Identifying the physical habitat characteristics associated with riverine freshwater mussel assemblages is challenging but crucial for understanding the causes of mussel declines. The occurrence of mussels in multispecies beds suggests that common physical factors influence or limit their occurrence. Fine-scale geomorphic and hydraulic factors (e.g., scour, bed stability) are predictive of mussel-bed occurrence, but they are computationally challenging to represent at intermediate or riverscape scales. We used maximum entropy (MaxEnt) modeling to evaluate associations between riverscape-scale hydrogeomorphic variables and mussel-bed presence along 530 river km of the Meramec River basin, USA, to identify river reaches that are fundamentally suitable for mussels as well as those that are not. We obtained the locations of mussel beds from an existing, multiyear dataset, and we derived river variables from high-resolution, open-source datasets of aerial imagery and topography. Mussel beds occurred almost exclusively in reaches identified by our model as suitable; these were characterized by laterally stable channels, absence of adjacent bluffs, proximity to gravel bars, higher stream power, and larger areas of contiguous water (a proxy for drought vulnerability). We validated our model findings based on model sensitivity using a set of mussel-bed locations not used in model development. These findings can inform how resource managers allocate survey, monitoring, and conservation efforts.

Understanding the status and distribution of species is fundamental for conservation. However, recent genetic work has challenged the known distributions of some unionid taxa. The recognized range of the Louisiana Fatmucket Lampsilis hydiana spans watersheds from east Texas northward to southern Arkansas and eastward to western Mississippi. Specimens with morphological similarities to L. hydiana have been observed in Illinois and were presumed to be Fatmucket Lampsilis siliquoidea based on known distributions of Lampsilis species in Illinois. We examined specimens from Illinois and completed comparative genetic analyses using the mitochondrial genes cox1 and nad1 for species resembling L. siliquoidea. Our results show two morphologically similar, yet genetically distinct, species in Illinois. One of these species was genetically similar to L. siliquoidea, and one of these species showed little-to-no genetic difference from topotypic L. hydiana. The confirmation of L. hydiana populations within Illinois is significant for documenting the faunal diversity of the state. The varying degree of phenotypic separation confirms the need for further morphological research within Lampsilis, as well as genetic research throughout the updated known range of L. hydiana.

Freshwater gastropod shells display a striking amount of variation. Shell characters are the foundation of most freshwater gastropod taxonomy and the basis for identifying most species. However, intraspecific shell variation is common, and the mechanisms that give rise to this variation are often unclear. One source of shell variation is phenotypic plasticity, in which one genotype gives rise to multiple phenotypes as a response to environmental cues. This phenomenon is often invoked as an explanation for intraspecific shell variation in gastropods, but its existence has not been confirmed experimentally or otherwise in many gastropod lineages. I review the evidence for phenotypic plasticity in freshwater gastropods, and I discuss areas of research needed for a better understanding of intraspecific shell variation. Phenotypic plasticity is well documented in the superorder Hygrophila, but evidence in other freshwater gastropod groups is limited or nonexistent because of the scarcity of common garden experiments for those groups. Despite statements to the contrary, studies that show correlations of shell traits with environmental factors, population genetic analyses, and phylogenetic inference fail to provide evidence of phenotypic plasticity. Researchers must be careful not to postulate about phenotypic plasticity without evidence. I argue that phenotypic plasticity should not be the default hypothesis for explaining intraspecific shell variation in freshwater gastropods and that more common garden experiments are needed to test its existence. Genomic studies of mantle gene expression and transgenerational epigenetic studies also will increase our understanding of gastropod shell variation.

Animal gut bacteria are involved in numerous critical functions. In snails, gut bacteria play crucial roles in organic material digestion and nutrient production and have been implicated in aspects of reproduction. Snail gut microbes are known to differ between species and even between anatomical compartments of the digestive tract; dietary changes are also known to alter snail gut flora. In an effort to better understand their diversity and function, we studied the gut microbial communities from two viviparid snails, Campeloma decisum and Cipangopaludina japonica. We were interested in whether significant differences in bacterial community composition existed between the two species, and whether differences in microbial diversity corresponded to differences in community function. Using next-generation sequencing of the bacterial 16S V4 region, we found no significant differences in alpha and beta diversity between Ca. decisum and Ci. japonica. Firmicutes and Proteobacteria were the most abundant bacterial phyla in both species, while Bacteroidetes had a higher mean abundance in Ci. japonica. Nine taxonomic groups were present in significantly different mean abundances between the snail species. Pseudomonads and Enterobacteriaceae were notably more abundant in Ca. decisum, while Proteobacteria and Chitinophagaceae were more abundant in Ci. japonica. Peptidoglycan synthesis, pyruvate fermentation, and aerobic respiration by cytochrome c were the three most abundant microbial pathways represented in the viviparid gut. Fourteen functional pathways differed significantly between Ca. decisum and Ci. japonica, potentially correlated with differences in bacterial community composition and snail life history. Our data fill in data gaps regarding gut microbes in Viviparidae and highlight future research paths examining the prevalence of Firmicutes and unidentified diversity in both snail species.

Black Carp (Mylopharyngodon piceus) has invaded the Mississippi River and is a potential threat to native mollusks. During prior diet research, we discovered that the fluke Aspidogaster conchicola, a mollusk parasite, occurs regularly in the gastrointestinal tract of Black Carp. The fluke remains in fish intestines for extended periods after the fish has consumed its host. Flukes were found in 33% of the wild Black Carp examined, and numbers ranged from 1 to 802, with no pattern evident across seasons of fish capture. Treating the flukes as indicators of prior mollusk consumption, we adjusted the percent occurrence of mollusks from 26.6% to 54.1%, indicating that the previously reported incidences for bivalves (22.8%) and gastropods (16.5%) in the diet of wild Black Carp are likely to be underestimated. Based on percent occurrences in Black Carp, larger fish (>791 mm) had significantly higher fluke occurrence (42.6%) and fish captured from lentic habitats had significantly greater fluke-adjusted mollusk occurrence (87.5%). These diet-occurrence estimates, coupled with the presence of gravid A. conchicola and evidence of their continued viability in Black Carp intestines, indicate that these fish retain evidence of mollusk consumption for extended periods after evacuation of the gastrointestinal tract. Consequently, Black Carp has the potential to disperse this parasite to other mollusks.