Mussel Clumps Research Articles

Storm resilience of subtidal soft‐bottom mussel beds: Mechanistic insights, threshold quantification and management implications

AbstractWith the projected escalation of extreme storm events, coastal ecosystems risk undergoing catastrophic shifts and losing essential ecosystem services. Subtidal soft‐bottom mussel beds, vital components of these ecosystems, are particularly vulnerable to hydrodynamically‐induced dislodgement (i.e., detachment of mussel clumps from the bed), especially during storms. However, the mechanisms underlying the resilience—comprising both resistance and recovery—of these beds to storms remain unclear, despite being essential for informed management.This study addresses this knowledge gap regarding subtidal soft‐bottom mussel beds by: (i) quantifying their dislodgement threshold (i.e., the hydrodynamics causing widespread dislodgement of mussel clumps) using novel in situ monitoring methodologies in a representative region, namely the Dutch Wadden Sea; (ii) unveiling the influence of prior life history (here, wave exposure extent) and storm durations on their dislodgement thresholds through a flume study; and (iii) assessing the impacts of repeated storms and prior life histories (here, wave exposure extent and substrate types) on their recovery (i.e. mussel re‐aggregation) through mesocosm experiments.Integrated experimental evidence indicates that: (i) hydrodynamic‐induced dislodgement is a sudden process characterized by distinct near‐bed orbital velocity thresholds, which were identified at our study site to be between 0.45 and 0.50 m s−1; (ii) peak storm intensity, rather than storm duration, primarily drives the dislodgement of subtidal soft‐bottom mussel beds, and prior wave exposure extent regulates the dislodgement threshold; (iii) repeated storms do not seem to affect the recovery of these beds following storm‐related disturbances when the conditions between storms are conducive to mussel re‐aggregation, whereas substrate type significantly impacts recovery.Synthesis and applications. Overall, concerns regarding subtidal soft‐bottom mussel beds degradation primarily stem from increasing storm intensity and their limited resistance to such events. The methodology we developed enables low‐cost quantification of mussel resistance thresholds across broad spatiotemporal scales, facilitating the pinpointing of vulnerable areas. Our findings inform strategic management by highlighting the influential role of prior life histories in shaping mussel bed resistance and the potential to accelerate mussel bed recovery through substrate modification (e.g., shell additions). Both our methodology and findings hold promise for application in comparable ecosystems, such as oyster and coral reefs.

The restoration potential of offshore mussel farming on degraded seabed habitat

AbstractThe United Kingdom's first large‐scale, offshore, long‐line mussel farm deployed its first ropes in 2013 in Lyme Bay, southwest United Kingdom, located in an area of seabed that was heavily degraded due to historic bottom‐towed fishing. It was hypothesised that due to the artificial structures that accumulate mussels and exclude destructive fishing practices, the seabed could be restored. To assess the restoration potential of the farm and its ecosystem interactions over time, a multi‐method, annual monitoring approach was undertaken. Here, we tested the effects of the farm trial stations on the seabed habitat, epifauna and demersal species over 5 years. Responses of % mussel cover, sessile and sedentary, and mobile taxa were measured using three video methods. Within 2 years of infrastructure deployment, mussel clumps and shells were detected below the headlines, increasing the structural complexity of the seabed. After 4 years, there was a significantly greater abundance of mobile taxa compared to the Controls that remained open to trawling. Commercial European lobster and edible crab were almost exclusively recorded within the farm. We discuss whether these findings can be considered a restoration of the seabed and how these data can be used to inform the future management of offshore mariculture globally.

Effects of habitat homogenisation on assemblages associated with mussel clumps.

