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The northern acorn barnacle (Semibalanus balanoides) is a robust system to study the genetic basis of adaptations to highly heterogeneous environments. Adult barnacles may be exposed to highly dissimilar levels of thermal stress depending on where they settle in the intertidal (i.e., closer to the upper or lower tidal boundary). For instance, barnacles near the upper tidal limit experience episodic summer temperatures above recorded heat coma levels. This differential stress at the microhabitat level is also dependent on the aspect of sun exposure. In the present study, we used pool-seq approaches to conduct a genome wide screen for loci responding to intertidal zonation across the North Atlantic basin (Maine, Rhode Island, and Norway). Our analysis discovered 382 genomic regions containing SNPs which are consistently zonated (i.e., SNPs whose frequencies vary depending on their position in the rocky intertidal) across all surveyed habitats. Notably, most zonated SNPs are young and private to the North Atlantic. These regions show high levels of genetic differentiation across ecologically extreme microhabitats concomitant with elevated levels of genetic variation and Tajima's D, suggesting the action of non-neutral processes. Overall, these findings support the hypothesis that spatially heterogeneous selection is a general and repeatable feature for this species, and that natural selection can maintain functional genetic variation in heterogeneous environments. publishedVersion

Background Natural populations inhabiting the rocky intertidal experience multiple ecological stressors and provide an opportunity to investigate how environmental differences influence microbiomes over small geographical scales. However, very few microbiome studies focus on animals that inhabit the intertidal. In this study, we investigate the microbiome of the intertidal barnacle Semibalanus balanoides. We first describe the microbiome of two body tissues: the feeding appendages, or cirri, and the gut. Next, we examine whether there are differences between the microbiome of each body tissue of barnacles collected from the thermally extreme microhabitats of the rocky shores’ upper and lower tidal zones. Results Overall, the microbiome of S. balanoides consisted of 18 phyla from 408 genera. Our results showed that although cirri and gut microbiomes shared a portion of their amplicon sequence variants (ASVs), the microbiome of each body tissue was distinct. Over 80% of the ASVs found in the cirri were also found in the gut, and 44% of the ASVs found in the gut were also found in the cirri. Notably, the gut microbiome was not a subset of the cirri microbiome. Additionally, we identified that the cirri microbiome was responsive to microhabitat differences. Conclusion Results from this study indicate that S. balanoides maintains distinct microbiomes in its cirri and gut tissues, and that the gut microbiome is more stable than the cirri microbiome between the extremes of the intertidal.

Human growth has caused an unprecedented increase in artificial light at night (ALAN). In coastal habitats, many species rely on day/night cycles to regulate various aspects of their life history and these cycles can be altered by this stressor. This study assessed the influence of ALAN on the early (cyprid) and late (spat) settlement stages of the acorn barnacle Semibalanus balanoides, a species widely distributed in natural and man-made coastal habitats of the North Atlantic. A newly designed settlement plate, originally for studies in rocky intertidal habitats in the southeast Pacific, was adapted to measure settlement rates on man-made habitats -wharf seawalls- located in Atlantic Canada. Plates equipped with a small LED diode powered by an internal battery (ALAN plates) were used to quantify settlement rates in comparison to plates lacking a light source (controls). These plates were deployed for 6 d in the mid-intertidal levels, where adult barnacles were readily visible. ALAN and control plates collected large number of settlers and showed to be suitable for this type of man-made habitats. The number of early settlers (cyprids) did not differ between plates but the number of late settlers (spat) was significantly lower in ALAN plates than in controls. These results suggest that light pollution has little influence on the early stages of the acorn barnacle settlement but is clearly detrimental to its late stages. As barnacles dominate in many natural and man-made hard substrates, it is likely that ALAN also has indirect effects on community structure.

Climate change has already altered the environmental conditions of the world’s oceans. Here we report declines in gastropod abundances and recruitment of mussels (Mytilus edulis) and barnacles (Semibalanus balanoides) over the last two decades that are correlated with changes in temperature and ocean conditions. Mussel recruitment is declining by 15.7% per year, barnacle recruitment by 5.0% per year, and abundances of three common gastropods are declining by an average of 3.1% per year (Testudinalia testudinalis, Littorina littorea, and Nucella lapillus). The declines in mussels and the common periwinkle (L. littorea) are correlated with warming sea temperatures and the declines in T. testudinalis and N. lapillus are correlated with aragonite saturation state, which affects rates of shell calcification. These species are common on shores throughout the North Atlantic and their loss is likely to lead to simplification of an important food web on rocky shores., Petraitis and Dudgeon report a decline in mussel and gastropod abundance and recruitment on rocky intertidal shores of the Gulf of Maine over a period of 20 years. These declines are correlated with increased water temperatures and aragonite saturation state, which affects rates of shell calcification, and suggests environmental impacts on the food webs of rocky shores.

Population connectivity plays a major role in the ecology and evolution of marine organisms. In these systems, connectivity of many species occurs primarily during a larval stage, when larvae are frequently too small and numerous to track directly. To indirectly estimate larval dispersal, ocean circulation models have emerged as a popular technique. Here we use regional ocean circulation models to estimate dispersal of the intertidal barnacle Semibalanus balanoides at its local distribution limit in Southwest England. We incorporate historical and recent repatriation events to provide support for our modeled dispersal estimates, which predict a recolonization rate similar to that observed in two recolonization events. Using graph theory techniques to describe the dispersal landscape, we identify likely physical barriers to dispersal in the region. Our results demonstrate the use of recolonization data to support dispersal models and how these models can be used to describe population connectivity.

A twenty-five year (1994–2018) survey at 10 rocky sites within Lough Hyne Marine Reserve (SW Ireland) showed that, despite considerable spatial and temporal patchiness, three barnacle species (Chthamalus stellatus, Semibalanus balanoides, and Austrominius modestus) declined in overall abundance in the early 2000s, particularly at three sites. There was no clear response of A. modestus or C. stellatus to the cold winters of 2010 and 2011. In 2004, however, S. balanoides declined suddenly, but showed a slight recovery after the cold winter of 2010 to maintain sparse populations in the lough, but remained abundant outside the lough. A fourth species, Chthamalus montagui, showed a different pattern, with wide fluctuations in abundance but no overall decline. Changes in the Atlantic Multidecadal Oscillation may have influenced the overall composition of barnacle populations. Rising maximum air temperatures, resulting in high body temperatures recorded for A. modestus even during mild days, are suggested to have impacted the barnacle populations as a result of the prolonged aerial exposure times caused by the lough's asymmetric tidal pattern.

