Barnacle Density Research Articles

Non-linear density-dependent effects of an intertidal ecosystem engineer

Ecosystem engineering is an important process in a variety of ecosystems. However, the relationship between engineer density and engineering impact remains poorly understood. We used experiments and a mathematical model to examine the role of engineer density in a rocky intertidal community in northern California. In this system, the whelk Nucella ostrina preys on barnacles (Balanus glandula and Chthamalus dalli), leaving empty barnacle tests as a resource (favorable microhabitat) for other species. Field experiments demonstrated that N. ostrina predation increased the availability of empty tests of both barnacle species, reduced the density of the competitively dominant B. glandula, and indirectly increased the density of the competitively inferior C. dalli. Empty barnacle tests altered microhabitat humidity, but not temperature, and presumably provided a refuge from wave action. The herbivorous snail Littorina plena was positively associated with empty test availability in both observational comparisons and experimental manipulations of empty test availability, and L. plena density was elevated in areas with foraging N. ostrina. To explore the effects of variation in N. ostrina predation, we constructed a demographic matrix model for barnacles in which we varied predation intensity. The model predicted that number of available empty tests increases with predation intensity to a point, but declines when predation pressure was strong enough to severely reduce adult barnacle densities. The modeled number of available empty tests therefore peaked at an intermediate level of N. ostrina predation. Non-linear relationships between engineer density and engineer impact may be a generally important attribute of systems in which engineers influence the population dynamics of the species that they manipulate.

Ecological Succession on Restored Intertidal Oyster Habitat in the Tidal Creeks of Coastal Georgia

ABSTRACT Microhabitat availability resulting from spatial complexity on oyster reef allows niche overlap and enhances benthic-pelagic coupling within a localized area; however, in Georgia, little is known regarding the temporal succession patterns of resident species on constructed reef. The purpose of this research is to evaluate the development and relative habitat value of man-made oyster habitat by monitoring oyster growth and the colonization by structurally important and resident oyster reef community species. Sixteen sampling units of commercial spat sticks in densities of 81/m2 were deployed prior to the oyster reproductive season during April 2004. Two sampling units were extracted every 3 mo starting July 2004 and assessed for biomass; oyster shell height and growth rate; oyster, barnacle, and mussel density per 0.01 m2; and species and phyla richness. Maximum oyster growth rate (0.39 mm/day) occurred during January 2005, with mean oyster shell height peaking at 83.56 ± 1.31 mm by May 2005. Oyst...

Regional decoupling between NW Atlantic barnacle recruit and adult density is related to changes in pelagic food supply and benthic disturbance

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.

Surf zone physical and morphological regime as determinants of temporal and spatial variation in larval recruitment

Larvae of intertidal species develop in the coastal ocean, and the last body of water they must cross while migrating back to shore is the surf zone. We hypothesized that the surf zone is a semipermeable barrier to this shoreward migration and that differences in water exchange across the surf zone result in temporal and spatial variation in larval delivery to the shore. We tested the hypotheses that larval delivery 1) should increase with larger waves and 2) should be higher on more dissipative beaches than on more reflective beaches. We found a significant positive correlation between the daily averaged ratio of wave height to wave period ( H/ T) and daily cyprid settlement at Dike Rock, California and Bastendorff Beach, Oregon, USA. We tested the second hypothesis by comparing populations of barnacles, limpets, and benthic algae on rocks on four more dissipative and six more reflective sandy beaches in northern California and southern Oregon. Newly recruited barnacles and limpets were significantly more abundant at more dissipative than reflective beaches, and the higher abundance was most likely due to differences in settlement rather than post-settlement mortality. The density and percent cover of barnacles and the density of limpets were significantly higher at more dissipative beaches. In contrast, the density and percent cover of algae were significantly higher at more reflective beaches. The results are consistent with the hypothesis that the surf zone is a semipermeable barrier to the shoreward migration of larvae and that differences in water exchange across the surf zone as function of the beach hydrodynamics result in temporal and spatial variation in larval delivery to the shore.

