Using external morphology as a proxy for stomach size in Hemigrapsus sanguineus.

Stomach morphology can provide insights into an organism's diet. Gut size or length is typically inversely related to diet quality in most taxa, and has been used to assess diet quality in a variety of systems. However, it requires animal sacrifice and time-consuming dissections. Measures of external morphology associated with diet may be a simpler, more cost-effective solution. At the species level, external measures of the progastric region of the carapace in brachyuran crabs can predict stomach size and diet quality, with some suggestion that this approach may also work to examine individual diet preferences and specialization at the individual level; if so, the size of the progastric region could be used to predict trends in diet quality and consumption for individuals, which would streamline diet studies in crabs. Here, we tested whether external progastric region size predicts internal stomach size across latitude and time of year for individuals of the invasive Asian shore crab Hemigrapsus sanguineus. We found that the width of the progastric region increased at a faster rate with body size than stomach width. In addition, the width of the progastric region followed different trends across sites and over time compared to stomach width. Our results therefore suggest that the progastric region may not be used as a proxy for stomach size variation across individuals.

Shift from income breeding to capital breeding with latitude in the invasive Asian shore crab Hemigrapsus sanguineus.

Organisms vary in the timing of energy acquisition and use for reproduction. Thus, breeding strategies exist on a continuum, from capital breeding to income breeding. Capital breeders acquire and store energy for breeding before the start of the reproductive season, while income breeders finance reproduction using energy acquired during the reproductive season. Latitude and its associated environmental drivers are expected to heavily influence breeding strategy, potentially leading to latitudinal variation in breeding strategies within a single species. We examined the breeding strategy of the Asian shore crab Hemigrapsus sanguineus at five sites spanning nearly 10° of latitude across its invaded United States range. We hypothesized that the primary breeding strategy of this species would shift from income breeding to capital breeding as latitude increases. We found that though this species' breeding strategy is dominated by capital breeding throughout much of the range, income breeding increases in importance at lower latitudes. This latitudinal pattern is likely heavily influenced by the duration of the foraging and breeding seasons, which also vary with latitude. We also found that reproductive characteristics at the northern and southern edges of the invaded range were consistent with continued range expansion. We suggest that the reproductive flexibility of the Asian shore crab is a key facilitator of its continued invasion success. Our results highlight the influence of latitude on the breeding strategy of a species and emphasize the need for further research regarding the ecological importance and implications of flexibility in breeding strategies within species.

Microbial communities are indicators of parasite infection status.

Growing evidence suggests that microbiomes have been shaping the evolutionary pathways of macroorganisms for millennia and that these tiny symbionts can influence, and possibly even control, species interactions like host-parasite relationships. Yet, while studies have investigated host-parasites and microbiomes separately, little has been done to understand all three groups synergistically. Here, we collected infected and uninfected Eurypanopeus depressus crab hosts from a coastal North Carolina oyster reef three times over 4 months. Infected crabs demonstrated an external stage of the rhizocephalan parasite, Loxothylacus panopaei. Community analyses revealed that microbial richness and diversity were significantly different among tissue types (uninfected crab, infected crab, parasite externae and parasite larvae) and over time (summer and fall). Specifically, the microbial communities from parasite externae and larvae had similar microbiomes that were consistent through time. Infected crabs demonstrated microbial communities spanning those of their host and parasite, while uninfected crabs showed more distinctive communities with greater variability over time. Microbial communities were also found to be indicators of early-stage infections. Resolving the microbial community composition of a host and its parasite is an important step in understanding the microbiome's role in the host-parasite relationship and determining how this tripartite relationship impacts coevolutionary processes.

Past energy allocation overwhelms current energy stresses in determining energy allocation trade-offs.

