Experimental test of biodeposition and ammonium excretion from blue mussels (Mytilus edulis) on eelgrass (Zostera marina) performance

Blue mussels and eelgrass have been found to coexist in many locations. However, knowledge of the interactions between these species is limited. Two experiments were conducted in the laboratory, a “Deposit” and an “Epiphyte” experiment. The Deposit experiment examined possible effects of increasing load of blue mussel (Mytilus edulis) biodeposits on sediment biogeochemistry and eelgrass (Zostera marina) performance. Z. marina mesocosms received normal or high loads of mussel biodeposits (Normal and High), while no biodeposits were added to the Control. High dosage had overall negative effects on Z. marina, which was reflected as lower leaf numbers and biomass and accumulation of elemental sulphur in rhizomes. The sediment biogeochemical conditions were altered, as the mussel biodeposits enhanced sulphate reduction rates and increased sulphide concentrations in the porewater, which may result in sulphide invasion and reduced growth of Z. marina.In the Epiphyte experiment effects of mussel excretion, with particular emphasis on ammonium, on the growth of Z. marina and their epiphytes were examined. A thick cover of epiphytes developed on Z. marina growing together with M. edulis, and the relative growth rate was reduced with 20% compared to plants from control without mussels. Overall the experiments showed negative effects on Z. marina growing together with M. edulis, thereby supporting a preceding field study by Vinther et al. [Vinther, H.F., Laursen, J.S., Holmer, M. 2008. Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord, Denmark. Est. Coast Shelf. Sci. 77, 91-103.].     

Three species of Mytilus and their hybrids identified in a Scottish Loch: natives, relicts and invaders?

Three species of the mussel, Mytilus, occur in the North Atlantic region, M. edulis, M. galloprovincialis and M. trossulus, and hybrid zones are present where their distributions overlap. M. edulis is a native species in the UK. M. galloprovincialis originated in the Mediterranean and its distribution extends northwards along the Atlantic seaboard to Scotland. Baltic Sea mussels have a M. trossulus ancestry but are highly introgressed by M. edulis. In recent decades, farming of mussels on long-line rope culture systems has been introduced into Scotland. On farms in Loch Etive, a form of mussel with a fragile shell and a different shape to either M. edulis or M. galloprovincialis has been increasing in frequency over recent years. Samples of fragile shelled, normal strong shelled and intermediate mussel types were sampled from two farms in 2006 and compared with samples of M. edulis, M. galloprovincialis and M. trossulus from other sources where their species identity is well established. Abundance relative to depth, shell strength, condition index and shell morphology were analysed together with 5 allozyme loci and one nuclear DNA genetic marker (Me 15/16). The fragile shelled mussels, and many of those classed as intermediate, were identified as a mixture of M. trossulus and M. trossulus x M. edulis hybrids. This identification was strongly supported by both morphological and genetic data and is the first record of the presence of M. trossulus in UK waters. M. trossulus in Loch Etive are most likely to be a post-glacial relict population restricted to the low salinity area of the Loch that has recently increased in abundance due to commercial mussel growing activity. In addition, individual mussels of all three species and their hybrids were detected amongst Loch Etive mussels. This is the first genetic demonstration of all three species and their hybrids occurring together in one location in the Atlantic region and provides a unique opportunity to study the processes of speciation, divergence, and introgression in the genus Mytilus.     

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.     

Mytilid mussels: global habitat engineers in coastal sediments

Dense beds of mussels of the family Mytilidae occur worldwide on soft-bottoms in cold and warm temperate coastal waters and have usually been considered hot spots of biodiversity. We examined intertidal mussel beds at four distant locations around the globe with the same sampling method, to find out whether this “hot spot” designation holds universally. We studied species assemblages within the matrices of byssally interconnected mussels engineered by Mytilus edulis in the North Sea, by mixed Perumytilus purpuratus and Mytilus chilensis at the southern Chilean coast, by Musculista senhousia in the Yellow Sea and by Xenostrobus inconstans at the coast of southern Australia. In all cases, species assemblages inside mussel beds were significantly different from those outside with many species being restricted to one habitat type. However, species richness and diversity were not generally higher in mussel beds than in ambient sediments without mussels. In the North Sea (M. edulis) and at the Chilean coast (P. purpuratus, M. chilensis), mussel beds have markedly higher species numbers and diversities than surrounding sediments, but this was not the case for mussel beds in Australia (X. inconstans) and the Yellow Sea (M. senhousia) where numbers of associated species were only slightly higher and somewhat lower than in adjacent sediments, respectively. In conclusion, although soft bottom mytilid mussels generally enhance habitat heterogeneity and species diversity at the ecosystem level, mussel beds themselves are not universal centres of biodiversity, but the effects on associated species are site specific.     

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.     