Shore level differences in barnacle ( Chthamalus dalli) recruitment relative to rock surface topography

Local recruitment of marine invertebrates is often variable and can be an important determinant of population structure and dynamics. Here, we examined recruitment of barnacles relative to rock surface topography (cracks) over 4 years at 5 sites in Hokkaido, Japan. We used recruitment over multiple years to test for differences in barnacle response to rock surface topography among shore levels under natural conditions and varying recruit densities. Our results showed that barnacles exhibit different recruitment patterns in relation to surface topography over small spatial scales (50 cm) with barnacles in upper intertidal associating more with cracks ( p < 0.0001). In general, we found that barnacles tend to recruit more frequently within cracks in the upper shore and this result was not a function of barnacle density or available crack sites.

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.

Effect of the substratum in the recruitment and survival of the introduced barnacle Balanus glandula (Darwin 1854) in Patagonia, Argentina

The barnacle Balanus glandula was introduced in Argentina in the 1970s, and today it dominates the high intertidal level in most Argentinean rocky shores. The aim of this work was to evaluate the effect of the type of substrata and intertidal height on a population of Balanus glandula by conducting field surveys and one-year field experiments in which we combined different substrata (hardness: hard and soft, and texture: smooth and rough) at two intertidal heights (mid and high). In natural populations, the highest density of adults and recruits occurred on soft–rough substratum and in the high intertidal. The different textures were important only on the soft substrata and high intertidal, and the density of barnacles of the soft–rough substrata was higher than soft and smooth ones. The most suitable experimental substratum was the soft–rough of the high intertidal, which had the highest recruitment, survival and final density of barnacles at the end of the experiment. In contrast, the hard and smooth of the high and middle intertidal were the least suitable in all cases. Although the recruitment of B. glandula occurred throughout the year, it was higher in the high intertidal, and it showed a recruitment peak in the winter and a second in the summer. While most studies on this barnacle investigated the effects of granite or other volcanic hard substrata, our study also focused on soft substrata. The effects of soft substrata are particularly important because soft sedimentary rocks characterise the southern Atlantic coast of South America and the presence of soft rocks appears to optimize the success of Balanus glandula.

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.

Establishment and persistence of species-rich patches in a species-poor landscape: role of a structure-forming subtidal barnacle

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 380:187-198 (2009) - DOI: https://doi.org/10.3354/meps07905 Establishment and persistence of species-rich patches in a species-poor landscape: role of a structure-forming subtidal barnacle A. R. Davis*, D. W. Ward Institute for Conservation Biology and School of Biological Sciences, University of Wollongong, New South Wales 2522, Australia *Email: adavis@uow.edu.au ABSTRACT: Some sessile invertebrates are capable of maintaining space in barren habitats produced by sea urchins, thereby creating species-rich patches in a species-poor landscape. We sought to determine the role of a large and common barnacle, Austrobalanus imperator, in the establishment and persistence of these species-rich patches. Barnacle density was modified in 2 experiments at sites in southeastern Australia. The first experiment concerned community establishment and involved the addition of barnacles in 4 densities (zero [control], low, medium and high) to plots on vertical rock surfaces. The addition of barnacles at ecologically realistic densities and spatial arrangements rapidly resulted in statistically significant increases in invertebrate cover and diversity. After 56 mo, the diversity of invertebrates was significantly higher on plots that received high densities of barnacles relative to controls. However, invertebrate cover no longer differed between treatment plots, despite evidence that barnacles modify the grazing intensity of sea urchins. The second experiment assessed the persistence of sessile invertebrates following the removal of barnacles from well-established assemblages dominated by sponges. The removal of barnacles did not hinder the rapid recovery of sponges to pre-manipulation levels, indicating that barnacles played an inconsequential role in well-established assemblages. We conclude that barnacles play an important functional role in this system, as they promote the recruitment of sessile invertebrates. It was also clear that the development of invertebrate assemblages on natural vertical surfaces was very slow, which raises the question of whether we are examining these important occupiers of space at appropriate temporal and spatial scales. KEY WORDS: Cirripedia · Echinoidea · Grazing activity · Habitat structure · Porifera · Sessile invertebrates · Structural heterogeneity · Rocky subtidal zone Full text in pdf format PreviousNextCite this article as: Davis AR, Ward DW (2009) Establishment and persistence of species-rich patches in a species-poor landscape: role of a structure-forming subtidal barnacle. Mar Ecol Prog Ser 380:187-198. https://doi.org/10.3354/meps07905 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 380. Online publication date: April 07, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Modeling variation in interaction strength between barnacles and fucoids.

