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[Excerpt] Understanding the life-history characteristics of endangered species is crucial to their conservation, management, and predicting their responses to environmental change (Stark et al., 2004). Host specificity is central to the evolutionary diversification and conservation of the Unionida (Barnhart et al., 2008; Modesto et al., 2018). In the North American mussels of the Ambleminae subfamily, specificity toward a restricted host fish range that shares the same microhabitat as the mussel has resulted in remarkable morphologies and behaviors that dramatically increase the likelihood of attachment and successful transmission of larvae (glochidia). This includes modification of mantle flaps into lures and packaging of glochidia into conglutinates that resemble insect larvae upon which the host fishes preferentially feed (Barnhart et al., 2008). [...]

Adaptation to temperature regimes and host presence may enhance fitness in parasites. In an experimental study, we evaluated the timing of glochidia release by Unio crassus subjected to three spring water temperature regimes in the presence and absence of the host fish Cottus gobio. The timing of glochidia release was delayed at (i) constantly low temperatures (<10°C), in contrast to earlier and pronounced releases at (ii) natural temperature increases that level off at intermediate temperatures (10–15°C), and (iii) higher-than-normal temperatures (10–20°C). Mussels from treatment (i) that had not released glochidia during the experiment did so soon after being moved to the temperature in (ii), indicating a temperature threshold for glochidia release. Neither host fish presence nor the combined effect of temperature and host fish presence significantly affected the timing of glochidia release. The treatment with natural spring water temperatures indicated possible fitness benefits for U. crassus through combined effects of high intensities of glochidia releases and high survival of released glochidia. The furthered understanding of climate change effects on mussel and host phenology in seasonal environments, potentially inducing temporal mismatches of glochidia release to host availability, is key to mussel conservation.

Understanding the life-history characteristics of endangered species is crucial to their conservation, management, and predicting their responses to environmental change (Stark et al., 2004). Host specificity is central to the evolutionary diversification and conservation of the Unionida (Barnhart et al., 2008; Modesto et al., 2018). In the North American mussels of the Ambleminae subfamily, specificity toward a restricted host fish range that shares the same microhabitat as the mussel has resulted in remarkable morphologies and behaviors that dramatically increase the likelihood of attachment and successful transmission of larvae (glochidia).

Freshwater mussels in the order Unionida are highly adapted to parasitize fish for the primary purpose of dispersal. The parasitic larval stage affixes itself to the gills or fins of the host where it becomes encysted in the tissue, eventually excysting to develop into a free-living adult. Research on the parasitic interactions between unionids and their host fishes has garnered attention recently due to the increase in worldwide preservation efforts surrounding this highly endangered and ecologically significant order. With the exception of heavy infestation events, these mussels cause minor effects to their hosts, typically only observable effect in combination with other stressors. Moreover, the range of effect intensities on the host varies greatly with the species involved in the interaction, an effect that may arise from different evolutionary strategies between long- and short-infesting mussels; a distinction not typically made in conservation practices. Lower growth and reduced osmotic potential in infested hosts are commonly observed and correlated with infestation load. These effects are typically also associated with increases in metabolic rate and behaviour indicative of stress. Host fish seem to compensate for this through a combination of rapid wound healing in the parasitized areas and higher ventilation rates. The findings are heavily biased towards Margaritifera margaritifera, a unique mussel not well suited for cross-species generalizations. Furthermore, the small body of molecular and genetic studies should be expanded as many conclusions are drawn from studies on the ultimate effects of glochidiosis rather than proximate studies on the underlying mechanisms.

In freshwater temperate environments, waterbodies warmed by power plants can serve as a stepping stone in the spread of tropical and subtropical species. This offers a unique opportunity to acquire an insight into the life history of organisms undergoing range expansions beyond their native habitat. Here, we studied the patterns of offspring production and growth in the Asian mussel Sinanodonta woodiana inhabiting the thermal plume of the Odra River in Central Europe. Compared to the S. woodiana males, females had more convex shells and had a greater tendency to continue post-maturation growth and thus follow an indeterminate growth pattern. Gravid females were observed all-year round, and individuals with either large or more convex shells brooded more offspring. The proportion of incubating females, brood size and glochidia size were linked to gonadal activity and changed seasonally. Females with a higher amount of nutritive substances in the ovaries produced more and larger offspring. The smallest and the largest offspring occurred in the summer and in the winter, respectively. Sinanodonta woodiana incubated different offspring generations simultaneously, constantly releasing them into the environment. Using a life history evolution perspective, we discuss our results in view of allocation trade-offs between growth and offspring production.