Biodiversity loss is considered one of the main threats to marine ecosystems. In this framework of biodiversity decline, organisms that provide biogenic habitat play a relevant role by their capacity to structure assemblages and influence ecological processes. The Mediterranean mussel Mytilus galloprovincialis is considered an ecosystem engineer because it alters local environmental conditions maintaining habitat suitability for other organisms, and enhancing local biodiversity. Although it is widely recognized that mussel beds increase diversity, the drivers shaping these assemblages are poorly explored. We investigate whether mussel size homogenisation shapes the abundance, richness and structure of macrobenthic assemblages associated with mussel beds in two shores of the Galician coast (NW Spain). At each shore, two sites, 10 m apart, were selected and at each site, faunal assemblages were compared between mussel clumps showing shells of various sizes (control), and mussel clumps with closely similar-sized mussels, considered as homogenised. Homogenised clumps showed, in general, higher values in total number of individuals and species than control clumps. Regarding the effect of mussel size homogenisation on the multivariate structure of the assemblages, significant differences between control and homogenised clumps were found in three out of the four sites. Most relevant associated species usually reached higher abundances in homogenised clumps than control ones. Therefore, mussel size homogenisation influenced the structure of the macrofaunal assemblages associated with mussel beds but, its effect was context dependent (i.e., varied with sites). Information about the species contribution to dissimilarities among homogenised and control clumps was provided and the potential influence of sediment and algae on mussel clumps was discussed.

The importance of stock selection for improving transplantation efficiency

The restoration of shellfish reefs to soft‐sediment environments often relies on the translocation of donor stock, typically from aquaculture, to the seafloor. In New Zealand, subadult green‐lipped mussels (Perna canaliculus) grown on subtidal aquaculture long‐lines are being considered over adult conspecifics to increase restoration efficiency but are currently limited by predation and hydrodynamic dislodgment following transfer to the seabed. In this study, the survival of subadult mussels from five different sources representing a range of growth conditions (i.e. subtidal aquaculture long‐lines 1 and 2, subtidal shellfish aquaculture baskets 1, wild intertidal reefs 1 and 2) were compared across separate translocations (1 and 2) to see if careful consideration of stock source could improve subadult mussel survival following transfer to the seafloor. Mussel morphology (shell strength, attachment thread structure) and clumping behavior (perimeter:area ratios, clump densities, clump complexity) were compared among populations to explain relationships among prior stock growth conditions, mussel size, and survival. Following experimental translocations, high survival (>90%) was closely related to decreases in perimeter:area ratios of mussel clumps and increases in clump complexity. High survival groups of mussels were selected from stock with high shell compression strength (subtidal aquaculture long‐line 2, 95.5% survival), greater attachment thread number and thickness (aquaculture basket 1, 92.3% survival), or some combination of the two (wild intertidal reef 2, 99.3% survival). This study supports further consideration for incorporating subadult mussels into restoration provided they are selected from sources that produce individuals with the resistant characteristics outlined in this study.

Spatial-temporal variability of Mytilus galloprovincialis Lamarck 1819 populations and their accumulated sediment in northern Portugal.

Mytilus galloprovincialis is an ecosystem engineer that provides habitat and generates environmental heterogeneity, increasing local biodiversity. Moreover, it is an economically important species representing 14% of the global production of marine bivalves. Natural drivers and the increase of anthropogenic pressures, such as sediment stress, influence its populations on rocky shores. The objective of this study was to explore the spatial-temporal patterns of different M. galloprovincialis attributes along the north of Portugal. For that purpose, six rocky shores were selected and sampled six times along the year 2019. The percentage of cover, density, spat density, condition index, clump thickness, size classes and clump sediment content were considered. Results showed the lack of seasonality in M. galloprovincialis along the north coast of Portugal. However, density, spat stage, clump thickness, condition index and size classes showed some variability among dates and shores. The percentage cover and sediment content only significantly differed among shores. Our results indicated an absence of seasonality for all the studied variables, probably because temperature was always within the optimum range for this species and the abundance of food supply in the study area independently of the season. The accumulated sediment on mussel clumps did not show any temporal variability with only significant differences among shores. The accumulated sediment was composed mainly by medium and coarse sand and it was correlated with mussel average size, condition index, but especially with the mussel clump thickness.

Community disruption in small biogenic habitats: A coastal invader overcomes habitat complexity to alter community structure.

