Regional variation in the spatial scale of selection at MPI* and GPI* in the acorn barnacle Semibalanus balanoides (Crustacea)

Elucidating the ecological processes by which adaptive genetic polymorphism is maintained in heterogeneous environments requires knowledge on the spatial scale at which alternate habitats affect genotype-specific fitness. The general objective of this study was to document patterns of temporal and spatial variation of genetic polymorphism in the acorn barnacle (Semibalanus balanoides) at MPI* and GPI* allozyme loci. A total of 7261 barnacles were sampled in the intertidal at various locations north and south of the Miramichi estuary, New Brunswick, Canada. The results of this study supported the hypothesis that both MPI* and GPI* are under the effect of strong directional selection south of the Miramichi, whereas neutrality cannot be ruled out at sampling sites located north of the estuary. Comparisons between this study and previous ones also question the generality of current hypotheses regarding ecological processes that are responsible for maintaining polymorphism at MPI* and GPI* in the acorn barnacle.     

Effects of herbivorous snails and macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.)

This study investigated the effects of grazing by the herbivorous snail Littorina littorea (L.) and the presence of a macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.) at four sites of various distances from the mouth of Toothacher Cove on Swans Island in the Gulf of Maine in 1998. Recruitment plates were attached to the substrate at each site in all possible treatment combinations, including either presence of absence of macroalgal canopy, presence or absence of the herbivore, and presence or absence of a cage. Significant differences in recruitment were observed among sites in April and May, but not in June. The average number of recruits also varied significantly among herbivore treatments in May and June, and in the May macroalgaexherbivore interaction. Survivorship did not differ among sites, but did vary significantly among herbivore treatments. These differences were due almost entirely to consistently low recruitment and survival in the uncaged or open controls. Comparisons of the caged and open treatments explained between 88 and 99% of the variation in the respective main effects tests. The presence or absence of L. littorea or of macroalgal canopy had little effect on S. balanoides recruitment or survivorship. Results support previous conclusions that recruitment is greater on more exposed shores and suggest that exclusion cages may protect barnacle recruits from factors such as desiccation and the whiplash effect of macroalgal fronds.     

Genetic variation in the acorn barnacle from allozymes to population genomics

Understanding the patterns of genetic variation within and among populations is a central problem in population and evolutionary genetics. We examine this question in the acorn barnacle, Semibalanus balanoides, in which the allozyme loci Mpi and Gpi have been implicated in balancing selection due to varying selective pressures at different spatial scales. We review the patterns of genetic variation at the Mpi locus, compare this to levels of population differentiation at mtDNA and microsatellites, and place these data in the context of genome-wide variation from high-throughput sequencing of population samples spanning the North Atlantic. Despite considerable geographic variation in the patterns of selection at the Mpi allozyme, this locus shows rather low levels of population differentiation at ecological and trans-oceanic scales (F(ST)?～?5%). Pooled population sequencing was performed on samples from Rhode Island (RI), Maine (ME), and Southwold, England (UK). Analysis of more than 650 million reads identified approximately 335,000 high-quality SNPs in 19 million base pairs of the S. balanoides genome. Much variation is shared across the Atlantic, but there are significant examples of strong population differentiation among samples from RI, ME, and UK. An F(ST) outlier screen of more than 22,000 contigs provided a genome-wide context for interpretation of earlier studies on allozymes, mtDNA, and microsatellites. F(ST) values for allozymes, mtDNA and microsatellites are close to the genome-wide average for random SNPs, with the exception of the trans-Atlantic F(ST) for mtDNA. The majority of F(ST) outliers were unique between individual pairs of populations, but some genes show shared patterns of excess differentiation. These data indicate that gene flow is high, that selection is strong on a subset of genes, and that a variety of genes are experiencing diversifying selection at large spatial scales. This survey of polymorphism in S. balanoides provides a number of genomic tools that promise to make this a powerful model for ecological genomics of the rocky intertidal.     

A comparative study of the anti-settlement properties of mytilid shells

Marine organisms have evolved defence mechanisms to prevent epibiosis. This study investigated the anti-settlement properties of natural periostracal microtopographies of two mytilid species, Mytilus edulis (from North, Baltic and White Seas) and Perna perna (from the SW Atlantic). Resin replicas of shells were exposed to cyprids of the barnacle Semibalanus balanoides. Replicas with intact isotropic microtopographies and smooth controls were much less fouled than roughened anisotropic surfaces. This indicates that in both M. edulis and P. perna the periostracum possesses a generic anti-settlement property, at least against S. balanoides cyprids, which is not regionally adapted. Such a potential globally effective anti-settlement mechanism possibly contributes to the invasive success of Mytilidae.     

