Invasive European Green Crab Research Articles

Variable Effects of Experimental Sea-level Rise Conditions and Invasive Species on California Cordgrass

Sea-level rise (SLR) will produce unprecedented changes in tidal marsh systems that already cope with daily tidal fluctuation, disturbances from storms, and salinity changes from droughts and runoff events. Additionally, negative impacts from non-native invasive species may alter marsh plants’ ability to respond to SLR stressors like increased inundation and salinity. Increasingly, tidal marsh communities must tolerate both changes in the physical environment from SLR and increased risk of invasion by non-native species. To assess the response of a threatened tidal marsh cordgrass (Spartina foliosa) to both stressors, we implemented a field experiment in San Francisco Bay, CA, USA, exposing cordgrass to a treatment that extended tidal inundation projected with SLR using a recently developed in situ method. At one of two field sites, we also enclosed the cordgrass with or without the invasive European green crab, Carcinus maenas. We found that cordgrass responded negatively to longer inundation, although these effects varied by site and year. In higher inundation treatments, cordgrass survival increased with increasing surface elevation of the plot. Cordgrass survival was lower in the presence of invasive green crabs relative to controls. We did not find interacting effects of responses to increased inundation and invasive species presence, which highlights the need to consider how latent or sequential effects of multiple stressors may affect ecosystems. This study demonstrates significant biological responses to invasive species presence and increased inundation. Evaluating relative effects and timing of multiple stressors, especially those induced by climate change and invasive species, will help us to manage threatened ecological communities in a changing world.

Dispersal pathways of European green crab larvae into and throughout the eastern Salish Sea

The invasive European green crab (Carcinus maenas) was first detected on the US west coast around 1989 and has expanded its range northward from central California to southern Alaska. The eastern Salish Sea was initially thought to be protected from invasion by the dominant seaward surface current in the Strait of Juan de Fuca (SJdF). However, this “oceanographic barrier” has been breached as established green crab populations have been detected in the eastern Salish Sea in recent years. Here we carried out particle-tracking simulations to understand possible natural pathways of green crab larvae invading the eastern Salish Sea. Both diel vertical migration and temperature-dependent mortality were considered in these simulations. Our results suggest that green crab larvae from the outer coast (outside the Salish Sea) and Sooke Basin (in SJdF) could be carried into the eastern Salish Sea in a narrow time window during the later cold season (esp. in March) when frequent flow reversals in SJdF occur and the seasonally rising water temperature becomes relatively favorable for green crab larvae. The major pathway for larvae to reach the eastern Salish Sea is along the southern coast of SJdF. The probability of live larvae reaching the eastern Salish Sea is highly sensitive to water temperature. Sensitivity simulations indicate that a temperature increase of 0.5–1 °C would double or quadruple the probability of successful arrival in the eastern Salish Sea. This suggests that invading green crabs might have taken advantage of the mild winter conditions in recent warm years. Our results also suggest that the warming climate in the near future may facilitate green crab larval exchange across the Salish Sea.

Invasive European green crab (Carcinus maenas) predation in a Washington State estuary revealed with DNA metabarcoding.

Predation by invasive species can threaten local ecosystems and economies. The European green crab (Carcinus maenas), one of the most widespread marine invasive species, is an effective predator associated with clam and crab population declines outside of its native range. In the U.S. Pacific Northwest, green crab has recently increased in abundance and expanded its distribution, generating concern for estuarine ecosystems and associated aquaculture production. However, regionally-specific information on the trophic impacts of invasive green crab is very limited. We compared the stomach contents of green crabs collected on clam aquaculture beds versus intertidal sloughs in Willapa Bay, Washington, to provide the first in-depth description of European green crab diet at a particularly crucial time for regional management. We first identified putative prey items using DNA metabarcoding of stomach content samples. We compared diet composition across sites using prey presence/absence and an index of species-specific relative abundance. For eight prey species, we also calibrated metabarcoding data to quantitatively compare DNA abundance between prey taxa, and to describe an 'average' green crab diet at an intertidal slough versus a clam aquaculture bed. From the stomach contents of 61 green crabs, we identified 54 unique taxa belonging to nine phyla. The stomach contents of crabs collected from clam aquaculture beds were significantly different from the stomach contents of crabs collected at intertidal sloughs. Across all sites, arthropods were the most frequently detected prey, with the native hairy shore crab (Hemigrapsus oregonensis) the single most common prey item. Of the eight species calibrated with a quantitative model, two ecologically-important native species-the sand shrimp (Crangon franciscorum) and the Pacific staghorn sculpin (Leptocottus armatus)-had the highest average DNA abundance when detected in a stomach content sample. In addition to providing timely information on green crab diet, our research demonstrates the novel application of a recently developed model for more quantitative DNA metabarcoding. This represents another step in the ongoing evolution of DNA-based diet analysis towards producing the quantitative data necessary for modeling invasive species impacts.