In intertidal communities, consumers (especially carnivorous gastropods) have historically been thought to exert strong top-down control on community composition by regulating competitively dominant mussels and barnacles. This paradigm was formulated based on wire mesh cage exclusion experiments, which have well-known artifacts such as altering hydrodynamics and excluding non-target, but potentially important, consumer species. Recent research highlights the potential importance of multiple consumers that are often subtle and transient, as well as the modifications of consumer pressure by spatial and temporal environmental variability and bottom-up processes. Manipulative experiments that target individual taxa will be essential to more clearly identify their roles in complex ecological communities. The predatory gastropod Nucella lapillus has long been considered an important consumer controlling the structure and dynamics of intertidal communities in the Gulf of Maine. To test the role of N. lapillus in shaping community structure, we manually reduced its densities for 2.33 years. Species composition, stable stage community composition (based on a Markov model), and transition probabilities (as measures of ecological pathway strength) were compared between treatments (control vs. density reductions), and among seasons (spring, summer, fall, winter). In contrast to previous studies, exclusions had no effect on community composition or long-term Markov model predictions of stable stage community structure. Reducing N. lapillus abundance increased the persistence and reduced the mortality of the barnacle Semibalanus balanoides in the model, but did not affect blue mussels, Mytilus edulis , in a similar way. Reducing N. lapillus abundance had additional indirect effects of increasing Ralfsia spp. mortality and increasing hydroid persistence. Despite differences in transition probabilities among treatments, compensatory changes in direct and indirect pathways led to communities that converged over the long-term. Our results contrast previous estimates of the role of N. lapillus in intertidal communities and might reflect low mussel recruitment, predation by alternate consumers (e.g., C. maenas , C. borealis , Tautogolabrus adspersus , birds), or spatial and/or temporal environmental variation that influenced the role of N. lapillus in structuring these communities. The role of N. lapillus may be more limited, or variable, than often assumed in the Gulf of Maine, and the methods manipulating broad functional groups such as “consumers”, though useful in developing basic conceptual models, blur the effects of individual species in community assembly and dynamics.

Space is one of the primary limiting resources for organisms on the intertidal rocky shore. This paper examined the effect of reduced density on key traits (mortality and growth) on the intertidal barnacles, Chthamalus montagui and Semibalanus balanoides, on the mid-shore in Plymouth, UK. Intra- and interspecific treatments comprising of C. montagui and S. balanoides were manipulated to reduce densities at two similar sites. Changes in mortality and operculum growth were assessed over an 8-week period using digital photography. Covariates of growth included nearest neighbour distance, competition between closest pairs and initial size. Conflicting patterns were observed when comparing growth rates between treatments and sites. At Site 1, interspecific treatments had a lower growth rate than intraspecific treatments, whereas at Site 2, interspecific growth rates were higher. ANCOVA showed that nearest neighbour distance had no significant effect on growth, but when comparing differences in growth of closest neighbouring pairs, C. montagui treatment showed evidence of competition whereas S. balanoides did not. ANCOVA analysis indicated no difference in growth between each outcome of pair competition, suggesting winners are initially bigger than losers. Comparisons of mortality between treatments indicated mortality over time with no significant differences observed between treatments, but response surface methodology (RSM) revealed no effects of competition on mortality of S. balanoides, but negative effects of both intra- and interspecific competition on C. montagui survivorship. Examination of natural populations of barnacles in the mid-shore indicated there was strong spatial variation in growth rates, perhaps driven by small-scale differences within sites.

Barnacle recruitment is often studied in rocky intertidal habitats due to the relevant role that barnacles can play in intertidal communities. In 2014, barnacle (Semibalanus balanoides) recruitment was measured at high elevations in wave-exposed intertidal habitats on the NW Atlantic coast in Nova Scotia, Canada. Values were considerably lower than previously reported for middle elevations in wave-exposed intertidal habitats on the NE Atlantic and NE Pacific coasts. To determine if such differences in recruitment may have resulted from elevation influences, I did a field experiment in 2019 in wave-exposed intertidal habitats in Nova Scotia to test the hypothesis that recruitment is higher at middle than at high elevations, based on known environmental differences between both elevation zones. Based on data from three locations spanning 158 km of the Nova Scotia coast, barnacle recruitment was, on average, nearly 200 % higher (and recruits were larger) at middle than at high elevations. However, even with this increase, barnacle recruitment on this NW Atlantic coast is still lower than for comparable habitats on the NE Atlantic and NE Pacific coasts, and also lower than previously reported for wave-exposed locations farther south on the NW Atlantic coast, in Maine, USA. Therefore, barnacle recruitment in wave-exposed intertidal environments in Nova Scotia appears to be only moderate relative to other shores. This difference in the supply of barnacle recruits might influence the intensity of interspecific interactions involving barnacles.

Barnacles are conspicuous members of rocky intertidal communities and settlement of the final larval stage, the cyprid, is influenced by the presence of biofilms. While modulation of cyprid settlement by biofilms has been studied extensively, the acquisition of a specific microbiome by the settling larva has not. This study investigated settlement in the field of Semibalanus balanoides in two consecutive years when the composition of the benthic bacterial community differed. In both years, settling cyprids adopted a specific sub-set of benthic bacteria that was distinct from the planktonic cyprid and the benthos. This microbiome was consistent, regardless of annual variability in the benthic community structure, and established within hours of settlement. The results imply that a natural process of selection occurs during the critical final transition of S. balanoides to the sessile form. The apparent consistency of this process between years suggests that optimal growth and survival of barnacles could depend upon a complex inter-kingdom relationship, as has been demonstrated in other animal systems.

Benthic–pelagic coupling refers to the ecological relationships between benthic and pelagic environments. Studying such links is particularly useful to understand biological variation in intertidal organisms along marine coasts. Filter-feeding invertebrates are ecologically important on marine rocky shores, so they have often been used to investigate benthic–pelagic coupling. Most studies, however, have been conducted on eastern ocean boundaries. To evaluate benthic–pelagic coupling on a western ocean boundary, we conducted a 5-year study spanning 415 km of the Atlantic coast of Nova Scotia (Canada). We hypothesized that the summer size of intertidal barnacles (Semibalanus balanoides) recruited in the preceding spring would be positively related to the nearshore abundance (biomass) of phytoplankton, as phytoplankton constitutes food for the nauplius larvae and benthic stages of barnacles. Every year between 2014 and 2018, we measured summer barnacle size in clearings created before spring recruitment on the rocky substrate at eight wave-exposed locations along this coast. We then examined the annual relationships between barnacle size and chlorophyll-a concentration (Chl-a), a proxy for phytoplankton biomass. For every year and location, we used satellite data to calculate Chl-a averages for a period ranging from the early spring (when most barnacle larvae were in the water) to the summer (when barnacle size was measured after weeks of growth following spring benthic recruitment). The relationships were always positive, Chl-a explaining nearly half, or more, of the variation in barnacle size in four of the five studied years. These are remarkable results because they were based on a relatively limited number of locations (which often curtails statistical power) and point to the relevance of pelagic food supply to explain variation in intertidal barnacle size along this western ocean boundary coast.

The abundant-centre hypothesis (ACH) predicts that the density of a species should peak at its distribution centre and decrease similarly towards distribution margins. The ACH has been deduced from a theory that postulates that environmental conditions should be most favourable for a species at the centre of its distribution. This idealised density pattern, however, has been supported by limited field studies, as natural patterns are often more complex. It is thus of interest to examine under what conditions compliance with the ACH could be favoured. Such conditions could be smooth environmental gradients with limited habitat patchiness throughout the distribution range of a species. Thus, we tested the ACH by measuring the density of an intertidal barnacle (Semibalanus balanoides) across its full vertical distribution range (from low to high elevations) on a rocky shore with similar substrate properties across elevations. To do a reliable test, we surveyed eight elevation zones applying an equal sampling effort in each zone. Average barnacle density conformed to the ACH, as it peaked at the middle of the vertical distribution range of this species. The same underlying theory predicts a similar unimodal pattern for maximum body size, but this trait was decoupled from density, as maximum barnacle size increased from low to high elevations. Overall, although the ACH is not a universal predictive tool as once envisioned, it may predict some cases well, as shown by this study. Therefore, the ACH should not be discarded completely, and its domain of application should be further evaluated.

