Direct and indirect effects of predation, herbivory and surface rugosity on mussel recruitment

A global synthesis of predation on bivalves

Predation is a dominant structuring force in ecological communities. In aquatic environments, predation on bivalves has long been an important focal interaction for ecological study because bivalves have central roles as ecosystem engineers, basal components of food webs, and commercial commodities. Studies of bivalves are common, not only because of bivalves' central roles, but also due to the relative ease of studying predatory effects on this taxonomic group. To understand patterns in the interactions of bivalves and their predators we synthesised data from 52 years of peer-reviewed studies on bivalve predation. Using a systematic search, we compiled 1334 studies from 75 countries, comprising 61 bivalve families (N = 2259), dominated by Mytilidae (29% of bivalves), Veneridae (14%), Ostreidae (8%), Unionidae (7%), and Dreissenidae and Tellinidae (6% each). A total of 2036 predators were studied, with crustaceans the most studied predator group (34% of predators), followed by fishes (24%), molluscs (17%), echinoderms (10%) and birds (6%). The majority of studies (86%) were conducted in marine systems, in part driven by the high commercial value of marine bivalves. Studies in freshwater ecosystems were dominated by non-native bivalves and non-native predator species, which probably reflects the important role of biological invasions affecting freshwater biodiversity. In fact, while 81% of the studied marine bivalve species were native, only 50% of the freshwater species were native to the system. In terms of approach, most studies used predation trials, visual analysis of digested contents and exclusion experiments to assess the effects of predation. These studies reflect that many factors influence bivalve predation depending on the species studied, including (i) species traits (e.g. behaviour, morphology, defence mechanisms), (ii) other biotic interactions (e.g. presence of competitors, parasites or diseases), and (iii) environmental context (e.g. temperature, current velocity, beach exposure, habitat complexity). There is a lack of research on the effects of bivalve predation at the population and community and ecosystem levels (only 7% and 0.5% of studies respectively examined impacts at these levels). At the population level, the available studies demonstrate that predation can decrease bivalve density through consumption or the reduction of recruitment. At the community and ecosystem level, predation can trigger effects that cascade through trophic levels or effects that alter the ecological functions bivalves perform. Given the conservation and commercial importance of many bivalve species, studies of predation should be pursued in the context of global change, particularly climate change, acidification and biological invasions.

Enhancing the settlement and attachment strength of pediveligers of Mytilus galloprovincialis by changing surface wettability and microtopography

Surface wettability and microtopography can either enhance or deter larval settlement of many sessile marine organisms. This study quantifies the effect of these surface properties on the settlement of pediveligers of Mytilus galloprovincialis, using polymers spanning a range of wettability and microtextured polydimethylsiloxane (PDMS). Furthermore, the adhesion strength of settled pediveligers on microtextured PDMS surfaces was quantified using a flow chamber. Settlement was enhanced at the hydrophilic end of the wettability spectrum, where mean settlement on nylon reached 33.5 ± 13.1%. In contrast, mean settlement on the most hydrophobic polymer (PDMS) was 4.2 ± 3.2%. Microtopography had a much stronger effect compared to wettability, where 400 μm textured PDMS enhanced settlement above 90%. Settlement preferences were also positively correlated to adhesion strength at flow rates of 4 knots, with all initially settled pediveligers on smooth PDMS detaching, while 79.9 ± 5.7% of pediveligers remained on the 400 μm texture.

Habitat selection, facilitation, and biotic settlement cues affect distribution and performance of coral recruits in French Polynesia

Habitat selection can determine the distribution and performance of individuals if the precision with which sites are chosen corresponds with exposure to risks or resources. Contrastingly, facilitation can allow persistence of individuals arriving by chance and potentially maladapted to local abiotic conditions. For marine organisms, selection of a permanent attachment site at the end of their larval stage or the presence of a facilitator can be a critical determinant of recruitment success. In coral reef ecosystems, it is well known that settling planula larvae of reef-building corals use coarse environmental cues (i.e., light) for habitat selection. Although laboratory studies suggest that larvae can also use precise biotic cues produced by crustose coralline algae (CCA) to select attachment sites, the ecological consequences of biotic cues for corals are poorly understood in situ. In a field experiment exploring the relative importance of biotic cues and variability in habitat quality to recruitment of hard corals, pocilloporid and acroporid corals recruited more frequently to one species of CCA, Titanoderma prototypum, and significantly less so to other species of CCA; these results are consistent with laboratory assays from other studies. The provision of the biotic cue accurately predicted coral recruitment rates across habitats of varying quality. At the scale of CCA, corals attached to the “preferred” CCA experienced increased survivorship while recruits attached elsewhere had lower colony growth and survivorship. For reef-building corals, the behavioral selection of habitat using chemical cues both reduces the risk of incidental mortality and indicates the presence of a facilitator.

