Direct and indirect effects of Littorina littorea (L.) on barnacles growing on mussel beds in the Wadden Sea

On the extensive sedimentary tidal flats of the Wadden Sea, beds of the blue mussel Mytilus edulis represent the only major hard substratum and attachment surface for sessile organisms. On this substratum, the barnacle Semibalanus balanoides is the most frequent epibiont. In summer 1998, it occurred on over 90% of the large mussels (>45 mm shell length) and the dry weight of barnacles reached 65% of mussel dry weight. However, the extent of barnacle overgrowth is not constant and differs widely between years. Periwinkles (Littorina littorea) may reach densities >2000 m^–2 on intertidal mussel beds. Field experiments were conducted to test the effect of periwinkle grazing on barnacle densities. An experimental reduction of grazing and bulldozing pressure by periwinkles resulted in increased recruitment of barnacles, while barnacle numbers decreased with increasing snail density. The highest numbers of barnacles survived in the absence of L. littorea. However, a lack of periwinkle grazing activity also facilitated settlement of ephemeral algae which settled later in the year. Field experiments showed that the growth rate of barnacles decreased in the presence of these ephemeral algae. Thus, L. littorea may reduce initial barnacle settlement, but later may indirectly increase barnacle growth rate by reducing ephemeral algae. It is suggested that periwinkle density may be a key factor in the population dynamics of S. balanoides on intertidal mussel beds in the Wadden Sea.     

Age-dependent zonation of the periwinkle Littorina littorea (L.) in the Wadden Sea

On sedimentary tidal flats near the island of Sylt (German Bight, North Sea) abundance and size distribution of periwinkles, Littorina littorea L., were studied in low intertidal and in shallow and deep subtidal mussel beds (Mytilus edulis L.). In low intertidal mussel beds, surveys revealed that high densities (1,369±571 m^–2) of juvenile snails (≤13 mm) were positively correlated with strong barnacle epigrowth (Semibalanus balanoides L. and Balanus crenatus Bruguière) on mussels. A subsequent field experiment showed that recruitment of L. littorea was restricted to the intertidal zone. Abundances of periwinkles (213±114 m^–2) and barnacles abruptly decreased in the adjacent shallow subtidal zone, which served as a habitat for older snails (>13 mm). L. littorea was completely absent from disjunct deep (5 m) subtidal mussel beds. Snail abundance varied seasonally with maxima of >4,000 m^–2 in low intertidal mussel beds in October and minima in July, just before the onset of new recruitment. I suggest that the presence of cracks and crevices among the dense barnacle overgrowth in intertidal mussel beds favoured recruitment and survival of juvenile snails. Larger (older) specimens are assumed to actively migrate to the less favourable adjacent subtidal. Therefore, intertidal mussel beds are considered as nurseries for the population of L. littorea in the Wadden Sea. Received in revised form: 25 September 2000 Electronic Publication     

Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication     

Adult Prey Neutralizes Predator Nonconsumptive Limitation of Prey Recruitment

Recent studies have shown that predator chemical cues can limit prey demographic rates such as recruitment. For instance, barnacle pelagic larvae reduce settlement where predatory dogwhelk cues are detected, thereby limiting benthic recruitment. However, adult barnacles attract conspecific larvae through chemical and visual cues, aiding larvae to find suitable habitat for development. Thus, we tested the hypothesis that the presence of adult barnacles (Semibalanus balanoides) can neutralize dogwhelk (Nucella lapillus) nonconsumptive effects on barnacle recruitment. We did a field experiment in Atlantic Canada during the 2012 and 2013 barnacle recruitment seasons (May–June). We manipulated the presence of dogwhelks (without allowing them to physically contact barnacles) and adult barnacles in cages established in rocky intertidal habitats. At the end of both recruitment seasons, we measured barnacle recruit density on tiles kept inside the cages. Without adult barnacles, the nearby presence of dogwhelks limited barnacle recruitment by 51%. However, the presence of adult barnacles increased barnacle recruitment by 44% and neutralized dogwhelk nonconsumptive effects on barnacle recruitment, as recruit density was unaffected by dogwhelk presence. For species from several invertebrate phyla, benthic adult organisms attract conspecific pelagic larvae. Thus, adult prey might commonly constitute a key factor preventing negative predator nonconsumptive effects on prey recruitment.     

