William G. Bissett1
Patricia A. Ramey-Balci
Pedro A. Quijón- 1Coastal Ecology Laboratory, Department of Biology, University of Prince Edward Island, Charlottetown, PE, Canada
- 2Department of Biological Sciences, University of Manitoba, Winnipeg, MN, Canada
By creating habitats or influencing the immediate physical environment, ecosystem engineers shape the diversity, function and services provided by ecosystems. Thus, the disruption of these species is relevant given their broad influence on native communities and ecosystems. As such, we review the effects (positive, negative, or neutral) of a widespread invasive species, the European green crab (Carcinus maenas) on key coastal ecosystem engineers. We examined the literature and focused on 53 published studies to assess reported impacts on well-known macrophytes, mussels, oysters and clams. Despite the wide range of response variables measured and reported, green crab effects were overwhelmingly negative. These effects were mediated by direct (through consumption and sediment burrowing) or indirect mechanisms (through seed consumption, alteration of habitat quality or effects on related species), and were often context dependent. These conclusions are limited by ongoing green crab expansions where possible impacts have not been yet documented, and by cases of neutral or minor impacts that remain unpublished. Green crab effects often result in disruption rather than the loss of local ecosystem engineers, but they clearly add to the ongoing effects of other global stressors.
1 Introduction and approach
Coastal ecosystems are exposed to multiple anthropogenic stressors, including the arrival of an increasing number of invasive species (Ruiz et al., 1997; Stachowicz et al., 2002; Byrnes et al., 2007; Bailey et al., 2020). While some invaders cause minor changes, others trigger cascading effects that amplify their ecological influence on communities or ecosystems. The extent of these effects depends on the nature of the invader (Capelle et al., 2015) and the species that they target upon establishment. The European green crab (Carcinus maenas) is a voracious omnivorous predator that has been labeled one of the world’s 100 worst invasive species (Lowe et al., 2000). This crustacean has spread to most coastal regions, and its diet includes a wide variety of prey (e.g., Ropes, 1968; Cohen et al., 1995; Baeta et al., 2006; Cordone et al., 2022; Fisher et al., 2024) including a key group of species that, given their role, are referred to as ecosystem engineers (Jones et al., 1994). These species create or transform the habitat (autogenic or allogenic engineers, respectively; Jones et al., 1994), enhancing diversity (Romero et al., 2015), and changing the function and services that communities and ecosystems provide (Tsuchiya and Nishihira, 1986; Bos et al., 2007; Barbier et al., 2011; Scherer and Reise, 1981).
Coastal ecosystem engineers encompass plants and animals operating from micro- to macro-benthic communities, but the groups that have gathered the most attention include macrophytes and a wide variety of bivalves (e.g., Gutiérrez et al., 2003; Matheson et al., 2016). We argue that examining the impacts of invasive species on ecologically important ecosystem engineers as a distinct group is timely and meaningful, as these effects may shape the influence that invaders ultimately have on native communities and ecosystems. Hence, using the green crab as an aggressive and widespread model invader (Baeta et al., 2006), this Minireview examines the main habitat-forming or modifier ecosystem engineers this species has come to interact with, the types of studies conducted, the nature of the effects commonly reported –whether direct or indirect and whether positive, negative or neutral–. In doing so, we aim to identify consistent findings across studies, species and regions, and highlight knowledge gaps that warrant further investigation.
We examined the published literature and identified 53 studies (Table 1) describing and quantifying green crab effects on ecosystem engineers. Studies were found through Google Scholar, available academic databases, and online networks (e.g., ResearchGate). We used a series of keywords (and their combinations), including but not restricted to, “Carcinus maenas”, “invasion”, “ecosystem engineer” “foundation species”, “seagrass”, “saltmarsh”, “mollusc”, “bivalve”, “clam”, “mussel”, “oyster”, “native macrofauna”, and “native community”, in addition to the species names of known ecosystem engineers, and articles’ cross-references. We therefore circumscribed the search of engineers to the groups best represented in the published literature, i.e., macrophytes (seagrass, saltmarsh and macroalgal species) and bivalves (mussels, oysters and clams). Moreover, a key step in the inclusion of a species in the list of ecosystem engineers was the confirmation (by published sources) of its status as such. For some well-researched species (e.g., the blue mussel, Mytilus edulis), the number of studies was purposedly limited to avoid unnecessary repetition of relatively well-known effects. In this case, only studies explicitly addressing a green crab effect on the engineer (e.g., reporting rates of interaction rather than simply including the species as part of a community invaded by green crabs) were retained. While most studies refer to invaded regions, a few refer to the effects of green crabs on their native range of distribution.