Adaptive maintenance of genetic polymorphism in an intertidal barnacle: habitat- and life-stage-specific survivorship of Mpi genotypes

In the northern acorn barnacle, Semibalanus balanoides, genotype frequencies of three genetic markers were tracked over time in four types of intertidal habitats. These habitats were selected to represent natural variation in several environmental parameters, specifically the degree of physical stress experienced by barnacles. Frequencies for one allozyme locus (Gpi) and a presumably neutral mtDNA marker were homogeneous among habitats in each temporal sample. Similarly, no temporal stratification in genotype frequencies was evident across the five sampling intervals: from planktonic larvae sampled in March to juveniles collected at the end of June. In contrast to the Gpi and mtDNA loci, Mpi genotypes significantly changed in frequency in two habitats in the high intertidal zone. On exposed substrate, the Mpi-FF homozygote increased in frequency, whereas the alternative homozygote, Mpi-SS, significantly decreased in frequency. Barnacles that were protected from environmental stress at high intertidal heights by the Ascophyllum nodosum algal canopy demonstrated the opposite pattern. In both habitats, the change in frequency of the heterozygote was intermediate to that of the homozygous genotypes. Furthermore, these patterns of genotype-by-environment association reflected a pulse of genotype-specific mortality that occurred over a two-week interval subsequent to metamorphosis from the larval to the adult form. These data indicate that each Mpi homozygote is the highest fitness genotype in some portion of the intertidal environment. Using the Levene (1953) model to evaluate the spatial variation in genotypic fitness, the stable maintenance of the Mpi polymorphism is predicted under certain subsets of conditions. Environmental heterogeneity in the intertidal zone translates to spatial variation in selection pressures, which may result in the active maintenance of the Mpi polymorphism in this species.     

Genetic structure and the North American postglacial expansion of the barnacle, Semibalanus balanoides

Population genetic characteristics are shaped by the life-history traits of organisms and the geologic history of their habitat. This study provides a neutral framework for understanding the population dynamics and opportunities for selection in Semibalanus balanoides, a species that figures prominently in ecological and evolutionary studies in the Atlantic intertidal. We used mitochondrial DNA (mtDNA) control region (N = 131) and microsatellite markers (～40 individuals/site/locus) to survey populations of the broadly dispersing acorn barnacle from 8 sites spanning 800 km of North American coast and 1 site in Europe. Patterns of mtDNA sequence evolution were consistent with larger population sizes in Europe and population expansion at the conclusion of the last ice age, approximately 20?000 years ago, in North America. A significant portion of mitochondrial diversity was partitioned between the continents (?(ST) = 0.281), but there was only weak structure observed from mtDNA within North America. Microsatellites showed significant structuring between the continents (F(ST) = 0.021) as well as within North America (F(ST) = 0.013). Isolation by distance in North America was largely driven by a split between populations south of Cape Cod and all others (P < 10(-4)). The glacial events responsible for generating allelic diversity at mtDNA and microsatellites may also be responsible for generating selectable variation at metabolic enzymes in S. balanoides.     

Density and shell-size variation of Nodilittorina lineolata (Orbigny, 1840) in the intertidal region in southeastern Brazil

Nodilittorina lineolata is a small littorinid characteristic of the supralittoral of rocky shores but extends its vertical range to the lower midlittoral. Shell-size and density of Nodilittorina were measured at two different levels, the supralittoral and barnacle zones, in six sites along the south-eastern Brazilian coast. The sites studied varied in the degree of wave exposure. Densities were lower in the supralittoral, and higher overall values occurred on the most exposed shores. Shell size increased significantly in an upshore direction at five of the six sites. However, mean-size and the magnitude of the difference between levels varied according to the site studied. The shell-size gradient can be explained by larval recruitment in the barnacle zone and subsequent ascending migration to the supralittoral zone, where the larger individuals were found. Reciprocal transfer experiments revealed that differential mortality of large individuals in the lower levels, possibly due to lack of refuges, may also influence the size-distribution. It was also shown that some individuals can be passively transported to upper levels of the shore by wave action during storms.     