Non-indigenous species are often identified as threats to native species and communities. Yet, the mechanisms that enable many of these invaders to thrive and alter their newly invaded habitats are still not fully understood. This applies to habitats such as widespread sedimentary shorelines characterized by the presence of scattered biogenic clumps of blue mussels (Mytilus edulis) structurally more complex than bare sediments. In Atlantic Canada, some of these shorelines are numerically dominated by native mud crabs (Dyspanopeus sayi) but have been gradually invaded by the European green crab (Carcinus maenas). This study describes between-habitat (mussel clump vs. bare sediment) differences in density and diversity of invertebrates. It also tests the impact of juvenile green crabs in comparison to native mud crabs using two approaches: First, measuring habitat-related differences in these crabs’ feeding rates on a common prey (soft-shell clams, Mya arenaria). Second, measuring their influence on invertebrate communities associated with mussel clumps. The results show that mussel clumps hold higher invertebrate density and diversity than surrounding sedimentary bottoms. In the laboratory, the feeding rates of native mud crabs were dependent on the type of habitat (sand flat > mussel clump), whereas those of green crabs were significantly higher and unrelated to the habitat in which predation occurred. In field experiments, juvenile green crabs were also the only predators that changed community structure in the mussel clump habitat. These results indicate that green crabs can cause a significant impact on native species and communities. Moreover, they suggest that the ability of this species to overcome the refuge provided by complex biogenic habitats for prey may represent an unexplored mechanism to explain this invader’s expansion here and elsewhere.

Colonisation of South African kelp-bed canopies by the alien mussel Mytilus galloprovincialis: extent and implications of a novel bioinvasion

The Mediterranean mussel Mytilus galloprovincialis is the most significant invasive alien marine species in South Africa and, although not normally found subtidally, has recently been observed colonising heads and stipes of the kelp species Ecklonia maxima in False Bay. We quantified this invasion and explored its ecological implications. Transects laid across kelp beds revealed that 10.34% of kelp individuals surveyed bore canopy mussels, with kelp heads (10.07%) being far more commonly infected than stipes (0.27%). Twenty kelp individuals with infected heads and 20 with infected stipes were separately collected for more-detailed examination. Wet mass of mussels on these kelp heads ranged from 2.5 to 2 462 g (median 86.4 g, interquartile range 14.8–353.8 g) and that of mussels on the stipes from 7.6 to 3 492 g (median 595.5 g, interquartile range 194.0–955.0 g). Mussel clumps consisted mostly of individuals <40 mm in length. Mussel clumps supported a rich biota of 80 invertebrate and 13 algal species. Larger clumps supported more epibiotic species, and those on stipes more species than those of comparable mass on kelp heads. The mussels and their associated epibiotic species negatively affected kelp buoyancy, but rarely enough to overcome natural buoyancy. Some kelp individuals that had been toppled by the weight of mussels and their epibiotic species, however, were encountered in situ. Implications of this invasion include large increases in animal biomass and species richness in the kelp canopy, plus reductions in kelp buoyancy and increased hydrodynamic drag on infected kelps, increasing their probability of being uprooted. Uprooted kelp individuals can raft long distances, potentially transporting both native and alien species to distant sites.

Patterning in Mussel Beds Explained by the Interplay of Multi-Level Selection and Spatial Self-Organization

Cooperation, ubiquitous in nature, is difficult to explain from an evolutionary perspective. Many modelling studies strive to resolve this challenge, but their simplifying assumptions on population and interaction structure are rarely met in ecological settings. Here we use a modelling approach that includes more ecological detail to investigate evolution of cooperation in spatially self-organized mussel beds. Mussels cooperate with each other through aggregative movement and attachment using byssal threads. These cooperative behaviours shape the spatial structure of the mussel bed, which can range from scattered distributions to labyrinth-like patterns and dense mussel clumps. The spatial pattern in turn impacts an individual’s fitness at two levels: (i) proper attachment to neighbouring individuals decreases predation risk, and (ii) attachment to a sufficiently large group prevents dislodgement by wave stress. Without this second level of selection, our simulations do typically not result in evolutionary attractors that lead to the labyrinth-like spatial patterns that are characteristic for natural mussel beds. Yet, when group-level selection is included, labyrinth-like patterns emerge under a wide range of conditions. Our model demonstrates that multiple selection factors working at different spatial scales – predation of individuals and dislodgement of entire mussel clumps – combinedly determine evolution of cooperative traits in mussels and thereby result in emergence of the labyrinth-like spatial patterns that we observe in natural mussel beds.