Regeneration of lost appendages is a gradual process in many species, spreading energetic costs of regeneration through time. Energy allocated to the regeneration of lost appendages cannot be used for other purposes and, therefore, commonly elicits energetic trade-offs in biological processes. We used limb loss in the Asian shore crab Hemigrapsus sanguineus to compare the strength of energetic trade-offs resulting from historic limb losses that have been partially regenerated versus current injuries that have not yet been repaired. Consistent with previous studies, we show that limb loss and regeneration results in trade-offs that reduce reproduction, energy storage, and growth. As may be expected, we show that trade-offs in these metrics from historic limb losses far outweigh trade-offs from current limb losses, and correlate directly with the degree of historic limb loss that has been regenerated. As regenerating limbs get closer to their normal size, these historical injuries get harder to detect, despite the continued allocation of additional resources to limb development. Our results demonstrate the importance of and a method for identifying historic appendage losses and of quantifying the amount of regeneration that has already occurred, as opposed to assessing only current injury, to accurately assess the strength of energetic trade-offs in animals recovering from nonlethal injury.

Parasites indicate trophic complexity and faunal succession in restored oyster reefs over a 22-year period.

Foundation species like the eastern oyster (Crassostrea virginica) create complex habitats for organisms across multiple trophic levels. Historic declines in oyster abundance have prompted decades of restoration efforts. However, it remains unclear how long it takes for restored reefs to resemble the trophic complexity of natural reefs. We used a space-for-time approach to examine community succession of restored reefs ranging in age from 3 to 22 years old in coastal North Carolina, surveying both free-living taxa and parasite communities and comparing them to natural reefs that are decades old. Trophically transmitted parasites can serve as valuable biodiversity surrogates, sometimes providing greater information about a system or question than their free-living counterparts. We found that the diversity of free-living taxa was highly variable and did not differ among new (<10 years), old (20 years), and natural reefs. Conversely, parasite diversity increased with elapsed time after restoration, and parasite communities in older restored reefs resembled those found in natural reefs. Our study also revealed that oyster toadfish (Opsanus tau) act as a key host species capable of facilitating parasite transmission and trophic ascent in oyster reef food webs. Overall, our results suggest that trophic complexity in restored oyster reefs requires at least 8 years to resemble that found in natural reefs. This work adds to a growing body of evidence demonstrating how parasites can serve as biodiversity surrogates, proxies for the presence of additional taxa that are often difficult or impractical to sample. Given the multiplicity of links formed with their hosts, parasites offer a powerful tool for quantifying diversity and trophic complexity in environmental monitoring studies.

Overview of renicolid digeneans (Digenea, Renicolidae) from marine gulls of northern Holarctic with remarks on their species statuses, phylogeny and phylogeography.

Renicolid digeneans parasitize aquatic birds. Their intramolluscan stages develop in marine and brackish-water gastropods, while metacercariae develop in molluscs and fishes. The systematics of renicolids is poorly developed, their life cycles are mostly unknown, and the statuses of many species require revision. Here, we establish based on integrated morphological and molecular data that adult renicolids from gulls Larus argentatus and Larus schistisagus and sporocysts and cercariae of Cercaria parvicaudata from marine snails Littorina spp. are life-cycle stages of the same species. We name it Renicola parvicaudatus and synonymized with it Renicola roscovitus. An analysis of the cox1 gene of R. parvicaudatus from Europe, North America and North Asia demonstrates a low genetic divergence, suggesting that this species has formed quite recently (perhaps during last glacial maximum) and that interregional gene flow is high. In Littorina saxatilis and L. obtusata from the Barents Sea, molecular analysis has revealed intramolluscan stages of Cercaria littorinae saxatilis VIII, a cryptic species relative to R. parvicaudatus. In the molecular trees, Renicola keimahuri from L. schistisagus belongs to another clade than R. parvicaudatus. We show that the species of this clade have cercariae of Rhodometopa group and outline morphological and behavioural transformations leading from xiphidiocercariae to these larvae. Molecular analysis has revealed 3 main phylogenetic branches of renicolids, differing in structure of adults, type of cercariae and host range. Our results elucidate the patterns of host colonization and geographical expansion of renicolids and pave the way to the solution of some long-standing problems of their classification.

Latitudinal and temporal variation in injury and its impacts in the invasive Asian shore crab Hemigrapsus sanguineus.