Freshwater unionid mussels, which have parasitic glochidia larvae, often occur in multispecies mussel beds where they likely compete for host fishes. The temporal dynamics of glochidia release was examined at 2-h intervals over ten, 24-h periods in the Sydenham River, Ontario, Canada from late August through September 2020 using a rosette water sampler. A total of 6104 glochidia from 17 species were identified morphometrically and these were dominated numerically by four species ( Eurynia dilatata ; Ortmanniana ligamentina ; Epioblasma triquetra ; and Cyclonaias tuberculata ). The total glochidia abundance was greatest during the night, 20:00 h (± 1 h) local solar time (LST) in part because of the dominant species. Conversely, the abundance of glochidia from individual species was not uniform temporally among days ( p  < 0.001) nor time-of-day ( p  < 0.001) but appeared to coincide with the reported diel activity periods (i.e., diurnal, crepuscular, and nocturnal) of the host fishes for 10 species. The abundance of glochidia peaked at different times in unionid species that shared hosts (five cases), suggesting an attempt to partition host fishes temporally. Whereas temporal partitioning of hosts could ultimately provide a mechanism by which multiple species of unionids coexist in the same habitat, this is remarkable because glochidia release occurs over longer temporal scales (i.e., weeks), and there is limited gill-space on host fish, especially small individuals that may not have acquired immunity. Recognizing the temporal dynamics of glochidial abundance in multispecies mussel beds will inform conservation approaches that address mussel-host fish relationships in this imperiled taxon.

The reproductive activity of females of the Chinese pond mussel (Sinanodonta woodiana) was investigated in a European population inhabiting cooling water. The study used histological and stereological methods to determine gonad structure, changes in reproductive follicles during gametogenesis, oocyte development, glochidia structure, and brooding periods. Water in the channel of the “Dolna Odra” power plant did not freeze during the winter, and its mean annual temperature was 18.4°C. The population sex ratio was female biased (χ2 = 25.70, df = 1, P < 0.0001). Ovaries in mussels were formed by reproductive follicles. Previtellogenic and vitellogenic oocytes were attached to the follicle wall via the cytoplasmic stalk, and mature ovulated oocytes were present in the follicle lumen. Individuals with mature oocytes in gonads were present over the whole two-year study period, which indicates the continuous activity of gonads. Hooked glochidia had a larval thread. Females incubated the offspring in gill marsupia of outer demibranchs and were characterized by multiple tachytictic brooding periods. All gravid individuals presented mature and spent stages of gonadal development. The study provides the first direct information about the reproductive dynamics of this invasive species outside its original Asiatic range.

Effect of freshwater mussels’ (Unionoida) glochidia on the growth of fish host has remained poorly studied. We compared the specific growth rate of the juvenile, PIT-marked brown trout (Salmo trutta) between uninfected controls to those experimentally infected (average initial intensity of infection 8000 fish−1) with Margaritifera margaritifera glochidia, kept in high and low feeding. Growth and mortality of fish were monitored for 10 months. Our hypothesis was that glochidiosis would impair the growth of fish. According to our hypothesis, infected fish gained statistically significantly less weight than the control fish throughout the experiment. A proportional increase in weight of control individuals was 11% higher than that of the infected fish. However, neither the feeding regime (high, low) nor the period (September–November, November–March, March–May), had a significant effect on the growth difference between control and infected fish. As the effect of infection on the growth of fish was subtle and no effect on host mortality was detected either, this may turn public opinion favorable for M. margaritifera conservation even if the salmonid host population is important for commercial or recreational fishing.

Freshwater mussels (Order Unionoida) can provide crucial ecological functions and services. However, many of them are endangered worldwide due to local extirpation and reproductive failure, which emphasizes the importance of mussel life cycle monitoring, including reproduction. Buldowskia iwakawai (Suzuki, 1939) is one such species native to the Ishikari floodplain oxbow lakes where its reproductive failure was recently reported. In this study, a species-specific environmental DNA (eDNA) assay was developed to monitor B. iwakawai abundance and winter reproduction in the laboratory and natural environment simultaneously even at times when natural lakes are covered by ice. A positive CPUE-eDNA correlation was observed across all the 14 lakes sampled prior to ice cover formation. The eDNA concentration significantly differed between gravid and non-gravid mussels in relation to their metabolic-rate differences in laboratory experiments, but the eDNA concentration increase was not detected in the intensively monitored lakes during the glochidia release period. PCR inhibition in natural water has been suggested, and reducing its effects combined with appropriate seasonal contexts has the potential to improve the applicability of this method. This research sheds new light on the applicability of the eDNA assay for monitoring mussel reproductive activities throughout their life cycle including the winter season.

Gill parasites can negatively affect hosts by altering behavior or causing adverse effects to host physiology. Most unionid mussel larvae (glochidia) are obligate parasites requiring fish hosts, but much of the literature concerning how these parasites affect their hosts has been limited to only a few study species of salmonids and mussels. Here, we test the effects of natural glochidia infection levels on resting metabolic rate and hypoxia tolerance of bluegill Lepomis macrochirus and largemouth bass Micropterus salmoides using glochidia of the southern fatmucket mussel Lampsilis straminea, a warm-water unionid. We quantified oxygen uptake, hypoxia tolerance, and ability to regulate metabolic rate via respiration rate, critical dissolved oxygen values (DOcrit), and a regulation index (RI), respectively, to compare the effects of glochidia infection between infected and uninfected fish over 11 wk after host inoculation. Hosts were infected with glochidia at levels similar to those seen in wild, naturally occurring infections. At these levels, we observed no effects of glochidia infection on metabolic rate, DOcrit, or RI of infected versus control fish of either species over our 11-wk experiment. Glochidia infection on fish gills at levels expected under field conditions may not always induce an organismal-level (i.e., individual organism) response or cause host respiratory stress. Preventing respiratory stress from infection would be beneficial for both host and parasite, given that glochidia survival and dispersal depend on host survival.