Competition and facilitation between the alien mussel Mytilus galloprovincialis and indigenous species: Moderation by wave action

As the alien species that most dominates space along the South African coast, the Mediterranean mussel Mytilus galloprovincialis has radically altered community composition on invaded shores. We experimentally assessed interspecific interactions between this invasive species and dominant indigenous species in conjunction with considering how wave action moderates such interactions. The density of both M. galloprovincialis and the limpet Scutellastra granularis increased with wave action. Conversely, the tube-building polycheate Gunnarea capensis was negatively affected by wave exposure, being most abundant on sheltered shores. The influence of wave action on the indigenous mussel Aulacomya ater, however, remains unclear. M. galloprovincialis outcompeted both G. capensis and A. ater at moderate to high exposure levels, whereas it had both positive and negative effects on S. granularis. It outcompeted adult limpets on primary rock space on semi-exposed and exposed shores, reducing densities of this portion of the population. However, recruitment of S. granularis was facilitated by M. galloprovincialis, as greater numbers recruited to the secondary substratum offered by mussel shells. Again this interaction intensified with wave action. Due to the extremely high density of recruits on secondary space, the net effect of M. galloprovincialis on S. granularis was positive. Thus, wave action not only influences the abundance of individual species, but also mediates both positive and negative interspecific interactions in rocky shore communities, including the impact of alien species such as M. galloprovincialis.     

Predation by small mobile aquatic predators regulates populations of the intertidal limpet Patella vulgata (L.)

Highly mobile aquatic predators are known to forage in the intertidal during periods of immersion. There is limited quantitative information, however, on the extent to which these predators influence the abundance of grazing molluscs which are known to have a key role in structuring intertidal assemblages. Our preliminary video observations revealed that crabs and small fish were abundant on shores in southwest England during high-tide. We then used manipulative experiments to quantify the effect of small mobile aquatic predators on the abundance of limpets (Patella vulgata L.). On the lower shore at two moderately sheltered rocky shores three treatments were established: complete cage, partial cage (cage control) and uncaged (natural condition). The complete cages excluded all predators. The partial cage treatment allowed full access to small predators and the uncaged treatment allowed access to all predators. After two months, limpet abundance in uncaged and partial cage treatments had declined by around 50% compared to the complete cage treatment. Population structure also changed with survival of larger individuals being greater than smaller individuals in the open and partial cage treatments compared to the complete cage treatment. The effects of excluding predators were consistent at small (meters) and large spatial scales (kilometres) and hence, it would appear that the outcomes of our research are generally applicable to similar shores in the region.To explore the mechanism behind the differential effects of predators according to prey size, we compared the detachment force required to remove limpets of differing sizes from the shore. This was around four times greater for larger individuals than for smaller ones indicating that smaller limpets were more vulnerable to predation. These effects were also consistent between locations. Subsequent laboratory observations showed that the crabs Carcinus maenas (L.), Necora puber (L.) and Cancer pagurus (L.) which are locally abundant predators of limpets, had differing handling behaviour but were all highly efficient at removing limpets from substratum. Hence, shell width and attachment force appeared to be critical factors influencing the vulnerability of limpets to predation by these crabs. Limpets are known to control the abundance of macroalgae on shores in the North-east Atlantic and so our conclusions about the role of mobile predators in regulating the abundance of these grazers are important to our broader understanding of the ecology of these shores.     

Cheating the Locals: Invasive Mussels Steal and Benefit from the Cooling Effect of Indigenous Mussels