Interspecies and spatial differences in the shell microbiome of Atlantic rock crab Cancer irrotatus and European green crab Carcinus maenas from Atlantic Canada

IntroductionThe native Atlantic rock crab (Cancer irrotatus) and the invasive European green crab (Carcinus maenas) are commercially and ecologically important crustacean species in Atlantic Canada. The importance of microbiomes for host health and ecology has been recognized in many species, although very few studies have focused on crustaceans or their external shell microbiome. This is the first-ever study to characterize and analyze the microbial communities associated with the external carapace of C. irrotatus and C. maenas.MethodsMicrobiome samples were collected from three locations in Atlantic Canada, processed using standard 16S Illumina MiSeq PE250 sequencing and analyzed with the open-access QIIME2 software.ResultsTaxonomic classification of the microbial compositions, as well as alpha- and beta diversities, reveal that the shell microbiome differs by host species between C. irrotatus and C. maenas sampled from the same location and between C. irrotatus sampled from different locations. Interestingly, the differences are greater between species at the same location than between locations for the same species.DiscussionThese are the first-ever results showing that the crustacean shell microbiome not only depends on geographical factors but also on intrinsic factors specific to the host species. This implies that crustaceans exert some impact on their shell microbiome, potentially selecting beneficial taxa. These are important findings that could elucidate contributing factors of crustacean shell diseases that are still poorly understood.

Comprehensive biorefinery of invasive European green crab (Carcinus maenas) into multiple high-value biomolecules

European green crab (Carcinus maenas) is an invasive species globally and is considered marine waste that needs to be eliminated. In this study, the green crab was biorefined via a clean enzymes-assisted bioprocess into multiple value-added bioproducts, i.e., crab oil, minerals, carotenoprotein, and chitin. The recovered crab oil was rich in unsaturated fatty acids such as EPA and DHA as per GC-MS analysis. Citric acid, as a replacement for conventional hash acids, was found to optimally demineralize the crab powder at 1 M and 4 °C for 18 h, where the recovered crab minerals identified using ICP-MS showed a high level of calcium, in form of nutritious calcium citrate. Crab carotenoprotein recovered using trypsin exhibited antioxidant properties. It contained 139.26 μg/g of carotenoids pigment, mainly astaxanthin and astaxanthin diester, as verified by thin layer chromatography, and the protein component in the carotenoprotein was rich in essential amino acids such as leucine via UPLC-MS. The produced crab chitin from papain-assisted deproteinization showed characteristic physicochemical properties following FTIR and XRD analysis. These biomolecules biorefined from the green crab possess wide applications in food, pharmaceutical, biomaterials, etc. This new biorefinery strategy enhances the sustainability for biowaste utilization.

Tracking an invasion front with environmental DNA.

Data from environmental DNA (eDNA) may revolutionize environmental monitoring and management, providing increased detection sensitivity at reduced cost and survey effort. However, eDNA data are rarely used in decision-making contexts, mainly due to uncertainty around (1) data interpretation and (2) whether and how molecular tools dovetail with existing management efforts. We address these challenges by jointly modeling eDNA detection via qPCR and traditional trap data to estimate the density of invasive European green crab (Carcinus maenas), a species for which, historically, baited traps have been used for both detection and control. Our analytical framework simultaneously quantifies uncertainty in both detection methods and provides a robust way of integrating different data streams into management processes. Moreover, the joint model makes clear the marginal information benefit of adding eDNA (or any other) additional data type to an existing monitoring program, offering a path to optimizing sampling efforts for species of management interest. Here, we document green crab eDNA beyond the previously known invasion front and find that the value of eDNA data dramatically increases with low population densities and low traditional sampling effort, as is often the case at leading-edge locations. We also highlight the detection limits of the molecular assay used in this study, as well as scenarios under which eDNA sampling is unlikely to improve existing management efforts.