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.     

Mussel and dogwhelk distribution along the north‐west Atlantic coast: testing predictions derived from the abundant‐centre model

Aim We performed the first test of predictions from the abundant‐centre model using north‐west Atlantic coastal organisms. We tested the hypotheses that the density of intertidal mussels (Mytilus edulis and M. trossulus) and dogwhelks (Nucella lapillus) and mussel age and size would peak at an intermediate location along their distribution range. We also assessed the latitudinal variation in critical aerial exposure time. Location North‐west Atlantic coast between Newfoundland (Canada) and New York (USA), covering 1800 km of shoreline. Methods Using a nested design, we measured mussel density, age and size and dogwhelk density in 60 wave‐exposed rocky intertidal sites spread evenly in six regions. Critical aerial exposure times were determined using online data. Results Mytilus edulis peaked in abundance in Maine and was much less abundant in the other regions. Mytilus trossulus peaked in abundance in southern Nova Scotia and Maine, was less abundant in the other regions to the north, and was absent in the southernmost region (New York). Both mussel species were least abundant in a northern region (Cape Breton), although not in the northernmost region (Newfoundland). Critical aerial exposure times were negatively correlated with overall mussel density. Mussel age and size were similar among regions. Dogwhelks peaked in abundance in Maine and were much less abundant in the other regions, being positively correlated with overall mussel density across regions. Main conclusions Density data for M. edulis and N. lapillus provide limited support for an abundant‐centre pattern, while M. trossulus shows a clear ramped‐south distribution. Critical aerial exposure times suggest that physiological stress during summer and winter low tides may be lowest in Maine and southern Nova Scotia, which might partially explain mussel predominance in those regions. Winter ice scour in Cape Breton may explain the abundance trough observed there. Mussel size and age may be more limited by wave exposure at our sites (as they all face open waters) than by regional differences in environmental stress. Dogwhelks, which prey on mussels, seem to respond positively to prey density at the regional scale. Our study supports the notion that, while the abundant‐centre model is a useful starting point for research, it often represents an oversimplification of reality.     

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.     

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.     

The site of settlement indicates commensalism between bluemussel and its epibiont

Summary The site of settlement of barnacles (Balanus improvisus) attached on shells of bluemussels (Mytilus edulis) was mapped from a sample of mussels collected in the Baltic Sea. Most barnacles had settled near the siphonal apertures of the mussel. An experiment was made to measure the disadvantages and advantages that living in close association brings to barnacles and mussels. The barnacles on shells of living mussels were shown to grow significantly faster than those on empty mussel shells. Presence of barnacles had no effects on growth of mussels. The two-species association under study was demonstrated to be a case of commensalism.     

Isolated and combined impacts of blue mussels (Mytilus edulis) and barnacles (Balanus improvisus) on structure and diversity of a fouling community