The indigenous South African mussel Perna perna gapes during periods of aerial exposure to maintain aerobic respiration. This behaviour has no effect on the body temperatures of isolated individuals, but when surrounded by conspecifics, beneficial cooling effects of gaping emerge. It is uncertain, however, whether the presence of the invasive mussel Mytilus galloprovincialis limits the ability of P. perna for collective thermoregulation. We investigated whether varying densities of P. perna and M. galloprovincialis influences the thermal properties of both natural and artificial mussel beds during periods of emersion. Using infrared thermography, body temperatures of P. perna within mixed artificial beds were shown to increase faster and reach higher temperatures than individuals in conspecific beds, indicating that the presence of M. galloprovincialis limits the group cooling effects of gaping. In contrast, body temperatures of M. galloprovincialis within mixed artificial mussel beds increased slower and exhibited lower temperatures than for individuals in beds comprised entirely of M. galloprovincialis. Interestingly, differences in bed temperatures and heating rates were largely dependent on the size of mussels, with beds comprised of larger individuals experiencing less thermal stress irrespective of species composition. The small-scale patterns of thermal stress detected within manipulated beds were not observed within naturally occurring mixed mussel beds. We propose that small-scale differences in topography, size-structure, mussel bed size and the presence of organisms encrusting the mussel shells mask the effects of gaping behaviour within natural mussel beds. Nevertheless, the results from our manipulative experiment indicate that the invasive species M. galloprovincialis steals thermal properties as well as resources from the indigenous mussel P. perna. This may have significant implications for predicting how the co-existence of these two species may change as global temperatures continue to rise.     

Responses of the limpet, Cellana grata (Gould 1859), to hypo-osmotic stress during simulated tropical, monsoon rains

Although heat stress is often cited as the dominant physical stress on tropical shores, intertidal organisms in regions with monsoonal climates are also regularly exposed to prolonged periods of heavy rainfall. Such events are predicted to have adverse physiological effects on individuals and may result in mortality. In a series of laboratory experiments, the impact of simulated monsoonal rains was investigated on the patellid limpet, Cellana grata. Sub-lethal responses in terms of body water content, body fluid osmolality and heart rate were measured in two different size cohorts maintained on horizontal and vertical substrata. Limpets were unable to achieve any effective behavioural isolation, and exposure to either simulated rainfall or diluted seawater resulted in both large and small C. grata gaining water with subsequent dilution of mantle water and haemolymph osmolalities. With increased duration of rainfall, dilution of body fluids increased with little difference between individuals on horizontal and vertical surfaces. Body fluids generally showed proportional dilution during prolonged rain, but in some individuals there was evidence for regulation of the haemolymph relative to the mantle fluid. Overall, smaller limpets were more susceptible to prolonged rainfall than large animals in terms of swelling of soft tissues and detachment and also had higher heart rates than large limpets. Both cohorts reduced heart rates with prolonged rainfall, suggesting a degree of metabolic depression, especially on horizontal surfaces. In small limpets, no difference in heart rate was found with substratum orientation, whereas large limpets had elevated heart rates on vertical as compared to horizontal substrata, when exposed to either simulated rainfall or washed with dilute seawater. This may reflect the increased energetic costs required to maintain a relatively larger body on a vertical surface under stressful conditions. Monsoonal rainfall during emersion, and subsequent dilution of seawater, therefore, have sub-lethal physiological and possible lethal effects on intertidal limpets. This influence has been largely overlooked, but coupled with the possible synergistic effects of thermal stress, monsoon rains are likely to play an important role in community dynamics on tropical shores.     

Short- and long-term movements of the patellid limpet Patella flexuosa within gaps in intertidal mussel beds

Short-term and long-term movements of the patellid limpet Patellaflexuosa Quoy & Gaimard were studied on an intertidal rockyshore where a mytilid, Hormomya mutabilis (Gould), formed anextensive mussel zone. At low tide, all the limpets were found restingwithin gaps in the mussel zone, which were formed after massdislodgement of the mussels by waves. The foraging areas ofall the marked limpets were restricted to within the gaps duringfour 24h diving observations. All of these limpets displayedhoming behaviour after short excursions with a maximum averagedistance of 6.7 cm from their homes. During short-term observationperiods of about 4–5 days, no marked limpets moved fromone gap to another gap across the mussel bed of H. mutabilis.During long-term observation periods of one month, however,10–30% of marked limpets shifted their homes to differentgaps that had a lower limpet density. The shell length of theseemigrants was, in general, significantly smaller than that ofnon-emigrants. The percentage of emigrants tended to be higherin summer and autumn than in winter and spring. The movementof P. flexuosa within patchy habitats is discussed with a focuson the difference in site fidelity between small and large limpets. (Received 14 April 1998; accepted 7 September 1998)     

