Anatomical Features of Modellnaia siamensis (Mollusca, Bivalvia, Unionidae)

Although conchologically an etheriid mussel, M. siamensis from northeastern Thailand is, according to anatomical and larval features, an unionid. Its unique character states among the Unionidae are: relatively distant spacing of septa in all four demibranchs in both, males and females, absence of a foot in adults, and generation of subelliptical glochidial larvae. This species is seemingly a phylogenetic relic.     

Geometrical and crystallographic constraints determine the self-organization of shell microstructures in Unionidae (Bivalvia: Mollusca)

Unionid shells are characterized by an outer aragonitic prismatic layer and an inner nacreous layer. The prisms of the outer shell layer are composed of single-crystal fibres radiating from spheruliths. During prism development, fibres progressively recline to the growth front. There is competition between prisms, leading to the selection of bigger, evenly sized prisms. A new model explains this competition process between prisms, using fibres as elementary units of competition. Scanning electron microscopy and X-ray texture analysis show that, during prism growth, fibres become progressively orientated with their three crystallographic axes aligned, which results from geometric constraints and space limitations. Interestingly transition to the nacreous layer does not occur until a high degree of orientation of fibres is attained. There is no selection of crystal orientation in the nacreous layer and, as a result, the preferential orientation of crystals deteriorates. Deterioration of crystal orientation is most probably due to accumulation of errors as the epitaxial growth is suppressed by thick or continuous organic coats on some nacre crystals. In conclusion, the microstructural arrangement of the unionid shell is, to a large extent, self-organized with the main constraints being crystallographic and geometrical laws.     

Sexual selection and simultaneous hermaphroditism among the Unionidae (Bivalvia: Mollusca)

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.     

Molecular genetics of peripheral populations of Nova Scotian Unionidae (Mollusca: Bivalvia)

Peripheral populations of eight species of freshwater bivalves (Unionidae.) extending their geographic ranges into Nova Scotia, Canada, were examined electrophoretically to determine both the extent of genetic variability within such populations, and whether the hypothesized pathway of colonization across the Isthmus of Chignecto is reflected in patterns of genetic resemblance among these populations. The Nova Scotian species examined could be separated into two groups based on levels of observed heterozygosity and levels of variability in allele frequencies. The first group is characterized by low levels of heterozygosity and polymorphism compared with north-eastern American populations, and in the case of one species, Elliptio complanala, considerable variability in allele frequencies among populations occurring in similar habitats in different drainages. Populations of E. complanata from Nova Scotia can be differentiated from conspecific populations on the southern Atlantic Slope by possession of fast alleles at two loci. Multivariate analyses define subgroups within populations of E. complanata consistent with hypothesis that the species invaded Nova Scotia by way of the Isthmus of Chignecto, and then split into two groups, one of which colonized Cape Breton to the north and the other of which colonized southern areas of the Province. The second group of Nova Scotian species is characterized by little reduction in heterozygosity and polymorphism compared with values observed among north-eastern American conspecifics or congeners, little variability in allele frequencies from population to population, and little evidence to suggest that these species were dependent on the land bridge to invade the Province. The type of dispersal is hypothesized to be responsible, in part, for these differences: larvae of species in the first group rely on a parasitic attachment to fish with territorial habits limited to fresh water, and are thus likely to invade new drainages separated by salt water by chance, in small numbers, and in stepping-stone fashion. Species in the second group parasitize anadromous or saltwater tolerant hosts, are likely to be introduced into new habitats in greater numbers and/or receive greater amounts of gene flow subsequent to colonization, and seem less dependent on land-bridges to colonize new habitats.     

Estimating the biomass of freshwater mussels (Bivalvia: Unionidae) from shell dimensions

Packs of autumn-shed maple leaves were placed at coal ash effluent-exposed and reference sites in streams on December 5, 1977 and removed after 27 and 96 days. Leaf surface area (cm²/leaf) and disc weight (ash-free dry wt/15 mm disc) were greater at the effluent-exposed site than at the reference site after 96 days (p < .001). ATP content of leaves from the reference stream quadrupled between 27 and 96 days while ATP content of effluent-exposed leaves remained low. Macroinvertebrates colonized the leaf packs in the reference site but were not found on or in effluent-exposed packs. We concluded that leaf processing beyond the leaching of soluble organics did not occur in the effluent-exposed packs owing to reduced colonization and decomposition by fungi. Since stream invertebrates prefer decomposed leaf material and animals grow faster on leaves colonized by microbes, the ash effuent appears to indirectly affect macroinvertebrates by interfering with leaf decomposition and thus reducing the quality of their food.     

Role of Unionidae and Sphaeriidae (Mollusca,Bivalvia) in the Eutrophic Lake Zbęchy and its Outflow

The abundance, biomass, respiration, and filtration rate of the Unionidae and Sphaeriidae and their significance to the phosphorus cycle was estimated in an eutrophic lake in Poland, situated in an agricultural area, and in its outflow canal. A decrease in Unionidae biomass was compensated for the Sphaeriidae in terms of both biomass and filtration rate. Depending on the developmental state of the Unionidae and Sphaeriidae populations, the volume of pumped water, the amount of removed and assimilated seston and the phosphorus excretion changed significantly.     