Spatial patterns in aggregations form as a result of the interplay between costs and benefits experienced by individuals. Such self-organisation of aggregations can be explained using a zonal model in which a short-range zone of repulsion and longer-range zone of attraction surrounding individuals leads to emergent pattern properties. The signal of these processes can be detected using spatial pattern analyses. Furthermore, in sessile organisms, post-settlement mortality reveals the relative costs and benefits of positions within the aggregation. Acorn barnacles are known to require contact with conspecifics for reproduction and are therefore believed to aggregate for this purpose; isolated individuals may also be more susceptible to abiotic stress and predation. At short distances, however, competition for space and resources is likely to occur. In this study spatial patterns of barnacles (Semibalanus balanoides L.) were analysed using pair-correlation functions. Individuals were dispersed at distances below 0.30 cm, but peak relative density occurred at a distance of 0.36 cm from conspecifics. This is much closer than required for reproductive access, implying a strong aggregative drive, up to the point of physical contact with neighbours. Nevertheless, analysis of dead barnacles illustrated that such proximity carries a cost as barnacles with many neighbours were more likely to have died. The inferences obtained from these patterns are that barnacles aggregate as closely as they can, and that local neighbourhood competition is a powerful determinant of mortality. These processes give rise to the observed pattern properties.

Barnacle recruitment is often studied in rocky intertidal habitats due to the relevant role that barnacles can play in intertidal communities. In 2014, recruitment of the barnacle Semibalanus balanoides was measured at high elevations in wave-exposed intertidal habitats on the NW Atlantic coast in Nova Scotia, Canada. Values were considerably lower than previously reported for middle elevations in wave-exposed intertidal habitats on the NE Atlantic and NE Pacific coasts. To determine if such differences in recruitment may have resulted from elevation influences, I did a field experiment in 2019 in wave-exposed intertidal habitats in Nova Scotia to test the hypothesis that recruitment is higher at middle than at high elevations, based on known environmental differences between both elevation zones. Based on data from 3 locations spanning 158 km of the Atlantic coast of Nova Scotia, barnacle recruitment was, on average, nearly 200% higher (and recruits were larger) at middle than at high elevations. However, even with this increase, barnacle recruitment on this NW Atlantic coast is still lower than for comparable habitats on the NE Atlantic and NE Pacific coasts, and also lower than previously reported for wave-exposed locations farther south on the NW Atlantic coast, in Maine, USA. Therefore, barnacle recruitment in wave-exposed intertidal habitats on the Atlantic coast of Nova Scotia is moderate relative to other shores. This difference in the supply of barnacle recruits might influence the intensity of interspecific interactions involving barnacles.

The ability to survive freezing has repeatedly evolved across multiple phyla. This suggests that the mechanisms of freeze tolerance must be readily evolvable from basal physiological traits. While several biochemical correlates to freeze tolerance have been described, the mechanism that confers freeze tolerance is still not well understood. To understand both the basic biochemical mechanisms of freeze tolerance as well as their role in local adaptation at the poleward range edge, we conducted a transcriptomic study on two populations (one from the poleward range margin in the White Sea, Russia, and one from the central coast of British Columbia, Canada) of the cold water acorn barnacleSemibalanus balanoideson a time series following a freezing event. We found that the British Columbia population (at the equatorward range margin) was significantly less freeze tolerant than the White Sea population (at the poleward range margin). After assembling and annotating ade novotranscriptome forS. balanoides, we found that the patterns of differential transcript expression following freezing were almost entirely non-overlapping between the two populations, with the White Sea population expressing a series of heat shock proteins in response to freezing stress as well as several aquaporins, while the British Columbia population expressed a series of proteases instead, indicating severe protein damage. We found strong evidence of purifying selection on the significantly upregulated transcripts in the White Sea population, suggesting local adaptation to freezing threat. Taken together, this shows the importance of freeze tolerance to population survival at the poleward range margin, and highlights the central roles of aquaporins and heat shock proteins to the trait of freeze tolerance across taxa.

Ice can be an important structuring factor physically removing intertidal flora and fauna. At high latitudes in particular, the removal of canopy-forming algae by ice scour may be important as their canopy may serve to modify the extreme environment for marine organisms at low tide. We simulated the effect of ice scouring by manipulating the biomass of the canopy-forming algae Ascophyllum nodosum in a sub-Arctic fjord [“Full canopy,” “Reduced canopy,” “Bare (start),” “Bare (annual)”]. Over a three-year period, we quantified key physical parameters and the recolonization of flora and fauna to test the hypothesis that A. nodosum and rock rugosity facilitate recolonization of sub-Arctic intertidal fauna and that potential facilitation could rely on an ability of A. nodosum canopy to modify air temperature and ice scour. Finally, we estimated the recovery period of A. nodosum canopy height to pre-disturbance levels based on estimated early growth rates. We found that A. nodosum canopy facilitated higher species richness and recolonization of dominating faunal species (Littorina saxatilis, Littorina obtusata, Mytilus edulis, and Semibalanus balanoides), and also significantly reduced the high temperatures in summer and raised the low temperatures in winter. The abundance of M. edulis and A. nodosum recolonization increased significantly with rock rugosity and the recovery of A. nodosum canopy height was estimated to a minimum of 15 years. We conclude that algal canopy and rock rugosity play key roles in structuring sub-Arctic intertidal communities, likely by modifying environmental stress such as extreme temperature, desiccation, and by increasing the settling surface and the habitat complexity. As the distribution of canopy-forming algae is expected to shift northward, they may act as a key habitat facilitating a northward colonization of intertidal fauna in the Arctic. We highlight the importance of considering scales relevant to biological communities when predicting impacts of climate change on distributional patterns and community structure in the Arctic intertidal.

A new species of tardigrade in the family Echiniscoididae, Echiniscoides wyethi, was found on the barnacle Semibalanus balanoides (L.) at several intertidal sites around Allen Island, Maine, USA. The new species is named after the American artist Andrew Wyeth and the Wyeth family who own the island on which this species was discovered. The new species is separated from all other Echiniscoides by the smooth cuticle, long flexible buccal tube, and the number of claws on the legs of the adult.

Understanding the role of larger-scale processes in modulating the assembly, structure, and dynamics of communities is critical for forecasting the effects of climate-change and managing ecosystems. Developing this comprehensive perspective is difficult though, because species interactions are complex, interdependent, and dynamic through space and time. Typically, experiments focus on tractable subsets of interactions that will be most critical to investigate and explain shifts in communities, but qualitatively base these choices on experience, natural history, and theory. One quantitative approach to identify the putative forces regulating communities, without reducing system complexity, is estimating transition probabilities among species occupying space (i.e., multispecies Markov chain models). Although not mechanistic, these models estimate the relative frequency and importance of ecological pathways in community assembly and dynamics, and can serve as a framework to identify how pathways change across large scales and which are most important to investigate further. Here, we demonstrate this method in the Gulf of Maine (GOM) intertidal zone, where research has largely focused on the local-scale processes that influence communities, while the mechanisms responsible for more regional shifts in communities are less clear. Transition probabilities of faunal elements were quantified bimonthly for ~2.5 yr in local intertidal communities at three replicate sites in the southern, mid-coast, and northern GOM. Transitions related to mortality, colonization, and replacement by mussels, barnacles, red algae, and encrusting corallines differed regionally, suggesting specific pathways related to consumer pressure and recruitment vary across the GOM with shifting intertidal community structure. Combined with species abundance data and insights from previous research, we develop and evaluate the pathways by which communities likely change in the GOM. Species interactions in local communities can be complex, and this complexity should be incorporated into hypothesis building, experiments, theory, interpretations, and forecasts in ecology. Such a comprehensive approach will be critical to understand how regional shifts in local interactions can drive large-scale community change.

