Barnacle Semibalanus Balanoides Research Articles

Spatial variation in size and density of adult and post-settlement Semibalanus balanoides: effects of oceanographic and local conditions

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 <30 km away. Patterns at the 3 shore levels were similar. Hierarchical partitioning (HP) showed that survey year had the biggest effect and improved the predictive power of other variables: wave fetch for adult and juvenile densities, chl a for juvenile size and openness for adult size. SST had little effect. Regression models selected using information theoretic measures included positive effects of chl a, varying among surveys, on average size of barnacles (R 2 from 0.5 to 0.6), and larger high shore barnacles in greater wave exposure. Population densities of adults and juveniles increased with wave fetch, with chl a only influencing density at high shore levels (R 2 from 0.1 to 0.4). Despite temporal and spatial variation in responses to chl a and wave fetch among surveys, relationships were consistent with growth and size in S. balanoides being limited by food supply, and increased recruitment and adult densities in increased wave exposure. Large-scale ecological patterns in this rocky intertidal species thus result from large-scale oceanographic effects on food concentration with habitat-scale wave-mediated effects on supply of food and larvae.

Habitat selection at settlement endures in recruitment time series

Metamorphosis from a pelagic to a benthic stage is a critical transition in the life cycle of sessile marine invertebrates. The barnacle Semibalanus balanoides attaches permanently during set- tlement, and once attached, its gross location in the adult habitat is fixed. The partitioning of benthic habitat among barnacles in a recruit cohort is often mediated by settlement timing because early set- tlement preempts later settlement. We found that barnacles that settled earlier had a higher chance of survival to reproductive age. Additionally, there was a marked preference for some settlement locations over others as indicated by the order that different quadrats were occupied. Location of set- tlement resolved on a daily timescale over 34 d within 1 site had an enduring influence on which indi- viduals survived to adulthood up to 12 mo later. Furthermore, adult density from a recruit cohort was highly correlated with the timing of settlement. Therefore, habitat selection behavior at settlement may account for a large degree of variability in recruit survival through adulthood in sessile marine invertebrates.

Relative influences of ocean acidification and temperature on intertidal barnacle post-larvae at the northern edge of their geographic distribution

The Arctic Ocean and its associated ecosystems face numerous challenges over the coming century. Increasing atmospheric CO 2 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.

Novel application of imaging surface plasmon resonance for in situ studies of the surface exploration of marine organisms

The surface interactions of exploring cyprids of the barnacle Semibalanus balanoides were studied in situ using imaging surface plasmon resonance. It was demonstrated how the deposition of a proteinaceous adhesive could be followed in real time as the cyprids explored and temporarily attached to a surface. Furthermore, the amount of protein left on the surface when the cyprids moved on could be quantified. Clear differences were demonstrated between an oligo(ethyleneglycol) coated surface and a bare gold substrate. It is anticipated that this technique will be a valuable tool in the development of novel surface chemistries that can prevent biofouling.

Future high CO2 in the intertidal may compromise adult barnacle Semibalanus balanoides survival and embryonic development rate

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 389:193-202 (2009) - DOI: https://doi.org/10.3354/meps08141 Future high CO2 in the intertidal may compromise adult barnacle Semibalanus balanoides survival and embryonic development rate Helen S. Findlay1,*, Michael A. Kendall1, John I. Spicer2, Stephen Widdicombe1 1Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, PL1 3DH, UK 2Marine Biology and Ecology Research Centre, School of Biological Sciences, University of Plymouth, PL4 8AA, UK *Email: hefi@pml.ac.uk ABSTRACT: The effects of CO2-induced acidification on survival, shell mineralogy, embryonic development and the timing of larval release were investigated in the intertidal barnacle Semibalanus balanoides 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 significant 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 ‘natural’ rates seen in populations found in similar locations. There was an estimated 19 d delay in development 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 detrimentally 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. KEY WORDS: Ocean acidification · pH · Barnacle · Embryonic development · Carbon dioxide · Climate change · Intertidal · Larvae Full text in pdf format PreviousNextCite this article as: Findlay HS, Kendall MA, Spicer JI, Widdicombe S (2009) Future high CO2 in the intertidal may compromise adult barnacle Semibalanus balanoides survival and embryonic development rate. Mar Ecol Prog Ser 389:193-202. https://doi.org/10.3354/meps08141 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 389. Online publication date: September 04, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Chemistry-Specific Interfacial Forces Between Barnacle (Semibalanus Balanoides) Cyprid Footprint Proteins and Chemically Functionalised AFM Tips