Challenges in eDNA detection of the invasive European green crab, Carcinus maenas

The early detection of invasive species is essential to cease the spread of the species before it can cause irreversible damage to the environment. The analysis of environmental DNA (eDNA) has emerged as a non-harmful method to detect the presence of a species before visual detection and is a promising approach to monitor invasive species. Few studies have investigated the use of eDNA for arthropods, as their exoskeleton is expected to limit the release of eDNA into the environment. We tested published primers for the invasive European green crab, Carcinus maenas, in the Gulf of Maine and found them not species-specific enough for reliable use outside of the area for which they were designed for. We then designed new primers, tested them against a broad range of local faunal species, and validated these primers in a field study. We demonstrate that eDNA analyses can be used for crustaceans with an exoskeleton and suggest that primers and probe sequences must be tested on local fauna at each location of use to ensure no positive amplification of these other species.

The Green Wave: reviewing the environmental impacts of the invasive European green crab (Carcinus maenas) and potential management approaches

The European green crab (Carcinus maenas), native to northwestern Europe and Africa, is among the top 100 most damaging invasive species globally. In some regions, including the Atlantic coast of North America, C. maenas has caused long-term degradation of eelgrass habitats and bivalve, crab, and finfish populations, while other areas are near the beginning of the invasion cycle. Owing to the high persistence and reproductive potential of C. maenas populations, most local and regional mitigation efforts no longer strive for extirpation and instead focus on population control. Long-term monitoring and rapid response protocols can facilitate early detection of introductions that is critical to inform management decisions related to green crab control or extirpation. Once C. maenas are detected, local area managers will need to decide on management actions, including whether and what green crab control measures will be implemented, if local invasion might be prevented or extirpated, and if population reduction to achieve functional eradication is achievable. Owing to the immense operational demands likely required to extirpate C. maenas populations, combined with the limited resources for monitoring and removal, it is unlikely that any single government, conservation, and (or) academic organization would be positioned to adequately control or extirpate populations in local areas, highlighting the importance of collaborative efforts. Community-based monitoring, and emerging methods such as environmental DNA (eDNA) may help to expand the spatial and temporal extent of monitoring, facilitating early detection and removal of C. maenas. While several C. maenas removal programs have succeeded in reducing their populations, to our knowledge, no program has yet successfully extirpated the invader, and the cost of any such program would likely be immense and unsustainable over the long-term. An alternative approach is functional eradication, whereby C. maenas populations are reduced below threshold levels such that ecosystem impacts are minimized. Less funding and effort would likely be required to achieve and maintain compared to extirpation. In either case, continual control efforts will be required because C. maenas populations can quickly increase from low densities and larval re-introductions.

Targeted next‐generation sequencing of environmental DNA improves detection of invasive European green crab (Carcinus maenas)

AbstractIn the northeast Pacific Ocean, there is high interest in developing eDNA‐based survey methods to aid management of invasive populations of European green crab (Carcinus maenas). Expected benefits are improved sensitivity for early detection of secondary spread and to assess the outcome of eradication efforts. A new eDNA‐based approach we term “targeted next‐generation sequencing (tNGS)” is introduced here and shown to improve detection relative to qPCR at sites with lower green crab CPUE values measured by trapping. DNA standards (gBlock) with starting molecule copies that were 10 to 100 times lower than the qPCR limit of detection returned significant numbers of sequencing reads, which in our field assessments translated to a 7%–10% increase in detection probability from tNGS relative to qPCR at sites with lower CPUE. We also found the number of sequencing reads from tNGS was significantly correlated with green crab CPUE whereas Ct values from qPCR were not. When sources of variation were partitioned for each assay, we found the difference between mean within‐site and mean between‐site variation was much larger and had non‐overlapping confidence intervals for tNGS relative to qPCR, suggesting the former may offer more power for detecting spatial variation in eDNA availability. Results presented here indicate this approach is suitable for species of known low abundances where a positive detection has high economic or environmental consequences, or for labs doing eDNA surveys for whom NGS‐based workflows are operationally more efficient than qPCR given its upward scalability. Any species with an existing qPCR assay can be easily tested with a tNGS assay using the approach presented here. We conclude with a discussion on the fitness for purpose of tNGS vs. qPCR for various applications and on how to best apply molecular surveys in management programs.