Recruitment is a key demographic process for population persistence. This paper focuses on barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. Lawrence coast of Nova Scotia (Canada), ice scour is common during the winter. At the onset of intertidal barnacle recruitment in early May (after sea ice has fully melted), mostly only adult barnacles and bare substrate are visible at high elevations in wave-exposed habitats. We conducted a multiannual study to investigate if small-scale barnacle recruitment could be predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult density range (ca. 0–130 individuals dm−2), the relationship between adult density and recruit density (scaled to the available area for recruitment, which excluded adult barnacles) was unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm−2), the relationship between adult and recruit density was positive and resembled the lower half of the unimodal relationship. Overall, adult barnacle density was able to explain 26–40% of the observed variation in recruit density. The unimodal adult–recruit relationship is consistent with previously documented intraspecific interactions. Between low and intermediate adult densities, the positive nature of the relationship relates to the previously documented fact that settlement-seeking larvae are chemically and visually attracted to adults, which might be important for local population persistence. Between intermediate and high adult densities, where population persistence may be less compromised and the abundant adults may limit recruit growth and survival, the negative nature of the relationship suggests that adult barnacles at increasingly high densities stimulate larvae to settle elsewhere. The unimodal pattern may be especially common on shores with moderate rates of larval supply to the shore, because high rates of larval supply may swamp the coast with settlers, decoupling recruit density from local adult abundance.

To explore how climate may affect the structure of natural communities, we quantified the role of thermal stress in setting the high intertidal borders of the acorn barnacle, Semibalanus balanoides. At sites north and south of Cape Cod, a major faunal and thermal boundary on the east coast of North America, we examined the interacting effects of thermal stress and recruit density on individual survivorship. At hotter southern sites, particularly in bays, high intertidal barnacle survivorship was enhanced by experimental shading or by neighbors which ameliorate heat and desiccation stresses. In contrast, at cooler northern bay and coastal sites, neither shading nor group benefits increased barnacle survival, and mortality patterns were driven primarily by predators with largely boreal distributions. Our field results, like recent laboratory microcosm studies, suggest that predicting even simple community responses to climate change may be more complex than is currently appreciated.

Experimental tests of the hypothesis that ecological communities can exist in “multiple stable states” are rare, and some argue, impossible, because of the unlikelihood that any system will meet the necessary criteria. These are that alternative states (1) are in the same location, (2) experience the same environment, (3) persist for multiple generations, (4) resist repeated perturbations, and (5) result from a pulse manipulation. In 1974, we initiated an experiment testing the ability of Ascophyllum nodosum-dominated rocky intertidal communities to recover from complete canopy removal. Manipulations were monitored frequently for 5 yr after clearance and resurveyed again after a 35-yr hiatus. After clearance, Ascophyllum was replaced immediately by another fucoid alga, Fucus spp, which continued to dominate the space through 1979 despite regular annual recruitment by Ascophyllum. Observations in 2009 revealed that Fucus spp. still dominated the cleared plots. Surveys in 2011 and 2013 demonstrated that Fucus persisted in experimental plots and Ascophyllum persisted in adjacent, unmanipulated plots. All criteria for testing an alternative state were met. Fucus persisted through multiple generations of both fucoids, a steady annual rain of recruits of both species, a high frequency of storm-driven perturbations, and it resulted from a pulse manipulation. Likely mechanisms include poor Ascophyllum recruitment directly under the Fucus spp. canopy despite abundant recruitment in adjacent areas, self-maintenance by Fucus spp. through high recruitment and fast growth, and recruitment facilitation of Fucus by the barnacle Semibalanus balanoides. Several lines of evidence indicate that other possible mechanisms including indirect facilitation of fucoids through predation on competitors (mussels), and positive or negative effects of littorine grazing, are unlikely. Although prior results in Maine suggested that the mussel Mytilus edulis was an alternative stable state, the new results suggest that Fucus spp. was the alternative state to Ascophyllum. During the nearly four decades of this experiment, a number of important species including fucoids, littorine grazers, and mussels all declined in abundance, most likely due to climate change. The presumed impact of climate change makes prediction of the long-term response of this system difficult, but it already differs dramatically from its structure in the 1970s.

Geographic distributions of species are frequently influenced by temperature and as a result, there is considerable interest in predicting the effect of climate change on biogeography. Previous studies have found strong correlations between sea surface temperature and recruitment in the acorn barnacle Semibalanus balanoides, an important member of temperate intertidal communities in the Northern Hemisphere. The underlying mechanism that drives this relationship is, however, unknown. The hypothesis tested here is that variation in recruitment is determined by temperature-induced variation in survival of brooded embryos. Brooding adults were held under several environmentally-relevant temperature conditions from 7 °C to 13 °C until eggs were ready to hatch. There was no significant effect of temperature on embryonic development rate. Brooding individuals held at colder temperatures had more than twice the reproductive mass of those held at warmer temperatures. This mass difference is caused by an over three-fold larger number of embryos surviving in the coldest treatment (7 °C) compared to the warmest treatment (13 °C). Temperature-induced variation in number of surviving embryos likely contributes to differences in recruitment following cold winters vs. warm winters. Such temperature-dependent variation in recruitment has been shown to cause year to year fluctuations in the positions of the geographic limits of the species in Southwest England, France, and Spain. This study illustrates the key role of adult input into the larval pool in generating variation in recruitment and its influence on biogeography.

Few links have been established between the Atlantic Multidecadal Oscillation and long-term dynamics of marine systems due to the scarcity of sustained biological time-series with sufficient multi-decadal coverage. The abundances of co-occurring boreal and Lusitanian species of barnacle have been recorded annually at a rocky shore in Devon, southwest England since 1953. Multidecadal cycles in relative abundances of the cold-water Semibalanus balanoides and warm-water Chthamalus spp. are strongly correlated with both local sea surface temperatures, and a ‘Warm Index’ of barnacle abundance shows strong links to the basin-scale Atlantic Multidecadal Oscillation. In contrast there are weak or no observed relationships with the North Atlantic Oscillation for either species. The shorter lifecycle of S. balanoides compared to the chthamalids and the increase in spring and summer temperatures to which newly settled S. balanoides recruits have been exposed during the last decade are likely mechanisms by which barnacle densities are responding to low-frequency temperature variability expressed in the Atlantic Multidecadal Oscillation.

Predators may have nonconsumptive effects (NCEs) on prey populations mediated by chemical cues detected by prey. We experimentally investigated dogwhelk (Nucella lapillus) NCEs on intertidal barnacle (Semibalanus balanoides) recruit density in Nova Scotia, Canada. Under a moderate abundance of coastal phytoplankton (food for barnacle larvae and recruits), barnacle recruitment was moderate and the nearby presence of dogwhelks limited barnacle recruit density at the end of the recruitment season. Under a high phytoplankton abundance, barnacle recruitment was high and neutralized dogwhelk NCEs on barnacle recruit density, likely through the chemical attraction that recruits exert on larvae seeking settlement.