Nonlethal injury is a pervasive stress on individual animals that can affect large portions of a population at any given time. Yet most studies examine snapshots of injury at a single place and time, making the implicit assumption that the impacts of nonlethal injury are constant. We sampled Asian shore crabs Hemigrapsus sanguineus throughout their invasive North American range and from the spring through fall of 2020. We then documented the prevalence of limb loss over this space and time. We further examined the impacts of limb loss and limb regeneration on food consumption, growth, reproduction, and energy storage. We show that injury differed substantially across sites and was most common towards the southern part of their invaded range on the East Coast of North America. Injury also varied idiosyncratically across sites and through time. It also had strong impacts on individuals via reduced growth and reproduction, despite increased food consumption in injured crabs. Given the high prevalence of nonlethal injury in this species, these negative impacts of injury on individual animals likely scale up to influence population level processes (e.g., population growth), and may be one factor acting against the widespread success of this invader.

Native tube-building polychaete prefers to anchor non-native alga over other macrophytes.

Novel facultative mutualisms that develop between native and non-native ecosystem engineers can lead to the retention of the non-native partner. In some cases, behavior plays an additional, but less understood, role in the development and persistence of mutualisms. In soft-sediment marine habitats along the western Atlantic, the native decorator worm Diopatra cuprea anchors the non-native red alga Gracilaria vermiculophylla to its tube cap in a mutualism. To understand whether the worm's usage of G. vermiculophylla could represent a preference, we first surveyed the species composition of macrophytes affixed to worm tube caps at three sites in coastal Virginia, USA using transect and quadrat sampling. These unmanipulated field surveys supported previous work revealing variable, but often high frequencies (31-98%) of D. cuprea decoration with G. vermiculophylla. We next used field manipulations and controlled laboratory experiments to test the consistency of individual D. cuprea decoration with G. vermiculophylla versus three common macrophytes (Ulva sp., Agardhiella sp., and Spartina alterniflora) found in our field surveys. Twenty-four hours after removing the worm's tube cap in the field, D. cuprea decoration was dominated by both G. vermiculophylla (39.6%) and S. alterniflora (25.9%). When provided a choice of macrophytes in the laboratory, individual D. cuprea consistently decorated with G. vermiculophylla (58.7%) over the other macrophytes, showing a preference for the non-native macrophyte. Our study suggests that preference can drive strong and steadfast interactions between native and non-native organisms, facilitating the latter's persistence and spread, change available habitat, and alter community interactions.

'Candidatus Mellornella promiscua' n. gen. n. sp. (Alphaproteobacteria: Rickettsiales: Anaplasmataceae): An intracytoplasmic, hepatopancreatic, pathogen of the flatback mud crab, Eurypanopeus depressus.

Bacterial pathogens are a long-standing threat to the longevity and survival of crustacean hosts. Their presence and continuing emergence require close monitoring to understand their impact on fished, cultured, and wild crustacean populations. We describe a new bacterial pathogen belonging to the Anaplasmataceae family (Alphaproteobacteria: Rickettsiales), providing pathological, ultrastructural, phylogenetic, and genomic evidence to determine a candidate genus and species ('Candidatus Mellornella promiscua'). This bacterium was found to infect the mud crab, Eurypanopeus depressus, on the North Carolina coastline (USA) at a prevalence of 10.8%. 'Candidatus Mellornella promiscua' was often observed in co-infection with the rhizocephalan barnacle, Loxothylacus panopaei. The bacterium was only found in the hepatopancreas of the mud crab host, causing cytoplasmic hypertrophy, tubule necrosis, large plaques within the cytoplasm of the host cell, and an abundance of sex-pili. The circular genome of the bacterium is 1,013,119 bp and encodes 939 genes in total. Phylogenetically, the new bacterium branches within the Anaplasmataceae. The genome is dissimilar from other described bacteria, with 16S gene similarity observed at a maximum of 85.3% to a Wolbachia endosymbiont. We explore this novel bacterial pathogen using genomic, phylogenetic, ultrastructural, and pathological methods, discussing these results in light of current bacterial taxonomy, similarity to other bacterial pathogens, and the potential impact upon the surrounding disease ecology of the host and benthic ecosystem.

Invasion of the body snatchers: the role of parasite introduction in host distribution and response to salinity in invaded estuaries.

