Community regulation: the relative importance of recruitment and predation intensity of an intertidal community dominant in a seascape context

Predicting the strength and context-dependency of species interactions across multiple scales is a core area in ecology. This is especially challenging in the marine environment, where populations of most predators and prey are generally open, because of their pelagic larval phase, and recruitment of both is highly variable. In this study we use a comparative-experimental approach on small and large spatial scales to test the relationship between predation intensity and prey recruitment and their relative importance in shaping populations of a dominant rocky intertidal space occupier, mussels, in the context of seascape (availability of nearby subtidal reef habitat). Predation intensity on transplanted mussels was tested inside and outside cages and recruitment was measured with standard larval settlement collectors. We found that on intertidal rocky benches with contiguous subtidal reefs in New Zealand, mussel larval recruitment is usually low but predation on recruits by subtidal consumers (fish, crabs) is intense during high tide. On nearby intertidal rocky benches with adjacent sandy subtidal habitats, larval recruitment is usually greater but subtidal predators are typically rare and predation is weaker. Multiple regression analysis showed that predation intensity accounts for most of the variability in the abundance of adult mussels compared to recruitment. This seascape-dependent, predation-recruitment relationship could scale up to explain regional community variability. We argue that community ecology models should include seascape context-dependency and its effects on recruitment and species interactions for better predictions of coastal community dynamics and structure.     

Prey selection and the functional response of sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin)) and blue mussels (Mytilus edulis Linnaeus)

Predators in nature include an array of prey types in their diet, and often select certain types over others. We examined (i) prey selection by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) when offered two prey types, juvenile sea scallops (Placopecten magellanicus) and blue mussels (Mytilus edulis), and (ii) the effect of prey density on predation, prey selection, and component behaviours. We quantified predation rates, behavioural components (proportion of time spent searching for prey, encounter probabilities) and various prey characteristics (shell strength, energy content per prey, handling time per prey) to identify mechanisms underlying predation patterns and to assess the contribution of active and passive prey selection to observed selection of prey. Sea stars strongly selected mussels over scallops, resulting from both active and passive selection. Active selection was associated with the probability of attack upon encounter; it was higher on mussels than on scallops. The probability of capture upon attack, associated with passive selection, was higher for mussels than for scallops, since mussels can not swim to escape predators. Sea stars consumed few scallops when mussels were present, and so did not have a functional response on scallops (the target prey). Rock crabs exhibited prey switching: they selected mussels when scallop density was very low, did not select a certain prey type when scallop density was intermediate, and selected scallops when scallop density was high relative to mussel density. The interplay between encounter rate (associated with passive selection) and probability of consumption upon capture (associated with both active and passive selection) explained observed selection by crabs. Scallops were encountered by crabs relatively more often and/or mussels less often than expected from random movements of animals at all scallop densities. However, the probability of consumption varied with scallop density: it was lower for scallops than mussels at low and intermediate scallop densities, but tended to be higher for scallops than mussels at high scallop densities. When mussels were absent, crabs did not have a functional response on scallops, but rather were at the plateau of the response. When mussels were present with scallops at relatively low density, crabs exhibited a type II functional response on scallops. Our results have implications for the provision of protective refuges for species of interest (i.e., scallops) released onto the sea bed, such as in population enhancement operations and bottom aquaculture.     

Predator (<Emphasis Type="Italic">Carcinus maenas</Emphasis>) nonconsumptive limitation of prey (<Emphasis Type="Italic">Nucella lapillus</Emphasis>) feeding depends on prey density and predator cue type

Predators often have nonconsumptive effects (NCEs) on prey. For example, upon detection of predator cues, prey can reduce feeding activities to hamper being detected by predators. Previous research showed that waterborne chemical cues from green crabs (Carcinus maenas, predator) limit the dogwhelk (Nucella lapillus, prey) consumption of barnacles regardless of dogwhelk density, even though individual predation risk for dogwhelks decreases with conspecific density. Such NCEs might disappear with dogwhelk density if dogwhelks feed on mussels, as mussel stands constitute better antipredator refuges than barnacle stands. Through a laboratory experiment, we effectively found that crab chemical cues limit the per-capita consumption of mussels by dogwhelks at low dogwhelk density but not at high density. The combination of tactile and chemical cues from crabs, however, limited the dogwhelk consumption of mussels at both dogwhelk densities. The occurrence of such NCEs at both dogwhelk densities could have resulted from tactile cues indicating a stronger predation risk than chemical cues alone. Overall, the present study reinforces the notions that prey evaluate conspecific density when assessing predation risk and that predator cue type affects their perception of risk.     