The role of the non-indigenous green crab (Carcinus maenas) in the decline of a unique strain of Irish moss (Chondrus crispus): direct and indirect effects

AbstractA unique strain of the red alga Irish moss (Chondrus crispus) is found solely amongst clumps of blue mussels (Mytilus edulis) in a coastal lagoon in Atlantic Canada. Since about 2000, its bed area has shrunk by &amp;gt;99.9%, coinciding with the arrival of the non-indigenous green crab (Carcinus maenas). This study tested two mechanisms by which green crabs may harm the Irish moss. The hypothesis that green crabs directly consume the alga was tested by exposing fronds and clumps to crabs. Crab interaction with the clumps caused limited fragmentation, consumption was very small, and the condition (visible grazing damage) of fronds did not change significantly. A second hypothesis, that during predation and handling of mussels green crabs indirectly displace the seaweed and remove its attachment substrate, was addressed by placing crabs with Irish moss-mussel clumps containing either large or small mussels. Green crabs removed and ate up to 100% of the small mussels but did not consume or displace large mussels. This study concludes that direct consumption is not a plausible mechanism for green crabs to harm this strain. Instead, green crabs harm could be mediated by mussels, whose removal deprives the giant Irish moss of positional stability.

Bivalves boost biodiversity

Bivalves boost biodiversity

Effects of harvesting on annelid assemblages associated with mussel beds

Event Abstract Back to Event Effects of harvesting on annelid assemblages associated with mussel beds Puri Veiga1*, Leandro Sampaio1, Catarina R. Oliveira1, Juan Moreira Da Rocha2, Jesus S. Troncoso3 and Marcos Rubal1 1 Interdisciplinary Center for Marine and Environmental Research, Abel Salazar Institute of Biomedical Sciences, University of Porto, Portugal 2 Autonomous University of Madrid, Spain 3 University of Vigo, Spain The mussel, Mytilus galloprovincialis Lamarck, 1819 is an ecologically and economically relevant species on European coasts. It is very abundant in the Atlantic intertidal fringe of the Iberian Peninsula where is harvested in natural systems and/or cultured intensively on rafts. Aquaculture production is dependent on their abundance in natural environments because juveniles are collected in rocky shores and then attached to ropes on rafts. Moreover, it provides habitat for many species, enhancing local biodiversity. The aim of this study was to explore effects of harvesting on the annelid assemblage associated with mussels. To achieve this, two rocky shores, where mussels are intensively harvested, were selected in Galicia (NW Iberian Peninsula). At each shore, two different sites were considered; at each site, 4 replicates (10x10cm) of harvested mussel clumps and 4 replicates of non-harvested clumps (control) were collected. Size and number of mussels at each replicate were compared between treatments (harvested-control). Moreover, annelids at each replicate were identified to species level. A total of 1351 individuals and 27 species were found. Abundance, taxon richness, Margalef index and assemblage composition were compared between treatments by means of univariate and multivariate techniques. Results pointed out that harvested clumps showed a lower and more homogenous mussel size than control clumps, as expected. Regarding annelids, the interaction between treatment and site was significant for the multivariate assemblage structure and for the total abundance. Therefore, the effect of harvesting is dependent on the spatial scale considered. Acknowledgements This research was developed under the Project No. 30181, co-financed by COMPETE 2020, Portugal 2020 and the European Union through the ERDF, and by FCT through national funds. Keywords: Mytilus galloprovincialis L., Harvesting, Annelids, Iberian Peninsula, Rocky shores Conference: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019. Presentation Type: Poster Presentation Topic: Ecology, Biodiversity and Vulnerable Ecosystems Citation: Veiga P, Sampaio L, Oliveira CR, Moreira Da Rocha J, Troncoso JS and Rubal M (2019). Effects of harvesting on annelid assemblages associated with mussel beds. Front. Mar. Sci. Conference Abstract: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) . doi: 10.3389/conf.fmars.2019.08.00078 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 13 May 2019; Published Online: 27 Sep 2019. * Correspondence: Mx. Puri Veiga, Interdisciplinary Center for Marine and Environmental Research, Abel Salazar Institute of Biomedical Sciences, University of Porto, Matosinhos, 4450-208, Portugal, puri.sanchez@fc.up.pt Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Puri Veiga Leandro Sampaio Catarina R Oliveira Juan Moreira Da Rocha Jesus S Troncoso Marcos Rubal Google Puri Veiga Leandro Sampaio Catarina R Oliveira Juan Moreira Da Rocha Jesus S Troncoso Marcos Rubal Google Scholar Puri Veiga Leandro Sampaio Catarina R Oliveira Juan Moreira Da Rocha Jesus S Troncoso Marcos Rubal PubMed Puri Veiga Leandro Sampaio Catarina R Oliveira Juan Moreira Da Rocha Jesus S Troncoso Marcos Rubal Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Biotic consequences of a shift in invertebrate ecosystem engineers: Invasion of New Zealand rocky shores by a zone-forming ascidian