Barnacles are dominant sessile invertebrates on many rocky shores worldwide. Hence, investigating the factors that affect their recruitment is important. Through field experiments done on the Atlantic coast of Canada, we investigated interspecific and intraspecific relationships affecting intertidal barnacle recruitment. Specifically, we evaluated the effects of seaweed canopies (Ascophyllum nodosum) and adult barnacles (Semibalanus balanoides) on the density of barnacle recruits at the end of the recruitment season. The effects of three canopy treatments on barnacle recruitment and understory environmental conditions allowed us to identify positive and negative effects of canopies. At mid-intertidal elevations subjected to a moderate wave action, we found that, during high tides, the flexible algal fronds damage wire sensors established on the substrate (whiplash effect) and limit barnacle recruitment. However, at low tide, algal canopies limit water loss and temperature extremes and enhance barnacle recruitment in understory habitats. The net effects of algal canopies on barnacle recruitment, however, were neutral, as the positive and negative influences balanced out. By manipulating the abundance of adult barnacles under the seaweed canopies, we found that adult barnacles enhance barnacle recruitment, likely due to the known attraction that adults exert on larvae seeking settlement and to the absence of post-settlement events that could otherwise have blurred such effects by the adults. The presence of adult barnacles, however, did not protect developing recruits from canopy whiplash effects. By understanding the contrasting influences that intertidal algal canopies have on understory abiotic conditions and barnacle recruitment, our ability to predict net canopy effects depending on the relative degree of physiological (e.g., high vs. low intertidal zone) and physical (e.g., sheltered vs. exposed shores) stresses should increase. This study also suggests that recruitment, considered as an important external factor in environmental models of community organization, can also be affected by components of the community itself, potentially triggering local feedbacks.

Gravid adults of the common intertidal barnacle Semibalanus balanoides (L.) brood fully developed larvae until individuals perceive some cue from the environment that triggers syn- chronous larval release. The prevailing hypothesis has been that phytoplankton blooms trigger release because they provide a food source for nauplius larvae. Through observations and field experiments, we tested the hypothesis that turbidity from any source, not just phytoplankton blooms, can trigger release. We documented 5 larval release events at 3 sites in the northeastern USA. Two events coincided with chlorophyll increases, and all 5 coincided with turbidity in creases. In experiments, the larval release response was equivalent when adults were exposed to diatoms or inert synthetic beads, and it was significantly higher than under exposure to filtered seawater. We also tested the hypothesis that turbidity can decrease the risk of cannibalism for newly released nauplii. Under experimentally manipulated conditions, adults consumed significantly fewer nauplii in a high-turbidity environment. We suggest that reproduction in this species may have evolved to coincide roughly with the local onset of winter/spring phytoplankton blooms, but the timing of larval release may have been fine-tuned further by cannibalism and predation pres- sures. The potential for turbid conditions to serve as a refuge for planktonic larvae of other marine organisms merits further investigation.

On the Gulf of St. Lawrence coast of Nova Scotia (Canada), recruitment of the barnacleSemibalanus balanoidesoccurs in May and June. Every year in June between 2005 and 2016, we recorded recruit density for this barnacle at the same wave-exposed rocky intertidal location on this coast. During these 12 years, mean recruit density was lowest in 2015 (198 recruits dm−2) and highest in 2007 (969 recruits dm−2). The highest recruit density observed in a single quadrat was 1,457 recruits dm−2(in 2011) and the lowest was 34 recruits dm−2(in 2015). Most barnacle recruits appear during May, which suggests that most pelagic larvae (which develop over 5–6 weeks before benthic settlement) are in the water column in April. An AICc-based model selection approach identified sea surface temperature (SST) in April and the abundance of phytoplankton (food for barnacle larvae, measured as chlorophyll-aconcentration –Chl-a–) in April as good explanatory variables. Together, April SST and April Chl-aexplained 51% of the observed interannual variation in recruit density, with an overall positive influence. April SST was positively related to March–April air temperature (AT). April Chl-awas negatively related to the April ratio between the number of days with onshore winds (which blow from phytoplankton-limited offshore waters) and the number of days with alongshore winds (phytoplankton is more abundant on coastal waters). Therefore, this study suggests that climatic processes affecting April SST and April Chl-aindirectly influence intertidal barnacle recruitment by influencing larval performance.

On the Gulf of St. Lawrence coast of Nova Scotia (Canada), recruitment of the barnacle Semibalanus balanoides occurs in May and June. Every year in June between 2005 and 2016, we recorded recruit density for this barnacle at the same wave-exposed rocky intertidal location on this coast. During these 12 years, mean recruit density was lowest in 2015 (198 recruits dm-2) and highest in 2007 (969 recruits dm-2). The highest recruit density observed in a single quadrat was 1457 recruits dm-2 (in 2011) and the lowest density was 34 recruits dm-2 (in 2015). Most barnacle recruits appear during May, which suggests that most pelagic larvae, which develop over five-to-six weeks and originate the recruits, are in the water column in April. A model selection approach identified sea surface temperature (SST) in April and the abundance of phytoplankton (food for barnacle larvae, measured as chlorophyll-a concentration –Chl-a–) in April as good explanatory variables. Together, April SST and Chl-a explained 51 % of the observed interannual variation in recruit density with an overall positive influence. April SST was positively related to March–April air temperature. April Chl-a was negatively related to the April ratio between the number of days with onshore winds (which blow from phytoplankton-limited offshore waters) and the number of days with alongshore winds (coastal phytoplankton is higher on coastal waters). Therefore, these observations suggest that climatic processes affecting April SST and Chl-a indirectly influence intertidal barnacle recruitment by influencing larval performance.

Through a field experiment, we show that a predator has negative nonconsumptive effects (NCEs) on different life-history stages of the same prey species. Shortly before the recruitment season of the barnacle Semibalanus balanoides (spring), we established experimental cages in rocky intertidal habitats in Nova Scotia, Canada. The cages were used to manipulate the presence and absence of dogwhelks, Nucella lapillus, the main predators of barnacles. At the centre of each cage, we installed a tile where barnacle pelagic larvae could settle and the resulting recruits grow. Mesh prevented caged dogwhelks from accessing the tiles, but allowed waterborne dogwhelk cues to reach the tiles. During the recruitment season, barnacle larvae settled preferentially on tiles from cages without dogwhelks. In the fall, at the end of the dogwhelk activity period and once the barnacle recruits had grown to adult size, barnacle body mass was lower in the presence of dogwhelks. This limitation may have resulted from a lower barnacle feeding activity with nearby dogwhelks, as found in a previous study. The observed larval and adult responses in barnacles are consistent with attempts to decrease predation risk. In the fall, dogwhelk cues also limited barnacle reproductive output, a possible consequence of the limited growth of barnacles. Overall, the results of this study suggest that a predator might influence trait evolution in a prey mediated by NCEs on different life-history stages.

We tested whether the patchiness of barnacle settlement was determined by spatial (local) differences in settling and mortality rates. During the period of 2004–2011, annual photo-registration of a barnacles Semibalanus balanoides population was conducted on six flat rocks in the White Sea intertidal zone (Arctic). The average adult population density was 48 ind dm−2, and the average barnacle age was 3–4 years. According to age structure, during the first year, the number of recruits dropped by three to fourfold; while in the following 2 years, the population density remained nearly the same. The effect of ice friction influence was not recorded, although the rocks were covered with ice in winter. Settling peaks were recorded in 2005, 2007, and 2009. No correlation was observed between the recruit and adult distribution during 2 years of intense settlement. The death rate was uneven on different quadrates along the transects but did not correlate with population density. Because high settling rates and mortality are not linked to specific sites, this suggests that population patchiness is not spatially stable.