Table 1. Summary of 53 studies reporting the influence of the green crab (Carcinus maenas) on prominent ecosystem engineers (EE), grouped as macrophytes (MP), mussels (MU), oysters (OY), and clams (CL).
2 Influence on seagrasses and other macrophytes
Green crabs have a broad diet (Ropes, 1968; Le Roux et al., 1990; Baeta et al., 2006), but their consumption of seagrass tissue is restricted to a few records of clipping and shredding (Neckles, 2015; Howard et al., 2019) or grazing upon tender shoot meristems of eelgrass (Malyshev and Quijón, 2011). Most reported impacts (Table 1) are the result of crab burrowing in the search for shelter or other sources of food, a process by which they damage roots and rhizomes (Prystay et al., 2023), impacting their stability, dislodging, or uprooting entire plants (Davis et al., 1998; Neckles, 2015; Matheson et al., 2016). As an example, in the northwestern Atlantic, green crabs have drastically reduced eelgrass (Zostera marina) shoot densities in areas of New Hampshire and Maine (USA), Nova Scotia and Newfoundland (Canada). In New Hampshire, green crabs reduced the survival of eelgrass transplants almost four times compared to green crab exclusions (Davis et al., 1998). In Nova Scotia, eelgrass declines reached up to 75% in a short (4-month) period (Garbary et al., 2014), while in Newfoundland, eelgrass beds saw a milder 27% decline over a 14-year period (Matheson et al., 2016). The latter study also showed that across 20 sites in Placentia Bay, four were found to be devoid of eelgrass due to crab burrowing, with one site experiencing up to a 90% reduction in shoot abundance, due primarily to the digging by large male crabs (Matheson et al., 2016).
Green crab indirect impacts include the consumption of seagrass seeds, limiting or reducing spread potential and the subsequent seasons’ survival (Unsworth et al., 2024; Infantes et al., 2016). While seed consumption seems opportunistic, appearing when alternative food is unavailable, at least one study conducted in Sweden reported signs of preference. Compared with two other consumers, a hermit crab and a sea urchin, green crabs consumed 2–7 times more seeds, and a single green crab was recorded to consume 73% of the available seeds over a week-long study period (Infantes et al., 2016). Schooler et al. (2022) also reported green crabs eating over 10 eelgrass seeds per day in Coos Bay, Oregon, USA, a behavior that also impairs the success of restoration seagrass initiatives (Infantes et al., 2016). Green crab activities cause resuspension of fine sediments, which covers eelgrass blades which either suffocates them (Neckles, 2015) or reduces the plant’s ability to photosynthesize (Garbary et al., 2014). Green crabs also consume macro- or meso-grazers that feed primarily on algae, causing a feeding release that prompts algal overgrowth on eelgrass beds deterring its condition and growth (Infantes et al., 2016). Similar indirect effects are likely common but have not been documented.
Sediment burrowing has been shown to have a strong effect on at least two saltmarsh species, Sporobolus foliosa and S. alterniflorus (formerly Spartina foliosa and S. alterniflora, respectively). In San Francisco Bay, USA, the green crab alone or in combination with stressors like sea-level rise, accounted for at least 60% of the loss of saltmarsh stems (in some cases reaching up to 90%; Gonzalez et al., 2024a, 2024b). Meanwhile, on the Atlantic coast Bertness and Grosholz (1985) showed that green crabs indirectly harm the stability of saltmarshes by consuming a second and closely associated ecosystem engineer, the ribbed mussel (Geukensia demissa). In sharp contrast, and among the few positive effects of green crabs, Bertness and Coverdale (2013) found that green crab predation on purple marsh crabs (Sesarma reticulatum), a grazer and bioturbator that degrades saltmarshes, facilitated the recovery of S. alterniflorus marshes. A similar positive mechanism may occur in the southwest Atlantic, where recently established populations of green crabs are becoming likely predators of Neohelice granulata. Like the purple marsh crab, N. granulata is detrimental to Patagonian marshes (S. alterniflorus and S. densiflorus), so if predation by green crabs is confirmed to be substantial, it may indirectly benefit these plants as well (Battini and Bortolus, 2020).
