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Intensive agricultural land use may have adverse impacts on freshwater mussel assemblages.Before European settlement, the Minnesota River Basin (MRB) was home to dense and diverse assemblages of freshwater mussels. Of the 40 species that occurred there, 43% have been extirpated. the St. Croix Basin (SCB) in Minnesota/Wisconsin, in contrast, has maintained a dense and diverse mussel assemblage, probably owing to the maintenance of a higher‐quality ecosystem.Mussels were sampled in four rivers representing an agricultural land‐use gradient. Timed searches were conducted in each of three rivers of the MRB, and in one river in the SCB in 2015. These were previously sampled by the Minnesota Department of Natural Resources (1998–2008).Mussel abundance and richness were lower in the MRB compared with the SCB. There were negligible changes in mussel density in the SCB, whereas abundances declined 17–83% in the MRB over the same period. Rates of decline were greatest for rivers with higher agricultural land use. Individuals of species that inhabited both basins were larger in tributaries with higher agricultural land use.MRB rivers with increased agricultural land use harboured a greater proportion of individuals displaying opportunistic (low life‐span, age and high fecundity) and periodic life‐history traits (moderate to high growth rates, low to intermediate fecundity, life span and age at maturity), whereas SCB assemblages had a greater percentage of mussels displaying equilibrium life‐history traits (long life‐span, late maturity and low reproductive effort). Also, rivers with higher amounts of suspended sediment contained more individuals classified as disturbance tolerant.This study provides evidence that agricultural land use contributes to the loss of mussel diversity and abundance, especially in mussel species that are long lived or slow to reproduce. These life‐history traits may be correlated with increased sensitivity to disturbances commonly observed in agricultural basins, including more and larger flood events. [ABSTRACT FROM AUTHOR]

Freshwater mussels are a diverse group of bivalves with equally diverse life-history traits. We examined host-attracting behaviors, larval-brooding phenology, and larva–host relationships for Ellipsaria lineolata, Leptodea fragilis, Potamilus alatus, and Truncilla truncata through extensive field and laboratory studies. Our data from laboratory trials and recovery and identification of juvenile mussels from naturally infected fish support the long-held assumption that freshwater drum (Aplodinotus grunniens) probably is the exclusive host for these species and is a suitable host for Potamilus ohiensis and Truncilla donaciformis. These species have subtle but distinctive host-attracting behaviors involving use of a mantle lure to facilitate larval transfer to the host. General lure morphology and movement behavior were similar between E. lineolata and T. truncata and between L. fragilis and P. alatus, species pairs that are closely related, whereas displaying orientation was variable among species. Ellipsaria lineolata, L. fragilis, and P. alatus were long-term larval brooders, but duration and correspondence with displaying activity were dissimilar. Truncilla truncata was a short-term larval brooder in the St Croix River and is among the few documented examples of short-term brooding in the Tribe Lampsilini. The range of morphological and behavioral characters in these mussels coupled with the relatively uncommon character of specializing on a single host species make this group a model for evolutionary, phylogenetic, and ecological studies. [ABSTRACT FROM AUTHOR]

Abstract: Freshwater systems are increasingly threatened by human impacts. To effectively monitor and manage these systems, reference or baseline conditions need to be documented to establish restoration or management goals. Freshwater mussel (Unionoida) communities are in decline in many river systems worldwide. Yet there are few long‐term quantitative studies in relatively “pristine” habitats that could serve as “reference” locations. Here, we present the results of a 20‐year quantitative monitoring study of mussel assemblages in a federally protected river in the United States. We found significant declines in mussel abundance (41%) and species richness (16%) over the 20‐year period, while mussel species evenness and Shannon's diversity index values increased over time (19.3% and 12.1%, respectively). This, combined with an analysis of community composition change trajectories, suggests the declines were disproportionately higher for common species than for rare species. Variation in life‐history traits of macroinvertebrates is often used to assess the degree of degradation of aquatic systems. Using Haag's (2012) classification of mussel life‐history traits, we found the proportion of opportunistic species (fast growth, high reproductive output, short lifespan) decreased by 65.3%, while the proportion of equilibrium species (slow growth, long lifespan, lower investment in reproduction) increased by 54.6% and the proportion of periodic species (moderate‐to‐high growth rates, low‐to‐intermediate fecundity, lifespan and age at maturity) increased by 82.4%. There were modest changes in human population and land use in the catchment during our study period, and a change in the operation of a hydroelectric dam reduced discharge variability. Sediment size changed at several study sites, increasing at some sites while decreasing at others. This study occurred in one of the best‐protected and most pristine rivers in the upper Midwest of the United States, harbouring one of the region's most intact mussel fauna that could be used as a “reference” to set goals for the restoration of mussel fauna in other river systems. Despite the high quality and protection of this river system, we found slow declines in the mussel community but found no evidence that changes in land use, flow regime, water quality, the spread of invasive species or increased sedimentation were responsible for these declines. The slow decline of mussels and lack of specific causes in such a river underscore the imperilment of this fauna, and the need for more aggressive conservation, management and research approaches. [ABSTRACT FROM AUTHOR]

