Range-extension of invasive and native brachyuran crabs in North-West Iceland

Picture: Svanhildur Egilsdóttir. Picture: Svanhildur Egilsdóttir.

Range-extension of invasive and native brachyuran crabs in North-West Iceland

A study conducted by Jón T. Magnússon, Stephen J. Hawkins, Lilja Gunnarsdóttir, Jörundur Svavarsson and Karl Gunnarsson, published in Journal of the Marine Biological Association of the United Kingdom reveals that Species distributions are changing in response to global warming and the introduction of non-native species often acting in combination. Karl Gunnarsson and Lilja Gunnarsdóttir are employed as marine biologist at the Marine and Freshwater Research Institute.

Ranges are expected to move poleward as temperatures rise, with effects likely to be greater in low diversity systems in higher latitude areas. Such spread can potentially have consequences for resident species, especially in disturbed systems.

First sightings of the Atlantic rock crab in 2006

One recent case of invasion by a non-native species in Iceland is that of the Atlantic rock crab (Cancer irroratus) Say, 1817 from the east coast of North. First sightings date back to 2006 in Hvalfjörður, South-West Iceland; since then C. irroratus has spread to North and East Iceland, primarily in the subtidal zone but also occurring intertidally. The European green crab Carcinus maenas (Linnaeus, 1758), native to South-West Iceland, was later also found further north, in the Breiðafjörður area as its range extended poleward, probably in response to warming waters as a result of climate change.

Collecting crab samples in Borgarfjordur. Picture: Svanhildur Egilsdóttir. 

Despite numerous studies of C. irroratus in the subtidal zone in Iceland, where it has been shown to outcompete its native counterparts, the prevalence of this invasive species in the intertidal zone is yet unknown. Furthermore, in the context of marine ecosystem dynamics, there is a significant knowledge gap concerning the effects of Ascophyllum nodosum (L.) Le Jolis harvesting on associated mobile predators, and specifically crabs. Having a holistic understanding of the ecological consequences of seaweed harvesting, such as influences on the survival and distribution of associated fauna, can contribute to sustainable resource use and management.

Here, the distribution is described, relative abundance and population structure of crab species occupying the intertidal zone at eight sites within Ascophyllum-dominated sheltered shores of Breiðafjörður, where industrial-scale harvesting (~15.000 tons of algae/year) occurs. They are compared to two sites further south in Faxaflói, South-West Iceland, with long-established populations of C. maenas and large adjacent subtidal populations of C. irroratus. Based on previous observations of population changes in the subtidal zone, the hypothesis is tested that H. araneus would diminish in abundance and show changed population structure in the face of spread by the other two species. The relationship between Ascophyllum harvesting, intertidal crab abundance and assemblage composition in Breiðafjörður is also examined as a preliminary test of whether such disturbance affected the spread of C. maenas and C. irroratus.


Quantifying and mapping species distribution are essential for understanding the interplay of anthropogenic impacts such as climate change, introduction of non-native species and harvesting of habitat-forming species. In the study it is demonstrated that, currently, the presence of C. irroratus in the intertidal zone was negligible and that C. maenas is advancing northwards. Successful establishment of C. maenas populations in the intertidal zone in Breiðafjörður bay has led to a displacement and substantial reduction of H. araneus probably by competition for resources. Given its displacement in the subtidal by C. irroratus, current squeezing of H. araneus may accelerate with warming and further spread of C. maenas. This warrants further tracking. Ascophyllum harvesting was not linked to effects on crab species. Nevertheless, a precautionary approach should always be practised, as juvenile individuals are susceptible to nursery habitat degradation should harvesting intensify.

Materials and methods

  • Surveys were made during the summer of 2020 at eight sites in Breiðafjörður, a large semi-enclosed bay (fig. 1A), North-West Iceland and at two reference sites in Faxaflói (fig. 1B), South-West Iceland in July 2022.
  • Eight sampling sites were chosen in Breiðafjörður based on ease of access and known history of harvesting, including some unharvested sites.
  • Sites were surveyed during low-water at spring tides either in the beginning of the afternoon or just after midnight, taking advantage of high latitude continuous daylight during summer.
  • At each of the ten sites, six to fifteen replicated 5-minutes timed searches were made along a transect parallel to the shore (1–5 m wide depending on slope and 5–20 m long depending on abundance) to estimate relative abundance as catch per unit effort (CPUE). The searches were equally split between mid-shore and low-shore.
  • During these 5-minute searches, stones were lifted, the algal canopy and holdfast complexes were thoroughly searched to find as many crabs as possible. After each search, all crabs found were measured (maximum carapace length of araneus and maximum carapace width of C. irroratusand C. maenas) to the nearest millimetre with a caliper, sex was determined, and the condition of the carapace and presence of eggs were noted. Crabs were released to from where they had been sampled.