Experimental confirmation of multiple community states in a marine ecosystem

Small changes in environmental conditions can unexpectedly tip an ecosystem from one community type to another, and these often irreversible shifts have been observed in semi-arid grasslands, freshwater lakes and ponds, coral reefs, and kelp forests. A commonly accepted explanation is that these ecosystems contain multiple stable points, but experimental tests confirming multiple stable states have proven elusive. Here we present a novel approach and show that mussel beds and rockweed stands are multiple stable states on intertidal shores in the Gulf of Maine, USA. Using broad-scale observational data and long-term data from experimental clearings, we show that the removal of rockweed by winter ice scour can tip persistent rockweed stands to mussel beds. The observational data were analyzed with Anderson's discriminant analysis of principal coordinates, which provided an objective function to separate mussel beds from rockweed stands. The function was then applied to 55 experimental plots, which had been established in rockweed stands in 1996. Based on 2005 data, all uncleared controls and all but one of the small clearings were classified as rockweed stands; 37% of the large clearings were classified as mussel beds. Our results address the establishment of mussels versus rockweeds and complement rather than refute the current paradigm that mussel beds and rockweed stands, once established, are maintained by site-specific differences in strong consumer control.

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.

Direct effects of physical stress can be counteracted by indirect benefits: oyster growth on a tidal elevation gradient

The paradigmatic gradient for intertidal marine organisms of increasing physical stress from low to high elevation has long served as the basis for using direct effects of duration of water coverage to predict many biological patterns. Accordingly, changes in potential feeding time may predict the direction and magnitude of differences between elevations in individual growth rates of sessile marine invertebrates. Oysters (triploid Crassostrea ariakensis) experimentally introduced at intertidal (MLW+0.05 m) and subtidal (MLW-0.25 m) elevations in racks provided a test of the ability to use duration of water coverage to predict changes in growth. During early-to-mid winter, a depression of 38-47% in shell growth of intertidal oysters matched the 36% reduction in available feeding time relative to subtidal oysters. In late winter as solar heating of exposed oysters increased, growth differences of 52-55% departed only slightly from the predicted 39%. In spring, however, duration of water coverage failed to predict even the correct direction of growth change with elevation as intertidal oysters grew 34% faster despite 39% less feeding time. Intense seasonal development of shell fouling by other suspension feeders like ascidians, mussels, and barnacles on subtidal (94% incidence) but not on aerially exposed intertidal (21-38% incidence) oysters may explain why duration of water cover failed to predict spring growth differences. Less intense fouling develops on intertidal oysters due to the physiological stress of aerial exposure on settlers, especially during higher temperatures and longer solar exposures of spring. Fouling by suspension feeders is known to reduce growth of the host through localized competition for food and added energetic costs. Thus, in springtime, indirect effects of aerial exposure providing a partial refuge from biological enemies overwhelmed direct effects of reduced duration of water coverage to reverse the expected pattern of slower intertidal growth of a marine invertebrate.

The influence of natural surface microtopographies on fouling

Multiple antifouling strategies of marine organisms may consist of combinations of physical, chemical and mechanical mechanisms. In this study, the role of surface microtopography (< 500 microns) of different marine organisms, such as Cancer pagurus, Mytilus edulis, Ophiura texturata and the eggcase of Scyliorhinus canicula, has been investigated as a possible component of their defence systems. High resolution resin replicates of these natural surface structures were exposed to natural fouling in field experiments. Abundances of recruits were determined and compared to those on untextured, but otherwise identical, control surfaces to quantify the influence of the different microtopographies on fouling rates. Antifouling effects of microtopographies varied with type of microtopography and coloniser species. The surface microtopography of C. pagurus significantly rejected macrofoulers. The surface structures of the eggcase and O. texturata had repellent effects on microfoulers. Barnacle settlement was temporarily reduced on surface microtopographies of M. edulis and the eggcase. These results emphasise the promising nontoxic antifouling properties of microtextured surfaces.