Managing a rare species can be improved with knowledge of its natural history. The sheepnose (Plethobasus cyphyus) is a freshwater mussel recently listed by the US as federally endangered. We used standard methods to studyP. cyphyusbrooding behaviour, host fishes in the laboratory and under natural conditions, and glochidial morphology. We monitored a population ofP. cyphyusin the Chippewa River, WI during spring and summer 2007–2009 and 2011 and found brooding animals between mid-May and early August. Gravid individuals ranged between 5 and 27 yr (mean age ± 1 s.d. = 13 ± 4 yr).Plethobasus cyphyusbrooded glochidia in outer gills, which varied in colour from red, orange, pink, cream, or white. We observed mature glochidia more commonly in individuals with cream or white gills and these glochidia were released in a clear, adhesive, mucus matrix. In laboratory trials we found several minnow and topminnow species (29 spp.) served as productive suitable native hosts. The mean number of juvenile mussels released per cyprinid per day was significantly higher for trials conducted at 22–25°C compared with those at 18–20°C, and 83% of trials conducted at 18–20°C using suitable host species produced no juveniles. Glochidia had a unique outline and shell morphometrics that distinguishedP. cyphyusfrom seven other Chippewa River mussel species that produce similar sized glochidia. Using morphometrics we determined that mimic shiners (Notropis volucellus) were natural hosts forP. cyphyus, round pigtoe (Pleurobema sintoxia), and Wabash pigtoe (Fusconaia flava). Releasing mucus-bound glochidia has evolved in a variety of mussel species and may be more common than is currently realized. Our data show thatP. cyphyusis a cyprinid host specialist, and propagation efforts for this species can be strengthened through improved access to mature glochidia by using females with cream-coloured gills and increased juvenile production through warmer fish holding temperatures. [ABSTRACT FROM PUBLISHER]

Dams have been shown to impact freshwater mussels. We examined how mussels respond to differently sized dams (18 vs. 4 m) on the St. Croix River and its tributary, the Sunrise River. We hypothesized that: mussel density and growth rate would be greater downstream of the smaller dam due to the relatively greater food subsidies and temperature effects of the reservoir above it; and the effects of the small dam would moderate downstream as the localized impacts of the dam were reduced. We quantitatively sampled mussels upstream and downstream of the dams. For a common species, Actinonaias ligamentina, we ascertained growth rates by measuring successive growth rings. The highest mussel richness and diversity were upstream and downstream of the large dam. Higher mussel density was found immediately below the small dam but declined downstream. A. ligamentina downstream of the small dam grew faster and were larger than individuals in other reaches. Food availability and temperature appeared to influence mussel density and growth rate for A. ligamentina downstream of the small dam. This study provides information that may help managers decide whether to remove small dams or to maintain them because of the unique mussel habitats below these structures. [ABSTRACT FROM AUTHOR]

It has been hypothesized that variation in shell shape and sculpture in freshwater mussels from streams is related to hydrologic variability. Using quantitative sampling, we found smooth-shelled species were more common in upstream portions of the St. Croix River, Minnesota and Wisconsin, which has coarser substrate while species with sculptured shells were more common downstream and in finer substrate. For four species distributed throughout the length of the river, individuals tended to have shells that increase in obesity (width/length) and relative height (height/length) downstream. These results support suggestions that mussels with sculptured or obese shells may have an anchoring advantage in areas of high discharge while smooth-shelled or slim forms may reburrow more readily after dislodgement in hydraulically flashy environments. One species that showed increased obesity downstream had juveniles with similar shell shape at all locations suggesting phenotypic plasticity in shell shape. The juvenile and adult valve shape for the offer three species examined varied among locations making it is less clear whether the differences noted are genetically fixed or phenotypically plastic. Understanding the complex interplay between shell shape and sculpture and disturbance regimes in rivers may provide useful information when developing conservation plans for these vulnerable animals. While both shell ornamentation and shape vary along river length, and may be correlated with substrate anchoring and stream flow, other factors such as behavior and physiology also contribute to mussels' success in maintaining their location in a river. [ABSTRACT FROM AUTHOR]