Cypris larvae of the barnacle Semibalanus balanoides leave proteinaceous footprints on surfaces during pre-settlement exploration. These footprints are considered to mediate temporary adhesion of cyprids to substrata and, as such, represent a crucial first step in the colonization of man-made surfaces by barnacles, a process known as biofouling. Interest in this system also stems from the potential for a synthetic reversible adhesion system, based on the strategy used by cyprids. Cyprid footprints were probed using atomic force microscopy (AFM) and nanomechanical data relating to interfacial adhesion forces were correlated with AFM tip chemistry. Commercial Si3N4-tips and chemically functionalized CH3-tips were chosen to mimic the interactions of cyprid footprints with hydrophilic and hydrophobic surfaces, respectively. Force-extension curves of protein bundles picked up by AFM tips exhibited a characteristic saw-tooth appearance for both types of tip, but demonstrated clear differences relating to pull-off force and pull-off length, based on tip chemistry. Additional (∼6 nN) interfacial adhesion forces between −CH3 functionalized tips and footprints were assigned to hydrophobic interactions. Footprint proteins adhered with greater tenacity to the hydrophobic tip. This may suggest conformational change and denaturing of the protein which would facilitate hydrophobic interaction by enhancing contact forces between −CH3 functionalized tips and hydrophobic groups in the footprint molecule(s). Neither tip removed proteins from the −NH2 substratum suggesting that specific chemical interactions, rather than simple wetting phenomena, govern the adhesion of footprint proteins to that surface.

Intertidal barnacles as indicators of the intensity of scour by sea ice

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.

Predators on rocky shores in the northern Atlantic: Can the results of local experiments be generalized on a geographical scale?

Experimental manipulations of invertebrate prey and predators on rocky shores have been done by many authors. In the northern Atlantic the predators involved are usually the green crab Carcinus maenas and/or the dogwhelk Nucella lapillus, and the prey species studied are acorn barnacles (balanid Cirripedia), mussels ( Mytilus spp.) and winkles ( Littorina spp.). Usually the predators are found to have a regulating “top–down” effect on the prey species. In Iceland the acorn barnacle Semibalanus balanoides, the blue mussel Mytilus edulis and the flat periwinkle Littorina obtusata (including to some extent Littorina mariae) are found on rocky shores all around Iceland in what would seem to be near-optimal physical conditions. The predators Carcinus maenas and Nucella lapillus, on the other hand, are relatively southern species that do not thrive on the colder coasts of Iceland. Thus general surveys of different coasts of Iceland would seem to offer opportunities to see whether the results of local experiments can be discerned on a geographical scale (hundreds of km). The roughly 4900 km of the rocky coastline in Iceland was in this study subdivided into four regions, I–IV, according to the commonness or presence of the two predators. With the hope of reducing compounding factors the surveys were confined to sheltered or semi-sheltered fucoid shores, which were further divided into Ascophyllum (more sheltered) and Fucus vesiculosus (less sheltered) shores. Estuaries or other low-salinity environments were avoided. The study was based on 761 stations distributed around the rocky coastline on these two types of shores. The results for barnacles and mussels, being generally more abundant in regions were predators were scarce or absent, and being less common in Ascophyllum than F. vesiculosus shores in contrast to the predatory dogwelks, were in large measure in accord with predictions from experiments indicating “top–down” regulation. The results for the periwinkles, however, suggested that “bottom–up” regulation might be of greater importance, as their density was closely linked to the biomass of macroalgae, while the abundance of predators appeared to have little effect. The periwinkle, although certainly eaten, may not be a preferred prey of the two predators in question in Iceland. This paper forms a link between experimental and observational study approaches.

PROPAGATION OF SCALE-DEPENDENT EFFECTS FROM RECRUITS TO ADULTS IN BARNACLES AND SEAWEEDS.

