Polymorphism in the aggressive mimicry lure of the parasitic freshwater mussel Lampsilis fasciola.

Unionoid freshwater mussels (Bivalvia: Unionidae) are free-living apart from a brief, obligately parasitic, larval stage that infects fish hosts, and gravid female mussels have evolved a spectrum of strategies to infect fish hosts with their larvae. In many North American species, this involves displaying a mantle lure: a pigmented fleshy extension that acts as an aggressive mimic of a host fish prey, thereby eliciting a feeding response that results in host infection. The mantle lure of Lampsilis fasciola is of particular interest because it is apparently polymorphic, with two distinct primary lure phenotypes. One, described as "darter-like", has "eyespots", a mottled body coloration, prominent marginal extensions, and a distinct "tail". The other, described as "worm-like", lacks those features and has an orange and black coloration. We investigated this phenomenon using genomics, captive rearing, biogeographic, and behavioral analyses. Within-brood lure variation and within-population phylogenomic (ddRAD-seq) analyses of individuals bearing different lures confirmed that this phenomenon is a true polymorphism. The relative abundance of the two morphs appears stable over ecological timeframes: the ratio of the two lure phenotypes in a River Raisin (MI) population in 2017 was consistent with that of museum samples collected at the same site six decades earlier. Within the River Raisin, four main "darter-like" lure motifs visually approximated four co-occurring darter species (Etheostoma blennioides, E. exile, E. microperca, and Percina maculata), and the "worm-like" lure resembled a widespread common leech, Macrobdella decora. Darters and leeches are typical prey of Micropterus dolomieui (smallmouth bass), the primary fish host of L. fasciola. In situ field recordings of the L. fasciola "darter" and "leech" lure display behaviors, and the lure display of co-occurring congener L. cardium, were captured. Despite having putative models in distinct phyla, both L. fasciola lure morphs have largely similar display behaviors that differ significantly from that of sympatric L. cardium individuals. Some minor differences in the behavior between the two L. fasciola morphs were observed, but we found no clear evidence for a behavioral component of the polymorphism given the criteria measured. Discovery of discrete within-brood inheritance of the lure polymorphism implies potential control by a single genetic locus and identifies L. fasciola as a promising study system to identify regulatory genes controlling a key adaptive trait of freshwater mussels.

First freshwater mussel-associated piscicolid leech from East Asia.

Parasites and symbionts of freshwater mussels are poorly understood, although a diverse assemblage of mussel-associated leeches (Glossiphoniidae) was recently described. Here, we report on the discovery of a fish leech (Piscicolidae) in the mantle cavity of the freshwater mussel Cristaria plicata (Unionidae) in the Russian Far East. It is the first member of this leech family being associated with freshwater molluscs. This leech does not match any known genus and species both morphologically and genetically, and is described here as Alexandrobdella makhrovi gen. & sp. nov. It uses mussels as shelter (and probably as a secondary host), while the Amur catfish Silurus asotus (Siluridae) seems to be the primary host. These novel findings indicate that mussel-associated leech assemblage contains at least one piscicolid species. Our fossil-calibrated phylogeny suggests that the crown group of Piscicolidae was originated in the Early Cretaceous. This primarily marine family shares at least five independent colonization events into freshwater environments.

Pathological Changes Associated with Eggs and Larvae of Unionicola sp. (Acari: Unionicolidae) Infecting Strophitus connasaugaensis (Bivalvia: Unionidae) from Alabama Creeks.

We detail gross and histopathological changes associated with infection by the eggs, larvae, and cuticular remnants of Unionicola sp. in the mantle, gill, and visceral mass of 25 Alabama creekmussels, Strophitus connasaugaensis, collected during May 2010 through July 2012 from 2 Alabama streams. A multitude (estimated mean intensity >100) of mite eggs and larvae typically infected mantle, gill, and visceral mass integument. Pathology associated with eggs (prevalence = 0.57) and larvae (prevalence = 0.39) typically consisted of localized distension of the infection site; a host response to these infections was indeterminate. However, larval mites embedded in suprabranchial connective tissues were typically encapsulated (prevalence = 0.89). Mite remnants (prevalence = 0.5) occurred in mantle, gill, visceral mass integument, foot, heart, pericardial gland, intestinal lamina propria, and were typically encapsulated. We speculate that S. connasaugaensis clears some infections but is recolonized by autoinfection or horizontal dispersal of mites in the stream. Noteworthy is that high-intensity infections seemingly do not markedly impact the histological picture of mussel tissues, indicating that mites are relatively benign symbionts that are of little concern to mussels under normal environmental conditions.

The life cycle and molecular phylogeny of a gorgoderid trematode recorded from the mussel Nodularia douglasiae in the Yodo River, Japan.

