Lobster Homarus Americanus Research Articles

Movement of american lobster Homarus americanus associated with offshore mussel Mytilus edulis aquaculture

Bivalve aquaculture sites attract a variety of large benthic species. Previous studies have shown that American lobsterHomarus americanusare more abundant in musselMytilus edulisfarms than in areas outside of them, suggesting that farms provide lobsters with adequate food and shelter. This study used acoustic telemetry to evaluate the influence of longline mussel farms on lobster movement behavior. In 2014, 60 lobsters were acoustically tagged on a boat and released in a mussel farm and at 2 reference sites outside the farm. Most lobsters (92%) left the monitored area within 1 d post-tagging; those released in reference sites moved northeast, whereas those released in the farm moved in random directions. Of the 16 lobsters that stayed or returned to the study area over the course of the 2 mo experiment, 10 displayed nomadic movements, 3 displayed small, local movements—presumably associated with foraging behavior, and 3 displayed both movements. The time lobsters spent within a site, distance travelled, and walking speed did not differ between the farm and reference sites. A second experiment was done in 2017 over 2 mo to evaluate tagging method (‘on boat’ andin situtagging) effects on lobster movement behavior. The experiment followed movements by 50 lobsters, half for each treatment, and showed that tagging method can affect walking speed during the first 24 h, but had no impact on the residence time and the distance travelled by the lobsters.

Spatial and temporal patterns in the sex ratio of American lobsters (Homarus americanus) in southwestern Nova Scotia, Canada

An approximate 1:1 sex ratio of American lobsters can be skewed due to environmental factors or fisheries management. Substantial skewness can impact mating behaviour and lower reproduction which could have far-reaching ecological and economic consequences. The aim was to investigate the sex ratio patterns of lobsters in two lobster fishing areas (LFAs) in southwestern Nova Scotia, Canada and identify factors associated with skewed sex ratios. This study analyzed biological data from more than 270,000 lobsters sampled over ten years (2010–2019) by the Fishermen and Scientists Research Society. A mixed effect logistic regression model evaluated the effect of spatial, temporal and environmental factors as well as size on the sex ratio of lobsters. There were significant temporal patterns in sex ratios that differed by LFA. After the effects of sampling month, year and LFA were accounted for, lower bottom temperature and deeper water depth were associated with a higher prevalence of females, especially in larger lobsters. We present the first long term analyses of sex ratio patterns in H. americanus in Atlantic Canada’s most commercially important region for this species and provide evidence that these patterns are influenced by environmental factors and fisheries. In view of future climate change scenarios, monitoring the population dynamics of this iconic fishery species is crucial to ensure sustainable fisheries and healthy lobster stocks.

Tolerant Larvae and Sensitive Juveniles: Integrating Metabolomics and Whole-Organism Responses to Define Life-Stage Specific Sensitivity to Ocean Acidification in the American Lobster.

Bentho-pelagic life cycles are the dominant reproductive strategy in marine invertebrates, providing great dispersal ability, access to different resources, and the opportunity to settle in suitable habitats upon the trigger of environmental cues at key developmental moments. However, free-dispersing larvae can be highly sensitive to environmental changes. Among these, the magnitude and the occurrence of elevated carbon dioxide (CO2) concentrations in oceanic habitats is predicted to exacerbate over the next decades, particularly in coastal areas, reaching levels beyond those historically experienced by most marine organisms. Here, we aimed to determine the sensitivity to elevated pCO2 of successive life stages of a marine invertebrate species with a bentho-pelagic life cycle, exposed continuously during its early ontogeny, whilst providing in-depth insights on their metabolic responses. We selected, as an ideal study species, the American lobster Homarus americanus, and investigated life history traits, whole-organism physiology, and metabolomic fingerprints from larval stage I to juvenile stage V exposed to different pCO2 levels. Current and future ocean acidification scenarios were tested, as well as extreme high pCO2/low pH conditions that are predicted to occur in coastal benthic habitats and with leakages from underwater carbon capture storage (CCS) sites. Larvae demonstrated greater tolerance to elevated pCO2, showing no significant changes in survival, developmental time, morphology, and mineralisation, although they underwent intense metabolomic reprogramming. Conversely, juveniles showed the inverse pattern, with a reduction in survival and an increase in development time at the highest pCO2 levels tested, with no indication of metabolomic reprogramming. Metabolomic sensitivity to elevated pCO2 increased until metamorphosis (between larval and juvenile stages) and decreased afterward, suggesting this transition as a metabolic keystone for marine invertebrates with complex life cycles.