The strength by which species interact can vary throughout their ontogeny, as environments vary in space and time, and with the density of their populations. Characterizing strengths of interaction in situ for even a small number of species is logistically difficult and may apply only to those conditions under which the estimates were derived. We sought to combine data from field experiments estimating interaction strength of life stages of the barnacle, Semibalanus balanoides, on germlings of Ascophyllum nodosum, with a model that explored the consequences of variability at per capita and per population levels to the abundance of year-old algal recruits. We further simulated how this interaction affected fucoid germling abundance as the timing of their respective settlements varied relative to one another, as occurs regionally across the Gulf of Maine, USA. Juvenile S. balanoides have a weak estimated per capita effect on germlings. Germling populations are sensitive to variation in per capita effects of juvenile barnacles because of the typically large population sizes of the latter. However, high mortality of juvenile barnacles weakens the population interaction strength over time. Adult barnacles probably weakly facilitate fucoid germlings, but greater survival of adults sustains the strength of that interaction at the population level. Germling abundance is positively associated with densities of adult barnacles and negatively associated with that of juvenile barnacles. Metamorphosing cyprid larvae have the strongest per capita effect on germling abundance, but the interaction between the two stages is so short-lived that germling abundance is altered little. Variation in the timing of barnacle and A. nodosum settlement relative to one another had very little influence on the abundance of yearling germlings. Interactions between barnacles and germlings may influence the demographic structure of A. nodosum populations and the persistence of fucoid-dominated communities on sheltered rocky shores in New England.

Salt marsh colonization by a rocky shore invader: Balanus glandula Darwin (1854) spreads along the Patagonian coast

Balanus glandula, an east Pacific acorn barnacle from rocky shores, was introduced to Mar del Plata, Argentina more than 40 years ago and has spread over 17 latitudinal degrees southward. Here we report the first record of this species living in a soft-bottom environment colonizing the salt marsh plant species Limonium brasiliense, Spartina densiflora, S. alterniflora and Sarcocornia perennis. In addition, we describe the size frequency distribution, density and spatial distribution of the barnacles colonizing the different plant species. The size frequency distribution of Balanus showed a bimodal pattern in all plants. Barnacles were mostly large in S. densiflora, but small in S. alterniflora, with more balanced distributions of small and large barnacles on S. perennis and L. brasiliense. The highest density of barnacles was observed on S. perennis (x = 35.8 ind/cm2, SD = 40.5) and S. alterniflora (x = 33.8 ind/cm2, SD = 23), while the lowest on L. brasiliense (x = 1.5 ind/cm2, SD = 1.18) and S. densiflora (x = 0.17 ind/cm2, SD = 0.09). More than 90% of the barnacles on any given plant were found living. While barnacles colonized only the first few centimeters above the soil surface level in S. alterniflora and L. brasiliense, they reached their highest point on S. perennis. The finding of a rocky shore species successfully colonizing soft-bottom marshes within an invaded region brings new perspectives to discussions in biological invasion ecology, and raises additional considerations for coastal environmental management.

Colonization by barnacles on fossil Clypeaster : an exceptional example of larval settlement

The presence of c. 1450 individuals of the balanid barnacle Balanus crenatus Bruguiere encrusting the test of a clypeasteroid sea urchin from the Late Miocene of the Guadalquivir Basin (southwestern Spain) allows proposing a settlement pattern linked to the growth of the encrusting organism. The possible influence of dip angle was controlled by dividing the test into four concentric zones ranging from lowest margin to apex (0–15°, 15–30°, 30–50° and 0°). Contour diagrams were prepared to identify areas of highest barnacle density as well as size categories distribution in relationship to the pitch of the sea urchin test. The orientation of balanid tests was recorded and plotted on rose diagrams from 0° to 180°. Four size categories of barnacles were distinguished: (1) < 1 mm, (2) 1–2 mm, (3) 2–3 mm and (4) 3–4 mm; these correspond to a growth sequence ranging from post-larval forms to juveniles. Two areas of maximum settlement density are situated on the posterior margin of the test, on aboral as well as oral surfaces. The aboral surface shows the maximum number of barnacles. Two groups of individuals are defined on the basis of their location, that is, those encrusting the posterior medium part of the urchin, and those located on the anterior half. The results suggest that larval settlement was initially controlled by the availability of free space and afterwards by an intensification effect. Orientation and dip of the test may have played a secondary role in the settlement of the larvae. Substrate colonization seems to have been closely related to the biostratinomic history of the sea urchin test and although several scenarios are possible, our data are congruent with a synchronous settlement of both surfaces (aboral and oral) by one spat or several.

