Resolving the 150 year debate over the ecological history of the common periwinkle snail, Littorina littorea, in northeast North America

Littorina littorea (common periwinkle snail) is highly abundant in both Europe and North America. A known native of Europe, its presence in North America has been the subject of a one-hundred year debate. Prior published work attempting to resolve its cryptogenic (=uncertain origin) status with historical, archaeological, ecological and/or genetic data were not successful. I therefore included novel parasite and molecular evidence to definitively resolve L. littorea\u27s North American cryptogenic status. First, I explored trematode species richness patterns in European versus North American L. littorea and two co-occurring native congeners, L. saxatilis and L. obtusata. Through extensive field and literature surveys, I found only L. littorea to possess significantly fewer trematode species in North America, while all Littorina sp. North American trematodes were a nested subset of Europe. Overall, these results suggest a recent invasion to North America for L. littorea and an older, natural expansion to North America for L. saxatilis and L. obtusata.. Second, I explored genetic founder effect signatures in North American L. littorea. I sequenced a ∼1200 base-pair region of mitochondrial DNA in nearly 400 Europe and North American snails. My results demonstrate a significant reduction in overall genetic diversity in North America versus Europe, nested and common haplotype frequencies in North America, and a divergence estimate of no greater than 450 years ago. Third, I explored genetic founder effect signatures in L. littorea\u27s most common trematode parasite, Cryptocotyle lingua. I sequenced a 1043 base-pair region of mitochondrial DNA and found Cr. lingua to show several signatures of a recent introduction to North America, including a significant reduction in haplotype diversity in North America, nested and common haplotype frequencies in North America, and a range of divergence estimates between 240-480 years ago. Altogether, these three corroborative pieces of evidence suggest a recent, likely human-mediated introduction of North American L. littorea from Europe. This work represents the first time ecological studies of parasites and genetics have been used in concert to resolve the cryptogenic status of an important marine species. The successful application of these techniques can therefore be useful in cryptogenic investigations worldwide

Systematic assessment of the Panopeidae and broader Eubrachyura (Decapoda: Brachyura) using mitochondrial genomics

Abstract This study provides a broad phylogenetic analysis for the Eubrachyura, with the inclusion of three new Panopeidae mitochondrial genomes: Eurypanopeus depressus (flatback mud crab) (15,854bp), Panopeus herbstii (Atlantic mud crab) (15,812bp) and Rhithropanopeus harrisii (Harris, or ‘white-fingered’ mud crab) (15,892bp). These new mitogenomes were analyzed alongside all available brachyuran mitochondrial genomes (n = 113), comprising 80 genera from 29 families, to provide an updated phylogenetic analysis of the infra-order Brachyura (“true crabs”). Our analyses support the subsection Potamoida within the Eubrachyura as the sister group to Thoracotremata. The family Panopeidae aligns with the family Xanthidae to form the Xanthoidea branch, which is supported by current morphological and genetic taxonomy. A unique gene arrangement termed ‘XanGO’ was identified for the panopeids and varies relative to other members of the subsection Heterotremata (within the Eubrachyura) via a transposition of the trnV gene. This gene arrangement is novel and is shared between several Xanthoidea species, including Etisus anaglyptus (hairy spooner crab), Atergatis floridus (brown egg crab), and Atergatis integerrimus (red egg crab), suggesting that it is a conserved gene arrangement within the Xanthoidea superfamily. Our study further reveals a need for taxonomic revision of some brachyuran groups, particularly the Sesarmidae. The inclusion of panopeid mitogenomes into the greater brachyuran phylogeny increases our understanding of crab evolution and higher level Eubrachyuran systematics

An evolutionary perspective on marine invasions

Species distributions are rapidly changing as human globalization increasingly moves organisms to novel environments. In marine systems, species introductions are the result of a number of anthropogenic mechanisms, notably shipping, aquaculture/mariculture, the pet and bait trades, and the creation of canals. Marine invasions are a global threat to human and non�human populations alike and are often listed as one of the top conservation concerns worldwide, having ecological, evolutionary, and social ramifications. Evolutionary investigations of marine invasions can provide crucial insight into an introduced species’ potential impacts in its new range, including: physiological adaptation and behavioral changes to exploit new environments; changes in resident populations, community interactions, and ecosystems; and severe reductions in genetic diversity that may limit evolutionary potential in the introduced range. This special issue focuses on current research advances in the evolutionary biology of marine invasions and can be broadly classified into a few major avenues of research: the evolutionary history of invasive populations, post�invasion reproductive changes, and the role of evolution in parasite introductions. Together, they demonstrate the value of investigating marine invasions from an evolutionary perspective, with benefits to both fundamental and applied evolutionary biology at local and broad scales

Are genes faster than crabs? Mitochondrial introgression exceeds larval dispersal during population expansion of the invasive crab Carcinus maenas

Biological invasions offer unique opportunities to investigate evolutionary dynamics at the peripheries of expanding populations. Here, we examine genetic patterns associated with admixture between two distinct invasive lineages of the European green crab, Carcinus maenas L., independently introduced to the northwest Atlantic. Previous investigations based on mitochondrial DNA sequences demonstrated that larval dispersal driven by advective currents could explain observed southward displacement of an admixture zone between the two invasions. Comparison of published mitochondrial results with new nuclear data from nine microsatellite loci, however, reveals striking discordance in their introgression patterns. Specifically, introgression of mitochondrial genomes relative to nuclear background suggests that demographic processes such as sex-biased reproductive dynamics and population size imbalances—and not solely larval dispersal—play an important role in driving the evolution of the genetic cline. In particular, the unpredicted introgression of mitochondrial alleles against the direction of mean larval dispersal in the region is consistent with recent models invoking similar demographic processes to explain movements of genes into invading populations. These observations have important implications for understanding historical shifts in C. maenas range limits, and more generally for inferences of larval dispersal based on genetic data

Founder effects and species introductions: A host versus parasite perspective

Species colonizations (both natural and anthropogenic) can be associated with genetic founder effects, where founding populations demonstrate significant genetic bottlenecks compared to native populations. Yet, many successfully established free�living species exhibit little reduction in genetic diversity—possibly due to multiple founding events and/or high propagule pressure during introductions. Less clear, however, is whether parasites may show differential signatures to their free�living hosts. Parasites with indirect life cycles may particularly be more prone to founder effects (i.e., more genetically depauperate) because of inherently smaller founding populations and complex life cycles. We investigated this question in native (east coast) and introduced (west coast) North American populations of a host snail Tritia obsoleta (formerly Ilyanassa obsoleta, the eastern mudsnail) and four trematode parasite species that obligately infect it. We examined genetic diversity, gene flow, and population structure using two molecular markers (mitochondrial and nuclear) for the host and the parasites. In the host snail, we found little to no evidence of genetic founder effects, while the trematode parasites showed significantly lower genetic diversity in the introduced versus native ranges. Moreover, the parasite's final host influenced infection prevalence and genetic diversity: Trematode species that utilized fish as final hosts demonstrated lower parasite diversity and heightened founder effects in the introduced range than those trematodes using birds as final hosts. In addition, inter�regional gene flow was strongest for comparisons that included the putative historical source region (mid�Atlantic populations of the US east coast). Overall, our results broaden understanding of the role that colonization events (including recent anthropogenic introductions) have on genetic diversity in non�native organisms by also evaluating less studied groups like parasites