homer Park, Illinois; and from the Wabash River at West York, Illinois, are the hosts of the Aspidogaster used in this study. A detailed list of the host species will be presented in another paper dealing more directly with relationships to the host.