Relationship between ocean ecosystem indicators and year class strength of the invasive European green crab (Carcinus maenas)

The annual abundance of the non-native European green crab, Carcinus maenas, in Oregon estuaries varies greatly with ocean conditions. Average numbers were high following the 1997–1998 El Niño, decreased and remained low (<0.3 per trap) until they increased (>2 per trap) following the extended anomalous warming in 2014–2016. The year class strength of young crabs is strongly linked to ocean indicators during their planktonic larval development. Many of the same physical and biological ecosystem indicators used in salmon forecasting are also correlated with green crabs, but in the opposite direction. While cold ocean conditions benefit salmon, warm ocean indicators are positively linked to green crab year class strength. Among the best indicators for green crab year class strength are winter water temperatures, the sign of the Pacific Decadal Oscillation index, the day of physical and biological spring transitions, and negative biomass anomalies of northern copepods. These correlations suggest that green crabs need (1) warm winters (temperature > 10 °C), which enable larvae to complete their development in the near-shore, (2) strong northward flow of coastal waters during winter, which allows larvae to be transported from established populations to the south and (3) coastal circulation patterns that keep larvae close to shore, where they can be carried by wind and tidal currents into estuaries to settle. By using a relatively simple stoplight approach of ranking indicators, we were able to explain 69% of the inter-annual variability in green crab year class strength, while a quantitative metric of a combination of indicators explained 64% of the variability. Recruitment in 2018 and 2019 exceeded what was expected from the suite of ocean indicators. We discuss the possible role of additional larval sources, from the north or from local estuaries, that may have contributed to the increased recruitment during these years. If breeding populations of green crabs in Oregon and Washington continue to build, the relationships between ocean conditions and recruitment we have developed based solely on larval sources from the south could be greatly underestimating recruitment in the future.

Targeted Next Generation Sequencing improves detection and quantification of rare species from eDNA

Targeted species detection from eDNA is central to identifying and quantifying rare (i.e. invasive or endangered) species to inform conservation and resource management. Here we introduce a new targeted Next Generation Sequencing (tNGS) assay that shows improved detection relative to quantitative (q)PCR at low eDNA concentrations and increased precision to detect spatial variation in eDNA concentration related to species abundance. We compare the tNGS and qPCR methods using invasive European green crab (Carcinus maenas) in the northeast Pacific Ocean as a test case, and find that crab abundance measured by traditional trapping is significantly correlated with eDNA concentration across multiple sites for both methods. However, the tNGS assay outperformed qPCR in all tests: (1) increased precision of eDNA concentration estimation; (2) a 7-10% increase in detection probability at low abundance sites; and (3) greater power to detect spatial variation in eDNA concentration. The accuracy of predicting green crab abundance from eDNA concentration increased with the number of field replicates sampled and did not change appreciably over a tidal cycle. Green crab eDNA concentration behaving similarly to abundance measured from trapping demonstrates great promise for this tool to be implemented for early detection and routine monitoring surveys. The tNGS assay is easily accessible for surveying other species with existing qPCR assays and can thus be potentially important for detection and quantification of any species of high interest to management.

Effects of life stage on eDNA detection of the invasive European green crab (Carcinus maenas) in estuarine systems

Early and efficient detection of rare and invasive species is critical for the effective management of their populations. Environmental DNA (eDNA) detection techniques have been used for monitoring soft-bodied organisms (e.g., fishes) and some invertebrates, primarily in freshwater systems, but there are limited examples of eDNA as a method for monitoring marine crustaceans. The present study evaluates the efficacy of applying eDNA methods for detecting the invasive European green crab (Carcinus maenas) in a dynamic estuarine environment, and the effect of crab life stage (sex, molt stage, ovigery, abundance) on eDNA detection rates. An initial field experiment conducted in a local salt marsh system detected no C. maenas eDNA in sediment samples associated with traps containing C. maenas. In subsequent laboratory trials, aquaria containing one or two C. maenas at different life stages (soft-shell, hard-shell, male, female, ovigerous) were evaluated in replicated treatments to test the hypothesis that C. maenas exudes eDNA at higher levels when ovigerous, soft-shell, or at higher abundances. Duplicate sediment slurry and water samples were collected from each aquarium (n = 23) prior to crab addition (T-0), and after 24 h (T-1), 4 days (T-2), and 7 days (T-3). Sediment slurry and water samples were filtered, extracted, and analyzed using a species-specific droplet digital PCR (ddPCR) assay. In all non-control aquaria, C. maenas eDNA was detected, but concentrations were low (<10 copies/µL) in non-ovigerous treatments. eDNA concentrations were significantly higher in sediment slurry versus water samples for male and ovigerous treatments. Overall, concentrations increased from T-0 to T-1 but did not significantly change from T-1 through T-3. The findings from this study indicate that during most of their life cycle, C. maenas shed low levels of DNA, highlighting the importance of considering life stage and sampling methodology when using eDNA to monitor crustaceans in estuarine environments.