It has been proposed that barnacle cyprids can maintain position in shoreward- propagating fronts by swimming upward against a downwelling flow, potentially mediating onshore transport of larvae toward intertidal habitat. This study developed a novel flume to characterize swimming behavior of barnacle cyprids in a laboratory downwelling flow. Seawater was pumped through a cylindrical observation chamber fitted with diffusers to produce a homogeneous down- welling velocity field. The flume generated plug flow with mean downwelling velocities (indicated by negative sign) of 0 to -47.3 mm s -1 . Behavior experiments were done with wild Semibalanus bal- anoides cyprids. Vertical swimming rates and behaviors were estimated from video observations, and a mixture model was used to estimate velocity distributions for distinct behavioral modes. Larvae exhibited multiple behaviors but typically swam upward in response to downwelling, with a maxi- mum estimated vertical velocity of 72.3 mm s -1 . When faced with downwelling flows, cyprids alter- nated between upward swimming and downward swimming to maintain their vertical position in the chamber. As downwelling velocities increased, cyprids that remained in the field of view of the cam- eras exhibited faster mean upward swimming velocities. It is unclear how long individual S. bal- anoides cyprids can maintain depth, but cyprids were able to maintain depth throughout the 2 min observation period. This study supports earlier hypotheses based on field observations by demon- strating that S. balanoides cyprids swim well enough to counter downwelling velocities characteris- tic of convergence zones. Swimming against downwelling flow could be an adaptive behavior that enables shoreward transport in the absence of any larval ability to swim toward shore or even to sense its direction.

We investigated the regional variation in barnacle (Semibalanus balanoides) recruit and adult abundance on the NW Atlantic coast. At the end of the recruitment season (June–July), we sampled wave-exposed rocky intertidal sites in two regions on the open Atlantic coast (Maine, AM, and Nova Scotia, AN) and in two regions on the Gulf of St. Lawrence coast (Northumberland Strait, GN, and Cape Breton Island, GC). Recruit density was highest in the southernmost region (AM), followed by GN and, then, by AN and GC. Regional values of nearshore primary productivity (satellite data of chlorophyll-a concentration, a surrogate for phytoplankton abundance) were highest for AM and GN, suggesting that food supply (barnacles are filter feeders) is an important factor determining regional recruitment patterns. Adult barnacle density was regionally decoupled from recruit density. Adults occurred in very low abundances on the Gulf of St. Lawrence coast (GN and GC) and were relatively abundant on the Atlantic coast (AM and AN), although always in much lower abundances than recruits. The low adult densities on the Gulf of St. Lawrence coast seem to result mainly from intense ice scour, as this coast freezes extensively every winter, as opposed to the ice-free Atlantic coast. Ice scour thus appears to override regional recruitment differences in determining adult density. Therefore, our data suggest that both pelagic food supply and benthic disturbance contribute to setting regional patterns in barnacle population structure on the NW Atlantic coast.

Wave action and low population density can strongly reduce the ability of sessile acorn barnacles to find mates and copulate. For Semibalanus balanoides, penis morphology varies with wave exposure and with characteristics of the mating neighborhood. Field experiments were conducted at five intertidal sites on Long Island, New York, USA from July to December 2005 to determine how wave exposure and aggregation structure influence the length, diameter, mass, and number of annulated folds of the penis. Sparsely crowded barnacles had more annulations in the penis and are inferred to have greater ability to stretch. At higher wave exposure, the diameter of the penis was greater, but the mass was not. This study identifies density of crowding as the most important cue that barnacles respond to when perceiving their mating group and details how penis morphology varies in response to wave exposure.

The Arctic Ocean and its associated ecosystems face numerous challenges over the coming century. Increasing atmospheric CO2 is causing increasing warming and ice melting as well as a concomitant change in ocean chemistry (“ocean acidification”). As temperature increases it is expected that many temperate species will expand their geographic distribution northwards to follow this thermal shift; however with the addition of ocean acidification this transition may not be so straightforward. Here we investigate the potential impacts of ocean acidification and climate change on populations of an intertidal species, in this case the barnacle Semibalanus balanoides, at the northern edge of its range. Growth and development of metamorphosing post-larvae were negatively impacted at lower pH (pH 7.7) compared to the control (pH 8.1) but were not affected by elevated temperature (+4 °C). The mineral composition of the shells did not alter under any of the treatments. The combination of reduced growth and maintained mineral content suggests that there may have been a change in the energetic balance of the exposed animals. In undersaturated conditions more mineral is expected to dissolve from the shell and hence more energy would be required to maintain the mineral integrity. Any energy that would normally be invested into growth could be reallocated and hence organisms growing in lowered pH grow slower and end up smaller than individuals grown in higher pH conditions. The idea of reallocation of resources under different conditions of pH requires further investigation. However, there could be long-term implications on the fitness of these barnacles, which in turn may prevent them from successfully colonising new areas.

Survival in the intertidal zone for sessile marine invertebrates means enduring the drastic changes in environmental conditions that comes with tide cycling. An influential repercussion for Semibalanus balanoides, the common barnacle, is changes in food accessibility. As a filter-feeder, this intertidal organism does not obtain the required nutrients for growth when exposed to air and, as a result, experiences a compromised growth rate along with a limited body size. A four-day (August 16th-19th, 2009) field-based study was conducted at Indian Point in St. Andrews, New Brunswick to examine how duration of aerial exposure affects rostro-carinal length. Twenty 10cm x 10cm sites were sampled within which the total number of S.balanoides individuals was counted and rostro-carinal lengths of fifty individuals were measured. In support of the predicted results, this study shows that aerial exposure limits size in S.balanoides. This negative relationship between duration of aerial exposure and size is largely correlative, as supported by the R=0.9205 value obtained through application and analysis of the linear regression model. This information of how S.balanoides responds to the out of water experience may bring to light the significance of results from lab-based studies regarding environmental conditions that S.balanoides can tolerate.

Regional oceanographic processes are emerging as strong influences on growth and recruitment of intertidal species, with important consequences for populations. Yet local conditions such as wave exposure are also important. To disentangle these effects for the intertidal barnacle Semibalanus balanoides, we surveyed 259 sites around Scotland in July and August over 6 yr (from 2001 to 2006). Scaled digital photographs at 3 shore levels gave sizes and densities of juvenile and adult barnacles for comparison with wave fetch, remotely sensed chlorophyll a (chl a) concentration and seasonal sea surface temperature (SST). Patterns were also compared with site 'openness': the area of connected open sea

Ocean acidification and global warming are occurring concomitantly, yet few studies have investigated how organisms will respond to increases in both temperature and CO2. Intertidal microcosms were used to examine growth, shell mineralogy and survival of two intertidal barnacle post-larvae, Semibalanus balanoides and Elminius modestus, at two temperatures (14 and 19°C) and two CO2 concentrations (380 and 1,000 ppm), fed with a mixed diatom-flagellate diet at 15,000 cells ml−1 with flow rate of 10 ml−1 min−1. Control growth rates, using operculum diameter, were 14 ± 8 μm day−1 and 6 ± 2 μm day−1 for S. balanoides and E. modestus, respectively. Subtle, but significant decreases in E. modestus growth rate were observed in high CO2 but there were no impacts on shell calcium content and survival by either elevated temperature or CO2. S. balanoides exhibited no clear alterations in growth rate but did show a large reduction in shell calcium content and survival under elevated temperature and CO2. These results suggest that a decrease by 0.4 pH(NBS) units alone would not be sufficient to directly impact the survival of barnacles during the first month post-settlement. However, in conjunction with a 4–5°C increase in temperature, it appears that significant changes to the biology of these organisms will ensue.