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

Summary The predatory gastropod Nucella lapillus, commonly preys upon the mussel, Mytilus edulis, and is thought to control the distribution and abundance of mussels on the rocky shores of New England, USA. In this study, done in Maine, USA, not only the presence of Nucella lapillus but also the roughness of the experimental surface and the presence of the herbivorous gastropod, Littorina littorea, were manipulated. Four types of surfaces were used as recruitment substrata for mussels: smooth bare granite, aggregations of the barnacle, Semibalanus balanoides, fiberglass resin castings of smooth bare granite and resin castings of aggregations of S. balanoides. To ensure that caged N. lapillus were not starving, barnacles were provided as alternative prey. Experiments showed no detectable effect of N. lapillus on the recruitment of M. edulis. Mussel recruitment was enhanced by surface rugosity and depressed by the activities of L. littorea. Analysis of covariance, using the number of algal species as the covariate, suggested that L. littorea reduced the number of newlyrecruited mussels by removing algae that provided recruitment sites, but no manipulations were done to test this conjecture. It is likely that previous reports of N. lapillus controlling mussel abundance are attributable to N. lapillus preying upon barnacles, which increase surface rugosity and enhance mussel recruitment. Review of literature on feeding preferences of N. lapillus supports this view. When handling times and prey availability are taken into account, Nucella shows a clear preference for barnacles over mussels.     

Intertidal mussel beds from the South-western Atlantic show simple structure and uniform appearance: does environmental harshness explain the community?

Communities of the rocky mid-intertidal zone of the South-western Atlantic are uniform in appearance, dominated by dense monocultures of small-size mussels (Brachidontes rodriguezii and Perumytilus purpuratus). To explain this, two hypotheses have been advanced in the literature: environmental harshness due to high potential evaporation and historical contingency after the Last Glacial Maximum. In this study of Uruguayan and Argentine shores, we address the implications and predictions of these two hypotheses from a biogeographic perspective by studying the regional distribution and composition of mid-intertidal mussels. We conducted an extensive latitudinal sampling survey (21 locations, 34–54°S), along with a compilation of available information on mussel bed composition and mussel predators present along the coastline. Then we constructed latitudinal profiles of ecologically significant environmental variables with specific emphasis on potential evaporation, a proxy for desiccation stress. The results show that mussel beds are composed of two species of small mussels, which coexist over a biogeographic transition zone (40–42°S) related to sea surface water temperature. The distribution of mussels along the coastline studied is not consistent with the environmental harshness hypothesis. In addition, in the Central Patagonian zone (44–50°S), two invertebrate predators also inhabit the intertidal rocky shores. However, these localities showed higher environmental harshness (potential evaporation rate) than non-Patagonian localities. We suggest that further attention should be given to historical contingency in order to advance towards a hypothesis consistent with current knowledge on the post-glacial biogeographic history of the South-western Atlantic.     

Dynamic measurements of black oystercatcher (Haematopus bachmani) predation on mussels (Mytilus californianus)

Intertidal zone mussels can face threats from a variety of predatory species during high and low tides, and they must balance the threat of predation against other needs such as feeding and aerobic respiration. Black oystercatchers (Haematopus bachmani) on the Pacific coast of North America can depend on the mussel Mytilus californianus for a substantial portion of their diet. Observations suggest that oystercatchers tend to focus on mussels beginning to gape their valves during rising tides, following periods of aerial emersion. We present detailed, autonomous field measurements of the dynamics of three such predation events in the rocky intertidal zone. We measured accelerations of up to 4 g imposed on mussels, with handling times of 115–290 s required to open the shell and remove the majority of tissue. In each case a single oystercatcher attacked a mussel that had gaped the shell valves slightly wider than its neighbors as the rising tide began to splash the mussel bed, but no other obvious characteristic of the mussels, such as body temperature or orientation, could be linked to the oystercatcher's individual prey choice.     