In 2009, a novel larval trematode of the family Gorgoderidae was found in the gonads of Nodularia douglasiae (Unionidae) from the lower reaches of the Yodo River, Osaka Prefecture, Japan. This is the first collection of trematodes in a unionid mussel in Japan. We investigated the morphology and life cycle of the trematode, and conducted a molecular phylogenetic analysis with other gorgoderid species, both those collected in the Yodo River water system and those reported in the literature. Immature adult worms were obtained from the ureters of the common carp Cyprinus carpio, the first known instance of a gorgoderid from these fish in Japan. Morphological characteristics and molecular data show that it belongs to the subfamily Gorgoderinae (genus Phyllodistomum sensu lato). Regarding the morphology, first intermediate host, and the infection site of adult worms, it resembles Phyllodistomum elongatum Nybelin, 1926 from Europe, but no comparable molecular data exist for Ph. elongatum. Three cytochrome c oxidase subunit I haplotypes were detected in the specimens analyzed, suggesting that the present species is indigenous to the Yodo River water system. The 28S ribosomal DNA data showed that this species is a member of the clade consisting of Ph. cf. symmetorchis, Ph. folium, Pseudophyllodistomum and Xystretrum. However, its phylogenetic position within the clade differs between the maximum likelihood and maximum parsimony trees, and the sister species of the present species remain unclear.

Patterns of species richness among assemblages of Unionicola spp. (Acari: Unionicolidae) inhabiting freshwater mussels (Bivalvia: Unionoida) of North America.

Water mites of Unionicola species are common symbionts of freshwater mussels, living on the gills or mantle and foot of their hosts and using these tissues as sites of oviposition. Although surveys of the mite fauna among North American mussels suggest that these mites represent highly diverse assemblages, there are currently no quantitative data characterizing Unionicola species diversity among their molluscan hosts. The present study addresses patterns of species richness of Unionicola assemblages from freshwater mussels, including the relationship between richness and host specificity among these mites. Results from this study indicate that mite species richness increased significantly with an increase in the number of host individuals sampled. When corrected for sampling effort, there was a positive relationship between host size and mite species richness. Results from this study also reveal a significant relationship between mite species richness and the geographical distribution of host mussels. Overall, the patterns of species richness observed for this study are consistent with those examining the richness of parasitic helminth communities. Because the phylogenetic history of host taxa can have a significant influence on patterns of parasite species richness, studies that correct for the phylogenetic history among host mussels will be required to better understand the role that evolutionary processes have in determining Unionicola species richness. The present study did not indicate a significant relationship between species richness and host specificity and, in not doing so, suggests that the dispersal ability of mites may also play a role in influencing Unionicola species richness. The host recognition behavior and swimming abilities for a larger sample of mites will be required to substantiate this hypothesis.

Depletion of Cryptosporidium parvum oocysts from contaminated sewage by using freshwater benthic pearl clams (Hyriopsis schlegeli).

The freshwater benthic pearl clam, Hyriopsis schlegeli, was experimentally exposed to Cryptosporidium parvum oocysts, and it was verified that the oocysts were eliminated predominantly via the fecal route, retaining their ability to infect cultured cells (HCT-8). The total fecal oocyst elimination rate was more than 90% within 5 days after exposure to the oocysts. H. schlegeli was able to survive in the final settling pond of a sewage plant for long periods, as confirmed by its pearl production. In the light of these findings, the clam was placed in the final settling pond in a trial to test its long-term efficacy in depleting oocysts contaminating the pond water. The number of clams placed was set to ensure a theoretical oocyst removal rate of around 50%, and the turbidity and the density of feed microbes in the overflow trough water of the pond were about 35% and 40 to 60% lower, respectively, than in the control water throughout the year. It was found that the clam feces containing oocysts were sufficiently heavy for them to settle to the bottom of the pond, despite the upward water flow. From these results, we concluded that efficient depletion of oocysts in the sewage water of small or midscale sewage treatment plants can be achieved by appropriate placement of H. schlegeli clams.

[Distribution of Aspidogaster conchicola (Aspidogastrea, Aspidogastridae) in the organism of Colletopterum spp. (Bivalvia, Unionidae) of different age from the Chivyrkuiski Gulf of Lake Baikal].

Distribution of Aspidogaster conchicola Baer, 1827 in the organisms of its hosts Colletopterum spp. from the Chivyrkuiski Gulf of Lake Baikal was investigated. The number of A. conchicola in the organism of Colletopterum spp. was found to decrease along the row pericardial cavity-mantle cavity-gills-kidney. The pericardial cavity of Colletopterum spp. is the most favorable habitat for A. conchicola with 72% of the helminthes parasitizing in it. In the pericardial cavity the largest number (61%) of A. conchicola was found in its posterior part.