Effects of two presumably dominant competitors, the blue mussel Mytilus edulis and the barnacle Balanus improvisus on recruitment, population dynamics and community structure on hard substrata were experimentally investigated in the subtidal Kiel Fjord, Western Baltic. The hypothesis that blue mussels and/or barnacles are local dominants and strongly influence succession and community structure was tested by monitoring succession in the presence and absence of simulated predation on either or both species. Manipulations included blue mussel removal, barnacle removal, combined blue mussel and barnacle removal, as well as a control treatment for natural (non-manipulated) succession. In the second part of the experiment, recovery from the treatments was monitored over 1 year.During the manipulative phase of the experiment, blue mussels had a negative effect on recruitment of species, whereas barnacles had no significant effect. Even so, a negative synergistic effect of blue mussels and barnacles was detected. Calculation of species richness and diversity H′ (Shannon Index) showed a negative synergistic effect of blue mussels and barnacles on community structure. Additionally, diversity H′ was negatively affected by the dominant competitor M. edulis. These effects were also detectable in the ANOSIM-Analysis. The non-manipulative phase of the experiment brought about a drastic loss of diversity and species richness. Blue mussels dominated all four communities. Barnacles were the only other species still being able to coexist with mussels. Effects of simulated predation disappeared fast.Thus, in the absence of predation on blue mussels, M. edulis within a few months dominates available space, and diversity of the benthic community is low. In contrast, when mussel dominance is controlled by specific predators, more species may persist and diversity remains high.     

Shell form and niche occupation in Nucella lapillus (L.) (Gastropoda)

Collections of Nucella lapillus (L.) from various eastern Atlantic areas have been measured, and shell shape has been analysed by D'Arcy Thompson's method in terms of whorl ratio, apical angle, and spiral angle. In southwestern Ireland N. lapillus varies greatly from tall and narrow in sheltered situations to short and wide at very wave-exposed sites. A local population is described for a bay near Crookhaven, County Cork. In northwestern Spain the open-sea form is developed only weakly. In southwestern Ireland N. lapillus effectively populates mussel beds in the lower littoral on open coasts, whereas in Spain it is sparse or absent from such situations but plentiful high up on the shore on barnacles in places where there is some local shelter from rocks or crevices. It is suggested that failure to develop the open-sea type in Spain is responsible for this difference in niche. Both in Spain and at a site in Scotland N. lapillus feeding on mussels in sheltered situations is of the tall narrow type, so that a diet of mussels cannot alone be responsible for the development of the open sea type, although it still might be a necessary condition. It is to be expected that the genetic composition of populations will vary regionally and will contribute to variation in shell form, but the part which it plays on a purely local scale remains uncertain.     

Clumping behavior as a strategy against drilling predation: Implications for the fossil record

Drilling gastropod predators are of particular interest to paleontologists, because predatory drill-holes in marine invertebrates serve as one of the rare sources of data for the study of ancient predator-prey interactions. Modern laboratory studies are an important part of predation research providing valuable ecological insight and constraining fossil evidence and interpretations. Previous studies have shown that mussels use clumping behavior against durophagous predation [Okamura, B., 1986. Group living and the effects of spatial position in aggregations of Mylitus edulis. Oecologia 69, 341-347.; Lin, J., 1991. Predator-prey interactions between blue crabs and ribbed mussels living in clumps. Estuar. Coast. Shelf Sci. 32, 61-69.], but its role against drilling predation had not been explored. In this study, we explore the effect of clumping on predator success (drill-hole frequency) and prey handling (drill-hole position) using the mussel, Mytilus trossulus, as prey and the gastropod, Nucella lamellosa, as drilling predator. We assigned mussels to two groups: in one, mussels were allowed to clump together with their byssal threads, and in the other, they were kept separate. We observed a significant decrease in the drilling frequency within the group containing clumped mussels, confirming that clumping acts as a successful anti-predatory strategy against drilling predators. The use of clumping as an effective strategy against multiple types of predators may relax the trade-offs associated with aggregated lifestyles [Bertness, M.D., Grosholz, E., 1985. Population dynamics of the ribbed mussel, Geukensia demissa: the costs and benefits of an aggregated distribution. Oecologia 67, 192-204.]. The increased benefit and unchanged metabolic cost of clumped living alters estimates of individual fitness with evolutionarily significant implications (e.g., eliminating the need to invoke group or species selection to explain the adaptive benefit of an aggregated lifestyle). In spite of potential differences in prey handling and grappling due to clumping, mean drill-hole placement and variation in drill-hole placement showed no significant differences between the two groups. These observations suggest that comparison of predation intensities across clumping and non-clumping taxa must consider the anti-predatory effect of this behavior.     

