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The Unionidae represent an excellent model taxon for unravelling the drivers of freshwater diversity, but, phylogeographic studies on Southeast Asian taxa are hampered by lack of a comprehensive phylogeny and mutation rates for this fauna. We present complete female- (F) and male-type (M) mitogenomes of four genera of the Southeast Asian clade Contradentini+Rectidentini. We calculate substitution rates for the mitogenome, the 13 protein-coding genes, the two ribosomal units and three commonly used fragments (co1, nd1 and 16S) of both F- and M-mtDNA, based on a fossil-calibrated, mitogenomic phylogeny of the Unionidae. Phylogenetic analyses, including an M+F concatenated dataset, consistently recovers a monophyletic Gonideinae. Subfamily-level topology is congruent with that of a previous nuclear genomic study and with patterns in mitochondrial gene order, suggesting Unionidae F-type 2 as a synapomorphy of the Gonideinae. Our phylogeny indicates that the clades Contradentini+Rectidentini and Lamprotulini+Pseudodontini+Gonideini split in the early Cretaceous (~125 Mya), and that the crown group of Contradentini+Rectidentini originated in the late Cretaceous (~79 Mya). Most gonideine tribes originated during the early Palaeogene. Substitution rates were comparable to those previously published for F-type co1 and 16S for certain Unionidae and Margaritiferidae species (pairs). [ABSTRACT FROM AUTHOR]

The Viviparidae, commonly known as River Snails, is a dominant group of freshwater snails with a nearly worldwide distribution that reaches its highest taxonomic and morphological diversity in Southeast Asia. The rich fossil record is indicative of a probable Middle Jurassic origin on the Laurasian supercontinent where the group started to diversify during the Cretaceous. However, it remains uncertain when and how the biodiversity hotspot in Southeast Asia was formed. Here, we used a comprehensive genetic data set containing both mitochondrial and nuclear markers and comprising species representing 24 out of 28 genera from throughout the range of the family. To reconstruct the spatiotemporal evolution of viviparids on a global scale, we reconstructed a fossil-calibrated phylogeny. We further assessed the roles of cladogenetic and anagenetic events in range evolution. Finally, we reconstructed the evolution of shell features by estimating ancestral character states to assess whether the appearance of sculptured shell morphologies was driven by major habitat shifts. The molecular phylogeny supports the monophyly of the three subfamilies, the Bellamyinae, Lioplacinae, and Viviparinae, but challenges the currently accepted genus-level classification in several cases. The almost global distribution of River Snails has been influenced both by comparatively ancient vicariance and more recent founder events. In Southeast Asia, Miocene dispersal was a main factor in shaping the modern species distributions. A recurrent theme across different viviparid taxa is that many species living in lentic waters exhibit sculptured shells, whereas only one strongly sculptured species is known from lotic environments. We show that such shell sculpture is habitat-dependent and indeed evolved several times independently in lentic River Snails. Considerably high transition rates between shell types in lentic habitats probably caused the co-occurrence of morphologically distinct shell types in several lakes. In contrast, directional evolution toward smooth shells in lotic habitats, as identified in the present analyses, explains why sculptured shells are rarely found in these habitats. However, the specific factors that promoted changes in shell morphology require further work. [ b iogeographical analyses; fossil-calibrated phylogeny; fossil-constrained analyses; Southeast Asia; stochastic character mapping.] [ABSTRACT FROM AUTHOR]

Risk assessment currently lacks reliable tools that can be applied across a range of environmental contexts to predict the success and impact of invasive species. A comparative functional response approach may offer insight and predictive power regarding the impacts of invasive bivalves. This could prove valuable in the management of invasive species that drive losses in ecosystem function and biodiversity. We conducted feeding experiments to assist in predicting the relative impact of two sympatric species of dreissenid mussel, the zebra and quagga (Dreissena polymorpha and D. rostiformis bugensis, respectively). Feeding rates and functional responses were calculated for each species under different 'contexts', including ranges of algal concentrations and temperatures. Clearance rates ranged from 10 to 44 ml/g mussel/h. Both species exhibited type I functional responses across all contexts studied. There were no differences in the clearance rates or functional responses between species at 4 °C and 12°C. Thus, the competitive exclusion of zebra mussels by quagga mussels seen in nature does not appear to be driven by disparities in feeding efficiencies. Both species demonstrated elevated functional responses at the highest temperature (24 °C) compared with the lowest temperature (4 °C). Zebra mussels showed a significantly higher clearance rate than quagga mussels at 24 °C. We conclude that total species abundance, rather than changes in species dominance, is likely to be more important for net species impact. Functional responses may offer considerable explanatory and predictive power under certain contexts; they are particularly fitting for sessile invasive bivalves due to their high feeding rates and the seasonal changes in algae abundance they experience. [ABSTRACT FROM AUTHOR]

Freshwater gastropods often contribute significantly to freshwater ecosystem biomass and are a vital component of many consumers' diets. They can also serve as a nutrient resource for terrestrial predators, representing an important example of nutrient transfer across a habitat boundary. This study uses bankside middens to provide evidence of predation by the brown rat Rattus norvegicus on the native freshwater gastropod Viviparus viviparus in a closed, lentic system in Ely, southeastern England. Conchological analysis of shell remains in middens compared with live snails in adjacent water sites suggested this predation could be size-selective for larger snails, and analysis of soft tissue dry weights and the mechanical force required to break shells suggested that this was driven predominantly by maximizing nutrient gain, rather than by optimizing handling effort. Aquatic gastropods represent a potentially important nutrient resource for brown rat populations; this resource may be used continuously through time or exhibit 'resource pulse' dynamics. Predation of this nature might also have important impacts on gastropod population structure, which may in turn impair ecosystem functioning. [ABSTRACT FROM AUTHOR]

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

This study aimed to elucidate the mechanism of glochidial release in Unio pictorum (L., 1758) and U. tumidus Philipsson, 1788, and thus interpret the confused reports of unionine reproduction in the literature. Both species release mature, hooked glochidia on mucous threads, resembling a pearl necklace. Individual threads can reach 15 cm in length and carry c. 500 glochidia. Under hypoxic conditions both species released conglutinate-like structures comprising c. 500 eggs and immature, unhooked glochidia. The conglutinate-like structures released by U. pictorum and U. tumidus are coloured yellow and white, respectively. Increased temperature did not induce the release of conglutinates. The conglutinate-like structures are unlikely to be involved in increasing reproductive success by attracting host fishes. Rather, the premature evacuation of the gill contents may relieve the respiratory burden on adult mussels under hypoxic stress. Ambient hypoxia during the reproductive season may help explain the irregular recruitment success of relatively long-lived mussels, such as U. pictorum and U. tumidus. [ABSTRACT FROM PUBLISHER]

The shell morphology and population dynamics of the five British Unionidae are compared within a sympatric population. Pseudanodonta complanata is distinguished from Anodonta anatina and A. cygnea by the hinge length-shell length relationship; this morphological distinction may serve as a useful tool in the identification of this threatened species. The shell length at a given annulus was remarkably similar for all five species, although the symptotic length is reached most quickly in P. complanata and Unio pictorum. P. complanata is relatively short-lived and attains the lowest maximum length, while A. cyanea lives more than twice as long and attains almost double the length of P. complanata. Unio spp. have a short gravid season over the summer, while Anodonta spp. have a long gravid period, lasting from Autumn through to Spring. Unlike other members of the Anodontinae, P. complanata has a short breeding season, overlapping with that of the Unio spp. [ABSTRACT FROM PUBLISHER]