Lastly, green, red, and brown macroalgae often appear in the diet of green crabs in variable amounts and proportions (see Ropes, 1968; Le Roux et al., 1990; Griffiths et al., 1992; Baeta et al., 2006). However, no article has yet coined green crabs as primarily herbivore species, so the consumption of macroalgae is most often deemed “occasional” or “secondary” to alternative prey like bivalves. One example is the consumption of proportionally small amounts of a variety of Irish moss (Chondrus crispus) in Atlantic Canada (Tummon Flynn et al., 2019). These authors showed that green crabs consume some biomass and physically disrupt the macroalgal fronds. However, the actual impact of the crab is mediated by its consumption of associated blue mussels (Mytilus edulis), which this variety of Irish moss uses for anchoring to the sea floor forming entangled clumps (Tummon Flynn et al., 2019; 2020; Gibbons et al., 2024).
3 Influence on bed-forming mussels
Strong green crab consumptive effects upon various species of mussels have been well-documented across various coastal regions, both in correlation-based and experimental studies (Table 1). In the northwest Atlantic, green crabs accounted for roughly 550 kg ha-1 of blue mussel losses (Mytilus edulis), with a peak daily loss of 25.92 kg ha-1 over a 40-month coverage experiment in the Menai Strait (Murray et al., 2007). Similar (but widely variable) impacts have been reported from Denmark, where green crabs reduced blue mussel biomass by ~4 kg m-2 (Banke et al., 2024), and from the Netherlands, where small mussel seed was consumed at rates of up to 19 seeds d-1 (Kamermans et al., 2009). Green crabs also feed on blue mussels across the Atlantic (Matheson and Mckenzie, 2014; Pickering and Quijón, 2011; Miron et al., 2005), at rates considerably higher than native crab species such as the rock crab (Cancer irroratus; Miron et al., 2005). As ectotherms, feeding rates are directly influenced by temperature changes, as shown in Newfoundland (Matheson and Mckenzie, 2014) and the UK (Murray et al., 2007), whereby the latter study reported feeding rates six times higher at 13°C compared to 6°C. Green crabs have also been shown to have negative effects on populations of at least three other closely related species of mussels: the Pacific blue mussel (M. trossulus), gallo mussels (M. galloprovincialis), and purple mussels (Perumytilus purpuratus). In the Northeast Pacific large green crabs prefer and consume large amounts of Pacific blue mussels (Behrens Yamada and Hunt, 2000) and gallo mussels (Curtis et al., 2012), whereas gallo mussels are heavily preferred over invasive bivalves in Portugal (Veiga et al., 2011). Juvenile green crabs are also effective consumers of early (<20 mm shell length [SL]) stages of this species in the northeast Atlantic (U.K.; Morton and Harper, 2008).