Survivorship of juvenile barnacles and mussels: spatial dependence and the origin of alternative communities

Mussels, barnacles, and rockweeds often form a distinct mosaic of patches on rocky intertidal shores, and it has been suggested that these communities may represent alternative community states. One way that alternative community states can arise is if early successional events are scale-dependent, but it is not known if juvenile survivorships of mussels and barnacles are, in fact, scale-dependent. Scale-dependence of barnacles (Semibalanus balanoides (L.)) and mussels (Mytilus edulis L.) was tested in the Gulf of Maine, USA. In winter 1997, clearings of 1, 2, 4 and 8 m in diameter and uncleared controls were made in stands of the rockweed Ascophyllum nodosum (L.) Le Jolis at 12 sites spread evenly across four bays on Swan's Island, ME. Summer and fall-winter survivorship of barnacles, which recruited in spring 1997, were estimated by tracking the 1997 cohort until late winter 1998. Survivorship of mussels was estimated from following the fate of transplanted juveniles over 4 days in late August 1997. Both barnacles and mussels showed better survival in 4 and 8 m clearings than in small clearings and controls. There was also significant variation in survivorship among sites. Densities of gastropods in the clearings did not reflect survivorship patterns of barnacles and mussels. Barnacle survivorship increased in fall and winter, and in large clearings was comparable to survivorship in barnacle-dominated habitats. Mussel survivorship was low in all clearing sizes suggesting that mussel beds develop slowly.     

Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects

The risk of predation can drive trophic cascades by causing prey to engage in antipredator behavior (e.g. reduced feeding), but these behaviors can be energetically costly for prey. The effects of predation risk on prey (nonconsumptive effects, NCEs) and emergent indirect effects on basal resources should therefore depend on the ecological context (e.g. resource abundance, prey state) in which prey manage growth/predation risk tradeoffs. Despite an abundance of behavioral research and theory examining state‐dependent responses to risk, there is a lack of empirical data on state‐dependent NCEs and their impact on community‐level processes. We used a rocky intertidal food chain to test model predictions for how resources levels and prey state (age/size) shape the magnitude of NCEs. Risk cues from predatory crabs Carcinus maenas caused juvenile and sub‐adult snails Nucella lapillus to increase their use of refuge habitats and decrease their growth and per capita foraging rates on barnacles Semibalanus balanoides. Increasing resource levels (high barnacle density) and prey state (sub‐adults) enhanced the strength of NCEs. Our results support predictions that NCEs will be stronger in resource‐rich systems that enhance prey state and suggest that the demographic composition of prey populations will influence the role of NCEs in trophic cascades. Contrary to theory, however, we found that resources and prey state had little to no effect on snails in the presence of predation risk. Rather, increases in NCE strength arose because of the strong positive effects of resources and prey state on prey foraging rates in the absence of risk. Hence, a common approach to estimating NCE strength – integrating measurements of prey traits with and without predation risk into a single metric – may mask the underlying mechanisms driving variation in the strength and relative importance of NCEs in ecological communities.     

Predation on barnacles of intertidal and subtidal mussel beds in the Wadden Sea

Balanids are the numerically dominant epibionts on mussel beds in the Wadden Sea. Near the island of Sylt (German Bight, North Sea), Semibalanus balanoides dominated intertidally and Balanus crenatus subtidally. Field experiments were conducted to test the effects of predation on the density of barnacle recruits. Subtidally, predator exclusion resulted in significantly increased abundances of B. crenatus, while predator exclusion had no significant effects on the density of S. balanoides intertidally. It is suggested that recruitment of B. crenatus to subtidal mussel beds is strongly affected by adult shore crabs (Carcinus maenas) and juvenile starfish (Asterias rubens), whereas recruits of S. balanoides in the intertidal zone are mainly influenced by grazing and bulldozing of the very abundant periwinkle Littorina littorea, which is rare subtidally. Thus, not only do the barnacle species differ between intertidal and subtidal mussel beds, but the biotic control factors do so as well. Electronic Publication     