Examining Scale Dependent Environmental Effects on American Lobster (Homarus americanus) Spatial Distribution in a Changing Gulf of Maine

The Gulf of Maine (GOM) is a highly complex environment and previous studies have suggested the need to account for spatial nonstationarity in species distribution models (SDMs) for the American lobster (Homarus americanus). To explore impacts of spatial nonstationarity on species distribution, we compared models with the following three assumptions : (1) large-scale and stationary relationships between species distributions and environmental variables; (2) meso-scale models where estimated relationships differ between eastern and western GOM, and (3) finer-scale models where estimated relationships vary across eastern, central, and western regions of the GOM. The spatial scales used in these models were largely determined by the GOM coastal currents. Lobster data were sourced from the Maine-New Hampshire Inshore Bottom Trawl Survey from years 2000–2019. We considered spatial and environmental variables including latitude and longitude, bottom temperature, bottom salinity, distance from shore, and sediment grain size in the study. We forecasted distributions for the period 2028–2055 using each of these models under the Representative Concentration Pathway (RCP) 8.5 “business as usual” climate warming scenario. We found that the model with the third assumption (i.e., finest scale) performed best. This suggests that accounting for spatial nonstationarity in the GOM leads to improved distribution estimates. Large-scale models revealed a tendency to estimate global relationships that better represented a specific location within the study area, rather than estimating relationships appropriate across all spatial areas. Forecasted distributions revealed that the largest scale models tended to comparatively overestimate most season × sex × size group lobster abundances in western GOM, underestimate in the western portion of central GOM, and overestimate in the eastern portion of central GOM, with slightly less consistent and patchy trends amongst groups in eastern GOM. The differences between model estimates were greatest between the largest and finest scale models, suggesting that fine-scale models may be useful for capturing effects of unique dependencies that may operate at localized scales. We demonstrate how estimates of season-, sex-, and size- specific American lobster spatial distribution would vary based on the spatial scale assumption of nonstationarity in the GOM. This information may help develop appropriate local adaptation measures in a region that is susceptible to climate change.

G protein-coupled receptors as candidates for modulation and activation of the chemical senses in decapod crustaceans.

Many studies have characterized class A GPCRs in crustaceans; however, their expression in crustacean chemosensory organs has yet to be detailed. Class A GPCRs comprise several subclasses mediating diverse functions. In this study, using sequence homology, we classified all putative class A GPCRs in two chemosensory organs (antennular lateral flagellum [LF] and walking leg dactyls) and brain of four species of decapod crustaceans (Caribbean spiny lobster Panulirus argus, American lobster Homarus americanus, red-swamp crayfish Procambarus clarkii, and blue crab Callinectes sapidus). We identified 333 putative class A GPCRs– 83 from P. argus, 81 from H. americanus, 102 from P. clarkii, and 67 from C. sapidus–which belong to five distinct subclasses. The numbers of sequences for each subclass in the four decapod species are (in parentheses): opsins (19), small-molecule receptors including biogenic amine receptors (83), neuropeptide receptors (90), leucine-rich repeat-containing GPCRs (LGRs) (24), orphan receptors (117). Most class A GPCRs are predominately expressed in the brain; however, we identified multiple transcripts enriched in the LF and several in the dactyl. In total, we found 55 sequences with higher expression in the chemosensory organs relative to the brain across three decapod species. We also identified novel transcripts enriched in the LF including a metabotropic histamine receptor and numerous orphan receptors. Our work establishes expression patterns for class A GPCRs in the chemosensory organs of crustaceans, providing insight into molecular mechanisms mediating neurotransmission, neuromodulation, and possibly chemoreception.

Size affects intraspecific aggression and response to predation threat in juvenile American lobsters