Green crabs recently arrived at the southwestern Atlantic (Patagonian coast), where purple mussels and Brachidontes rodriguezii, form rocky intertidal beds of “scorched mussels”. Green crabs have been observed feeding heavily on purple mussels (Hidalgo et al., 2007; Cordone et al., 2022), but no reports of predation on the second species have been documented yet, although it is likely to occur. In the South Pacific (Australia), green crabs feed heavily on two other mussels: Xenostrobus inconstans and X. securis (Campbell et al., 2019; Bateman, 2017). For both male and female green crabs, X. inconstans is a preferred prey (> 82% in preference trials) over cockles (Katelysia peronii) due to its softer shell (Campbell et al., 2019). Meanwhile, X. securis is consumed at higher rates than that of native predators, threatening a potential overconsumption of this species’ local populations (Bateman, 2017). Green crabs have also been reported to consume non-indigenous populations of X. securis in the Northeast Atlantic (Portugal). However, feeding rates in this region are lower compared to those measured on native gallo mussels, possibly favoring the establishment of S. securis. As stated above, one additional mussel known to be predated upon by green crabs is the ribbed mussel (G. demissa; Peterson et al., 2014), which is closely associated with saltmarsh species in the Atlantic and Pacific sides of North America (S. alterniflorus and S. foliosa, respectively). Predation on G. demissa becomes important at high predator densities (Peterson et al., 2014) on small mussels (Bertness and Grosholz, 1985; Watt et al., 2011), which reflect prey size preferences (e.g., Kamermans et al., 2009).
4 Influence on bed- and reef- forming oysters
Green crabs are eager consumers of various species of oysters (Table 1), in some cases in much higher proportions than co-occurring native predators (e.g., the rock crab; Miron et al., 2005; Schooler et al., 2022). In the northwest Atlantic, small Eastern oysters (Crassostrea virginica) face up to a 74% mortality in coastal sites colonized by high green crab densities (Poirier et al., 2017). Rates of 14–43% Eastern oyster mortality are more common, but those measured in crab exclusion cages are strikingly lower <1% (Poirier et al., 2017). In this region, the greatest impacts on Eastern oysters are due to large (adult) green crabs (Pickering et al., 2017), although these quickly diminish with an increase in oyster size, until a refuge size is reached at about 35 (Miron et al., 2005) or 40 mm SL (Pickering et al., 2017). In the Northeastern Pacific, a related species (the Pacific flat oyster, Magallana gigas, formerly known as Crassostrea gigas) is consumed by expanding populations of green crabs (Ruesink et al., 2005), at rates of nearly four oysters d-1 (Ens et al., 2021). Green crabs in this region have been shown to use a logistic (type II) functional response, which has the potential to be highly detrimental to oyster beds in the absence of alternative prey for the crabs (Lipcius and Hines, 1986). Magallana gigas is also present in the southwest Atlantic (Patagonian coast), along with populations of Ostrea puelchana (Malvé et al., 2024). Both oyster species are likely to be targeted by green crabs currently expanding in that region, but no studies have quantified these potential impacts yet. Three congeners of the latter species (Ostrea lurida, O. edulis and O. conchaphila) are also heavily consumed by green crabs in the Pacific northwest (Palacios and Ferraro, 2003; Snyder, 2004; Ruesink et al., 2005).
Although green crabs have been reported to consume oysters as well as mussels and clams indiscriminately and irrespective of size (Miron et al., 2005), most studies indicate that this predator shows a preference for mussels and clams over oysters (Mascaro and Seed, 2000; Behrens Yamada and Hunt, 2000; Pickering and Quijón, 2011). This is due in most cases to shell thickness (strength) differences, and therefore profitability (the net energy return beyond effort invested on shell breaking; Juanes, 1992). Profitability also explains the preference of green crabs for small to mid-size oysters and other bivalves (Tan and Beal, 2015; Campbell et al., 2019; Poirier et al., 2017; Matheson and Mckenzie, 2014; Murray et al., 2007; Richards et al., 1999; Mascaró and Seed, 2001). Moreover, Campbell et al. (2019) established that green crabs can consume any bivalves with a shell strength <140 Newtons. Unlike clams (see below), oysters do not have the option of digging into the sediment to reach refuge depths, thus refuge strategies rely on size, shell thickness, and in the case of oyster commercial growth operations (see Poirier and Quijón, 2022), on the physical association with other oysters.