Non-trophic Interactions Control Benthic Producers on Intertidal Flats

The importance of positive effects of ecosystem engineers on associated communities is predicted to increase with environmental stress. However, incorporating such non-trophic interactions into ecological theory is not trivial because facilitation of associated species is conditional on both the type of engineer and the type of abiotic stress. We tested the influence of two allogenic ecosystem engineers (lugworms, Arenicola marina L. and cockles, Cerastoderma edule L.) on the main primary producers (microphytobenthos) of the tidal flats, under different abiotic stresses controlled by reefs of blue mussels (Mytilus edulis L.). We added 25,000 cockles or 2,000 lugworms to 5 × 5 m plots, both in a muddy site with high sedimentation rates located coastward of a mussel bed, and in a sandy site without mussels and characterized by high hydrodynamic stress. After a year, cockles increased algal biomass in the sandy area, but not in the mussel bed site, where high values were measured in all plots. However, lugworms did not affect algal biomass in any of the sites. Field measurements suggest that cockles outweighed negative effects of water currents in the site without mussels by locally increasing sediment stability, whereas mussels overruled the effects of cockles in the wake of the reefs through hydrodynamic stress alleviation and/or biodeposition. Our results suggest that non-trophic interactions by ecosystem engineering bivalves control primary production of intertidal areas, and that the sediment-stabilizing effect of cockles plays a crucial role where the overruling effects of mussel beds are not present.     

Estuarine habitats protect hybrid mussels from selection

The marine mussels, Mytilus edulis and Mytilus galloprovincialis, form an extensive hybrid zone in Europe where F₂ hybrids and mussels of mixed genetic ancestry are often locally abundant. Hybrid zones are maintained by the interplay of dispersal and selection on hybrid genotypes but there has been vigorous debate on the form of selection that may occur in these systems. Tension zone models argue that selection is against hybrids because of developmental misregulation and is independent of the external environment. Exogenous selection models argue that selection is habitat-dependent and the structure of the hybrid zone is due to the distribution of habitat patches that vary in selection intensity. We test this prediction by comparing the genetic structure of mussel populations in open-coast habitats, where selection on hybrids is strong, to those within two independent estuaries. We show that mussels within these estuaries are protected from selection and thus selection is strongly dependent on habitat, which supports the exogenous selection hypothesis. Hybrid mussel populations on the open-coast experience strong selection against M. edulis-like genotypes, which has been postulated to be the result of differential dislodgment by waves. This hypothesis is supported by our results since mussels within sheltered habitats are protected from selection. There was, despite previous suggestions, no evidence of selection in favor of M. edulis-like mussels within either estuary.     

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.     

Impact assessment of an invasive flatworm, Convoluta convoluta, in the Southern Gulf of Maine

Convoluta convoluta (Abildgaard 1806) is a small (2-3 mm long) acoellous turbellarian flatworm from Europe that has invaded the Gulf of Maine within the last 5 years. Although it has been reported in densities of up to 19 individuals/cm², its ecological impact remains unknown. In its native habitat, it consumes harpacticoid copepods and primary settling mussels <0.5-mm shell length. This study estimated the impact of C. convoluta on juvenile blue mussel populations (Mytilus edulis Linnaeus 1758) around the Isles of Shoals in the southern Gulf of Maine, USA as well as looking at their distribution in their new habitat. We surveyed worm densities at sites of differing wave exposure over three substrates (hard substrates, bladed algae, filamentous algae) to quantify patterns of worm abundance. We found worms on all substrate types with their highest abundances occurring in areas of maximal sunlight exposure and minimal physical disturbance. We showed a definite pattern of consumption of mussels in the lab and found C. convoluta to consume up to 35% of primary settling mussels in the field, but only under certain conditions. Per capita impact on juvenile mussels was found to vary greatly in correlation with mussel recruitment rate and water temperature, but not with the consumption of harpacticoids. Our results also suggested that per capita interaction strength was reduced by intraspecific density-dependent competition and water temperature. The overall impact of C. convoluta on mussel populations in the southern Gulf of Maine is therefore estimated to be minimal.     

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.     