Intraspecific aggression and competition can govern access to food and shelter in many marine organisms, but these interactions can be impacted by life stage and presence of predators. Juvenile lobsters (Homarus americanus) have social hierarchies that govern access to valuable resources like food and shelter, but these hierarchies could be impacted by the presence of invasive green crabs (Carcinus maenas). The goal of this study was to determine whether the presence of green crab effluent affected aggressive behavior in different sizes of juvenile lobsters, with the hypothesis that all juvenile lobsters would be more likely to share shelter under threat of predation. Small juveniles (15 mm carapace length) and larger juveniles (45 mm) had substantially different interactions and responses to predator threat cues, with small individuals more likely to both fight over shelter and share shelter when exposed to green crab effluent. There were also several size-based behavioral differences in control treatments, as small juvenile lobsters challenged each other more frequently for shelter and spent vastly more time in the shelter, less time stationary outside the shelter, and less time fighting. Despite being vulnerable to green crab predation, larger juvenile lobsters were generally unaffected by crab chemical cues. This suggests that life history plays a strong role in predator avoidance behavior and that both lethal and nonlethal effects of invasive species on native prey can vary markedly with life stage. Future research should examine these crab-lobster interactions in lobster nursery habitats in the field, as invasive green crabs in these habitats could change behavior, activity patterns and survival of juvenile lobsters.

Comparative study of the hemolymph microbiome between live and recently dead American lobsters Homarus americanus.

Lobsters and other crustaceans do not have sterile hemolymph. Despite this, little is known about the microbiome in the hemolymph of the lobster Homarus americanus. The purpose of this study was to characterize the hemolymph microbiome in lobsters. The lobsters were part of a larger study on the effect of temperature on epizootic shell disease, and several died during the course of the study, providing an opportunity to examine differences in the microbiomes between live and recently dead (1-24 h) animals. The hemolymph microbiomes of live lobsters was different from those in dead animals and both were different from the tank microbiome in which the animals had been held. The microbiomes of live lobsters were more diverse and had a different suite of bacteria than those from dead animals. The dominant taxa in live lobsters belonged to Flavobacteriaceae and Rhodobacteraceae, whereas Vibrionaceae and Enterobacteriaceae were predominant in the dead lobsters. Although aquarium microbiomes overlapped with the hemolymph microbiomes, there was less overlap and lower abundance of taxa in comparison with hemolymph from live lobsters. Previous studies reporting bacteria in the digestive tract of lobsters suggested that Vibrionaceae and Enterobacteriaceae had invaded the hemolymph via the gut. Our study suggests that hemolymph bacteria abundant in live lobsters do not originate from the tank milieu and comprise a rich, natural, or native background of bacterial constituents.

An evaluation of temporal changes in the trophic structure of Gulf of Maine ecosystem

The Gulf of Maine (GOM) ecosystem has experienced significant changes in the last few decades with a great increase in the American lobster Homarus americanus population and substantial reduction in some groundfish populations like Atlantic cod Gadus morhua. This study evaluated and compared the energy flows and trophic structures of the GOM ecosystem in the mid-2000s and mid-2010s. Following an early study, two mass-balance ecosystem models, each of which include 25 functional groups, were updated for the mid-2000s and mid-2010s using Ecopath. Fishing efforts were used as forcing drivers to simulate the variation of the GOM ecosystem in the mid-2000s to mid-2010s. Similar to the GOM ecosystem from the 1980s to 1990s found in a previous study, dominant species in both the mid-2000s and mid-2010s ecosystems were transferred from top predators to crustacean species in the last decade. However, food web structure of the GOM ecosystem became more complex, stable, and flexible to perturbations, indicating that the GOM ecosystem is moving toward a more mature stage. Simulation results indicated that fishing activities were considered to be the main driver to the variations dynamics of most species in the GOM ecosystem, while some species like Atlantic herring Clupea harengus might also suffer from changes in environmental conditions. This study provides a comprehensive overview on the changes in the GOM ecosystem in the last two decades.

Movements of egg-bearing American lobsters Homarus americanus during late stage brooding and hatching

Although the large-scale movements of ovigerous (egg-bearing) American lobsters have been well-studied, their finer scale movements before, and during, the time when their eggs hatch are poorly understood. In this study, acoustic telemetry was used to track 12 ovigerous lobsters, carrying late-stage eggs, near the Isles of Shoals in the southern Gulf of Maine. In the spring of 2016 and 2017, lobsters were captured, fitted with acoustic tags that transmitted both depth and activity data, and released within an array of receivers. Eleven of the 12 lobsters moved from relatively shallow water (12 m) to deeper water (>29 m) approximately 2 wk prior to the time when their eggs were predicted to hatch. Four of these lobsters were within the array when their eggs most likely hatched, while 8 left the array towards even deeper water (>35 m). These data, taken together with previous data from research traps, suggest that egg-bearing lobsters tend to move to deep water prior to the time when their eggs are due to hatch, which may be beneficial for the dispersal and survival of their larvae.