The invasion of the acorn barnacle Balanus glandula in the south-western Atlantic 40 years later

The acorn barnacle Balanus glandula (Darwin 1854) is native to the Pacific coast of North America and was accidentally introduced in Argentina in the early 1970s. Here the invasion status of this species is reviewed in the south-western Atlantic focusing on geographical patterns of density, cover, biomass, size and recruitment, and also estimating its rate of spread. Field surveys along rocky shores and a literature review show that B. glandula invaded most of the rocky shores of Argentina at a high rate of spread. Density and cover of barnacles do not follow a latitudinal trend; instead both variables show a bimodal pattern with the highest values in two distant locations (Puerto Lobos and Bahía Bustamante). However, the size of the barnacles increases with latitude, and is positively related to biomass. Recruitment of this species varies between wave exposed and protected areas, and over time. At lower latitudes barnacles recruit in winter, while at higher latitudes they recruit in spring and summer. The differences observed in density and recruitment suggest that along the Argentinean coast, oceanographic processes have a stronger influence in the distribution and success of the barnacles than the gradient in wave exposure. Balanus glandula is a successful invader which has completely re-shaped the native intertidal landscape. Moreover, considering the wide temperature range that this barnacle tolerates in native and invaded regions, the entire coastline of South America could be colonized by this species in the future.

Invasion of an indigenous Perna perna mussel bed on the south coast of South Africa by an alien mussel Mytilus galloprovincialis and its effect on the associated fauna

The introduced mussel Mytilus galloprovincialis is progressively increasing in abundance along the south coast of South Africa. Quantitative 0.1 m2 samples were collected in the mid-zone of an indigenous Perna perna mussel bed in the 1980s prior to the arrival M. galloprovincialis (12) and in the 2000s during the M. galloprovincialis invasion (16). In addition, in situ counts of M. galloprovincialis were done on eight occasions between 1993 and 2005, and in the low- and high-zones on four occasions. In the mid-zone M. galloprovincialis was absent until 1987, its mean densities were low (< 15 individuals/0.1 m2) between 1993 and 1996, but thereafter increased steadily, peaking in 2004 (at 721 individuals/0.1 m2), before declining in 2005 (331 individuals/0.1 m2). The greatest densities of M. galloprovincialis were recorded at the high-zone (1121 individuals/0.1 m2) and the smallest in the low-zone. As M. galloprovincialis numbers increased, there was an associated, but smaller decline in P. perna numbers and the overall density of mussels increased significantly (P < 0.05). No major change was recorded in the size composition of P. perna. The density of associated fauna differed significantly (P < 0.01) between sampling dates with the lowest and highest values being recorded near the ‘beginning’ (2001) and ‘end’ (2005) of the invasion period respectively. These differences were largely due to variations in the density of barnacles, and the toothed barnacle Chthamalus dentatus appeared to be the only associated faunal species that was directly affected by the M. galloprovincialis invasion, experiencing a significantly (P ≤ 0.05), but temporary decline in density and biomass values.

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.