Effects ofFucus vesiculosus covering intertidal mussel beds in the Wadden Sea

The brown algaFucus vesiculosus formamytili (Nienburg) Nienhuis covered about 70% of mussel bed (Mytilus edulis) surface area in the lower intertidal zone of Königshafen, a sheltered sandy bay near the island of Sylt in the North Sea. Mean biomass in dense patches was 584 g ash-free dry weight m^?2 in summer. On experimental mussel beds, fucoid cover enhanced mud accumulation and decreased mussel density. The position of mussels underneath algal canopy was mainly endobenthic (87% of mussels with >1/3 of shell sunk into mud). In the absence of fucoids, mussels generated epibenthic garlands (81% of mussels with <1/3 of shell buried in mud). Mussel density underneath fucoid cover was 40 to 73% of mussel density without algae. On natural beds, barnacles (Balanidae), periwinkles (Littorina littorea) and crabs (particularly juveniles ofCarcinus maenas) were significantly less abundant in the presence of fucoids, presumably because most of the mussels were covered with sediment, whereas in the absence of fucoids, epibenthic mussel clumps provided substratum as well as interstitial hiding places. The endobenthic macrofauna showed little difference between covered and uncovered mussel beds. On the other hand, grazing herbivores — the flat periwinkleLittorina mariae, the isopodJaera albifrons and the amphipodsGammarus spp. — were more abundant at equivalent sites with fucoid cover. The patchy growth ofFucus vesiculosus on mussel beds in the intertidal Wadden Sea affects mussels and their epibionts negatively, but supports various herbivores and increases overall benthic diversity.     

The importance of predation and competition in organizing the intertidal epifaunal communities of Barnegat Inlet,New Jersey

Summary Community organization was studied by experiment and observation from October 1972–October 1974 in the marine epifaunal assemblages at each end of Barnegat Inlet, New Jersey. The rock jetty at the wave-exposed eastern end of the inlet possesses an intertidal community with the following attributes: (1) a high intertidal zone dominated by the barnacle, Balanus balanoides, but also occupied by the blue mussel, Mytilus edulis, in rock crevices, (2) a mid and low intertidal zone with usually <10% free space and extreme numerical dominance by Mytilus edulis (usually >85% cover) during summer and fall, and (3) almost no intertidal predators or herbivores. The predatory seastar, Asterias forbesi, is abundant subtidally. Controlled removal experiments indicate that in the mid and low intertidal underlying barnacles perish as a consequence of the establishmentof extensive secondary cover by Mytilus, probably because Mytilus outcompetes Balanus through suffocation or starvation. Mytilus transplants demonstrate that the mussels do not survive outside of crevices in the high intertidal, which thus may represent for Balanus a refuge from competition by Mytilus.The pilings on docks at the protected western end of Barnegat Inlet possess an intertidal epifaunal community with the following characteristics: (1) a high intertidal zone that includes Balanus balanoides, a second barnacle, Balanus eburneus, and an herbivorous gastropod, Littorina littorea, (2) a mid and low intertidal zone with usually >40% free space in the summer and fall and the remaining area covered by several abundant species with no extreme dominant, and (3) abundant predators, chiefly the oyster drill, Urosalpinx cinerea, the blue crab, Callinectes sapidus, and a mud crab, Neopanope texana sayi. Asterias forbesi, while abundant subtidally, is also occasionally present on intertidal surfaces. Controlled exclusion of predators by caging several replicate pilings at the western end of the inlet reveals that predation prevents monopolization of mid and low intertidal space by the apparent competitive dominant, Mytilus. Predation appears to be a direct cause of the relatively great temporal and spatial heterogeneity in the mid and low intertidal of these pilings.Thus, although the Barnegat Inlet intertidal system appears to follow closely the patterns of community organization described for several other rocky intertidal coastlines, this organizational pattern is noteworthy because it is repeated here in a far more seasonal environment and with a new cast of interacting competitors and predators. That crabs play an important role as predators is novel for North America, but only perhaps because all previous North American studies have ignored the rocky intertidal zones of quiet, estuarine waters where in Europe predatory crabs are known to be extremely significant.This paper is dedicated to the memory of Robert H. MacArthur     