Hierarchical spatial structure in soft-bottom mussel beds

Mussels (Mytilus edulis L.) are unusual because they thrive in both rocky shore and soft-bottom habitats. Despite their ecological and economic importance, little is known about their spatial structure. Mussels do not generally recruit to bare soft substrate because larvae and postlarvae cannot attach to a bottom of small sediment particles. They attach to hard objects on the sediment surface (especially other mussels), so soft-bottom mussel beds may be spatially organized in ways that are fundamentally different from those on rocky shores. The purpose of our study was to characterize the scales of spatial variability for several mussel abundance parameters in soft-bottom, intertidal M. edulis beds in coastal Maine. We used a random factor nested-ANOVA design of 200 cm² Cores within 1 m² Quadrats within 6 m Transects within Positions within bed Sites along 70 km (euclidean distance) of the Maine coast. Based on the literature and our field observations, we hypothesized that Sites and Positions account for most of the spatial variance in soft-botttom mussel beds. We rejected this hypothesis. Sites and Positions were not important in explaining variation in total mussel density, density of new recruits, or density of larger mussels. Although most of the variance in surface silt-clay fraction did occur at these levels, most mussel variation occurred at smaller spatial scales, specifically at the Quadrat scale for new recruits and total mussels and at the Transect scale for larger mussels. Variance in mussel parameters was not closely linked to the silt-clay fraction of surface sediment or to Site rankings of wind exposure and tidal flow. Variance in total mussel density was due primarily to variance in recruitment. No single scale explained more than about half the mussel variance, and no single scale was best at explaining all the mussel parameters. Greater knowledge about mussel bed spatial variability would be useful because it can help direct scale-dependent sampling regimes, field experiments, and coastal management practices.     

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.     

The influence of predation by herring gullsLarus argentatus and oystercatchersHaematopus ostralegus on a newly established musselMytilus edulis bed in autumn and winter

Predation by herring gullsLarus argentatus and oystercatchersHaematopus ostralegus was evaluated on a newly established musselMytilus edulis bed on tidal flats of the German Wadden Sea. The mussel bed covered an area of 2 ha and showed a decrease in biomass of 40% in the most densely covered parts from August to January. Synchronously, the extent of the mussel bed was reduced, resulting in a decrease of average biomass of 98% over the whole mussel bed. From the beginning of August 1994 to mid January 1995, the average size of mussels increased from 10.7 to 20.3 mm. The P/B-ratio was 0.68 in August and 0.18 between September and November. Herring gulls and oystercatchers were the most important mussel predators. On average, 266 herring gulls and 63 oystercatchers were present on the mussel bed during one low tide; 34% of the herring gulls and 78% of the oystercatchers were observed to be feeding. Herring gulls fed at a rate of 4.2 mussels per minute and oystercatchers at a rate of 1.3 mussels per minute. While herring gulls took the most common mussel sizes (mean: 20 mm), oystercatchers searched for the largest mussels available (mean: 25 mm). Herring gulls consumed 13 mussels/m² (0.3g AFDW) during one day and oystercatchers 1.7 mussels/m² (0.1 g AFDW). Predation by birds was compensated by 33% of the production. The proportion removed by bird predation amounted to 10% of abundance and to 16% of biomass (including production). Oystercatchers were responsible for 1% of the reduction in abundance and for 3% of biomass. Removal was highest in the most common size classes of mussels, mainly caused by herring gulls. However, the highest proportion of mussels was eaten in the largest size classes, mainly by oystercatchers. *** DIRECT SUPPORT *** A03B6035 00004     

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.     

Effect of the excretory-secretory products of some marine invertebrates on byssus production of the blue mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> (Bivalvia: Mytilidae)

Under laboratory conditions, we investigated byssus production in the blue mussel Mytilus edulis, as affected by the excretory-secretory products (ESPs) of the mussel itself and some marine invertebrates: the predatory starfish Asterias rubens, and organisms competing with mussels in White Sea fouling communities—a bivalve Hiatella arctica, the solitary ascidians Styela rustica and Molgula citrine, and a sponge Halichondria panicea. The number of attachment disks produced by a mussel per day and the thickness of byssal threads were estimated. Excretory-secretory products of H. arctica and M. citrine had no effect on the number of attachment disks, while ESPs of S. rustica, H. panacea and A. rubens stimulated mussels to produce attachment plaques. The activity of the mussel was slightly increased at low levels of its own ESPs in seawater. The thickness of byssal threads decreased with an increase in the ESPs of mussels in seawater, but it increased in experiments with the ESPs of any other species tested.     

Increasing seaweed crop yields through organic fertilisation at the nursery stage

Extractive seaweed aquaculture is gaining attention in the western Baltic Sea and in particular the co-cultivation with other species for bioremediation or nutrient delivery. However, there are still limitations to viable seaweed production yields in a brackish habitat with a short production period for Saccharina latissima. This investigation presents the specific growth-enhancing effect of Mytilus edulis on the seaweed early nursery stages during the hatchery and during the grow out period at sea in a Baltic fjord. Gametogenesis and juvenile sporophyte development were evaluated with and without blue mussels during 9 weeks of seaweed hatchery. The presence of mussels resulted in a significantly higher abundance of large multicellular sporophytes. After the hatchery period, seedling lines were transferred into the field and installed both in the direct vicinity of and 25 m away from mussel culture ropes. The previously observed supporting effect of mussel co-culture on seaweed development during the hatchery period was still visible after 6 months at sea. Sporophytes were larger, had a higher biomass and had higher carbon content if previously combined with mussels in the hatchery. This investigation suggests that the co-cultivation of seaweed and mussels during seaweed hatchery can increase seaweed crop yields in the following grow out period at sea, with the possibility of being certified organic.     