Surface structures and sense organs of the cypris larva of Balanus balanoides as seen by scanning and transmission electron microscopy.

Various features of the settlement stage larva (cyprid) of the barnacle, Balanus balanoides (L.), were studied using scanning and transmission electron microscopy. The cuticle of the valves is pitted and in section has a characteristic ultrastructure. Small sensory setae protrode from the surface of this cuticle and are probably mechanoreceptors able to sense water movement around the larva. Each of the pair of caudla appendages which protrude from between the larval valves posteriorly, is made up of several sensory setae. These appendages are able to sense settlement surface topography. Certain other features of the larva are alos described and their roles discussed; such features include the frontal filaments, antennules and thoracic limbs.     

The vertical distribution of Chthamalus montagui and Chthamalus stellatus (Crustacea,Cirripedia) in two areas of the NW Mediterranean Sea

The Mediterranean Sea is characterised by a small tidal range (0.3–1 m). Despite this, intertidal communities are well established and their upper limits often extend above mean high water level. Organisms living in the intertidal region and in the supralittoral zone rely on both tides and wave action to perform their biological functions. Lack of food, desiccation and predation are common stresses in such a harsh environment. The present study deals with the vertical distribution of two species of intertidal barnacles, Chthamalus montagui Southward and Chthamalus stellatus (Poli), which are the main constituents of the barnacle belt along Mediterranean rocky shores. Previous work, carried out in the Atlantic, showed that where the distribution ranges of the two Chthamalus species overlap, C. montagui is more common in the upper barnacle zone while C. stellatus is dominant lower down. The main aims of our study are: (1) to establish if there is a relationship between position and extension of the barnacle belt on the shore and tidal range and/or wave exposure, (2) to test the hypothesis that in the study areas C. montagui is more abundant than C. stellatus high on the shore, and that the pattern is reversed lower down. Barnacle populations were monitored in summer 1998 in the Gulf of Genoa (Ligurian Sea) and in the Gulf of Trieste (North-Adriatic Sea). The two areas differ in tidal range and hydrodynamism, the former presenting quite strong wave action and a tidal range of 30 cm, the latter having limited wave action and 1 m tidal range. Three shores were randomly selected in each gulf and two transects on each shore. Counts of barnacles in 10 ^* 10 cm quadrats were done at different shore heights along each transect. The data was subjected to analysis of variance. Results showed that a more pronounced hydrodynamic regime corresponded to a shift of the barnacle belt towards the higher shore (Gulf of Genoa), while in more sheltered areas (Gulf of Trieste), the barnacle distribution was confined to the intertidal region. The relative spatial distribution of C. montagui and C. stellatus within the barnacle belt varied locally, even between transects on the same shore, and this obscured the distribution pattern along the vertical gradient. Nevertheless, it was still possible to conclude that at mid and high shore in Genoa, C. stellatus was more abundant than C. montagui, while in Trieste the pattern was reversed.     

Dynamics of species interaction strength in space, time and with developmental stage

Quantifying species interaction strengths enhances prediction of community dynamics, but variability in the strength of species interactions in space and time complicates accurate prediction. Interaction strengths can vary in response to density, indirect effects, priority effects or a changing environment, but the mechanism(s) causing direction and magnitudes of change are often unclear. We designed an experiment to characterize how environmental factors influence the direction and the strength of priority effects between sessile species. We estimated per capita non-trophic effects of barnacles (Semibalanus balanoides) on newly settled germlings of the fucoid, Ascophyllum nodosum, in the presence and absence of consumers in experiments on rocky shores throughout the Gulf of Maine, USA. Per capita effects on germlings varied among environments and barnacle life stages, and these interaction strengths were largely unaltered by changing consumer abundance. Whereas previous evidence shows adult barnacles facilitate fucoids, here, we show that recent settlers and established juveniles initially compete with germlings. As barnacles mature, they switch to become facilitators of fucoids. Consumers caused variable mortality of germlings through time comparable to that from competition. Temporally variable effects of interactors (e.g. S. balanoides), or spatial variation in their population structure, in different regions differentially affect target populations (e.g. A. nodosum). This may affect abundance of critical stages and the resilience of target species to environmental change in different geographical regions.     