The interplay between local and large spatial scale processes in open systems is often dependent upon ecological context and species specific factors such as longevity, dispersal capability, or vulnerability to predation. When disturbance clears patches in open systems, the successful reestablishment of adult colonizers and the trajectory of succession may depend upon both the scale of the disturbance event and scale of life history characteristics. Here we examine the links between the size of a disturbance event and long term patterns of variation in recruitment, density, and percent cover in a relatively short-lived but long-range disperser, the acorn barnacle Semibalanus balanoides, and a relatively long lived and short range disperser, the fucoid alga Fucus vesiculosus. Further, a model of linked scale-dependent processes is developed and we explore how long term patterns of scale-dependent recruitment are related to successful establishment of adults. Recruitment, densities, and cover were monitored for five years for both species in a single experiment using clearings of different sizes spread over two north-facing and two south-facing bays on Swan's Island, Maine, USA. Barnacle recruitment was particularly variable in small clearings and was lower overall at more interior sites within bays. While local-scale factors also strongly influenced patterns of variation in fucoid recruitment, fucoids dominate surface cover in large clearings and in south-facing bays. Recruitment was a poor predictor of density, whereas density itself was a good predictor of percent cover for both species. These results indicate that scale dependencies, location-specific factors, and life history traits contribute to patterns of community development on rocky shores and may ultimately determine whether an open patch converges to or diverges from its initial community state.

Ecological hindcasting of biogeographic responses to climate change in the European intertidal zone

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 re analysis 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.

Temporal changes in the strength of density‐dependent mortality and growth in intertidal barnacles

1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated and density-dependent mortality is sufficiently strong to reshape patterns established at settlement. 2. We examined temporal variation in the way density-independent and density-dependent mortality interact in a typical sessile marine benthic invertebrate, the acorn barnacle Semibalanus balanoides (L.), over a 2-year period. 3. Recruitment was manipulated at two high recruitment sites in north Wales, UK to produce recruit densities covering the range naturally found in this species. Following manipulation, fixed quadrats were monitored using digital photography and temporal changes in mortality and growth rate were examined. 4. Over a 2-year period there was a clear, spatially consistent, over-compensatory relationship between the density of recruits and adult abundance indicating strong density-dependent mortality. The strength of density dependence intensified with increasing recruitment. 5. Density-dependent mortality did not operate consistently over the study period. It only operated in the early part of the benthic phase, but the pattern of adult abundance generated was maintained throughout the whole 2-year period. Thus, early life-history processes dictated adult population abundance and dynamics. 6. Examination of the natural recruitment regime in the area of study indicated that both positive and negative effects of recruitment will occur over scales varying from kilometres to metres.

DENSITIES AND COVER DATA FOR INTERTIDAL ORGANISMS IN THE GULF OF MAINE, USA, FROM 2003 TO 2007

Experimental clearings in macroalgal (Ascophyllum nodosum) stands were made in 1996 to determine if mussel beds and macroalgal stands on protected intertidal shores in New England represent alternative community states. Uncleared control plots and four sizes of circular clearings, which mimicked ice scour events, were established in A. nodosum stands at 12 sites on Swan's Island, Maine, USA. The purpose of this data set is to provide access to data on densities and percent cover in the 60 experimental plots from 2003 to 2007 and to update data from 1996 to 2002 that are already published in Ecological Archives. Data include densities of mussels (Mytilus edulis and Modiolus modiolus), an herbivorous limpet (Tectura testudinalis), herbivorous snails (Littorina littorea, Littorina obtusata), a predatory snail (Nucella lapillus), a barnacle (Semibalanus balanoides), and fucoid algae (Ascophyllum nodosum and Fucus vesiculosus), and percent cover by mussels, barnacles, fucoids, and other sessile organisms.