The pervasive effects of invasive ecosystem engineers, that is those species that modify their environment, are well documented, but rarely have the broader impacts of one foundation invertebrate species being replaced by another been examined. In New Zealand, green‐lipped mussels, Perna canaliculus, commonly dominate wave‐exposed rocky shores. The recent appearance of an invasive ecosystem engineer, the ascidian Pyura doppelgangera, at the very northern tip of New Zealand now threatens to exclude these bivalves from this habitat. Here, we report major shifts in assemblages associated with the invader and chronicle its continued spread. We examined epibiota associated with clumps of mussels and clumps of Pyura from two rocky shore habitats—pools and emergent substrata at two locations. We detected some differences in species richness in biota associated with the two foundation species, but faunal abundance only differed between the locations. These minor changes were dwarfed by the shift in species composition within clumps of each foundation species. Molluscs, particularly gastropods, and crustaceans dominated the assemblage within mussels. In contrast, tubicolous polychaetes dominated the fauna associated with the ascidian. Sessile epifauna, notably barnacles and calcareous tube‐dwelling polychaetes, were common on mussels, but never encountered on the ascidian. Multivariate analysis revealed marked dissimilarity (>80%) between the characteristic mussel and ascidian faunas with virtually no overlap. This biotic shift overshadowed any differences between habitats and locations. The broader implications of these faunal shifts for local and regional patterns of biodiversity, as well as ecosystem function, remain unclear, but deserve further attention.

Predation on competing mussel species: Patterns of prey consumption and its potential role in species coexistence

Theoretical and empirical studies have demonstrated that predators can modulate coexistence between prey species and can modify broad patterns of community structure. Although the keystone predator effect (i.e., local coexistence and species diversity mediated by predator consumption) is commonly reported, negative effects of predators on species coexistence are also common. The balance depends critically on the asymmetry of competition, predator preferences for different prey, and the effective mortality rates exerted by predator populations on prey per capita growth rates. Here we studied the role of predation on two competing mussels, the dominant competitor Perumytilus purpuratus that dominates in abundance at most shores, and the subordinate Semimytilus algosus, which is common, but persistently abundant only at a smaller set of shores. We examined the potential for keystone effects by quantifying and evaluating the patterns of mussel consumption rates of sea star (Heliaster helianthus) and crab (Acanthocyclus gayi) predators in laboratory experiments, and under natural field conditions. Field sites with contrasting mussel abundance and recruitment (i.e., different local prey population growth rates) were chosen. Laboratory experiments showed that crabs would first consume Semimytilus over Perumytilus in mixed mussel clumps, while the larger sea star predators made no distinction between mussel species. Regardless of the order in which mussel species were first ingested, consumption rates of the two mussel species through the experiment were not different for either predator species. A similar temporal pattern was partly observable in the field through specific differences in the order of mussel consumption; however, experimental mussels of both species where consumed after the first two months of exposure to predators at both sites. Thus, in this system predation could not foster coexistence between mussel species and it may instead favor the occurrence of local extinction of the subordinate competitor, especially at sites where the smaller and more selective crab predators are relatively more abundant and recruitment of the subordinate species is low. Existing patterns of locally abundant Semimytilus beds are probably related to high site-specific recruitment rates of this species and should be further explored.