Effects of epibiotic algae on the survival, biomass and recruitment of mussels, Mytilus L. (Bivalvia: Mollusca)

Habitats composed of living ‘ecosystem engineers’, such as mussels, are subject to direct and indirect interactions with organisms that live among them. These interactions may affect the presence and structure of habitat, and hence, the associated taxa. We examined the direct effects of epibiotic algae on the survival, biomass and recruitment of mussels (Mytilus L.) on the west coast of Ireland. A field experiment showed that the presence of epibiotic fucoid algae reduced the likelihood of survival of mussels during storms. We also found that the strength of attachment of mussels did not increase in the presence of epibionts. Another in situ experiment revealed that the presence of ephemeral epibiotic algal mats had no effect on the biomass of host mussels, suggesting no effect on mussel growth or production. The abundance of small mussels (< 5 mm) covaried with cover of epibiotic algae at one location, but experimental results suggest no direct effect of the algae on recruitment unless through the action of chemical cues which linger after the removal of algae. We have identified a negative direct interaction between epibiotic algae and mussels on exposed rocky shores, which may often be characterised by bottom-up regulation. It is thought that positive interactions may be more important on more sheltered or sedimentary shores where top-down processes are more likely to dominate.     

Wadden Sea mussel beds invaded by oysters and slipper limpets: competition or climate control?

Introduced species are often considered to be a threat to residents, but not all reciprocal trends may reflect species interaction. In the northern German Wadden Sea, native mussel Mytilus edulis beds are declining and overgrown by introduced Pacific oysters Crassostrea gigas and slipper limpets Crepidula fornicata. We review the population development of the three species and analyse whether the invading species may be responsible for the decline of native mussels. The Pacific oyster predominately settles on mussel beds in the intertidal and the slipper limpet dominates around low water line. We compare the development of mussels and invaders in two subregions: mussel beds near the islands of Sylt and Amrum decreased both in the presence (Sylt) and absence (Amrum) of the two invading species and more detailed investigations could not confirm a causal relationship between the increasing invaders and decreasing mussel beds. There is evidence that the decline of mussel beds is mainly caused by failing spatfall possibly due to mild winters, whereas the increase in slipper limpets and oysters is facilitated by mild winters and warm summers, respectively. We conclude that changing species composition is a result of the climatic conditions in the last decade and that there is no evidence yet that the exotic species caused the decline of the natives. It remains an open question whether the species shift will continue and what the consequences for the native ecosystem will be.     

Benefit from an invader: American slipper limpet <Emphasis Type="Italic">Crepidula fornicata</Emphasis>reduces star fish predation on basibiont European mussels

Introduced species have recently become a major concern in ecological research and aquatic conservation. This is due to an increasing appearance of introduced species at a global scale and a multitude of negative impacts on native biota. However, impacts of introduced species are not necessarily only negative. The epizootic American slipper limpet Crepidula fornicata, native at North American Atlantic shores, was introduced to Europe in the 1870s and is now widespread along the Atlantic coast of Europe. Negative effects like trophic and spatial competition have been reported. In its major basibiont in the Wadden Sea, the blue mussel Mytilus edulis, attached limpets reduce survival and growth. However, a laboratory experiment also showed sea star (Asterias rubens) predation on mussels with limpet epigrowth to be three times lower than in unfouled mussels. Hence, although negatively affected by C. fornicatain one way, this epigrowth is beneficial for fouled mussels in another. This indicates that the actual impact of an introduced species is a complex interplay of positive and negative effects which may only be revealed experimentally.     