The effects of CO2-induced acidification on survival, shell mineralogy, embryonic devel- opment and the timing of larval release were investigated in the intertidal barnacle Semibalanus bal- anoides using an intertidal microcosm system. Compared to that in the control (CO2 = 344 ppm, pH = 8.07), adult survival was 22% lower in the high-CO2 treatment (CO2 = 922 ppm, pH = 7.70) and sig- nificant changes in the mineral structure of the adult shell were observed. Embryonic development rate was significantly slower in the high-CO2 treatment than in the control but still resembled 'nat- ural' rates seen in populations found in similar locations. There was an estimated 19 d delay in devel- opment under high-CO2 conditions, which resulted in a 60% reduction in the number of nauplii reaching hatching stage at the time when over 50% of the control nauplii had hatched. We conclude that ocean acidification could potentially further compromise embryonic development in a species already stressed by temperature, which could in turn impact naupliar development and recruitment. S. balanoides, the adults of which live in a highly variable environment, has been shown to be detri- mentally impacted by a chronic change in chemical conditions (pH lowered beyond the current range) over a crucial period in their life cycle. Under experimental high-CO2 conditions, some adults were able to survive and larvae were able to hatch. This may indicate that there is still potential for organisms to find suitable habitats and for populations to develop and survive.

We investigated the potential utility of intertidal barnacles as ecological indicators of the intensity of ice scour on the Gulf of St. Lawrence coast of Nova Scotia, Canada. This coast is exten- sively covered by sea ice in winter. Shortly after ice melt, between May and June 2007, we quantified the density of adult barnacles Semibalanus balanoides in the high intertidal zone in 8 rocky locations distributed along nearly 25 km of coastline. At each location, we took measurements in 2 types of habitat that differ in ice scour intensity: habitats facing open waters, where ice scour is intense, and habitats facing a rocky land mass between a few meters and 10s of meters away, where ice scour is moderate. Adult barnacle density was significantly higher in sheltered sites than in exposed sites by a factor of 3, on average. Ice scour intensity is likely the main factor determining such a pattern, as we previously found that barnacle recruitment is similar in both types of habitat and that the poten- tial competitors (algae and mussels) and predators (whelks) are virtually absent in the high intertidal zone. Thus, high-intertidal barnacles might serve as indicators of differences in ice scour intensity among coastal sites in our region. Future studies should determine the spatial scales of applicability of this approach.

Barnacle cypris larvae respond to many cues when selecting a settlement site. The settlement of over a million larvae on tiles of different textures, orientations and densities of incumbent settlers was measured on the rocky intertidal at Great Cumbrae, Scotland. Half of the tiles were replaced every tide whereas the others simultaneously accumulated settlers. Factor effects varied on each tide, and converged in the accumulating deployment. Increasing incumbent density led to net loss of settlement, which was less probable on the textures on which fastest settlment occurred ('very fine'), and more probable on those on which settlement was slowest ('smooth'). More settlement occurred on down-facing orientations during daylight and vice versa. Cue ranks were non-linear, so a path analysis model quantified the relative influence of each factor. Gregariousness was the most influential cue measured, although unmeasured factors had greater effects, highlighting the complexity of settlement influences in this species.

Intertidal organisms are often assumed to live close to their thermal limits, and have emerged as potential early indicators of the effects of climate change. We compared our survey of the 2004–2006 geographic distribution of the barnacle Semibalanus balanoides to its distribution in 1872, 1955, 1963, 1971, and 1985, from surveys by Fischer, Crisp, Fischer-Piette, Barnes, Powell, and Southward. The southern geographic limit has retreated 300 km in France since 1872, at a rate of 15 to 50 km per decade. We compared our 2006 survey of the geographic distribution of the polychaete Diopatra neapolitana to its distribution in 1893–1923, from surveys by Saint-Joseph and Fauvel, and its distribution in 1969–1976 from surveys by Glemarec. The northern geographic limit of this species has advanced 300 km in France since 1893 at similar rates to Semibalanus. We used NOAA weather reanalysis data and our mechanistic simulation model of intertidal animal body temperatures to hindcast the thermal environmental change near historical geographic limits in Europe for the past 55 years. Results indicate that changes in the southern limit of S. balanoides are due to intolerance of winter body temperatures above 10°C, leading to reproductive failure. Results for Diopatra are ambiguous: based on the northern extension of its range, either cold winters or cool summers limit its range, while gaps in its distribution are consistent with limitation by cooler summer conditions. The parallel shifts of D. neapolitana on sedimentary shores and Semibalanus on rocky shores suggest that similar climatic factors control the geographic limits of both species. The intertidal zone is a model system for examining the effects of climate change on biogeographic change both because of the rapidity of its response, and because the rich historical record allows direct tests of hypotheses.

The intertidal serrated wrack, Fucus serratus L. (Fucales: Phaeophyceae), has become an abundant canopy-forming alga along a ca. 100 km stretch of shore in southwestern Iceland in the last century. Its distribution has not changed noticeably since a survey in 1975/1976. Many experimental studies have shown that canopy-forming algae can have profound effects on the community structure. Although such experiments involving F. serratus are few it was decided to test predictions from these experiments on a geographical scale by comparing community compositions within the area where F. serratus is a dominant algae (since at least 1975/1976) with an adjacent area of similar size where F. serratus is absent (with a single exception without consequences). The work is based on measurements on some 372 stations in the F. serratus area, termed region A, and 227 stations from the F. serratus -free area, termed region B. Percentage cover of algae and sessile invertebrates was estimated on 2 m 2 on each station and animals collected from 800 cm 2 from each station. The vertical distribution of F. serratus was most similar to that of Fucus distichus and the two species often grew intermingled on the lower part of the shore. F. serratus appeared to have reduced the cover of F. distichus in the lowermost part of the shore, while having little or no effect on other canopy-forming species. The low abundance of Semibalanus balanoides in the F. serratus area (region A) is in line with experiments showing detrimental effect on this barnacle by whiplash of F. serratus . In general algal grazers were more abundant in region A, consistent with the greater attractiveness of F. serratus than other canopy-forming species to grazers, although this may be partly explained by the abundance of understorey algae in region A.

We describe spatial trends in population structure of the barnacle, Semibalanus balanoides, across gradients of environmental stress on the Gulf of St. Lawrence coast of Nova Scotia, Canada. In the early summer of 2006, we examined vertical gradients of intertidal elevation and horizontal gradients of exposure to wave action and winter ice scour. Recruits were considerably less abundant than adults at all sites, suggesting a high post-recruitment mortality. Although adult barnacles seemed less abundant in areas subjected to strong winter ice scour than in sheltered areas, such a trend lacked statistical support. Adult barnacles were more abundant at high elevations than at low elevations, possibly as a result of vertical differences in the intensity of competition and predation. Size structure was better developed at the high intertidal zone than at the low intertidal zone, with larger organisms occurring at high elevations. In general, it appears as though biotic interactions at the low intertidal zone may play a greater role than abiotic factors at the high intertidal zone in determining the population structure of S. balanoides, although field experimentation will be necessary to confirm this notion. Describimos tendencias espaciales en la estructura poblacional del balano Semibalanus balanoides a traves de gradientes de estres ambiental en la costa del golfo de San Lorenzo en Nova Scotia, Canada. En el verano temprano de 2006, examinamos gradientes verticales de elevacion intermareal y gradientes horizontales de exposicion al oleaje y al raspado por hielo marino invernal. Los reclutas fueron considerablemente mas abundantes que los adultos en todos los sitios, sugiriendo una alta mortalidad luego del reclutamiento. Aunque los balanos adultos parecian ser menos numerosos en areas sujetas a un raspado intenso por hielo marino invernal que en areas protegidas, tal tendencia no mostro un sustento estadistico. Los balanos adultos fueron mas numerosos en el intermareal superior que en el inferior, posiblemente como resultado de diferencias verticales en la intensidad de la competencia y predacion. La estructura de tamanos estuvo mejor desarrollada en el intermareal superior que en el inferior, con organismos mas grandes ocurriendo en el alto intermareal. En general, parece que las interacciones bioticas fueran mas importantes en el intermareal inferior que los factores abioticos en el intermareal superior en la determinacion de la estructura poblacional de S. balanoides, aunque experimentos de campo seran necesarios para confirmar esta nocion.