Population dynamics of the ribbed mussel,Geukensia demissa: The costs and benefits of an aggregated distribution

Summary Although mussel beds are common in many intertidal habitats, the ecological significance of the aggregated distribution of mussels has not been examined. The ribbed mussel, Geukensia demissa, is found in dense aggregations on the seaward margin of many salt marshes in New England. Here, we examine the population structure of G. demissa in a New England salt marsh and investigate experimentally the costs and benefits of aggregation.Size, growth rate, and settlement rates of mussels decrease with increasing tidal height, whereas survivorship and longevity increase with increasing tidal height. Winter ice dislodges mussels from the substratum, resulting in mortality over all size classes, whereas crab predation results in the mortality of smaller mussels. The intensity of each of these mortality agents decreases with increasing tidal height. Effects of intraspecific competition on individual growth and mortality also decrease with increasing tidal height.At high densities, individual growth rates were reduced, with depression of growth rates most pronounced on smaller individuals. Mortality from sources other than intraspecific crowding, however, was reduced at high mussel densities, including mortality due to winter ice and crab predators. As a result, our data suggest that the mussel population at our study site would be reduced by 90% in only five years and no juveniles would survive through their second year without an aggregated distribution.Juveniles settle gregariously with or without adults present. The aggregated distribution of settlers and the postsettlement movement of smaller mussels to favorable microhabitats result in size and age class segregation within the population. This probably reduces intraspecific competition for food, while maintaining the survivorship advantages of an aggregated distribution.     

Generalizing from experiments: is predation strong or weak in the New England rocky intertidal?

Summary Petraitis (1990) recently critized previous generalizations regarding the effects of predation in the New England rocky intertidal region (e.g., Menge 1976; Lubchenco and Menge 1978). Contrary to Lubchenco's and my conclusions, Petraitis concluded that (1) barnacles and not mussels are the favored prey of dogwhelks and (2) barnacles and not dogwhelks control mussel abundances in the mid and low rocky intertidal region. I provide evidence that these criticisms are unwarranted. First, Lubchenco and I never claimed that diet composition reflected prey preference. Moreover, predators can influence prey abundance without preferring the prey. Hence, claims regarding preferences have no bearing on our conclusions. Second, Petraitis' experiments do not invalidate Lubchenco's and my experimental results. Reanalyses of our experimental data support the earlier conclusion that at wave-sheltered sites, whelks reduce the abundance of mussels independently of barnacle abundances. Further, at all but one of Lubchenco's and my study sites, predator densities were higher than at Petraitis' site. Thus, the absence of a predator effect in Petraitis's study was most likely due to low predator density rather than a lack of generality of our earlier results. This reevaluation therefore suggests that within a broader conceptual framework, Petraitis' apparently divergent results are actually consistent with ours.     

Mussel attachment on rocky shores: the effect of flow on byssus production

Mussels rely on a strong byssal attachment to persist in a range of habitats with differing rates of water flow. Recent studies, however, suggest that the ability of one mussel species to sense and respond adaptively to the flow in its environment is limited under even modest flow conditions because the process of byssal thread formation is disrupted. This study extends these findings to four mussel species, Mytilus trossulus, M. galloprovincialis, M. californianus, and Modiolus modiolus. Collectively, the response of byssal thread formation decreased with rates of flow above ～25 cm/s and the critical flow threshold was estimated to be <50 cm/s. How can mussels persist on shores where flow is an order of magnitude higher? Using a combination of techniques for measuring flow, velocity profiles were obtained above and within mussel aggregations in the laboratory and in the field. Flow was greatly reduced within mussel aggregations, ranging from 0.1% to 10% of free-stream velocity. These results suggest one key to the success of mussels in habitats with high rates of flow is the ability to form aggregations that ameliorate flows to a level that is conducive to byssal thread formation.     

Predator size interacts with habitat structure to determine the allometric scaling of the functional response

While both predator body size and prey refuge provided by habitat structure have been established as major factors influencing the functional response (per capita consumption rate as a function of prey density), potential interactions between these factors have rarely been explored. Using a crab predator (Panopeus herbstii) – mussel prey (Brachidontes exustus) system, we examined the allometric scaling of the functional response in oyster (Crassostrea virginica) reef habitat, where crevices within oyster clusters provide mussels refuge from predation. A field survey of mussel distribution showed that mussels attach closer to the cluster periphery at high mussel density, indicating the potential for saturation of the refuge. In functional response experiments, the consumption rate of large crabs was depressed at low prey density relative to small crabs, while at high prey density the reverse was true. Specifically, the attack rate coefficient and handling time both decreased non‐linearly with crab size. An additional manipulation revealed that at low prey densities, the ability of large crabs to maneuver their claws and bodies to extract mussels from crevices was inhibited relative to small crabs by the structured habitat, reducing their attack rate. At high prey densities, crevices were saturated, forcing mussels to the edge of clusters where crabs were only limited by handling time. Our study illuminates a potentially general mechanism where the quality of the prey refuge provided by habitat structure is dependent on the relative size of the predator. Thus anthropogenic influences that alter the natural crab size distribution or degrade reef habitat structure could threaten the long‐term stability of the crab –mussel interaction in reefs.     