Spatial patterns of abundance and growth of Mytilus edulis on boulders in the Northern Baltic Sea proper

Small scale spatial patterns of abundance, growth and condition of the mussel Mytilus edulis on sub-littoral boulders (approx. 1–2 m high) were investigated at the island of Askö in the northern Baltic proper. The effect of side (exposed/sheltered with respect to wave action and sunlight) of boulder and position (up/down) on boulder was investigated. A large spatial variability in abundance of M. edulis between boulders and between various sites within boulders were found. The highest numbers of mussels were found on the wave exposed side, near the top of boulders. Shell growth was favoured by a sheltered side and a down position. The body condition (meat weight/shell weight) of mussels was, on the other hand, affected only by position, the condition of mussels being better at the up position. Consequently, there seems to be temporal differences in the condition for growth within a spatial position. The body condition of the mussels was best near the top of boulders in the spring, but long-term shell growth was favoured by a sheltered side and a down position. This may reflect changes in the composition and availability of food during the year with phytoplankton as the major food source during the spring bloom and resuspension of benthic production and detritus as relatively more important during the rest of the year.     

Hard rock versus soft bottom: the fauna associated with intertidal mussel beds on hard bottoms along the coast of Chile,and considerations on the functional role of mussel beds

The fauna associated with hard bottom mussel beds along the exposed Pacific coast of Chile was examined. The abundance of adult (>10 mm body length) purple mussels Perumytilus purpuratus varied between 32 and 75 individuals per 50 cm², and their biomass between 4.8 and 8.6 g AFDW per 50 cm² at eight sampling sites between Arica (18°S) and Chiloé (42°S). At all sampling sites, the associated fauna was dominated by suspension-feeding organisms (cirripeds, spionid and sabellid polychaetes, a small bivalve) followed by grazing peracarids and gastropods. Predators and scavengers also reached high abundances while deposit- and detritus-feeding organisms were of minor importance. The majority of organisms associated with these hard bottom mussel beds feed on resources obtained from the water column or growing on the mussels rather than on materials deposited by the mussels. This is in contrast to the fauna associated with mussel beds on soft bottoms, which comprises many species feeding on material accumulated by mussels (faeces and pseudofaeces) and deposited within the mussel bed. Many of the organisms dwelling between mussels both on hard bottoms and on soft bottoms have direct development, but organisms with pelagic development also occur abundantly within mussel beds. We propose that species with direct development are disproportionately favoured by the structurally complex habitat with diverse interstitial spaces between the mussels, which provides ample shelter for small organisms. We conclude that mussels on hard-bottoms primarily provide substratum for associated fauna while mussels on soft bottoms provide both substratum and food resources. Electronic Publication     

Development of a real-time PCR assay for detection of Mytilus species specific alleles: Application to a sampling survey in Scotland

Shellfish aquaculture is a growing industry in Scotland, dominated by the production of the mussel Mytilus edulis, the native species. Recently the discovery of Mytilus galloprovincialis and Mytilus trossulus together with M. edulis and all 3 hybrids in cultivation in some Scottish sea lochs led to questions regarding the distribution of mussel species in Scotland. The establishment of an extensive sampling survey, involving the collection of mussels at 34 intertidal sites and 10 marinas around Scotland, motivated the development of a high-throughput method for identification of Mytilus alleles from samples. Three Taqman®-MGB probes and one set of primers were designed, based on the previously described Me 15/16 primers targeting the adhesive protein gene sequence, and samples were screened for the presence of M. edulis, M. galloprovincialis and M. trossulus alleles using real-time PCR. Mytilus edulis alleles were identified in samples from all 44 sites. Mytilus galloprovincialis alleles were found together with M. edulis alleles extensively in northern parts of the west and east coasts. Mytilus trossulus alleles were identified in samples from 6 sites in the west and south-west of Scotland. Because M. trossulus is generally undesirable in cultivation and therefore preventing the geographical spread of this species across Scotland is considered beneficial by the shellfish aquaculture industry, these 6 samples were further analysed for genotype frequencies using conventional PCR. Although distribution of the non-native species M. galloprovincialis and M. trossulus have proven to be more widespread than previously thought, there is no evidence from our study of either M. trossulus or M. galloprovincialis acting as an invasive species in Scotland. The real-time PCR method developed in this study has proven to be a rapid and effective tool for the identification of M. edulis, M. galloprovincialis and M. trossulus alleles from samples and should prove useful in future surveys, ecological or aquaculture management related studies in both unispecific and mixed species areas of these species.     

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.