Identification of the hatching factor of the barnacle Balanus balanoides as the novel eicosanoid 10,11,12-trihydroxy-5,8,14,17-eicosatetraenoic acid

Barnacle egg hatching factor which is released into the mantle cavity where the egg masses are brooded and stimulates embryonic musculature resulting in hatching and liberation of the larvae into the sea has been isolated in a purified form from a common barnacle Balanus balanoides. Derivatised hatching factor has been analysed by GC-MS and identified as 10,11,12-trihydroxy-5,8,14,12-eicosatetraenoic acid, a novel eicosanoid probably formed from endogenous eicosapentaenoic acid (20:5w3). Hatching factor activity is the first specific physiological function to be established for this type of compound.     

Competing species in a changing climate: effects of recruitment disturbances on two interacting barnacle species

1. The climate is changing and data-based simulation models can be a valuable tool for predicting population response to such changes and investigate the mechanisms of population change. In this study, a data-based two-species matrix model was constructed to explore the possible effects of elevated sea surface temperature (i.e. climate change) on the interaction between open populations of the south Atlantic barnacle species Chthamalus montagui and the boreal species Semibalanus balanoides in the north-east Atlantic. 2. First, the model was used to perform an elasticity analysis to determine the relative importance of recruitment and survival in the interaction. Further, three scenarios of changes in recruitment, related to climate change, were investigated with model simulations: (i) increased frequencies of low recruitment for S. balanoides; (ii) increased frequencies of high recruitment for C. montagui; (iii) a combination of (i) and (ii). 3. Model simulations showed that in present environmental conditions, S. balanoides occupied most of the space and dominated the interaction through high recruitment and survival. These results matched independent field observations, which validated the model for further analyses. 4. The elasticity analyses showed that although free space was available there was competition for space during recruitment intervals. It was also shown that both populations were sensitive to changes in recruitment. 5. Introducing the three scenarios of recruitment disturbances led to large changes in species abundance and free space. The most significant changes were found when scenario (i) and (ii) were combined, producing a shift in species dynamics towards C. montagui dominance. This demonstrates that recruitment can be an important mechanism in the interaction between populations and that the population response to changes in recruitment depends on the added response of interacting species. 6. In a more general context, this model shows that increased sea surface temperature could rapidly lead to increased competition from southern species at higher latitudes. This might accelerate the effects of climate change on the species distribution at these latitudes and eventually lead to changes in community dynamics on temperate and subarctic shores.     

Fluctuating asymmetry of <Emphasis Type="Italic">Amphibalanus</Emphasis> (<Emphasis Type="Italic">Balanus</Emphasis>) <Emphasis Type="Italic">amphitrite</Emphasis> (Cirripedia: Thoracica) in association with shore height and metal pollution

The level of fluctuating asymmetry (FA), which is defined as random deviations from perfect bilateral symmetry in the morphological traits of an organism, increases with increasing developmental instability, and it may be used as an indicator of environmental and/or genetic stresses. This study attempted to relate FA levels in the opercular plates of the barnacle Amphibalanus (Balanus) amphitrite with shore height and body trace metal concentrations. Barnacles were collected from both low and mid shores at six coastal locations with various degrees of marine pollution in Hong Kong. Four opercular traits, namely scutum length, tergum length, scutum width, tergum width were measured in the specimen while concentrations of five common trace metals (Cd, Cu, Cr, Mn and Zn) were determined in their body tissues using inductively coupled plasma-atomic emission spectrophotometry. Among the four traits, only tergum length and scutum width fulfilled the assumption of FA while their measurement errors were low. Across all sites, mid-shore A. amphitrite consistently exhibited a significantly higher FA level in scutum width than that in low-shore barnacles, but FA levels in tergum length were similar between the two shore heights. These results suggested that FA was trait-dependent, and mid-shore barnacles were possibly under high physical stress, such as desiccation and high temperature, leading to high developmental instability. Although no positive association was observed between FA and metal contamination in low-shore A. amphitrite, FA level in tergum length significantly increased with decreasing body concentration of manganese (Mn) in these barnacles. Such a negative relationship may be explained by the mechanism of bioaccumulation and physiological role of Mn in A. amphitrite with respect to the formation of barnacle shell plates. Handling editor: T. P. Crowe     

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.     