In dynamic systems, organisms are faced with variable selective forces that may impose trade-offs. In estuaries, salinity is a strong driver of organismal diversity, while parasites shape species distributions and demography. We tested for trade-offs between low-salinity stress and parasitism in an invasive castrating parasite and its mud crab host along salinity gradients of two North Carolina rivers. We performed field surveys every six to eight weeks over 3 years to determine factors influencing parasite prevalence, host abundance, and associated taxa diversity. We also looked for signatures of low-salinity stress in the host by examining its response (time-to-right and gene expression) to salinity. We found salinity and temperature significantly affected parasite prevalence, with low-salinity sites (less than 10 practical salinity units (PSU)) lacking infection, and populations in moderate salinities at warmer temperatures reaching prevalence as high as 60%. Host abundance was negatively associated with parasite prevalence. Host gene expression was plastic to acclimation salinity, but several osmoregulatory and immune-related genes demonstrated source-dependent salinity response. We identified a genetic marker that was strongly associated with salinity against a backdrop of no neutral genetic structure, suggesting possible selection on standing variation. Our study illuminates how selective trade-offs in naturally dynamic systems may shape host evolutionary ecology.

Invasion history shapes host transcriptomic response to a body-snatching parasite.

By shuffling biogeographical distributions, biological invasions can both disrupt long-standing associations between hosts and parasites and establish new ones. This creates natural experiments with which to study the ecology and evolution of host-parasite interactions. In estuaries of the Gulf of Mexico, the white-fingered mud crab (Rhithropanopeus harrisii) is infected by a native parasitic barnacle, Loxothylacus panopaei (Rhizocephala), which manipulates host physiology and behaviour. In the 1960s, L. panopaei was introduced to the Chesapeake Bay and has since expanded along the southeastern Atlantic coast, while host populations in the northeast have so far been spared. We use this system to test the host's transcriptomic response to parasitic infection and investigate how this response varies with the parasite's invasion history, comparing populations representing (i) long-term sympatry between host and parasite, (ii) new associations where the parasite has invaded during the last 60 years and (iii) naïve hosts without prior exposure. A comparison of parasitized and control crabs revealed a core response, with widespread downregulation of transcripts involved in immunity and moulting. The transcriptional response differed between hosts from the parasite's native range and where it is absent, consistent with previous observations of increased susceptibility in populations lacking exposure to the parasite. Crabs from the parasite's introduced range, where prevalence is highest, displayed the most dissimilar response, possibly reflecting immune priming. These results provide molecular evidence for parasitic manipulation of host phenotype and the role of gene regulation in mediating host-parasite interactions.

Trends in illegal wildlife trade: Analyzing personal baggage seizure data in the Pacific Northwest.

The illegal import of wildlife and wildlife products is a growing concern, and the U.S. is one of the world's leading countries in the consumption and transit of illegal wildlife and their derivatives. Yet, few U.S. studies have analyzed the illegal wildlife trade (IWT) on a national or local scale. Moreover, to our knowledge, no studies have specifically examined the trends associated with IWT moving through personal baggage. This work aimed to better understand the magnitude of illegal wildlife importation into U.S. ports of entry by determining trends associated with illegal wildlife products from personal baggage seizures, using the Pacific Northwest (PNW) as a specific case study. To identify the most influential factors determining the numbers and types of personal baggage seizures into PNW, we analyzed 1,731 records between 1999 and 2016 from the Fish and Wildlife Service's (FWS) Law Enforcement Management Information System (LEMIS) database. We found five significant contributors: taxonomic classification of wildlife, categorical import date, wildlife product, source region, and the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) status. While wildlife seizures across taxonomic categories have generally decreased in the PNW since 2008, other findings provide a reason for concern. More specifically, mammals were identified as the largest animal group of seized wildlife, and temporal trends indicate increases in seizures for this and several other taxonomic groups. Many of the seizures originated from overseas, with East Asia serving as the largest source. Our PNW case study can be a model for how large-scale geographical seizure data can be used to inform about the major factors that have historically and presently contribute to IWT, with conservation implications globally.

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.

Recent introductions reveal differential susceptibility to parasitism across an evolutionary mosaic.