Consumer Aggregations Drive Nutrient Dynamics and Ecosystem Metabolism in Nutrient-Limited Systems

Differences in animal distributions and metabolic demands can influence energy and nutrient flow in an ecosystem. Through taxa-specific nutrient consumption, storage, and remineralization, animals may influence energy and nutrient pathways in an ecosystem. Here we show these taxa-specific traits can drive biogeochemical cycles of nutrients and alter ecosystem primary production and metabolism, using riverine systems that support heterogeneous freshwater mussel aggregations. Freshwater unionid mussels occur as distinct, spatially heterogeneous, dense aggregations in rivers. They may influence rates of production and respiration because their activities are spatially concentrated within given stream reaches. Previous work indicates that mussels influence nutrient limitation patterns, algal species composition, and producer and primary consumer biomass. Here, we integrate measures of organismal rates, stoichiometry, community-scaled rates, and ecosystem rates, to determine the relative source–sink nutrient dynamics of mussel aggregations and their influence on net ecosystem processes. We studied areas with and without mussel aggregations in three nitrogen-limited rivers in southeastern Oklahoma, USA. We measured respiration and excretion rates of mussels and collected a subset of samples for tissue chemistry and for thin sectioning of the shell to determine growth rates at each site. This allowed us to assess nutrient remineralization and nutrient sequestration by mussels. These rates were scaled to the community. We also measured stream metabolism at three sites with and without mussels. We demonstrated that mussel species have distinct stoichiometric traits, vary in their respiration rates, and that mussel aggregations influence nutrient cycling and productivity. Across all mussel aggregations, we found that mussels excreted more nitrogen than they sequestered into tissue and excreted more phosphorus than they sequestered except at one site. Furthermore, gross primary productivity was significantly greater at reaches with mussels. Collectively, our results indicate that mussels have ecosystem-level impacts on nutrient availability and production in nutrient-limited rivers. Within these streams, mussels are affecting the movement of nutrients and altering nutrient spiralling.     

Mussels matter: postlarval dispersal dynamics altered by a spatially complex ecosystem engineer

This study investigated postlarval dispersal of soft-bottom macrofauna at a spatially complex intertidal mudflat comprising patches of bare sediment and an ecosystem engineer, the mussel Mytilus edulis. At each of four sites in Guard Point Cove, Maine, USA, we took core samples and deployed bedload traps in bare sediment and mussel bed habitats to estimate ambient densities, rates of sediment flux, and several measures of postlarval dispersal. Univariate and multivariate nonmetric multidimensional scaling (nMDS) results showed few significant site effects and no habitat×site interactions. In contrast, there were numerous significant habitat effects. Compared to the bare sediment, the mussel bed habitat had: fewer species; higher ambient density and proportional abundance of the oligochaete Tubificoides benedeni (the dominant species in both habitats); lower ambient densities and proportional abundances of major taxa and the nonoligochaetes as a group; and higher sediment flux and relative (i.e., per capita) dispersal of nonoligochaetes. Macrofauna species dispersed in relative proportions that were different from those in the ambient assemblage. Per capita T. benedeni transport rates were low in mussel beds compared to those for nonoligochaetes, consistent with the view that beds represent favorable habitat for oligochaetes. The number of total macrofauna individuals trap⁻¹ day⁻¹ was negatively correlated with ambient density and positively correlated with sediment flux in both habitats, but these relationships were significant only in the mussel bed. The results indicate that altered transport rates of sediment and postlarvae are important mechanisms by which mussels act as ecosystem engineers to modify soft-bottom habitats. Differential transport rates caused by aggregations of mussels and other foundation species must be considered in explanations of spatial pattern in soft-bottom communities.     