The effects of diet and physiological stress on the evolutionary dynamics of an enzyme polymorphism

In the northern acorn barnacle Semibalanus balanoides, polymorphism at the mannose-6-phosphate isomerase (Mpi) locus appears to be maintained by distinct selection regimes that vary between intertidal microhabitats. The goal of the present experiment was to elucidate the mechanism of selection at the Mpi locus by examining the relationship between genotype and fitness-related life-history traits in laboratory manipulations. When barnacles were cultured on a mannose-supplemented diet and exposed to thermal stress, different Mpi genotypes exhibited differences in the rate of growth that predicted survivorship. In contrast, no such relationship was observed in control or fructose-supplemented dietary treatments either in the presence or in the absence of stress. Similarly, the phenotype and survivorship of genotypes at another allozyme locus and a presumably neutral mitochondrial DNA marker were homogeneous across all treatments and unaffected by experimental manipulations. These results suggest that the differential survivorship of Mpi genotypes in the field and laboratory results from a differential ability to process mannose-6-phosphate through glycolysis. The widespread polymorphism at Mpi observed in marine taxa may reflect the interaction between dietary composition and environmental heterogeneity in intertidal habitats.     

Experimental tests of sex allocation theory with two species of simultaneously hermaphroditic acorn barnacles

Sex allocation theory for simultaneous hermaphrodites predicts increases in relative allocation to male-specific function as competition for fertilizations increases. Theoretical models developed specifically for competing acorn barnacles predict that the proportional allocation to male function increases toward an asymptote of 50% as the number of competitors for fertilizations increases. Experimental manipulations were used to investigate how mate competition affected both relative and absolute allocation to the sex functions for two species of acorn barnacle: Semibalanus balanoides and Balanus glandula. The ratio of male to female allocation did not increase with the number of competitors for either species. However, both species showed increased allocation to male function (estimated as total mass of sex-specific tissues) with increased crowding. Allocation to female function seemed to be limited by other factors and did not vary with mating group size as predicted. Allocation to male and female function were both positively related to body size, but a trade-off between male and female function, a key assumption of prior models, was not observed.     

GEOGRAPHIC VARIATION IN THERMAL TRAITS IN DIGENEA SIMPLEX AND CHAMPIA PARVULA (RHODOPHYTA) IN RELATION TO PRESENT AND GLACIAL TEMPERATURE REGIMES

Geographic variation in temperature responses (survival and growth) was investigated in two red algae: Digenea simplex (Wulfen) C. Agardh and Champia parvula (C. Agardh) Harvey. D. simplex has a tropical to warm temperate distribution; C. parvula extends from the tropics into the cold temperate zone. Ecoclinal variation was found in both species but was much stronger in C. parvula than in D. simplex. The former species showed variation in upper and lower tolerance limits as well as in the upper and lower limits for growth. The latter species showed variation mainly in its lower tolerance limit. Ecoclinal variation was related to the amount of present and glacial selection pressure along the climate gradient. In both species, isolates from the colder localities had insufficient cold tolerance to have survived low glacial winter temperatures, so these locations must have been colonized after the end of the glaciation. Eastern Mediterranean and Atlantic populations were probably isolated during the glaciation by a thermal barrier at the entrance of the Mediterranean. In C. parvula, evidence existed for a trade-off between the performance at high and at low temperatures, which would enhance selection pressure in opposite directions at either end of the climatic range. No evidence for such a trade-off was found in D. simplex.     

Identification and egg hatching activity of monohydroxy fatty acid eicosanoids in the barnacle Balanus balanoides.

Monohydroxy fatty acids (MHFAs) were isolated from homogenates of the barnacle Balanus balanoides and identified by gas chromatography-mass spectrometry (GC-MS) as 14- and 17-hydroxy docosahexaenoic acids, 8-, 11-, 12-, 15- and 18-hydroxy eicosapentaenoic acids, 13- and 16-hydroxyoctadecatrienoic acids and 9-, 13- and 15-hydroxyoctadecadienoic acids. Each monohydroxy fatty acid was tested for egg hatching activity in a bioassay using Elminius modestus egg masses, but 8-hydroxy-5, 9, 11, 14, 17-eicosapentaenoic acid (8-HEPE) was the only MHFA with barnacle egg hatching activity. Studies on the egg hatching activity of MHFAs prepared from the oxidation of polyunsaturated fatty acids showed that activity was confined to the 8-hydroxy isomer of eicosapentaenoic acid and arachidonic acid, and that unsaturation at C5 and C14, but not C17, was essential for activity. In addition, the 8(R) conformation is necessary for activity, as 8(R)-HEPE caused egg hatching at 10(-7) M whereas the enantiomer 8(S)-HEPE was inactive.