Cross-scale variation in top-down and bottom-up control of algal abundance

Previous studies of top-down and bottom-up effects in Oregon rocky intertidal communities suggested that, unlike sea star–mussel dominated food chains, grazer-alga food chains deviated from simple food web predictions. However, this conclusion lacked generality due to limited replication at the site level. We explored how grazing and algal colonization and growth in different zones and multiple sites in intertidal communities varied in relation to processes varying on local to mesoscales, including oceanographic variation. Our approach employed the comparative-experimental method, in which identically-designed and replicated experiments are performed at multiple sites spanning the environmental gradient of interest. In a limpet–algae–barnacle interaction web, we tested the hypothesis that top-down and bottom-up effects were inversely related along the Oregon coast. To distinguish between the alternatives of large-scale oceanographic forcing vs. local control, our experiments spanned scales of meters (between mid and low zones), kilometers (seven sites), 10's to 100's of km (three regions), and ∼ 260 km (central and southern coasts). Identically designed and executed experiments on effects and rates of grazing were conducted in a southern coast region of persistent upwelling and two central coast regions, one with intermittent upwelling in an area of weak offshore currents, and another with intermittent upwelling in an area of strong offshore currents. Grazer effects and grazing rates varied on all scales, indicating a complex interplay of processes operating at zone, site, regional, and coastal scales. Grazing was generally highest in all mid zones and in the low zone at the middle region (Cape Perpetua) on the central coast. Growth of early successional colonists tended to be higher at the northern and southern regions (Capes Foulweather and Blanco). Grazers had no effect on barnacle cover, but their foraging was inhibited by high densities of barnacles. Differences in grazing appear to be a complex consequence of direct and indirect effects of variable oceanographic conditions, limpet recruitment, and barnacle abundance. In contrast to positive associations between predation strength and the magnitude of prey subsidies, higher grazing strength was generally associated with lower magnitude of bottom-up effects and vice versa. Hence in this system predator–prey (sea star–filter feeder) and herbivore–plant (limpet–microalgae) interactions evidently respond differently to ecological subsidies.

Density‐Dependent Effects in Jehlius cirratus (Darwin, 1854) (Cirripedia: Chthamalidae) under Different Growth Conditions

Abstract. We analyse the effects of crowding (‘hummocking’) on the growth, reproduction and mortality of the chthamaloid barnacle Jehlius cirratus (Darwin, 1854) on the coast of Chile. Recruitment intensity, ranging from solitary individuals to dense groups, generates different growth conditions. Density‐dependent effects are reflected in the shape, growth parameters, fecundity, sexual maturity and mortality of solitary and crowded specimens. The skewing of size distribution during growth was not significant at the beginning of the growth period, irrespective of initial density. Nevertheless, skewness in size increased with time, both at high and low densities, indicating that the dominance and suppression exerted by larger specimens over smaller ones generate a density‐dependent effect on size during growth. Mortality was greatest when high‐density recruitment occurred immediately before the period when water temperature and food quantity are highest (spring), when it is assumed that growth is greatest; this seasonal effect was not detected at lower levels of recruitment density. Similarly, no differences in mortality were detected between recruitment densities during the period when temperatures are lowest (autumn) and growth is probably slower. In the middle‐high intertidal zone, where J. cirratus co‐exists with Notochthamalus scabrosus (Darwin, 1854), the proportion of dead J. cirratus specimens was directly related both to the relative abundance of N. scabrosus and the total barnacle density. Although the mortality of J. cirratus was density dependent, the increase in the number of N. scabrosus towards the lower section of the middle intertidal zone could have a greater effect on J. cirratus mortality than intra‐specific competition.

Population consequences of mutual attraction between settling and adult barnacles

Summary Spatial patterns of recruitment were compared between populations of the rocky shore barnacles (Crustacea: Cirripedia) Semibalanus balanoides, an obligate cross‐fertilizer, and Chthamalus montagui, which can self‐fertilize. We tested the hypothesis that recruitment depends on a behaviourally mediated interaction at settlement between the effects of adult background density and adult spacing, which limit free space for settlement and mating opportunities, respectively. Recruitment to patches of cleared rock (10‐ or 30‐cm diameter) was compared between replicate shores with background densities of adult barnacles classed as low (≤20 per 25 cm2) and high (&gt;4× low). Replicate patches were cleared of all barnacles surrounding a remnant cluster, comprising 0, 1, 2, 4, 8, 16 or 32 adults. For S. balanoides, settlement and subsequent recruitment over 5 months varied in direct proportion to remnant cluster size, except on the shore with the highest background density where recruitment was inversely proportional to cluster size. We interpret this inversion to indicate attraction to potential mates at low adult densities (positive density dependence, an Allee effect) switching to attraction to free space at high density (negative density dependence). The strengths and slopes of the regressions increased from shores with the lowest to the highest overall recruitment of barnacles, but retained significance over a five‐fold range in recruitment. Positive effects of cluster size on recruitment were consistent between consecutive years, despite considerable variation in recruitment densities. In contrast, recruitment of C. montagui was generally more weakly proportional to cluster size, except for a strong positive correlation at the shore with the highest recruitment. Dispersion of recruits within treatment patches was accurately modelled by a computer simulation that allowed each barnacle to settle at random between fixed minimum and maximum distances from the nearest other settled barnacle. The model estimated threshold distances by maximum likelihood fit to observed recruitment into concentric annuli around the central adult cluster. Upper thresholds of separation corresponded to penis length for 65% of S. balanoides and 42% of C. montagui patches. Lower thresholds were ∼2× cyprid length for 75% of S. balanoides patches, but were larger than this for C. montagui patches.