Small scale variability of benthic assemblages: biogenic neighborhood effects

In this study, patterns of community development were investigated within vs. outside 'habitats'. These habitats represented five different monospecific assemblages of one of the following species: the brown alga Fucus serratus, the red alga Delesseria sanguinea, the green alga Enteromorpha intestinalis, the seagrass Zostera marina and the blue mussel Mytilus edulis. Natural assemblages were allowed to develop on paired artificial substrata-separated by ca. 1 m-within (treatment) vs. outside (control) of habitats. The same colonizer species settled on treatment and control substrata for given habitats. However, after 5 months of settlement and post-settlement dynamics, their proportional abundance and the structure of treatment and control assemblages differed in many instances. Variability among replicates of a given treatment, seperated by up to 50 m, was large, indicating a patchy spatial distribution of organisms. Despite this spatial heterogeneity among within-treatment replicates, analysis of similarity revealed that in most instances significantly different assemblages developed between treatments on a small spatial scale depending on whether substrata were positioned within as compared to outside a given habitat.Consequently, the algae, seagrass or mussels constituting a habitat seem to control the structure of the benthic assemblage developing in their vicinity by one or more possible mechanisms: reduction of larval advection, exudation of metabolites that influence settlement and/or post-settlement survival, and/or-in the case of mussel assemblages-predation on larvae.In addition to spatial variability in larval supply, stochasticity in succession, substratum heterogeneity, competition and predation effects, this investigation reveals the potential of a further assemblage structuring factor: the impact of neighboring organisms.

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.

Scales of mussel bed complexity: structure, associated biota and recruitment

Hierarchically scaled surveys were carried out on beds of the brown mussel Perna perna (Linnaeus) on the South coast of South Africa. The object was to assess spatial and temporal variations in the complexity of mussel beds and to investigate relationships between mussel bed complexity and mussel recruitment. Complexity was divided into three components: physical complexity; demographic complexity; associated biota. A series of variables within each component were recorded at two different scales (10 and 50 cm) within nested quadrats on three separate occasions. The nested ANOVA design explicitly incorporated spatial scale as levels of the ANOVA. These scales were: shores (areas 1 km in length separated by 25 km); transects (areas 20 m in length separated by 100s of meters); 50x50-cm quadrats separated by meters and 10x10-cm quadrats separated by cm) This approach was intended to generate hypotheses concerning direct associations between recruitment and complexity versus co-variation due external processes. Three main questions were addressed: (1) At what scale does each variable of complexity exhibit greatest significant variation? (2) At these scales is there similar ranking of variables of complexity and recruitment? (3) Within this/these scales, is there any significant relationship between the variables measured and mussel recruitment? On two occasions (Nov. 97 and Mar. 98) the majority of variables showed greatest significant variation at the transect-scale. On a third occasion (Oct. 97) most variables showed greatest significant variation at the quadrat-scale and the site-scale. On all occasions a markedly high percentage of the variation encountered also occurred at the smallest scale of the study, i.e., the residual scale of the ANOVA analyses. Some similarity in the ranking of variables occurred at the transect scale. Within the transect-scale, there was little indication of any relationship between variables of complexity and recruitment. Relationships were inconsistent either among transects or among sampling occasions. Overall, the results suggest that a high degree of variation in mussel bed complexity consistently occurs at very small scales. High components of variance generally also occur at one or more larger scales; however, these scales vary with season. Mussel recruitment does not appear to be directly affected by complexity of mussel beds. Instead it appears external factors may influence both complexity and recruitment independently. In addition recruitment may influence complexity rather than vice versa.

Structural complexity in mussel beds: the fractal geometry of surface topography

Seafloor topographic complexity is ecologically important because it provides habitat structure and alters boundary-layer flow over the bottom. Despite its importance, there is little agreement on how to define and measure surface complexity. The purpose of this investigation was to utilize fractal geometry of vertical cross-section profiles to characterize the surface topography of the soft-bottom mussel bed (Mytilus edulis L.) at Bob's Cove, ME, USA. Mussels there have been shown previously to have spatially ordered fractal characteristics in the horizontal plane. Two hypotheses were tested. The first was that the bed surface is fractal over the spatial scale of 1.44-200 mm, with fractal dimension less than or equal to 1.26, the value for the Koch curve, our model for bed profiles. The second was that bed surface topography (i.e., in vertical profile) is less complex than the mussel bed spatial pattern (i.e., aerial view in the horizontal plane). Both hypotheses were supported. Cross-sections of plaster casts of the bed produced 88 surface profiles, all of which were fractal over the entire spatial scale of more two orders of magnitude employed in the analysis. Fractal dimension values (D) for individual profiles ranged from 1.031 to 1.310. Fractal dimensions of entire casts ranged up to mean (1.242+/-0.046) and median (1.251) values similar to 1.26, the theoretical value of the Koch curve. The bed surface was less complex than the bed spatial pattern because every profile had D<1.36, the smallest value previously obtained from aerial views of the bed. The investigation demonstrated for the first time that surface topography of a soft-bottom mussel bed was fractal at a spatial scale relevant to hydrodynamic processes and habitat structure important for benthic organisms. The technique of using cross-section profiles from casts of the bed surface avoided possible underestimates of fractal dimension that can result from other profiling methods reported in the literature. The results demonstrate that fractal dimension can be useful in the analysis of habitat space and water flow over any irregular seafloor surface because it incorporates the size, shape, and scale of roughness elements into a simple, numerical metric.