An American in the Aegean: first record of the American lobster Homarus americanus H. Milne Edwards, 1837 from the eastern Mediterranean Sea

An American in the Aegean: first record of the American lobster Homarus americanus H. Milne Edwards, 1837 from the eastern Mediterranean Sea

Size at Maturity, Shell Conditions, and Morphometric Relationships of Male and Female Jonah Crabs in the Middle Atlantic Bight

AbstractThe Jonah crab Cancer borealis is a data‐poor species with an unknown stock status. They were historically caught as bycatch in American lobster Homarus americanus traps. The decline of the southern New England lobster stock since 1990 helped stimulate a targeted fishery and a sixfold increase in both the landings and ex‐vessel value of Jonah crab. Current knowledge on Jonah crab growth and reproduction is scarce and dated. Therefore, this research focused on updating fundamental biological information for Jonah crabs in the Middle Atlantic Bight. Morphometric analyses determined that male crabs reach size at 50% sexual maturity (SM50) at 98.3 ± 1.4 mm carapace width (CW; ±95% CI), and female crabs reach SM50 at 88.2 mm CW. These values are below the current minimum legal size. An increase in the proportion of female crabs in a new‐shell condition during the spring and summer months suggests a seasonal mating period. Expanding available information will substantiate accurate appointment of minimum legal sizes, minimize discard mortality, and increase understanding of stock dynamics, thus improving management and long‐term sustainability for the Jonah crab fishery.

A review of American lobster (Homarus americanus) records around the British Isles: 2012 to 2018

Although native to eastern Canada and the USA, American lobsters (Homarus americanus) have been reported from British coastal waters in recent years. The introduction of most have been as escapees from holding facilities or releases by members of the public for animal welfare or religious reasons. Life-history traits of H. americanus put them at a potential competitive advantage over and hybridisation with the European lobster (Homarus gammarus), the 5th most valuable seafood to the UK. While yearly reported numbers of H. americanus found in British waters have been low, a mass release of 361 H. americanus into the English Channel off Brighton, southern England, took place in June 2015. This review identified 162 records of H. americanus in British waters between 2012 and 2018, with specimens caught mainly in or around Brighton, although additional H. americanus released may have gone undetected. While this evidence suggests a growing potential for establishment of the species, further information is provided evidencing hybridisation between released H. americanus and native H. gammarus via genetic analysis. This review highlights the need for a more robust means of detecting H. americanus and other non-native species in British waters.

Bycatch in a Commercial Lobster Fishery: Effects on Two Benthic Predators, Sea Raven and Longhorn Sculpin

AbstractStudying the species‐specific responses to fishing capture is critical for effective management and conservation of bycatch species given that acute stress incurred from capture and handling may ultimately lead to mortality. While species of low commercial value are often overlooked, having accurate information on the effects of capture on all species is necessary for ecosystem‐based management. Sea Raven (SR) Hemitripterus americanus and Longhorn Sculpin (LHS) Myoxocephalus octodecemspinosus are routinely captured in the commercial American lobster Homarus americanus fishery in the Gulf of Maine, and they are discarded due to low commercial value. Despite a lack of economic value, these predatory species play important roles in shaping the benthic communities that they inhabit, highlighting the need to study their stress and mortality due to capture and handling. To help understand the effects of the lobster fishery on these species, the current study evaluated the physical, behavioral, and physiological stress responses of SR and LHS to capture in the state of Maine Zone G commercial lobster fishery. Collectively, our results suggest that although these species appeared to be resilient to capture based on an overt injury assessment, stress responses occurred based on reflex impairment and physiological perturbations, and these responses were species‐specific. Given the prevalence of behavioral and physiological stress in this study, further research into the survival outcomes of SR and LHS following release in the commercial lobster fishery is warranted.

Evidence that mud seafloor serves as recruitment habitat for settling and early benthic phase of the American lobster Homarus americanus H. Milne Edwards, 1837 (Decapoda: Astacidea: Nephropidae)