Demography, competitive interactions and grazing effects of intertidal limpets in southern New Zealand

Population dynamics and the effects of intraspecific competition on limpet growth and maintenance of bare patches were investigated for the intertidal limpet Cellana ornata (Dillwyn) at a boulder-dominated site and on a rocky platform near Kaikoura (South Island), New Zealand. Distribution and abundance patterns of C. ornata were described in relation to other biota and tidal height. C. ornata occurred almost exclusively in patches devoid of macroalgae, particularly in the mid-tidal zone. Both adult and juvenile limpets were most abundant on the tops of boulders, where their numbers were positively correlated with barnacle cover, which averaged 77%. The size structure and growth patterns of C. ornata were different between populations. Mark-recapture studies showed that the slopes of annual growth increments regressed on initial sizes were similar at both sites but that the annual increments on the platform were about 6 mm greater than on boulders. Growth virtually ceased at 27 mm for limpets at the barnacle-dominated boulder site and at 40 mm at the platform site. Recruiting cohorts had 20% survival on boulders and 37% on the platform during their first year. The largest size classes at both sites had around 57% annual survival. To test the effects of varying limpet densities on the growth and mortality of limpets and the maintenance of bare patches, densities of C. ornata were experimentally increased at both sites. Beyond a density of 4 per 0.25 m², sizes and survival of limpets were reduced at both sites, but the effect was more pronounced at the boulder site. Limpets at the boulder site were more effective at maintaining bare space than those on the reef platform. Enclosing limpets in plots with and without barnacles showed that C. ornata and a co-occurring species (Cellana denticulata (Martyn)) grazed more effectively and had greater growth in cleared plots. Overall, there was considerable variation in the demographics of C. ornata between populations driven by site-specific characteristics.     

Intertidal assemblages associated with natural<Emphasis Type="Italic">corallina</Emphasis> turf and invasive mussel beds

There is considerable concern about conservation of biodiversity in highly disturbed and urbanized environments, although a very large proportion of biodiversity (i.e. the small and cryptic invertebrates) have been little studied in this regard. Many biogenic structures (e.g. coral reefs, mussel beds, foliose algae) provide habitat for a large number of small invertebrates. The features of these habitats to which these animals respond are complex and poorly documented. Invasive species are increasing in abundance and diversity in many disturbed estuaries, but most previous studies have concentrated on effects of invasive species on surrounding macroscopic assemblages. This study examines the assemblages of small invertebrates and algae living in natural patches of coralline turf and in patches of the invasive mussel, Mytilus galloprovincialis, on seawalls in Sydney Harbour. Although most taxa identified were common to both habitats, they were generally more abundant in turf than in the mussels, especially the more widespread and numerous taxa. Few taxa were unique to either habitat and those were generally sparse and patchy. In addition, there were relatively more smaller animals in the algal turf than in the mussels, although it is not known whether these were juveniles of adults present in both habitats, or different species. These data show that coralline turf and mussel beds do not provide similar intertidal habitat for associated assemblages and that overgrowth of natural biota by mussels may have strong indirect effects on associated assemblages. These warrant further experimental investigation, so that the effects of invasive species on local biodiversity can be better understood and managed.     

Lift as a mechanism of patch initiation in mussel beds

The mussel Mytilus californianus is the dominant competitor for space in the mid-intertidal zone of wave-swept rocky shores in the Pacific Northwest where it forms extensive tightly packed beds. The rate at which patches are formed in these beds, can play an important rôle in community ecology by controlling the establishment and persistence of fugitive species. Despite the biological importance of physical disturbance, the mechanism of patch initiation has not been adequately explained. Battering by logs can create patches, but is the predominant mechanism only on shores near active logging sites. In other areas, it has been speculated that the hydrodynamic forces associated with storm waves somehow cause patches to form. However, the forces acting along the direction of flow — drag and the acceleration reaction — are unlikely to initiate patch formation. Here, it is suggested that fluid-dynamic lift forces imposed on mussel beds by breaking waves are sufficient to dislodge individual mussels and trigger patch formation. Arguments are presented suggesting that the likelihood of dislodgment by lift is consistent with the observed rate of patch formation in the absence of log battering.     