An ultrastructural study of shell regeneration in Mytilus edulis (Mollusca: Bivalvia)

Female budgerigars, provided with nest-boxes and exposed to male vocalizations, lay more rapidly under a 14 L: 10 D photoregime than under regimes involving shorter light periods, though some birds will lay even when kept under continuous darkness. The photoperiod affects principally the interval between nest-box entry and egg-laying. Birds with previous breeding experience lay sooner than naive birds.     

Tissue Hemoglobins in Bivalvia (Mollusca)

Distribution and properties of tissue hemoglobins in bivalves are reviewed. The mollusk gills are given particular attention; data on their functions, relative weight, and surface area as well as the content, properties, and functional importance of gill hemoglobin are reviewed.     

Testing hypotheses on the dispersal and evolutionary history of freshwater mussels (Mollusca: Bivalvia: Unionidae)

The relationship between dispersal and differentiation of the European freshwater mussel Unio pictorum (Linnaeus, 1758) was studied with molecular genetic methods. Forty‐two populations from France, Italy and central Europe were analysed. Genetic relationships were assessed from the geographical distribution of allele frequencies at 17 enzyme loci. Neighbouring groups of populations show small to moderate mean genetic distances (0.020 < D_mean < 0.263). With a few exceptions the genetic affinities of the populations are the closest within the same drainage basin. In central Europe and Northern Italy genetic differences between drainage systems are relatively large. Populations from north‐eastern Italy are genetically similar to Danubian populations. Mussels from the islands of Corsica and Sardinia are more closely related to populations from the Italian peninsula than to French populations from the Rhône drainage system. Genetic relationships within U. pictorum from central Europe reflect palaeogeographical relationships between river systems during the Pliocene and Pleistocene. Literature data on two North American unionid species and one European fish species show the same relationship between genetic diversity and the history of drainage systems, although the correlations are less strong. In France and Italy this correspondence is much less evident. Population dynamic processes and human activities leading to populational bottlenecks might have obscured it.     

On the relationships between absolute and allometric shell growth in unionid mussels (Bivalvia,Unionidae) from European Russia

The heuristic analysis of the relationship between the parameters of equations of linear (L. von Bertalanffy) and allometric shell growth was carried out on Unio pictorum (Linnaeus, 1758), U. tumidus Retzius, 1778, and U. crassus Retzius 1778 (Unionidae) from European Russia. Growth constants of the shell, k, were studied for length (k _L ), height (k _h ), and convexity (k _C ). It was shown that the ratios of growth constants (k _h /k _L , k _C /k _h , and k _C/k _L ) and parameter bfrom the allometric equations (h=aL ^b ;B=ah ^b ; C=aL ^b ) are connected by a strong negative correlation. If the b parameter is more than 1 (positive allometry), the growth constant relations are less than one, if b < 1 (negative allometry), the growth constant proportions are more than 1, and, in case when b is close to 1, (isometry), the growth constant proportions are also close to 1. All three types of allometric growth (isometry and both positive and negative allometry) are observed in the studied mussels.     

Ciliary ultrastructure of protobranchs (Mollusca, Bivalvia)

Abstract. The external epithelial cilia and other surface structures of the nuculoid protobranchs Nuculana pernula and Nucula nitidosa were studied. The gill lamellae and labial palps are partly covered with very long cilia. These have a modified slender distal portion, an ordinary metazoan-type basal body, a basal foot. and a single, long cross-striated rootlet. In cilia on the gills of N. nitidosa , the basal foot is thick and attaches to the next basal body directly behind. Unciliated surface areas on the gills, labial palps, and foot are covered with a dense brushborder of microvilli. We observed no specific homologies between the cilia of the protobranchs studied and the epidermal cilia of the enigmatic Xenoturbella bocki , hence the recent hypothesis of a close connection of the latter to the protobranch bivalves is questioned.     

A multilocus phylogeny of archiheterodont bivalves (Mollusca,Bivalvia, Archiheterodonta)