Ecological interactions modulate responses of two intertidal mussel species to changes in temperature and pH

Predicted climate change scenarios for the end of this century suggest a substantial warming and ocean acidification of sea surface waters. Physiological performance of marine coastal organisms will be affected leading to changes in whole communities. In particular, ocean acidification poses an important threat for shelled molluscs. Nevertheless, climate change impacts are species-specific and responses may even differ among species that play a similar role in the ecosystems. Currently, two mussel species, the native Mytilus galloprovincialis Lamarck 1819 and the non-indigenous Xenostrobus securis Lamarck 1819, share the same habitat along shores in the inner part of the Galician Rias Baixas (NW Spain) forming mixed patchy clumps. Here, we used an experimental approach in mesocosms to evaluate the effects of increasing temperature and acidification on mussel clumps of different composition (monospecific and mixed clumps). Our model systems were artificially constructed clumps resembling those found on rocky shores. Responses to environmental stressors were measured on several functioning variables at the individual (i.e. survivorship, growth, condition index and composition of shell) and clump level (i.e. respiration, ammonia and phosphate excretion rates). The decrease of pH had a lethal effect and reduced growth on the native mussels. The invader was more resilient although individual responses were very often shaped by the composition of the clump. The reduced pH and, especially the high temperature conditions, tended to cause an increase in respiration rates of all types of clumps. Interactions between the two species seemed to modulate many effects at both individual and clump levels highlighting the complexity of responses when considering multiple stressors.

Community convergence and recruitment of keystone species as performance indicators of artificial reefs

An experimental artificial reef was constructed in Strangford Lough, Northern Ireland as part of trials to regenerate damaged biogenic reefs formed by the horse mussel Modiolus modiolus. Experimental reef plots were constructed using Pecten maximus shell as cultch. Clumps of live adult M. modiolus were translocated from nearby natural reefs into cultch with a high profile (elevated cultch), cultch with a low profile (flattened cultch), as well as directly into the seafloor. The aim of the study was to test the hypothesis that translocated mussel clumps would increase habitat complexity thus accelerating community succession and enhancing natural recruitment of M. modiolus spat. These effects were predicted to be greater on elevated cultch due to greater protection from predators and increased accessibility to food resources. Within the artificial reef array the translocated clumps had a significant positive effect on recruitment compared to cultch without mussels with average densities of spat settled on the translocated M. modiolus clumps ranging from 100 to 200individualsm−2 compared to 4 to 52spatm−2 on cultch without mussels. Recruitment of M. modiolus spat was also significantly higher on translocated horse mussels when compared to natural reefs where densities of 8–36spatm−2 were recorded. Reef elevation appeared to provide some degree of protection from predators but differences in translocated M. modiolus survival on the different elevation treatments were not significant. In total, 223 taxa were recorded 12months after reef construction. The presence of translocated clumps of M. modiolus was the main driver of the increases in faunal diversity and species abundance. Application of objective criteria to assess the performance of artificial reefs suggested that translocation of M. modiolus clumps alone achieved most of the restoration objectives. Consequently this pilot study demonstrates a straightforward and realistic intervention technique that could be used to kick start the regeneration and expansion of impacted mussel and similar biogenic reefs elsewhere.

Density effects on the clearance rate of the zebra mussel Dreissena polymorpha: flume study results