Natural cyclic degeneration by a sequence of programmed cell death modes in Semibalanus balanoides (Linnaeus, 1767) (Crustacea, Cirripedia Thoracica)

The male reproductive system of the barnacle Semibalanus balanoides (Linnaeus, 1767) is a perfect model system for investigating naturally occurring cyclic nonpathological degeneration and programmed cell death (PCD). Every year after copulation, the barnacle degenerates its gonads, eggs, spermatids and penis initiated by severe environmental constraints. This is apparently a useful strategy for saving energy. By careful ultrastructural and immunohistochemical analysis of the male reproductive organs, we identified autophagic cell death in the penis and spermatids, both morphologically on ultrathin sections as well as histochemically for the first time on semithin tissue sections with the new antibody against microtubule-associated protein 1 light chain 3 (LC3). Apoptosis in the testis and vesicula seminalis was determined based on the morphological characters on the apoptosis-specific antibody against the apoptosis inducing factor (AIF), and on positive TUNEL (terminal in situ nick end labeling) nuclei. Secondary necrosis in late degrading stages occurred in all tissues investigated, based on morphological characters and weak TUNEL staining. Moreover, the ultrastructural screening showed transient forms of cell death in the ductus ejaculatorius epithelium, the epidermis, and the longitudinal muscles of the penis. According to the immunohistochemical tests, the cyclic degeneration of the male reproductive system follows a sequence of programmed cell death modes from autophagic cell death via apoptosis to secondary necrosis.

Effect of predation by the invasive crab Hemigrapsus sanguineus on recruiting barnacles Semibalanus balanoides in western Long Island Sound, USA

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.

Timing of successful settlement: demonstration of a recruitment window in the barnacle Semibalanus balanoides

Recruitment is a key factor in benthic population dynamics, and spatial and temporal processes that affect settlement may determine recruitment; however, temporal processes are not well understood. We tested whether the date that recruits settle is a random sample within the settle- ment season by measuring daily settlement of the barnacle Semibalanus balanoides throughout the entire settlement season. A total of 2721 barnacle larvae settled during 89 d on 12 quadrats. Individ- ual settlers were tracked to reproductive age (11 mo after settlement); only 8 survived to reproduc- tion. Survivors settled within a narrow 21 d recruitment window, a period shorter than expected by chance. The concept of a recruitment window has broad implications in studying benthic recruitment and population dynamics. Focus on the recruitment window when it is narrow could simplify the study of recruitment, since fewer factors would have to be considered.

Stable genetic polymorphism in heterogeneous environments: balance between asymmetrical dispersal and selection in the acorn barnacle

Elucidating the processes responsible for maintaining polymorphism at ecologically relevant genes is intimately related to understanding the interplay between selection imposed by habitat heterogeneity and a species' capacity for dispersal in the face of environmental constraints. In this paper, we used a model-based approach to solve equilibria of balanced polymorphism, given values of fitness and larval dispersal among different habitats in the acorn barnacle Semibalanus balanoides from the Gulf of St Lawrence. Our results showed that allele frequencies observed at both MPI* and GPI* loci represented stable equilibria, given empirical estimates of fitness values, and that considerably more larvae dispersed from one region (north) to the other (south) than vice versa. Dispersal conditions were predicted to be similar for the maintenance of polymorphism at both loci. Moreover, the values of asymmetrical dispersal required by the model to reach stable equilibria were compatible with empirical estimates of larval dispersal and oceanic circulation documented in this system. Overall, this study illustrated the usefulness of a modified and computable version of Bulmer's model (1972) in order to test hypotheses of balanced polymorphism resulting from interactions between spatial selection and asymmetrical dispersal.

Field evidence for an association between growth and protein polymorphism in the acorn barnacle Semibalanus balanoides

Organisms living in highly heterogeneous environments are useful for examining the effects of associations between phenotypes and ecologically relevant genes in the field. Here, we tested the null hypothesis of no association between the variation in 2 fitness-related traits (growth and fecundity) and polymorphism at 2 enzymatic loci, MPI* and GPI*, known to be subject to strong spatial selection. We also tested if such an association was related to the intensity of genotype selec- tion observed for both allozymes at sampling locations in 2 barnacle cohorts. For both cohorts, indi- viduals with the GPI*286/286 genotype were larger in size than individuals with the GPI*100/100 genotype, particularly in sites and microhabitats south of the Miramichi Estuary, Gulf of St. Lawrence, Canada. This coincided with a reduced GPI*100/100 frequency in this region. In contrast, the growth-MPI* genotype association showed no clear pattern. Overall, our results indicate that the phenotype-genotype association differed strikingly between the 2 genes, and that both the growth-GPI* genotype association and genotype selection occur in localities south of the Miramichi Estuary only, and at different times during development, first a few days after settlement (genotype selection) and later through growth-genotype association.