Do mussel patches provide a refuge for algae from grazing gastropods?

On rocky shores, cover of macroalgae is often greater growingepibiotically on mussels compared to algae growing directlyattached to rock. A survey of two shores on the east coast ofIreland confirmed that mussel beds contained greater percentagealgal cover and more diverse algal assemblages compared to thoseon rock. The reasons for this difference are not clear. It hasbeen suggested that mussel beds provide a refuge for algae fromgrazing gastropods. Surprisingly, we found no evidence to supportthis. Using wax discs, gastropod grazing patterns were foundto be similar within the mussel beds as on rock. The musselbeds do not appear to provide a refuge for algae from grazingactivity at this scale and we suggest other possible mechanismsfor the prevalence of epibiotic algal cover on mussels. Intertidalgrazers may in fact affect the epibiotic algae on mussels andthereby affect indirectly the persistence of mussel beds. (Received 14 May 2007; accepted 20 October 2007)     

Effects of environmental stress on intertidal mussels and their sea star predators

Consumer stress models of ecological theory predict that predators are more susceptible to stress than their prey. Intertidal mussels, Mytilus californianus, span a vertical stress gradient from the low zone (lower stress) to the high zone (higher thermal and desiccation stress), while their sea star predators, Pisaster ochraceus, range from the low zone only into the lower edge of the mussel zone. In summer 2003, we tested the responses of sea stars and mussels to environmental stress in an experiment conducted on the Oregon coast. Mussels were transplanted from the middle of the mussel bed to cages in the low and high edges of the mussel bed. Sea star predators were added to half of the mussel cages. Mussels and sea stars were sampled between June and August for indicators of sublethal stress. Mussel growth was measured, and tissues were collected for heat shock protein (Hsp70) analyses and histological analyses of reproduction. Sea stars were weighed, and tissues were sampled for Hsp70 analyses. Mussels in high-edge cages had higher levels of total Hsp70 and exhibited spawning activity earlier in the summer than mussels in the low-edge cages. Sea stars suffered high mortality in the high edge, and low-edge sea stars lost weight but showed no differences in Hsp70 production. These results suggest that stress in the intertidal zone affected the mobile predator more than its sessile prey, which is consistent with predictions of consumer stress models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Subtidal-intertidal trophic links: American lobsters [Homarus americanus (Milne-Edwards)] forage in the intertidal zone on nocturnal high tides

Homarus americanus (Milne-Edwards), the American lobster, is a predator in New England subtidal communities, feeding on ecologically important grazers (sea urchins), mesopredators (crabs), and basal species (mussels). In this study, we provide the first report of adult American lobsters foraging in rocky intertidal habitats during nocturnal high tides. Censuses by SCUBA divers in the low intertidal (Chondrus crispus Stackhouse) zone showed mean densities of 2.2 lobsters/20 m² on nocturnal high tides, with contrasting low densities of 0.18/20 m² during diurnal high tides. Nocturnal high-tide intertidal densities were 62% of those reported in a previous study of lobsters in nearby subtidal rocky areas (Novak, 2004). The average carapace length of lobsters in the intertidal at night was > 50 mm. These lobsters were actively foraging in the intertidal with collected individuals having a mean stomach fullness of 67%. Prey found in the stomach contents primarily consisted of crabs, mussels and snails. Field experiments showed that lobsters rarely fed on medium to large size individuals of the common intertidal snail, Littorina littorea (L.). In contrast, experiments with local crab species demonstrated that lobsters actively and readily prey on Cancer irroratus (Say) and Carcinus maenas (L.), but were significantly less likely to consume Cancer borealis (Stimpson). The abundance of Carcinus maenas and blue mussels (Mytilus edulis L.) in the intertidal zone may explain the upshore movement of lobsters. Since nocturnal migration of Homarus americanus into the intertidal zone has not been documented before, our understanding of the dynamics of New England intertidal communities needs to be expanded to include this predator.     

Optimal foraging versus shared doom effects: interactive influence of mussel size and epibiosis on predator preference

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts.There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size.Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor ‘mussel size’. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles.We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts.     