Contrasting spatial heterogeneity of sessile organisms within mussel (Perna perna L.) beds in relation to topographic variability

We examined the spatial heterogeneity in three sessile rocky shore organisms, the mussel Perna perna, the barnacle Octomeris angulosa (Sowerby) and the red alga Gelidium pristoides (Turn.) at a range of continuous local scales along horizontal transects within mid- and upper mussel beds of South African shores. We also examined the relationships between variability of organisms and topographic features (rock depressions, slope, aspect), and between mussel, barnacle and algal variability over the same scales. To estimate spatial heterogeneity, we analyzed scaling properties of semivariograms using a fractal approach. Relationships between different variables at the different scales were examined by cross-semivariograms. Spatial dependence of P. perna variability increased with spatial dependence of topographic variability, so that scaling regions of mussel and topographic distributions corresponded well. This relationship often improved with larger local scales (mussel cover increased with depressions, steeper slope and aspect towards waves), while at smaller spatial scales, variability in mussel cover was less well explained by variability in topography. The variability of the barnacle O. angulosa exhibited spatial dependence, even on topographically unstructured shores. In contrast, the distribution of the alga G. pristoides revealed high fractal dimensions, showing spatial independence on topographically unstructured shores. Algae also showed a very strong negative relationship with mussels at most local scales, and a negative relationship with barnacles in upper zones, especially at larger local scales. Barnacles may show clear spatial dependence because of hydrodynamics (at larger local scales) and the need to find a future mate in close proximity (at smaller local scales), while algae may show a strong negative relationship with mussels because of competition for space.     

Phenotypic flexibility in the metabolic response of the limpet Cellana tramoserica to thermally different microhabitats

Temperature determines all physiological responses by limiting cellular reaction rates. Daily temperature variation differs between microhabitats, which means that subpopulations of the same species may respond differently to temperature. The aim of this study is to determine how physiological responses to temperature of the limpet Cellana tramoserica differ between limpets from variable and from stable thermal environments. Oxygen consumption and anaerobic and aerobic metabolic capacities were measured over a range of temperatures in limpets from thermally stable and variable field sites in summer and winter, and in laboratory acclimation treatments. Limpets from both variable and stable sites, showed acclimatisation of anaerobic and aerobic potentials. Limpets from stable environments, but not from variable environments, showed increased oxygen consumption in winter. Comparison of field and laboratory data showed that temperature was the signal for acclimatisation. The physiological response of C. tramoserica to temperature depends on season and microhabitat. Care must therefore be taken when conducting interspecies comparisons of response to temperature to address the confounding effects of phenotypic plasticity. Differences in physiological response to temperature in phenotypically flexible species like C. tramoserica may simply reflect individual reactions to immediate environmental conditions.     

Larval settlement can explain the adult distribution of Mytilus californianus Conrad but not of M. galloprovincialis Lamarck or M. trossulus Gould in Moss Landing, central California: Evidence from genetic identification of spat

We investigated the spatial distribution of adult and newly settled mussels (Mytilus galloprovincialis Lamarck, Mytilus trossulus Gould and Mytilus californianus Conrad) on the shore at Moss Landing, California to test the hypothesis that adult distributions are a result of settlement patterns. Adult M. californianus were most abundant on a wave-exposed rocky jetty and adults of Blue mussels (M. trossulus and M. galloprovincialis) were more abundant inside the protected Moss Landing harbor. Using taxon-specific polymerase chain reactions, we monitored recruitment during continuous 1-2 week intervals on fibrous scrubbing pads for 12 months in 2002-2003. All mussel species settled in greatest numbers on the exposed jetty, and Blue mussels settled in greater numbers there than did M. californianus. Because Blue mussels settled abundantly where their adults were rare, post-settlement mortality appeared to be the strongest influence on adult distribution. In contrast, M. californianus settled mostly in their adult habitat.     

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.     