Zebra mussel filtration rates and regulating factors have been addressed earlier in a number of studies. Still, only a few of them have taken into consideration the refiltration phenomenon, and therefore the direct extrapolation of experimental results may only give the potential filtering capacity, and hence, over- or underestimate the actual amount of seston being removed by zebra mussels in an ecosystem. The current experimental study aimed to gain insight into the refiltration effect on the clearance rate of the zebra mussels at relatively high seston concentrations, and its potential role in controlling the filtration efficiency of the zebra mussel population. The experiment was conducted in a laboratory flume following the Latin squares design with one fixed (mussel density) and three random factors (initial total particulate matter (TPM) concentration, flume “wall effect” and distance from the flume inflow area) considered. The results showed the significant effects of mussel density and the TPM concentration on the effective clearance rate (ECR) of zebra mussels. The higher ECR values were obtained at denser mussel clumps and lower TPM concentrations. The flume “wall effect” had no significant effect on the ECR, whereas the distance from the flume inflow area appeared to have a significant impact. A positive relationship between ECR and the zebra mussel density was most evident in the proximity of the TPM source. Based on the results, we assume that at high TPM concentration, refiltration may assert itself by the elevated net clearance rate of mussels within dense clumps compared to that of mussels at relatively low individual densities. This should be taken into consideration while modelling and assessing the role of the zebra mussel in energy flow and redistribution of organic matter in an ecosystem.

Extraordinarily low mortality rates reported in juvenile Pacific oysters in the German Wadden Sea

In a recent publication in Helgoland Marine Research, Schmidt et al. (2008) report the rapid spread of the Pacific oyster (Crassostrea gigas) into the East Frisian Wadden Sea. They conclude that the observed rapid increase in population size was facilitated by low-mortality rates in juveniles. Particularly for the period between *0.5 and *1.5 years after settlement, they report extremely low mortality when compared with the next year, when instantaneous mortality rates in the same area and in the same cohort were an order of magnitude higher (compare Table 1). Detailed observations on this oyster cohort in another part of the German Wadden Sea by Diederich (2006) revealed even higher mortality rates for the first half year after settlement, i.e. in the period preceding the one of the supposedly low mortality. Generally, mortality rates in bivalves decline from high values in fragile spat to much lower levels in robust adult stages. Several examples of such declining trends in age– mortality relationships in marine bivalves are available from the literature, e.g. Walne (1961) in Ostrea edulis, Brousseau (1978) in Mya arenaria, Nakaoka (1996) in Yoldia notabilis, and Van der Meer et al. (2001a, b) in Macoma balthica. A reverse pattern of a strongly increasing mortality rates from an age of *1 to an age of *2 years as reported by Schmidt et al. (2008) appears to be unusual and asks for a specific explanation. Consistent sampling errors in the assessment of numerical densities at the start and/or end of the period of observation are bound to bias estimates of mortality: underestimating initial numbers or overestimating final numbers would both cause an underestimation of mortality. Underestimates of initial numbers could arise from overlooking part of the individuals present. Particularly in the summer of 2003, when initial numbers were estimated, Schmidt et al. (2008) made an enormous effort by determining numbers of small oysters at 15 mussel beds that were each sampled at no less a number than 100 sites of 1 m each, resulting in records of many thousands of juvenile oysters. At the time of first sampling, the shell lengths of the oysters mostly amounted to around 40 and 20 mm, respectively, in the two frequently sampled mussel beds, but they ranged down to only 5–10 mm in both beds. Searching for these small oysters on the mussel clumps was entirely done in the field, as different from the procedure followed by Diederich (2006), who searched the spat with a stereomicroscope in the laboratory. Apparently, Schmidt et al. did not check their field counts in any way. Overestimates at the end of the observation period might arise from wrongly including individuals of a younger cohort. Allocation to a specific cohort was done using length–frequency distributions. At the time of sampling, the lengths of the individuals of the two cohorts concerned showed substantial overlap. Again, a check is not stated. Another source of errors might arise from changes in mussel distribution. As oysters were counted exclusively within mussel beds, any change in the size of these beds or in the numbers of mussels within these beds would also affect estimates of oyster numbers. Mussels might move away from beds or immigrate into beds and such moving mussels may or may not bear attached oysters. Although we are not aware of any quantitative studies of oyster Communicated by H.-D. Franke.