5 Influence on habitat-modifier clams
While suspension and deposit feeding clams do not create physical reefs, they can form dense, widespread beds whereby their engineering activities alter the physical and chemical properties of the local habitat, and green crabs can exert high predation pressure on them (Table 1). The reported impacts of green crabs on clams vary widely and depend on habitat type (see Wong, 2013; Malyshev et al., 2020), even in well-studied species like softshell clams (Mya arenaria) and cockles (Cerastoderma edule). Early records of green crab impacts on the softshell clam in the northwest Atlantic (Maine, USA), date to the 1950s: Correlative studies linked the green crab with a 50% decline in the clam population over the course of four years (Glude, 1955; Welch, 1968). In the same region, Tan and Beal (2015) found that softshell clam survival was seven times higher when protected from green crabs, whereas further north in Atlantic Canada, green crabs targeted primarily small clams (<17 mm SL) and removed nearly 80% in the field (Floyd and Williams, 2004), about 80% in the laboratory (Malyshev et al., 2020), and 45–58% in hatchery tanks (Miron et al., 2005). In the latter two studies, consumption by native predators was much lower. In response to predation risk (green crab presence or odor cues), softshell clams have been shown to dig 12% (Tan and Beal, 2015) or 15% (Whitlow et al., 2003) deeper in the seafloor, and up to two times deeper in laboratory-prepared sediments (Flynn and Smee, 2010). This behavioral response increases clam survival at least three times relative to shallower sediment layers (Whitlow et al. (2003). The balthic clam (Macoma balthica) uses the same escape strategy and digs twice as deep into the sediment when exposed to green crabs (Griffiths and Richardson, 2006).
Another widespread ecosystem engineer, that is heavily preyed on by green crabs in the northeast Atlantic (Wales, UK) is the cockle (Cerastoderma edule), with feeding rates following recruitment events of six cockles d-1 (Mascaro and Seed, 2000) and 30 cockles d-1 (Sanchez-Salazar et al., 1987a) which roughly correspond to 2,360 cockles m-2 (Sanchez-Salazar et al., 1987b). Similarly, in the Dutch Wadden Sea, green crabs accounted for 26.1% of juvenile cockle mortality over one recruitment season (Jensen and Jensen, 1985). The preference of green crabs for small-sized softshell clams (Campbell et al., 2019) and cockles (Mascaró and Seed, 2001) is commonly reported. Green crab effects on other clams have also been observed, although not always quantified. Noticeable examples include two related species of Nutricola (N. confusa and N. tantilla), which are part of the diet of green crabs in California, USA (Grosholz, 2005; Grosholz et al., 2000), juveniles of Katelysia scalarina and Fulvia tenuicostata in Tasmania (Walton et al., 2002; Ross et al., 2004), juveniles of quahogs or hard clams (Mercenaria mercenaria) in New Jersey, USA (Quijón, 2024; Quijón et al., 2025), the California softshell clam (Cryptomya californica) in California, USA (Palacios and Ferraro, 2003), in addition to varnish clams (Nuttallia obscurata) and Manila clams (Venerupis philippinarum) both targeted by green crabs in British Columbia, Canada (Curtis et al., 2012). The continued spread of green crabs makes many additional clam species that are considered as ecosystem engineers likely targets for this predator (e.g., Darina solenoides and Ardeamya petitiana, in the Argentinian Patagonia; Malvé et al., 2024).
6 Common effects, limitations, and further studies
We found that a large majority of the studies reporting green crab effects (51 out of 53) describe a negative influence on ecosystem engineers. In the couple of instances in which neutral or positive effects were reported, these were driven by indirect interactions, in which green crabs targeted herbivores or bioturbator species that were detrimental to ecosystem engineers (e.g., Bertness and Coverdale, 2013). The strength of green crab effects was also variable and difficult to compare given the diverse approaches used and the type of ecosystem engineers studied (i.e., habitat-forming seagrasses and bivalves as opposed to non-habitat forming clam populations). Despite that, some consistent mechanisms became evident. Effects on seagrasses and other macrophytes were primarily mediated by burrowing and sediment disturbance (e.g., Garbary et al., 2014; Gonzalez et al., 2024a), and to a much lesser degree by seed or plant tissue consumption (e.g., Infantes et al., 2016). Likewise, interactions with mussels, oysters and clams were primarily direct (consumptive) effects (e.g., Miron et al., 2005; Campbell et al., 2019), although indirect (non-consumptive) effects were also present (e.g., Flynn and Smee, 2010). The latter was not surprising considering the complexity of oysters and mussels as habitat-forming species (e.g., Cordone et al., 2022), and the ability of clams to engage in escape strategies by e.g. burrowing into the sediment up to refugial depths (Tan and Beal, 2015). As a result, green crabs harm or disrupt (in some cases heavily) local populations of ecosystem engineers, beds or reefs, although there are no reports of losses of ecosystem engineers that could be attributed solely to green crabs. Despite this, green crabs’ wide range of effects on such a diverse group of species clearly contributes to ongoing changes driven by other global stressors (Holland et al., 2021). The examination of their combined effects (additive or synergistic in nature; see Crain et al., 2008) clearly warrants further research.