Parasitism can represent a potent agent of selection, and introduced parasites have the potential to substantially alter their new hosts' ecology and evolution. While significant impacts have been reported for parasites that switch to new host species, the effects of macroparasite introduction into naïve populations of host species with which they have evolved remain poorly understood. Here, we investigate how the estuarine white-fingered mud crab (Rhithropanopeus harrisii) has adapted to parasitism by an introduced rhizocephalan parasite (Loxothylacus panopaei) that castrates its host. While the host crab is native to much of the East and Gulf Coasts of North America, its parasite is native only to the southern end of this range. Fifty years ago, the parasite invaded the mid-Atlantic, gradually expanding through previously naïve host populations. Thus, different populations of the same host species have experienced different degrees of historical interaction (and thus potential evolutionary response time) with the parasite: long term, short term, and naïve. In nine estuaries across this range, we examined whether and how parasite prevalence and host susceptibility to parasitism differs depending on the length of the host's history with the parasite. In field surveys, we found that the parasite was significantly more prevalent in its introduced range (i.e., short-term interaction) than in its native range (long-term interaction), a result that was also supported by a meta-analysis of prevalence data covering the 50 years since its introduction. In controlled laboratory experiments, host susceptibility to parasitism was significantly higher in naïve hosts than in hosts from the parasite's native range, suggesting that host resistance to parasitism is under selection. These results suggest that differences in host-parasite historical interaction can alter the consequences of parasite introductions in host populations. As anthropogenically driven range shifts continue, disruptions of host-parasite evolutionary relationships may become an increasingly important driver of ecological and evolutionary change.

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.

Strong genetic structure in a widespread estuarine crab: A test of potential versus realized dispersal.

Aim: Genetic structure has proven difficult to predict for marine and estuarine species with multi-day pelagic larval durations, since many disperse far less than expected based on passive transport models. In such cases, the gap between potential and realized dispersal may result from larval behaviours that evolved to facilitate retention and settlement in favourable environments. Behaviour is predicted to play a particularly key role in structuring truly estuarine species, which often moderate their behaviour to remain within their natal estuaries. In such systems, this restricted dispersal may lead to high divergence, local adaptation and eventual speciation across their range. Here, we test whether a geographically widespread estuarine crab, known to have behaviour promoting larval retention, exhibits high population structure despite a 2- to 4-week larval duration. Location: Atlantic and Gulf Coasts of North America. Taxon: White-fingered mud crab, Rhithropanopeus harrisii. Methods: Population genomic analyses across nine estuaries from New Hampshire to Louisiana using 12,638 transcriptome-derived SNPs. Results: We found highly differentiated genetic signatures among all nine estuaries, separated by 200-5,000 km of coastline. Estimates of gene flow suggest that migration is low and largely symmetrical between sites. We also observed deep phylogenetic divides corresponding to major biogeographical breaks. Main conclusions: These results indicate substantial and longstanding constraints to dispersal in the species' native range, likely arising from the emergence of geological and oceanographic barriers and sustained by behaviour that promotes estuarine retention during larval development. This work supports the idea that larval behaviour promoting estuarine retention can be reflected in substantial genetic structure even in species with multi-week pelagic larval durations. Such behaviour-restricted dispersal has implications for predicting adaptation and spread in estuarine species, many of which have been introduced outside their native ranges.

Circular Single-Stranded DNA Virus (Microviridae: Gokushovirinae: Jodiemicrovirus) Associated with the Pathobiome of the Flat-Back Mud Crab, Eurypanopeus depressus.

A single-stranded DNA (ssDNA) virus is presented from a metagenomic data set derived from Alphaproteobacteria-infected hepatopancreatic tissues of the crab Eurypanopeus depressus The circular virus genome (4,768 bp) encodes 14 hypothetical proteins, some similar to other bacteriophages (Microviridae). Based on its relatedness to other Microviridae, this virus represents a member of a novel genus.

Incorporating Ploidy Diversity into Ecological and Community Genetics.