Love Thy Neighbour: Group Properties of Gaping Behaviour in Mussel Aggregations

By associating closely with others to form a group, an animal can benefit from a number of advantages including reduced risk of predation, amelioration of environmental conditions, and increased reproductive success, but at the price of reduced resources. Although made up of individual members, an aggregation often displays novel effects that do not manifest at the level of the individual organism. Here we show that very simple behaviour in intertidal mussels shows new effects in dense aggregations but not in isolated individuals. Perna perna and Mytilus galloprovincialis are gaping (periodic valve movement during emersion) and non-gaping mussels respectively. P. perna gaping behaviour had no effect on body temperatures of isolated individuals, while it led to increased humidity and decreased temperatures in dense groups (beds). Gaping resulted in cooler body temperatures for P. perna than M. galloprovincialis when in aggregations, while solitary individuals exhibited the highest temperatures. Gradients of increasing body temperature were detected from the center to edges of beds, but M. galloprovincialis at the edge had the same temperature as isolated individuals. Furthermore, a field study showed that during periods of severe heat stress, mortality rates of mussels within beds of the gaping P. perna were lower than those of isolated individuals or within beds of M. galloprovincialis, highlighting the determinant role of gaping on fitness and group functioning. We demonstrate that new effects of very simple individual behaviour lead to amelioration of abiotic conditions at the aggregation level and that these effects increase mussel resistance to thermal stress.     

Defensive responses to predatory seastars by two specialist limpets, Notoacmea insessa (Hinds) and Collisella instabilis (Gould), associated with marine algae

The limpets Notoacmea insessa (Hinds) and Collisella instabilis (Gould), which are associated specifically with certain algae, were tested for defensive responses to seastars. Contact with predatory seastars elicited responses by both species of limpets, in contrast to previous work reporting a lack of defensive responses by specialist limpets associated with plants. Non-predatory seastars normally did not elicit defensive responses. The form of the response by Notoacmea insessa differed, depending on whether the limpet was on or off its grazing scar. When the limpet was off its grazing scar, the most common response was movement away from the point of contact. When on the scar, the limpet usually elevated its shell and rocked from side-to-side, but rarely moved from the scar. The responses of Collisella instabilis almost always involved rapid movement away from the point of contact and tended to be more vigorous than those of Notoacmea insessa.     

What triggers the “bail out” behaviour in the limpet Lottia gigantea?

Manipulating objects (small rocks, mussel shells, etc.) to which limpets are clinging will cause many spontaneously to release their hold ("bail out"). Experiments demonstrated that in the owl limpet (Lottia gigantea) bail out was only triggered by rotation of the limpet around a horizontal axis at rates of 0.13 to 1.00rps. Limpets reattached more vigorously to a surface following a “bail out” than when they were forcibly removed from the substratum. In the field a variety of intertidal gastropods were observed to “bail out”. “Bail out” appears to be an adaptive response allowing an individual to abandon a dislodged substratum for a more stable one.     

Genetics and taxonomy of Chilean smooth-shelled mussels, Mytilus spp. (Bivalvia: Mytilidae)

It has been previously established that native smooth-shelled mussels in southern South America possess close evolutionary affinities with Northern-Hemisphere Mytilus edulis L. 1758 (McDonald et al. (1991) [5]). This result has since been challenged by authors claiming that Chilean mussels should be considered a local subspecies of M. galloprovincialis Lmk. 1819. Moreover, morphological, physiological, ecotoxicological and molecular genetic studies on Chilean smooth-shelled mussels still frequently refer to ‘M. chilensis’ Hupé 1854, even though the previous discovery of alien M. galloprovincialis and considerable heterogeneity in shell morphology among samples collected along the Chilean shores raise concerns that different Mytilus spp. species might have been included under ‘M. chilensis’. Here we reviewed the molecular and morphological data available on smooth-shelled mussels from Chile in an attempt to clarify both their genetic composition and their taxonomic status. Using multivariate analysis on sample × allozyme-frequency matrices, we confirmed the widespread occurrence of the Southern-Hemisphere form of M. edulis along the shores from the North Patagonia region of Chile to the southern tip of the South American continent. The populations sampled in southern central Chile showed some evidence of slight introgression from Southern-Hemisphere M. galloprovincialis. Morphological characterization of a sample from Dichato in southern central Chile was consistent with its previous genetic identification as Mediterranean M. galloprovincialis. The occurrence of Southern-Hemisphere M. galloprovincialis in Punta Arenas at the southern tip of the South American continent was also reported. Southern-Hemisphere M. edulis, including native Chilean smooth-shelled Mytilus, should be assigned subspecific rank and named M. edulis platensis d’Orbigny 1846.     

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.