Effects of perceived competition and water temperature on the functional responses of invasive and native crabs

As the frequency of species introductions increases globally, the need for predicting the ecological impacts of invaders becomes ever more urgent. Functional responses, i.e. how resource uptake by consumers changes as a function of resource density, describe the species-specific ability to deplete resources under standardized conditions, and their shape has been used as a tool to predict the impact of non-native species. However, functional responses are usually derived from individuals foraging alone, which overlooks the important roles of intra- and interspecific interactions in shaping per capita consumption rates. We tested the extent to which the functional responses of invasive European green crabCarcinus maenasand native graceful rock crabMetacarcinus gracilisare affected by the perceived presence of con- and heterospecific individuals. Overall, crabs did not consume significantly more prey in the perceived presence of either a conspecific or novel heterospecific, but did demonstrate significantly higher attack rates in warmer water in the presence of a conspecific. Regardless of competitor type, green crabs consumed, on average, 16% more prey than the native crabs. This was largely due to their higher attack rate and lower prey handling time in warmer water. Green crab prey consumption increased significantly with increasing water temperature while that of the native crab species was unaffected (12 vs. 3.5% per 1°C increase, respectively). Higher maximum feeding rates are congruent with green crab impacts throughout their invaded range and suggest that green crabs might be undeterred from feeding in the presence of this co-occurring native crab species.

Assessing Intertidal Populations of the Invasive European Green Crab

Assessing Intertidal Populations of the Invasive European Green Crab

Assessing Intertidal Populations of the Invasive European Green Crab.

Invasive species have caused major disruptions to ecosystems worldwide. The European green crab invaded North America in the 1800s and is considered one of the world's 100 worst invaders by the IUCN. Observations of spatiotemporal green crab population dynamics are essential for predicting and managing the ecological and economic impacts of this harmful invasive species. This protocol was developed in an effort to create a standardized method for assessing green crab population dynamics in the rocky intertidal zone of New England and Atlantic Canada. The protocol was designed to be accessible to multiple users including researchers, educators, students, and citizen scientists. Although it was designed for surveying crab populations, this protocol is easy to adapt and could be used for any number of intertidal species. The resulting data collected using this protocol has a wide range of uses, including to inform ecological research, conservation efforts, mitigation strategies, and fishery development, as well as for educational outreach purposes.

Analytical validation and field testing of a specific qPCR assay for environmental DNA detection of invasive European green crab (Carcinus maenas)

AbstractEnvironmental DNA (eDNA) methods are providing tools for detecting invasive species in aquatic environments. Targeted qPCR assays applied to eDNA samples promise to overcome limitations of traditional methods, especially for early detection. The European green crab (Carcinus maenas) is considered one of the most successful invasive species globally due to the large range it has invaded and negative impacts on native species, marine habitats, and shellfish industries. We developed, laboratory‐validated, and field‐tested a specific qPCR assay for the detection of green crab from eDNA samples. We also show that the assay can detect green crab in bulk DNA extracted from plankton samples. Assay design, optimization, sensitivity, and specificity testing generally followed the validation pathway recommended by the World Organization for Animal Health for assays used to manage global aquatic animal health and infectious disease. Assay specificity was verified in silico and in vitro by laboratory testing 26 nontarget species, none of which showed potential for amplification. Assay sensitivity was appropriately high, with the limit of detection approaching two gene copies/μl. The assay was field‐tested on eDNA samples collected from filtered seawater at five sites on the Pacific coast of Canada known to harbor green crab based on historical monitoring data; green crab DNA was amplified from all sites. We also present early pilot field testing of the assay done on bulk DNA extracted from plankton samples from four sites from Australia, two sites with and two sites without reported records of green crab presence. Green crab was detected at both sites with known green crab records. Significant inhibition was recorded for some plankton samples but not for eDNA samples. This is the first qPCR assay for detection of European green crab, providing researchers and managers with a valuable new tool to aid early detection and ongoing monitoring.