Factors organizing rocky intertidal communities of New England: Herbivory and predation in sheltered bays

In New England, U.S.A., shores exposed to severe wave action are dominated by the common blue mussel Mytilus edulis L. while moderately protected areas are covered with perennial algae. It is thought that algae are limited by mussels which are a superior competitor. Because the effectiveness of predators is inhibited by wave activity, it is assumed that the rate of predation, which varies across this environmental gradient, accounts for the observed distribution of mussels and algae.Shores along sheltered bays appear to be an exception to this pattern and this study addresses some of the possible causes. In New England bays, mussels and barnacles Semibalanus balanoides (L.) are the most common organisms on the solid surfaces in the lower intertidal zone. Perennial macroalgae, such as Chondrus crispas Stackhouse and Fucus vesiculosus L., are rare. The distribution and abundance of species differs from that on moderately protected shores and is similar to very exposed shores which are dominated by mussels and barnacles.Herbivory by the common periwinkle Littorina littorea (L.) limits the abundance of F. vesiculosus and indirectly affects the success of mussels. During 4 years of experimental manipulations, F. vesiculosus rarely recruited in the presence of periwinkles but dominated experimental surfaces if periwinkles were excluded. When experimental surfaces with F. vesiculosus, which had been protected from herbivory for > 1 year, were exposed to natural conditions, herbivores cleared most of the surfaces within several months. Recruitment by barnacles and mussels was higher when periwinkles were excluded. However, the effect of periwinkles on mussels was indirect; the snails reduced barnacle success and thus reduced mussel recruitment which was enhanced by the surface irregularities provided by barnacles.The occurrence of mussels in sheltered bays is not due to a lack of predators. Predators were commonly seen at all sites. Most mussels on experimental surfaces were removed <4 wk when surfaces were exposed to natural levels of predation. Experiments do not provide an explanation for the occurrence of mussels, although the enhancement of mussel recruitment by barnacles suggests that the availability of settlement sites may be important.     

Desiccation,predation, and mussel-barnacle interactions in the northern Gulf of California

Summary Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.     

Prey selection and the impact of the starfish Marthasterias glacialis (L.) and other predators on the mussel Choromytilus Meridionalis (Krauss)

The diet of the starfish, Marthasterias glacialis (L.), consists of a variety of mollusc species, as well as ascidians and barnacles. Starfish densities are maximal where mussels, Choromytilus meridionalis (Krauss), are abundant and in such areas mussels form the bulk of the diet. Laboratory feeding experiments indicate that Marthasterias glacialis select mussels of particular sizes and that the length of prey taken is an increasing function of predator arm length. The time taken to consume each mussel is determined by the ratio of shell length to starfish size. The number of mussels consumed per day increases only slightly with starfish size, but because the prey taken increase in size, energy consumption is maintained at a relatively consistent 1% of predator body energy per day. Using prey selection and feeding rate data for different sized starfish, predictive three dimensional predation surfaces are developed for a natural starfish population feeding on either one or two cohort Choromytilus meridionalis populations. The models indicate that predatory effort should be concentrated on the smallest mussels when a single adult cohort is present, but on recruiting mussels just above the minimum prey size limit where two cohorts are present. Other major predators of mussels, the rock lobster, Jasus lalandii (Milne Edwards), and the whelk, Natica tecta Anton, appear to select similar size-ranges of prey to starfish, despite their differing body forms and feeding methods. Since the juveniles of all three predators can only take small mussels, predator recruitment may well depend upon the successful settlement of strong mussel cohorts. Evidence for such entrainment of predator cohorts to settlements of mussels is presented.     

Mussel beds in deeper water provide an unusual situation for competitive interactions between the seastars Leptasterias polaris and Asterias vulgaris

The common seastars Leptasterias polaris and Asterias vulgaris show competitive interactions in shallow subtidal communities in the northern Gulf of St. Lawrence, particularly during summer when aggregations of the two seastars forage on mussel beds at 1-2 m in depth. We examined interactions between the two seastars in a different situation, in a mussel bed at 6 m in depth (a rare situation in this region). In the deeper mussel bed, seastars were three times more abundant than in the shallower beds, and the mussels were larger. The deeper bed disappeared rapidly due to the intense predation. Although decreased prey abundance should have favored interference interactions, we did not detect either partitioning of mussels by size or avoidance of A. vulgaris by L. polaris as previously reported when mussels are in short supply in shallower water. The lack of an avoidance behavior by L. polaris, together with the higher proportion of L. polaris than A. vulgaris that were feeding, suggests that in this situation, the dominance of A. vulgaris (observed in shallower water) is attenuated, or that L. polaris may dominate.