On NW Atlantic rocky shores, the main basal organisms in intertidal communities are seaweeds (Ascophyllum nodosum, Fucus spp. and Chondrus crispus) and filter feeders (barnacles, Semibalanus balanoides, and mussels, Mytilus spp.). Their ecology has been extensively studied in New England (United States), but knowledge gaps exist for northern shores, which are subjected to stronger environmental stress. Therefore, we studied the above organisms on Canadian shores. We quantified the summer abundance of these seaweeds and filter feeders across full vertical (intertidal elevation) and horizontal (wave exposure and winter ice scour) environmental gradients on the Gulf of St. Lawrence and open Atlantic coasts of Nova Scotia. At the regional scale along the open Atlantic coast, seaweeds showed similar abundances in Nova Scotia than values reported for New England. However, both filter feeders were considerably less abundant in Nova Scotia. At the local scale in Nova Scotia, intense winter ice scour (which only occurs on the Gulf of St. Lawrence coast) was associated with a very low abundance of all species except barnacles. Spatial trends in Nova Scotia were similar to patterns known for certain species elsewhere, such as A. nodosum being almost restricted to sheltered habitats, regardless of elevation, and C. crispus being almost restricted to low elevations, regardless of exposure. Other trends were, however, characteristic of Nova Scotia, such as C. crispus being frequent at low elevations in exposed habitats, unlike in New England, where mussels often predominate there because of competitive advantages. In Nova Scotia, mussels were always restricted to cracks and crevices, unlike in New England, where they form extensive intertidal beds on exposed shores. The direct effects of increased environmental stress and indirect effects through altered interspecific interactions might explain the regional differences in local species distribution, which will require experimental validation.

Since its introduction fewer than 20 yr ago, the spread of the Asian shore crab Hemigrapsus sanguineus along the east coast of the US has been remarkable. By many accounts it is now the numerically dominant brachyuran in rocky intertidal sites in Long Island Sound. Evidence from several laboratory studies has suggested that H. sanguineus could have significant consequences for barnacle abundance and distribution. In this study we used field caging experiments to determine the effect of this invader on recruitment (sampled at weekly intervals) of the indigenous barnacle Semibalanus balanoides. We found that although H. sanguineus readily consumed settling cypris larvae and early post-settlement juveniles, crab predation did not have a negative impact on barnacle recruitment when predator density was low (15 crabs 0.25 m–2). We also found a measurable decrease in barnacle density at medium (45 crabs 0.25 m–2) and high (90 crabs 0.25 m–2) predator densities after the larval settlement period had ended, but this effect was short-lived. Based on available estimates of Asian shore crab abundance in Long Island Sound, we conclude that crab predation will not have a significant impact on the recruitment success of S. balanoides populations.

We investigated the nonconsumptive effects (NCEs) of predatory dogwhelks (Nucella lapillus) on intertidal barnacle (Semibalanus balanoides) recruitment through field experiments on the Gulf of St. Lawrence coast and the Atlantic coast of Nova Scotia, Canada. We studied the recruitment seasons (May-June) of 2011 and 2013. In 2011, the Gulf coast had five times more nearshore phytoplankton (food for barnacle larvae and recruits) during the recruitment season and yielded a 58% higher barnacle recruit density than the Atlantic coast at the end of the recruitment season. In 2013, phytoplankton levels and barnacle recruit density were similar on both coasts and also lower than for the Gulf coast in 2011. Using the comparative-experimental method, the manipulation of dogwhelk presence (without allowing physical contact with prey) revealed that dogwhelk cues limited barnacle recruitment under moderate recruit densities (Atlantic 2011/2013 and Gulf 2013) but had no effect under a high recruit density (Gulf 2011). Barnacle recruits attract settling larvae through chemical cues. Thus, the highest recruit density appears to have neutralized dogwhelk effects. This study suggests that the predation risk perceived by settling larvae may decrease with increasing recruit density and that prey food supply may indirectly influence predator NCEs on prey recruitment.

In recent years, offshore wind energy in the shelf seas of the southern North Sea is experiencing a strong growth. Foundations are introduced in mainly sandy sediments, and the resulting artificial reef effect is considered one of the main impacts on the marine environment. We investigated the macrobenthic fouling community that developed on the concrete foundations of the first wind turbines built in Belgian marine waters. We observed a clear vertical zonation, with a distinction between a Telmatogeton japonicus dominated splash zone, a high intertidal zone characterised by Semibalanus balanoides, followed by a mussel belt in the low intertidal–shallow subtidal. In the deep subtidal, the species turnover was initially very high, but the community was soon dominated by few species (Jassa herdmani, Actiniaria spp. and Tubularia spp.), and only seasonal dynamics within this species assemblage were observed after 1–1½ years. Ten non-indigenous species (NIS) were found. In the intertidal, eight out of the seventeen typical intertidal species observed were NIS, while only two out of a species pool of 80 species were NIS in the deep subtidal. NIS were found to use the foundations to expand their range and strengthen their strategic position in the area.

It is generally assumed that larvae of benthic species are thoroughly mixed in the plankton and distributed randomly at settlement. Yet, it has also been hypothesized that a combination of larval gregarious behaviour coupled with particular oceanographic conditions may prevent larvae from mixing completely, and result in nonrandom spatial distributions following settlement. Using microsatellite markers, the main objective of this study was to investigate the occurrence of statistical connections between relatedness and settlement in the intertidal acorn barnacle from the Gulf of St Lawrence, Canada. A second objective was to test the hypothesis that patches of kin-related individuals came from a common parental site. Our results indicated that a significant number of barnacles within a given sample were more closely related than expected by chance despite the enormous potential for admixture during the planktonic phase. Thus, eight out of 37 samples analysed had relatedness values significantly higher than expected from random settlement. Moreover, analyses of sibship network construction and network complexity tests provided evidence for the occurrence of networks within several samples that were characterized by strong connections among individuals. Thus, nonrandom planktonic dispersal associated with relatively stable oceanic currents, as well as additional ecological factors to be rigorously investigated (e.g. behavioural mechanisms), may be more important in determining patterns of genetic structure in marine benthic invertebrates than generally assumed. Therefore, documenting genetic patterns associated with kin aggregation should be a fruitful and an important avenue for future studies in marine invertebrates.

We utilized microcosms that mimicked the rocky intertidal community to examine the predatory impacts of two introduced crab species, Carcinus maenas and Hemigrapsus sanguineus, on naturally occurring assemblages of organisms on rocks in the laboratory and in the field. The two crab species had similar consumption patterns causing significant declines in Semibalanus balanoides (barnacles), juvenile Mytilus edulis (mussels), Spirorbis sp. (polychaetes), and ephemeral algae. In both two-day and fourteen-day field experiments, the decline in S. balanoides in the H. sanguineus treatments was significantly greater than in the C. maenas treatments suggesting that H. sanguineus may have a higher per capita impact on barnacles than C. maenas. As H. sanguineus increases in abundance in northern New England, it may have greater impacts on prey species than C. maenas did when it became established.