Barnacle larvae in ice: Survival, reproduction, and time to postsettlement metamorphosis

Late stage larvae (cyprids) of the barnacle Semibalanus balanoides frequently encounter freezing conditions along the northwest Atlantic coast. S. balanoides cyprids survived for more than 4 weeks embedded in sea ice, and a significant fraction of larvae held in ice up to 2 weeks successfully settled and metamorphosed after thawing. Larvae that completed metamorphosis continued to develop and reproduce. In settlement experiments with cyprids of known age and where settled cyprids were removed every other day from the experimental containers, cyprids held in ice for 2 weeks settled and metamorphosed more than nonfrozen larvae. Mean time to metamorphosis was longer for frozen cyprids than for nonfrozen ones, and maximum time to metamorphosis was 38 d for cyprids held in sea ice for 2 weeks and 26 d for cyprids in nonfrozen treatments. Larval tolerance to freezing conditions greatly expands the environmental tolerance repertoire of marine invertebrates and may help explain the ecological success of this widespread intertidal species.

Divergent succession and implications for alternative states on rocky intertidal shores

It has been hypothesized that rockweed stands and mussel beds in sheltered bays in the Gulf of Maine, USA, are alternative community states. As a test of this hypothesis, experimental clearings of different sizes were established in stands of the rockweed Ascophyllum nodosum (L.) Le Jolis to determine if successional changes in large clearings developed species assemblages distinctly different from the surrounding A. nodosum stands. Clearings ranging from 1 to 8 m in diameter were created at 12 sites in 4 bays on Swan's Island, Maine, in 1996 to mimic the effects of ice scour, and abundances of gastropods, barnacles, mussels and fucoid algae were monitored until 2002. ANOVAs and MDS showed strong effects of clearing size and divergent successional changes in large clearings. Large clearings were quickly filled in and remained dominated by the alga Fucus vesiculosus L. and the barnacle Semibalanus balanoides (L). There was no evidence for site-specific effects, and Mantel tests showed clearing size was a better predictor of species composition than geographic distances among sites. Results suggest that large pulse disturbances using clearings of 8 m in diameter can initiate divergent successional pathways and have a protracted effect on species composition. Results are also consistent with the hypothesis that mussel beds and rockweed stands in sheltered bays may be alternative community states.

Doses of darkness control latitudinal differences in breeding date in the barnacle Semibalanus balanoides

This paper reports the first study of breeding in the boreo-arctic barnacle Semibalanus balanoides in which latitudinal variation in timing of egg mass hardening has been examined simultaneously over the geographical scale involved, thereby excluding temporal confounding of the data. The timing of autumn egg mass hardening on the middle shore was established in 2002 and 2003 at ten stations ranging latitudinally from Trondheim (63°24′N) to Plymouth (50°18′N). To assess variation at local scale (&lt;10 km), breeding was studied on three shores at each of two Irish locations (Cork and Galway). At Oban (Scotland) and Cork, the effect of shore height on timing of breeding was investigated. A strong influence of latitude and day length on timing of breeding was found in both 2002 and 2003. In both years, barnacles bred much earlier (when day length was longer) at high rather than low latitudes. No significant effect of environmental temperature or insolation on timing of breeding was detected. Shores no more than 10 km apart showed minimal difference in middle shore breeding date (&lt;4 days). However, upper shore barnacles bred significantly earlier (by 7–13 days) than middle shore animals. The data indicate that breeding is controlled by period of daily darkness, with high shore animals encountering longer effective ‘nights’ because of the opercular closure response to emersion (which will reduce light penetration to tissues). Predictions concerning the effects of global changes in climate and cloud cover on breeding and population distribution are made. It is suggested that increased cloud cover in the northern hemisphere is likely to induce earlier breeding, and possibly shift the present southern limit of Semibalanus southwards.