The bivalve clade Heterodonta encompasses more than half of the extant bivalve species and is presently considered a derived group of the modern bivalves (Newell 1965 ; Waller 1998 ). Heterodonta is subdivided into two major lineages, the hyperdiverse Euheterodonta and Archiheterodonta. The latter comprises four relatively small extant families: Astartidae, Carditidae, Condylocardiidae and Crassatellidae, whose relationships and internal phylogeny are poorly understood. We assessed the phylogeny of archiheterodont bivalves using a multilocus data set comprised of molecular sequence data from six loci (18S rRNA, 28S rRNA, cytochrome c oxidase subunit I, cytochrome b, internal transcribed spacer 2 and histone H3). Resultant data sets of ~4 Kb of concatenated molecular sequence data were analysed using probabilistic approaches (maximum likelihood and Bayesian inference) and parsimony direct optimization. We recovered strong support for the monophyly of Archiheterodonta, within which Astartidae is the sister group of Crassatellidae, and these two constitute the sister clade of Carditidae, which is paraphyletic with respect to Condylocardiidae. The relationships among the constituent species groups were evaluated in the context of the archiheterodont fossil record through the estimation of divergence times. Diversification times of archiheterodont families were congruent with bounded estimates of origins based on palaeontological data: Archiheterodonta diversified during the Devonian, 373.1 Ma (95% highest posterior density interval [HPD] 325.8–428.2); Crassatelloidea around the Carboniferous, 330.1 Ma (95% HPD 291.0–372.7); Crassatellidae around the Triassic, 224.0 (95% HPD 140.6–320.2); Astartidae around the Permian, 288.2 Ma (95% HPD 269.2–307.3); and Carditoidea around the Jurassic, 178.8 Ma (95% HPD 120.9–228.3).     

Individual Growth of Anodonta beringiana (Unionidae,Bivalvia) in Postlarval Ontogenesis

Russian Journal of Developmental Biology - Individual linear growth in 26 specimens of Anodonta beringiana is studied by measuring the successive annual rings on the surface of the shells. It is...     

Morphological and functional specializations of the shell, musculature and pallial glands in the Lithophaginae (Mollusca: Bivalvia)

The structure of the pallial glands in limestone and coral boring species of Lithophaga (Bivalvia: Lithophaginae) is examined and related to evolutionary trends in habitat specialization. Boring glands of all species occur in the middle mantle fold but display a progressive degree of complexity from simple epithelial to ducted sub-epithelial structures coinciding with increasing specialization of habitat from natural limestone to species specific live coral dwellers. The size of this gland is relatively reduced anteriorly and along the entire length of the mantle in live coral borers.     

Freshwater mussels (genus Anodonta) from Anatolia and adjacent areas (Bivalvia,Unionidae)

Abstract

A review is given of the species of Anodontinae in Anatolia and the adjacent areas, based mainly on the author’s collections. Five species (A. anatina, A. cygnae, A. palustris, A. pseudodopsis, A. vescoiana) and several subspecies are recognized. Their distribution and the patterns of subspeciation are discussed.     

Molecular phylogeny,taxonomy, and distribution of French Unio species (Bivalvia,Unionidae)

A plethora of unionid names was established in the nineteenth century by the “Nouvelle Ecole”. Although naiad morphological plasticity is well documented, the currently recognized fauna, with 17 species and subspecies included in the French checklist for the Unio genus, is still based upon morphological characters only. Insights have been provided from molecular data elsewhere in Europe and North Africa, but the French fauna remains unstudied. We present a molecular phylogeny of the Unio genus in France based on COI, 16S and 28S genes; taking up all available data in Europe plus 273 specimens collected in all main French drainages. The results show that there are either three valid species in France, with U. pictorum and U. mancus synonymized, or five, with the subspecies U. crassus courtillieri elevated to species level. Subspecies were generally not recovered, which questions the evolutionary units tacitly implied by subspecific names. Although sampling topotypes is the most reliable way to evaluate the status of a nominal subspecies, major human-induced changes in aquatic hydrosystems challenge the method. Nevertheless, operational taxonomy has to rely on ground-truthed data and we propose to reduce the actual number of valid taxa in France to the seven observed operational taxonomic units.     

Silurozodus,new genus,the oldest known member of the Trigonioida (Bivalvia,Mollusca)

The trigonioid bivalveSilurozodus gotlandicus n. gen. et n. sp. is described in terms of functional morphology, ecology, and life habit. The excellently preserved material has made it possible to reconstruct the nature of the foot and its muscles and the position of the gills.Silurozodus gotlandicus is interpreted to have been a nonsiphonate, actively burrowing, infaunal suspension-feeder that lived in a high-energy and unstable environment.Silurozodus gotlandicus is of late Wenlockian (Homerian) age and is the oldest known representative of the Trigonioida. It is probably ancestral to all younger members of this order.     

The Morphology, Growth and Reproduction of Unionidae (Bivalvia) in a Fenland Waterway

The shell morphology and population dynamics of the five BritishUnionidae are compared within a sympatric population. Pseudanodontacomplanata is distinguished from Anodonta anatina and A. cygneaby the hinge length–shell length relationship; this morphologicaldistinction may serve as a useful tool in the identificationof this threatened species. The shell length at a given annuluswas remarkably similar for all five species, although the asymptoticlength is reached most quickly in P. complanata and Unio pictorum.P. complanata is relatively short-lived and attains the lowest maximumlength, while A. cygnea lives more than twice as long and attainsalmost double the length of P. complanata. Unio spp. have ashort gravid season over the summer, while Anodonta spp. havea long gravid period, lasting from Autumn through to Spring.Unlike other members of the Anodontinae, P. complanata has ashort breeding season, overlapping with that of the Unio spp. (Received 4 March 1998; accepted 23 April 1998)