Abstract Structurally complex cobble seafloor protects against predators, and is generally assumed to be the only meaningful habitat for settlement and benthic recruitment of American lobsters (Homarus americanus H. Milne Edwards, 1837). Accordingly, historical surveys on featureless substrates such as mud have found few and only older juveniles. Mud, however, is far more common than cobble across the lobster’s range, and may be of increasing importance in regions where the lobster population has been growing over the past few decades. As a first step to determining whether mud seafloor serves as meaningful recruitment habitat for juvenile lobsters, we deployed artificial habitats (bio-collectors) at five locations varying from 100% mud to mostly cobble, in Maces Bay, Bay of Fundy, NB, Canada, to capture young lobsters. A broad size range of lobsters, from new settlers up to adolescents, colonized bio-collectors in all locations, suggesting that lobsters settle and spend at least some of their juvenile life on mud. Differences in body condition (length-standardized mass) of lobsters sampled from the different locations suggest that some individuals must reside on mud for at least months to years, long enough to show differential growth between habitats. The greater relative abundance of adolescents in bio-collectors on mud also suggests their net movement from densely populated cobble beds into mud habitat. We propose that mud seafloor may be a more important habitat for settlement and early survival of American lobsters than is currently appreciated, especially given the current potential saturation of scarce cobble nursery habitat by growing lobster populations and, in some regions, reduced predation on mud seafloor due to overfishing of groundfish.

Contribution of mussel fall-off from aquaculture to wild lobster Homarus americanus diets

Anthropogenic subsidies to natural systems can influence the diet of mobile omnivore species and co-occurring species. This study assessed if fall-off from mussel aquaculture subsidized wild populations of mobile scavengers and predators, such as the commercially important lobster Homarus americanus. A Bayesian stable isotope-mixing model with parameters determined from the literature and from a 105 days laboratory feeding experiment was applied to wild lobsters to determine how important the various food sources were in these lobsters, especially mussel fall-off. Isotopic values were mainly affected by lobster size with model outputs indicating that large lobsters (>80 mm cephalothorax) fed mainly on mussels from the mussel farm (46% of the diet) while small ones fed mostly on the rock crab Cancer irroratus (99%). The contribution of mussel subsidies to the lobster's diet was thus size-specific and direct (i.e. through mussel fall-off and not through co-occurring species such as rock crab). The absence of a link between food sources and lipid energy content in lobsters suggested that the reduction of rock crab consumption would have to be more drastic to affect the general health of large lobsters in the short term.

Decrease in size at maturity of female American lobsters Homarus americanus (H. Milne Edwards, 1837) (Decapoda: Astacidea: Nephropidae) over a 50-year period in Maine, USA

ABSTRACTThe size at which female American lobsters Homarus americanus (H. Milne Edwards, 1837) reach maturity was determined for females collected from inshore (< 3 nautical miles, or 5.5 km) Boothbay Harbor (BBH), Maine, USA in 2018. A total of 272 females were collected during a three-week period in the spring and each female was assigned a maturity status (immature or mature) based on ovarian staging. These determinations were then compared to two similar female-maturity studies undertaken in BBH by the Maine Department of Marine Resources over the last 50 years. The comparison revealed that the length at which 50% of females reach maturity has decreased by 5 mm over the last 25 years and a significant difference between maturity ogives generated over time in BBH. Cement-gland stage was also recorded for all females and further analysis revealed no significant difference between maturity ogives generated using ovarian and cement-gland staging. Results indicate that cement-gland staging is an effective maturity assessment method in female lobsters from this region. Because there is a well-documented relationship between temperature and size at maturity in crustaceans, we also examined changes in sea-surface temperature during this period in BBH. We found that the region has warmed significantly over this period of time with extreme high temperatures occurring more frequently during the last 25 years. Our results can be used to update key parameters in the stock-assessment model related to growth and egg production in the Gulf of Maine/George’s Bank stock. These findings can also be used to inform future management decisions related to the carapace length of harvested lobsters and the preservation of mature females.

Functional anatomy of the female reproductive system of the American lobster (Homarus americanus).

Light microscopy studies of the female American lobster Homarus americanus reproductive system are essentially nonexistent or outdated. Based on samples taken in the spring, summer, and autumn from the southern Gulf of St. Lawrence between 1994 and 2014, and using a combination of histological and scanning electron microscope techniques, we propose an ovarian cycle with 10 stages, identifying for the first time a recovery stage. Also, an atypical resorption stage, characterized by massive reabsorption of mature oocytes, is occasionally observed during summer months. The oviducts are composed of connective tissue (elastic and collagen fibers) with no muscle or secretory activities. Their epithelium shows a cyclic pattern and phagocytosis activities linked to spawning. Although the role of the seminal receptacle is to store and protect semen, free spermatozoa (i.e., without the spermatophoric wall and the acellular gelatinous substance that constitute the semen) were also observed in its posteriolateral grooves immediately prior to spawning, which is consistent with an external fertilization mechanism at the seminal receptacle. Unexpectedly, free spermatozoa were observed externally near two pore-like structures located on the gonopore's operculum, not at the seminal receptacle, after spawning; hence, more work is needed to fully understand the fertilization mechanism for the American lobster.