Comparison of catch per unit effort of invasive European green crab (Carcinus maenas) across four bait types

Restoration of marine ecosystems impacted by invasive, non-native species usually depends on reducing the density of the invader through a targeted cull. The effectiveness of these programs depends, in part, on using a selective fishing gear that maximizes catch per unit effort (CPUE). In this study, we examine how using different types of bait affects CPUE of European green crab (Carcinus maenas), an invasive species impacting ecosystems on the south and west coasts of the island of Newfoundland, Canada. We compared both CPUE and mean body size of green crabs caught in Fukui traps baited with Atlantic herring (Clupea harengus), short-fin squid (Illex illecebrosus), blue mussels (Mytilus edulis), and Atlantic cod (Gadus morhua). We found that squid and cod produced substantially higher catch rates, and also caught larger-bodied crabs. Despite green crabs being generalist predators, our results show that bait choice affects capture efficiency, and thus managers should preface removal programs with controlled studies designed to identify locally-effective and ecologically-appropriate baits.

Habitat alteration by invasive European green crab (Carcinus maenas) causes eelgrass loss in British Columbia, Canada

Dominant, habitat-forming plant species, such as seagrasses, are key components of coastal ecosystems worldwide. Multiple stressors, including invasive species that directly alter, remove, or replace the foundation plant species, threaten these ecosystems. On the Atlantic coast of North America, ecosystem engineering by invasive European green crab (Carcinus maenas) has been linked to the loss of some eelgrass (Zostera marina) beds. However, the interaction of the same co-occurring species on the Pacific coast has not been investigated. We conducted an enclosure experiment in Barkley Sound, British Columbia, to determine if the engineering impacts of green crabs on Pacific eelgrass ecosystems mirror those previously identified on the Atlantic coast. Eelgrass shoot density declined rapidly over 4 weeks, with a 73–81% greater loss in enclosures with high crab density compared to the low-density and control treatments. The low ratio of eelgrass blades to rhizomes in the high-density treatment suggests that blade shredding, rather than bioturbation of whole plants, was the main mechanism of eelgrass loss. Eelgrass was detected in green crab stomach contents, consistent with observations from the Atlantic coast. Crab density did not have a detectable effect on the biomass or community composition of benthic fauna associated with eelgrass over the duration of the experiment. The eelgrass loss we observed was consistent with losses observed on the Atlantic coast, which raises management concerns on the Pacific coast, particularly in areas where green crabs co-occur with other coastal stressors and with ecologically and economically important species such as salmon.

Improving the efficiency of the Fukui trap as a capture tool for the invasive European green crab (Carcinus maenas) in Newfoundland, Canada.

The European green crab (Carcinus maenas) is a crustacean species native to European and North African coastlines that has become one of the world’s most successful marine invasive species. Targeted fishing programs aimed at removing green crabs from invaded ecosystems commonly use Fukui multi-species marine traps. Improving the efficiency of these traps would improve the ability to respond to green crab invasions. In this study, we developed four distinct trap modifications that were designed to facilitate the successful capture of green crabs, with the goal of improving the performance of the Fukui trap. We tested these modifications in situ during the summer of 2016 at two locations in Placentia Bay, Newfoundland. We discovered that three of our modified Fukui trap designs caught significantly more green crabs than the standard Fukui trap, increasing catch-per-unit-effort (CPUE) by as much as 81%. We conclude that our top-performing modifications have great potential for widespread use with existing Fukui traps that are being used for green crab removal efforts.

Invasion dynamics: interactions between the European Green Crab Carcinus maenas and the Asian Shore Crab Hemigrapsus sanguineus

As ecosystems are increasingly inhabited by multiple invasive species, interactions among exotic species have the potential to negatively impact native communities and habitats, often in counterintuitive ways. In rocky intertidal habitats along southern New England, the recently introduced Asian Shore Crab, Hemigrapsus sanguineus, has been implicated in the displacement of the more established invasive European Green Crab, Carcinus maenas. We coupled faunal sampling with two field experiments to examine whether H. sanguineus is also displacing C. maenas in the Gulf of Maine. First, we investigated natural recruitment of the two species across an elevation gradient, using cage exclusion units to manipulate predator access. Excluding H. sanguineus adults significantly increased the density of C. maenas recruits, but not conspecific recruit density. Next, to simulate the transition that occurred from pre-invasion to the establishment of H. sanguineus, we conducted a predator inclusion experiment manipulating the density of adult H. sanguineus to examine the effects on C. maenas and conspecific recruit densities. In contrast to the previous experiment, we found that the density of H. sanguineus included in cages negatively affected conspecific recruit density but had no effect on C. maenas recruit density. We hypothesize that juxtaposing results from the two experiments may be a consequence of the considerably greater abundance of C. maenas recruits relative to H. sanguineus recruits during the first experiment. Our work highlights the importance of considering the multitude of factors that influence competitive and intraguild predation interactions, including density-dependent effects, ontogeny and life history characteristics.