Group living and the effects of spatial position in aggregations of Mytilus edulis

Summary The mussel Mytilus edulis typically occurs in aggregations and several consequences of living in groups were studied. Isolated individuals and individuals associated in relatively small groups (6–9 mussles/group) grew more and therefore had greater reproductive output than mussels associated with relatively large groups of 21–28 individuals. Mussels located in the centers of groups exhibited reduced growth and thus lower reproduction relative to mussels located on the edges of groups whose growth and reproduction was similar to that of isolated individuals. Sampling from natural populations indicated that most mussels grow within the matrix of very large groups and hence will experience reduced growth and reproduction. Patterns of growth exhibited by mussels in association with living and model mussels showed that the adverse effects on growth exhibited by mussels in relatively large groups are not a function of the mere physical relief of a mussel clump, but are caused by some property of living neighbors.Laboratory experiments on mussel predation by the crab Pachygrapsus crassipes indicated that crabs prey disproportionately on mussels growing on the edges of groups.The consequences of group living in mobile and nonmobile organisms are considered, and it is suggested that a greater number of negative effects will arise in groups as mobility decreases. In addition, the noted ecological similarity between groups of sessile organisms and spreading clones and its evolutionary implications are discussed.This is contribution no 171 from the Smithsonian Marine Station at Link Port     

Using polyclonal antibodies to detect predation by dogwhelks Nucella lapillus (L.)

Interactions between predators and their multiple prey species can vary greatly among locations where they coexist. As a method to assess spatial variation in predation by intertidal dogwhelks on their dominant prey, immunoassays of dogwhelk gut contents from experimental populations and field collected individuals were evaluated using polyclonal antibodies raised separately to soluble proteins from Mytilus edulis L. mussels and Semibalanus balanoides (L.) barnacles. Both antisera produced strong reactions against their homologous antigens but no cross reactions between prey species. Experimental trials tested the critical hypothesis that prey species had equal detection intervals in dogwhelk guts. Two groups of 225 dogwhelks were starved for 14 days, provided with either mussels or barnacles for five days, and then sampled over 22 days. Independent immunoassays of dogwhelk gut contents against each antibody revealed a consistent, weak cross reaction between the anti-mussel antibody and dogwhelk gut tissues. After accounting for this cross reaction, the strength of immunoassays against both prey species declined exponentially and at similar rates. The proportions of dogwhelks that tested positive for their provided prey species declined linearly through time and were not significantly influenced by prey type. Prey were detectable throughout the sampled post-feeding period and were projected to have detection limits of 24.4 days (barnacles) and 26.5 days (mussels), demonstrating that immunoassay results are not biased by dissimilar prey detection intervals. Reactions against the antibody from the non-provided prey were time invariant and occurred at relatively low frequencies. Immunoassays of dogwhelks collected from five intertidal sites on Swans Island, Maine, USA revealed patterns similar to field observations, though immunoassays classified far fewer individuals as non-feeders and more as barnacle feeders than indicated by direct field observations. Unlike single observations, immunoassays also revealed the presence of both prey in dogwhelks from four sites, though most individuals tested positive for only a single prey type. Immunoassays facilitate concurrent collections of predation data from many individuals and will enable further local- to regional-scale assessments of dogwhelk predation at additional sites around the Gulf of Maine.