This review is a first approach to the study of green crab effects on ecosystem engineers. So even though this group of key species is taxonomically much wider, we were not fully comprehensive and focused on a subset of the best-known coastal engineers: seagrass, macroalgae, mussels, oysters and clams. This entailed overlooking a series of other ecosystem engineers (e.g., herbivorous and carnivorous gastropods; Quinn et al., 2012; Wells et al., 2023) which play clearly important roles in their ecosystems. We also must point out two intrinsic practical limitations on the study of invasive species like the green crab. First, several populations of this species are currently expanding their ranges or invasion (e.g., Malvé et al., 2024), and therefore, an unknown number of new interactions with local ecosystem engineers may be taking place but have yet to be documented. Second, the limited number of neutral or positive interactions reported here could be partially related to a lack of reporting of this type of result. The finding of “negative impacts” often gathers more attention, as discussed before in the context of other invasive species (e.g., Quijón et al., 2017). However, it applies to the reporting of ecological interactions in general (Weintraub, 2016), where neutral or positive effects have been less consistently published, despite their recognized importance (Bruno et al., 2003). Moreover, among the negative results that are published, there is also a bias towards reporting the outcome of trophic interactions, disregarding non-trophic interactions (including competition), which are often more difficult to quantify or remain simply overlooked (Kéfi et al., 2012). While a large majority of the effects described in this Minireview are direct (consumptive or not), the examination of indirect effects is gaining growing attention. In fact, under a different context, green crabs have already become a useful model species for the study of trait- or behaviorally mediated indirect interactions (Quinn et al., 2012; Vriends et al., 2024). So, it is reasonable to suggest that for each direct effect reported here, there are likely several indirect interactions that may need to be examined, and that are likely to contribute to the function and services provided by these species and their coastal ecosystems.
Author contributions
WB: Writing – original draft, Writing – review & editing, Investigation, Formal Analysis, Data curation. PR-B: Data curation, Investigation, Writing – original draft, Formal Analysis, Writing – review & editing. PQ: Methodology, Data curation, Formal Analysis, Conceptualization, Supervision, Investigation, Writing – original draft, Funding acquisition, Writing – review & editing.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. Funding sources have been stated in the Acknowledgments.
Acknowledgments
The authors thank the feedback received from Handling Editor and Reviewers. WGB and PAQ thank the support provided by the Natural Sciences and Engineering Research Council, Canada (NSERC), and Fisheries and Oceans Canada during the preparation of the manuscript. PRB thanks the University of Manitoba for start-up funds/seed grant and research study leave.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.
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Keywords: ecosystem engineer, invader, habitat-modifier, macrophytes, bivalves
Citation: Bissett WG, Ramey-Balci PA and Quijón PA (2025) Undermining the foundation: a brief overview of the effects of a widespread invader on coastal ecosystem engineers. Front. Mar. Sci. 12:1614368. doi: 10.3389/fmars.2025.1614368
Received: 18 April 2025; Accepted: 14 May 2025;
Published: 03 June 2025.
Edited by:
Clara Belen Giachetti, CONICET Instituto de Biología de Organismos Marinos (IBIOMAR), ArgentinaReviewed by:
Georgina Florencia Cordone, CONICET Centro de Estudios de Sistemas Marinos (CESIMAR), ArgentinaCopyright © 2025 Bissett, Ramey-Balci and Quijón. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Pedro A. Quijón, cHF1aWpvbkB1cGVpLmNh