Energy metabolism and survival of the juvenile recruits of the American lobster (Homarus americanus) exposed to a gradient of elevated seawater pCO2

The transition from the last pelagic larval stage to the first benthic juvenile stage in the complex life cycle of marine invertebrates, such as the American lobster Homarus americanus, a species of high economic importance, represents a delicate phase in these species development. Under future elevated pCO2 conditions, ocean acidification and other elevated pCO2 events can negatively affect crustaceans. This said their effects on the benthic settlement phase are virtually unknown. This study aimed to identify the effects of elevated seawater pCO2 on stage V American lobsters exposed to seven pCO2 levels. The survival, development time, metabolic and feeding rates, carapace composition, and energy metabolism enzyme function were investigated. Results suggested an increase in mortality, slower development and an increase in aerobic capacity with increasing pCO2. Our study points to potential reduction in juvenile recruitment success as seawater pCO2 increases, thus foreshadowing important socio-economic repercussions for the lobster fisheries and industry.

Impacts of a multi-trap line on benthic habitat containing emergent epifauna within the Mid-Atlantic Bight

Abstract Alteration and degradation of benthic structure by fishing gear can impede efforts to manage fish stock sustainably. Although the impacts of mobile gear are well known, effects of passive gear (e.g. fish traps) upon structure have been little studied. We modified commercial traps for American lobster Homarus americanus and black sea bass Centropristis striata by attaching GoPro® cameras to ascertain the degree and nature of impacts to seafloor habitats. Customized traps were included within a line of 20 traps, deployed and retrieved according to standard commercial fishing practice. Less than 5% of traps landed directly on bedforms when deployed. However, during retrieval traps dragged along the ocean floor, increasing trap–habitat contact rate to 50%, and causing traps to collide with corals, bryozoans, and other epifauna. Drag time of traps depended on the position in the trap line. Experimentally extending the trap line reduced drag time during retrieval for traps near the distal end of the line. Our results show that impacts of commercial trap fishing can be substantial during trap retrieval, and that the impact depends on their location on a trap line. Fishing practices should be developed that minimize effects of trap retrieval on structural benthic habitat.

Competitive and agonistic interactions between the invasive Asian shore crab and juvenile American lobster.

Invasive species can have profound ecological and evolutionary impacts on native fauna, particularly those from overlapping guilds. Intraguild predation and competition often occur simultaneously, and ontogenetic shifts in competitive strength can dictate the magnitude and direction of species interactions. The recent introduction of the Asian shore crab Hemigrapsus sanguineus to the Atlantic coast of North America has resulted in the potential for considerable intraguild interactions with juvenile American lobsters Homarus americanus, with which it now co-occupies rocky intertidal and shallow subtidal habitats. We present data from 5 yr of monthly (May-October) field sampling revealing a significant decline in lobster density as H.sanguineus density increased in the low intertidal. To investigate potential mechanisms behind this pattern, we conducted three mesocosm experiments designed to examine whether competitive interactions between H.americanus and H.sanguineus are size- or density-dependent. Larger early benthic phase lobsters (16-34mm CL) outcompeted H.sanguineus for both food and shelter resources. These lobsters fed faster and more in the presence of H.sanguineus, effectively defended shelter, were responsible for the majority of agonistic interactions, and, in multiple instances, killed and consumed H.sanguineus. Effects on sheltering and agonstic interactions by these lobsters were independent of H.sanguineus density; however, prior shelter residency increased lobster agonistic behavior towards crabs. In contrast, H.sanguineus outcompeted smaller, early benthic phase lobsters (7.2-11.2mm CL) for shelter, and these interactions were density dependent. Displacement of lobsters from shelter by crabs and cumulative agonistic interactions instigated by crabs towards lobsters scaled positively with H.sanguineus density. Given the similarity of the invasion documented here and others occurring globally, these experiments demonstrate the importance of considering how factors such as ontogeny, density-dependence, and primacy influence the outcomes of interactions between intraguild predators. Disentangling how these factors structure intraguild interactions between invasive and endemic species will both advance our fundamental understanding of community ecology and enhance efforts to conserve and manage natural resources.