  • A total of 914 crabs were captured across all eight study sites in Breiðafjörður bay and the two reference sites further south in Faxaflói bay from nearly 900 minutes of searching (~1 individual on the average per minute of searching).
  • The commonest species across all sites was araneus (found at all sites in Breiðafjörður but absent from Bolaklettar, Faxaflói). C. maenas was dominant at the southern sites in Faxaflói and at the southernmost site in Breiðafjörður. Very few C. irroratus were found intertidally.
  • araneus was numerically dominant at all sites except in the southern-most part of Breiðafjörður, where C. maenas was advancing and becoming dominant, especially on the mid-shore.
  • Within Breiðafjörður, there was higher crab abundances (as CPUE) on the south side than at sites in the north side. The more abundant southern sites had patchier distributions, reflected in larger SE. Abundances in the innermost part of Breiðafjörður were intermediate to those found to the south and north shores of Breiðafjörður.
  • More crabs were found lower on the shore, except at the reference sites in Faxaflói.
  • The effect of harvesting was only assessed in Breiðafjörður to avoid geographically confounding factors. When testing the species separately, there was not a significant correlation of abundance of araneus with time since last harvest, but a significant positive correlation with years since last harvest was found for C. maenas. This is driven largely by high numbers at the unharvested site at the front-edge of its poleward advance, combined with limited presence at other sites, leading to many zero scores. Correlation analysis for C. irroratus was not attempted because of their infrequent occurrences.
  • There were no significant differences when comparing crab abundances of each search of both shore levels at sites with different harvesting histories and there were also no differences when comparing a control area with many years since last harvest with nearby sites more recently harvested.
  • At sites where araneus was the only species present in the intertidal zone, two different patterns in size structure were found: large numbers of small individuals in the 10–12 mm size class, indicating a strong recruitment event and similar mean and medians as all other sites but with a larger dominating size class (24–26 mm).
  • The references sites in Faxaflói and Maðkavík [3] were dominated by maenas, showing large numbers of small crabs, indicating strong and recent recruitment, as well as some mature individuals. These sites had the lowest numbers of H. araneus.
  • Significant differences in size frequency distributions were found for araneus in sites where it is the sole crab species versus sites where it is found with C. maenas and or C. irroratus indicating negative interactions with these newcomers.


The study was the first to successfully show considerable tide-out occupancy in the Ascophyllum-dominated intertidal zone in Iceland by three brachyuran crabs (namely H. araneus, C. maenas and C. irroratus). Populations of the most common crab, H. araneus were composed of new recruits, immature individuals and juvenile males with fewer females, suggesting a nursery function for a species that is found subtidally.

Prior to the research, there were no records of C. irroratus nor C. maenas being present at these sites in Breiðafjörður. Introduction new brachyuran species into an ecosystem can have a wide arrange of effects, such as alteration of population dynamics (Sigurdsson & Rochette, 2013; Matheson et al., 2016), resource competition (Epifanio, 2013), biodiversity loss (Falk-Petersen et al., 2011; Clavero et al., 2022) or impact on prey populations and predator-prey relationships (Lohrer & Whitlatch, 2002). Documenting the abundance and population structure of these crabs expands our understanding of global change in high latitude intertidal ecosystems by providing valuable insights into the changing distribution patterns.

Within Breiðafjörður, H. araneus dominated all sites visited except Maðkavík [3] where C. maenas was the most abundant, especially in the mid shore level, where it has completely replaced H. araneus. It is confirmed that C. maenas is advancing poleward at the northern edge of its natural range in Europe and Western Iceland. While C. maenas is considered native to Icelandic waters, its current expansion due to increasing temperatures raises the question about its ecological role in these receiving ecosystems, turning it into a potential “climate migrant”, with implications reminiscent of an invasive species. The extent of the impact the species will have in the area will depend on many factors, such as interspecific interactions, rate of expansion, community adaptability and resilience or other changing ecological conditions. As C. maenas expansion continues, it is crucial that these changes are monitored to better understand further ecological consequences.

The occupancy of C. irroratus in the intertidal zone was limited. Despite being present in six out of ten sites in Breiðafjörður and at one of the reference sites, abundances were always low. Even with these low densities, the study found some changes in the size structure of H. araneus when comparing sites with the presence of C. irroratus to those where it was absent. These findings suggest that interspecific interactions between both species exist, underscoring the importance of considering such ecological interaction when examining community dynamics and population structure in the future.

This leads us to hypothesize that, in near future, H. araneus may be excluded from the intertidal zone at progressively northern sites by advancing C. maenas, particularly at mid-shore levels. Such a pattern reflects what is found further south in Europe and around the British Isles, where H. araneus is primarily subtidally distributed (Hartnoll, 1963), only occasionally found intertidally usually in low-shore pools, and never under Ascophyllum canopies (Gollety et al., 2011, S. J. Hawkins, unpublished observations over four decades of observations on Manx, Welsh and south-west shores). Additionally, its subtidal niche could also be compromised as C. irroratus proliferates; essentially being squeezed from below by C. irroratus, a proliferating non-native in the subtidal zone and from above by C. maenas, an advancing native in the intertidal zone.

The overall impact of harvesting on the benthic community will highly depend on frequency, tools, seasonality and magnitude of harvesting as well as the phenological attributes of the community. Moderate harvesting of Ascophyllum nodosum canopy has little long-term effects on the algal and associated benthic faunal community. However, more intense harvesting can have longer-lasting negative effects on the number of species, or animal and algal cover. Nevertheless, the results show undetectable effects – albeit with low power and some confounding factors – are consistent with studies and reports from Ireland which found very minor effects of Ascophyllum harvesting on megafauna.

Harvesting strategies should be in line with the phenological characteristics of the canopy species as well as the ecological characteristics of the community to allow fast recovery, maintain a sustainable biomass and avoid long-term degradation of the benthic community.

Link to the journal in Journal of the Marine Biological Association of the United Kingdom

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