The ‘ghost of settlement past’ determines mortality and fecundity in the barnacle, Semibalanus balanoides

Intraspecific competition is common in many organisms. For many sessile marine invertebrates high settlement densities can lead to competition with conspecifics that can affect the probability of mortality, morphology, and reproductive output. This work aims to determine the affect of recruit density on shell morphology and fecundity and the temporal nature of intraspecific competition for the acorn barnacle, Semibalanus balanoides. Two tile arrays were used; experiment 1 was deployed at the start of the settlement season (43,264 recruits, 4.1% survivorship to adult), experiment 2 was deployed in the middle of the season (5,454 recruits, 42.8% survivorship). Experiment 2 displayed density dependent mortality between recruits and juveniles and adults, whereas no relationship was found for exp. 1. Experiment 2 had higher egg production per individual, but also higher egg production per area of tile compared to exp. 1. A negative relationship was found between recruit density and egg production, however during the summer period, this relationship switched to a positive relationship between juvenile density and egg production, due to high mortality on tiles with high recruit density. Cumulative population density (CPD) was calculated as the cumulative mean density of barnacles per cm‐2 of tile and was used as an index of intraspecific competition. There was a linear negative relationship between mass of eggs per individual and CPD. However, CPD over the summer period more fully explained egg production per individual. A time window was identified when cumulative intraspecific competitive effects were shown to have a strong negative effect on egg production and an increase in mortality, causing a flip from density independent fecundity to density dependent fecundity. These results suggest that recruit density is an important driver of subsequent population processes.

Maintenance of species dominance through pulse disturbances to a sessile marine invertebrate assemblage in Port Shelter, Hong Kong

Pollution events can be considered as disturbances to an ecosystem and have the potential to be an important structuring force within assemblages of plants and animals. Our understanding of pollution impacts will benefit from the investigation of important aspects of these disturbances such as their timing and frequency. We investigated the effect of pulse copper pollution events on the development of hard-substrate invertebrate assemblages in Port Shelter, Hong Kong. Using a field dosing technique, we created single or repeated copper pollution events of less than 100 µg l -1 close to settlement plates. In short-term experiments, pulses were delivered in either the first or the second week of substrate immersion. For longer term experiments, pulses were delivered every 2, 4, 8 or 16 wk for the duration of the 16 wk experiments and the assem- blages allowed to develop under these dosing regimes. Pulse pollution events had a dramatic effect on densities of the dominant colonizer and space occupier in short-term experiments, Hydroides elegans. The impact was observed as a change in the size structure of the population with large organisms eliminated close to copper sources while new recruits were increased in densities in the same area. Reductions in densities of bryozoans and barnacles were also observed, while a feather hydroid doubled in densities on plates exposed to copper pulses. Young assemblages recovered rapidly from the impact of a copper pulse and reduced impacts were observed from a copper pulse in Week 1 of the experiments compared to exposure in Week 2. In longer-term experiments, copper pulses also dramatically altered assemblages predom- inantly through a direct negative effect on the densities of large competitively superior colonial and solitary ascidians. In the absence of a regular pollution event, colonial and solitary as- cidians and sponges were able to establish, grow-over and/or push out the smaller, more brittle H. elegans. These good competitors were unable to establish and grow to any great size on plates exposed to copper pulses every 2 or 4 wk and under the influence of repeated pulse copper exposures H. elegans maintained its initial dominance. Assemblages exposed every 8 or 16 wk were not significantly different from undosed controls. The results provide information on the type and magnitude of sessile invertebrate responses to transient pollution events as well as an assess- ment of the time expected to 'assemblage recovery' from single or repeated pulse disturbances. This is discussed in relation to the results of identical experiments conducted in temperate cli- mates.