Studies in ecological and community genetics have advanced our understanding of the role of intraspecific diversity in structuring communities and ecosystems. However, in near-shore marine communities, these studies have mostly been restricted to seagrasses, marsh plants, and oysters. Yet, macroalgae are critically important ecosystem engineers in these communities. Greater intraspecific diversity in a macroalgal ecosystem engineer should result in higher primary and secondary production and community resilience. The paucity of studies investigating the consequences of macroalgal intraspecific genetic variation might be due, in part, to the complexity of macroalgal life cycles. The majority of macroalgae have seemingly subtle, but in actuality, profoundly different life cycles than the more typical animal and angiosperm models. Here, we develop a novel genetic diversity metric, PHD , that incorporates the ratio of gametophytic to sporophytic thalli in natural populations. This metric scales from 0 to 1 like many common genetic diversity metrics, such as genotypic richness, enabling comparisons among metrics. We discuss PHD and examples from the literature, with specific reference to the widespread, red seaweed Agarophyton vermiculophyllum. We also discuss a sex diversity metric, PFM , which also scales from 0 to 1, but fewer studies have identified males and females in natural populations. Nevertheless, by incorporating these novel metrics into the repertoire of diversity metrics, we can explore the role of genetic diversity in community and ecosystem dynamics with an emphasis on the unique biology of many macroalgae, as well as other haplodiplontic taxa such as ferns, foraminiferans, and some fungi.

Temporal dynamics of genetic clines of invasive European green crab (Carcinus maenas) in eastern North America.

Two genetically distinct lineages of European green crabs (Carcinus maenas) were independently introduced to eastern North America, the first in the early 19th century and the second in the late 20th century. These lineages first came into secondary contact in southeastern Nova Scotia, Canada (NS), where they hybridized, producing latitudinal genetic clines. Previous studies have documented a persistent southward shift in the clines of different marker types, consistent with existing dispersal and recruitment pathways. We evaluated current clinal structure by quantifying the distribution of lineages and fine-scale hybridization patterns across the eastern North American range (25 locations, ~39 to 49°N) using informative single nucleotide polymorphisms (SNPs; n = 96). In addition, temporal changes in the genetic clines were evaluated using mitochondrial DNA and microsatellite loci (n = 9-11) over a 15-year period (2000-2015). Clinal structure was consistent with prior work demonstrating the existence of both northern and southern lineages with a hybrid zone occurring between southern New Brunswick (NB) and southern NS. Extensive later generation hybrids were detected in this region and in southeastern Newfoundland. Temporal genetic analysis confirmed the southward progression of clines over time; however, the rate of this progression was slower than predicted by forecasting models, and current clines for all marker types deviated significantly from these predictions. Our results suggest that neutral and selective processes contribute to cline dynamics, and ultimately, highlight how selection, hybridization, and dispersal can collectively influence invasion success.

Distribution and population structure in the naked goby Gobiosoma bosc (Perciformes: Gobiidae) along a salinity gradient in two western Atlantic estuaries.

Many species of fish produce larvae that undergo a prolonged dispersal phase. However, evidence from a number of recent studies on demersal fishes suggests that the dispersal of propagules may not be strongly correlated with gene flow. Instead, other factors like larval behavior and the availability of preferred settlement habitat may be more important to maintaining population structure. We used an ecologically important benthic fish species, Gobiosoma bosc (naked goby), to investigate local and regional scale population structure and gene flow along a salinity gradient (∼3 ppt to ∼18 ppt) in two North Carolina estuaries. G. bosc is an abundant and geographically widespread species that requires complex but patchy microhabitat (e.g. oyster reefs, rubble, woody debris) for reproduction and refuge. We sequenced 155 fish from 10 sites, using a common barcoding gene (COI). We also included recent sequence data from GenBank to determine how North Carolina populations fit into the larger biogeographic understanding of this species. In North Carolina, we found a significant amount of gene flow within and between estuaries. Our analysis also showed high predicted genetic diversity based upon a large number of rare haplotypes found within many of our sampled populations. Moreover, we detected a number of new haplotypes in North Carolina that had not yet been observed in prior work. Sampling along a salinity gradient did not reveal any significant positive or negative correlations between salinity and genetic diversity, nor the proportion of singleton haplotypes, with the exception of a positive correlation between salinity standard deviation and genetic diversity. We also found evidence that an introduced European population of naked gobies may have originated from an Atlantic source population. Altogether, this system offers a compelling way to evaluate whether factors other than dispersal per se mediate recruitment in an estuarine-dependent species of fish with a larval dispersal phase. It also demonstrates the importance of exploring both smaller and larger scale population structure in marine organisms to better understand local and regional patterns of population connectivity and gene flow.