Determining when and where the ecological impacts of invasive species will be most detrimental and whether the effects of multiple invaders will be superadditive, or subadditive, is critical for developing global management priorities to protect native species in advance of future invasions. Over the past century, the decline of freshwater bivalves of the family Unionidae has been greatly accelerated by the invasion of Dreissena. The purpose of this study was to evaluate the current infestation rates of unionids by zebra (Dreissena polymorpha) and quagga (D. rostriformis bugensis) mussels in the lower Great Lakes region 25 years after they nearly extirpated native unionids. In 2011–2012, we collected infestation data for over 4000 unionids from 26 species at 198 nearshore sites in lakes Erie, Ontario, and St. Clair, the Detroit River, and inland Michigan lakes and compared those results to studies from the early 1990s. We found that the frequency of unionid infestation by Dreissena recently declined, and the number of dreissenids attached to unionids in the lower Great Lakes has fallen almost ten-fold since the early 1990s. We also found that the rate of infestation depends on the dominant Dreissena species in the lake: zebra mussels infested unionids much more often and in greater numbers. Consequently, the proportion of infested unionids, as well as the number and weight of attached dreissenids were lower in waterbodies dominated by quagga mussels. This is the first large-scale systematic study that revealed how minor differences between two taxonomically and functionally related invaders may have large consequences for native communities they invade.

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Ligumia nasuta (Say, 1817; Eastern Pondmussel) is an imperiled freshwater mussel (Unionidae) in eastern North America. Population declines in the Laurentian Great Lakes resulting from the introduction of dreissenid mussels and habitat destruction in the 20 th Century have greatly reduced and limited its distribution. To properly inform restoration and management efforts for L. nasuta, fine-scale genetic analyses must be performed on the remnant populations. This study used Illumina paired-end shotgun sequencing to identify potential microsatellite loci for L. nasuta, utilizing two samples to develop the Illumina paired-end shotgun library. Forty-eight primer pairs were tested on the remaining 24 samples. Twenty-nine of the 48 microsatellite primer sets screened were successfully amplified using 24 L. nasuta samples collected from the Great Lakes watershed. The estimated fragment size ranged from 167-445 base-pairs (bp) and the number of alleles per locus varied between 5 and 16 (mean = 9.7). Only five of the loci deviated significantly from Hardy-Weinberg expectations after Bonferroni corrections. Highlights  Microsatellite loci identified from two specimens using Illumina sequencing  48 primer pairs tested for amplification success on 24 L. nasuta specimens  29 loci were successfully screened and characterized

1. The eastern pondmussel, Sagittunio nasutus (Bivalvia: Unionida), has declined in abundance and distribution in eastern North America over the last few decades.The declines are predominantly the result of infestation by invasive dreissenid mussels and changes in habitat. The species is now considered imperilled across large portions of its distribution, especially in the Laurentian Great Lakes region.2. The genetic diversity and structure of the remnant populations in the Great Lakes region were assessed using 10 newly developed microsatellite DNA loci for S. nasutus. Understanding the remaining populations can inform future management projects and determine whether the remnant populations have experienced a genetic bottleneck or a founder effect. We hope that this will inform the conservation of other species in regions founded by, and isolated from, a more genetically diverse source population or with disjointed geographical distributions.3. Focusing on the Great Lakes region, samples (n = 428) from 62 collection sites in 28 water bodies were collected. Across the locations sampled for S. nasutus 11 genetic populations were identified, with significant genetic differentiation among them. The genetic structure of the species was assessed, with genetic similarities among populations compared and with geographical routes of colonization and gene flow evaluated. 4. Initial colonization from the Atlantic coast into Lake Erie and Lake Ontario is evident, followed by colonization events into nearby inland rivers and lakes. Analyses found evidence of inbreeding in all but one population and evidence of past genetic bottlenecks or strong founder effects in all but four populations. This study deepens our understanding of the genetic past and present of this imperilled species, providing conservation suggestions for the future management of the species. K E Y W O R D S founder effects, genetic bottlenecks, Ligumia nasuta, management units, microsatellite DNA, phylogeography, population structure, rescue effect, Sagittunio nasutus, Unionidae