Distribution and spatial variation of hydrothermal faunal assemblages at Lucky Strike (Mid-Atlantic Ridge) revealed by high-resolution video image analysis

Whilst the fauna inhabiting hydrothermal vent structures in the Atlantic Ocean is reasonably well known, less is understood about the spatial distributions of the fauna in relation to abiotic and biotic factors. In this study, a major active hydrothermal edifice (Eiffel Tower, at 1690 m depth) on the Lucky Strike vent field (Mid-Atlantic Ridge (MAR)) was investigated. Video transects were carried out by ROV Victor 6000 and complete image coverage was acquired. Four distinct assemblages, ranging from dense larger-sized Bathymodiolus mussel beds to smaller-sized mussel clumps and alvinocaridid shrimps, and two types of substrata were defined based on high definition photographs and video imagery. To evaluate spatial variation, faunal distribution was mapped in three dimensions. A high degree of patchiness characterizes this 11 m high sulfide structure. The differences observed in assemblage and substratum distribution were related to habitat characteristics (fluid exits, depth and structure orientation). Gradients in community structure were observed, which coincided with an increasing distance from the fluid exits. A biological zonation model for the Eiffel Tower edifice was created in which faunal composition and distribution can be visually explained by the presence/absence of fluid exits.

Phenotypic variability in byssus thread production of intertidal mussels induced by predators with different feeding strategies

Predator-induced defenses have a significant influence on the expression of morpholog- ical and behavioral traits of marine species. In mussels, common responses to predators include thick- ening of the shell, enlargement of the adductor muscle and increases in byssus production. We hypothesize that predators with different feeding strategies have different effects on byssus produc- tion of the common intertidal mussels Perumytilus purpuratus and Semimytilus algosus in central Chile. Predators that dislodge prey mussels before killing them, such as crabs and seastars, should elicit increased byssus production in their prey compared to other predators such as whelks, which use different feeding mechanisms. Laboratory experiments with the seastar Heliaster helianthus, the crab Acanthocylus gayi, and the muricid gastropods Concholepas concholepas and Acanthina mon- odon showed that only A. gayi induced significant increases in byssus production, causing remark- ably similar responses in both mussel species. Further experiments in which individual mussels with different attachment strengths were offered to A. gayi showed that the crabs first tried different mus- sels and then selected those with the weakest attachment, leading to consumption rates of weakly attached mussels that were 5 to 6 times higher than those of mussels with strong attachment to the substratum. Measurements of mussel attachment strengths in the field showed that, where A. gayi is abundant, both mussel species are more strongly attached than in habitats where this predator is scarce. While responses of mussels to crabs seem to be adaptive, the lack of a response to H. helianthus is intriguing, because it is one of the most important mussel predators in the system which can dislodge entire clumps of mussels at a time. It is possible that increased byssus production is inef- fective in reducing predation by this large predator; however, this adaptive explanation requires fur- ther studies. These results highlight the predator-specific nature of many prey phenotypic responses and the importance of considering the multiplicity of predators typically present in most habitats.

Significant fish predation on zebra mussels Dreissena polymorpha in Lake Champlain, U.S.A.

The objectives of this study were to: (1) determine which fishes were consistently eating zebra mussels Dreissena polymorpha in Lake Champlain and document their feeding behaviour and (2) quantify the diet composition of the fish predators that were found to consume zebra mussels. From 2002 to 2005, freshwater drum Aplodinotus grunniens, pumpkinseed Lepomis gibbosus, yellow perch Perca flavescens and rock bass Ambloplites rupestris all consumed zebra mussels at varying frequencies and amounts. Aplodinotusgrunniens and L. gibbosus chewed clumps of zebra mussels, expelling shells, whereas P. flavescens and A. rupestris swallowed small individuals whole. Lepomis gibbosus consumed zebra mussels at the highest frequency (65–89% of prey consumed) and zebra mussels comprised a large part of this fish’s diet (up to 40% by dry mass). Zebra mussels were also an important component of the diet of A. grunniens (up to 59% of the diet by dry mass, 40–63% frequency of consumption). The percentage of the diet comprising zebra mussels in P. flavescens and A. rupestris varied significantly from year to year but never exceeded 10%. Because A. grunniens and L. gibbosus crushed zebra mussels, the nutritional return from consuming zebra mussels would be similar to other prey; for P. flavescens and A. rupestris zebra mussels were only partially digested and the nutritional return would probably be low. As predation on zebra mussels is widespread and significant, it is possible that fish predators could contribute to regulating the population of zebra mussels in Lake Champlain.