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.     

Will the invasive mussel Mytilus galloprovincialis Lamarck replace the indigenous Perna perna L. on the south coast of South Africa?

The mussel Mytilus galloprovincialis is invasive worldwide, has displaced indigenous species on the west coast of South Africa and now threatens Perna perna on the south coast. We tested the hypothesis that Mytilus will replace Perna by examining changes in their distribution on shores where they co-exist. Total cover, adult density, recruit density, recruit/adult correlations and mean maximum lengths of both species were measured in 2001 at two contrasting sites (Plettenberg Bay and Tsitsikamma) 70 km apart, each including two locations 100 m apart. Cover and density were measured again in 2004. Total mussel abundance was significantly lower in Tsitsikamma, and recruit density was only 17% that of Plettenberg Bay. Abundance and cover increased upshore for Mytilus, but decreased for Perna, giving Mytilus higher adult and recruit density and total cover than Perna in the upper zones. Low shore densities of recruits and adults were similar between species but cover was lower for Mytilus, reflecting its smaller size, and presumably slower growth or higher mortality there. Thus, mechanisms excluding species differed among zones. Recruitment limitation delays invasion at Tsitsikamma and excludes Perna from the high shore, while Mytilus is excluded from the low shore by post-recruitment effects. Recruitment limitation also shapes population structure. Recruit/adult correlations were significant only where adult densities were low, and this effect was species-specific. Thus, at low densities, larvae settle or survive better near adult conspecifics. After 3 years, these patterns remained strongly evident, suggesting Mytilus will not eliminate Perna and that co-existence is possible through partial habitat segregation driven by recruitment limitation of Perna on the high shore and post-settlement effects on Mytilus on the low shore.     

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.     

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.     

Mytilus galloprovincialis Lmk. in Southern Africa

The Mediterranean mussel, Mytilus galloprovincialis Lmk. has been identified on the west coast of southern Africa using morphological and biochemical-genetic comparisons with samples of “pure” M. edulis L. from Denmark and M. galloprovincialis from the Mediterranean coast of Spain. Our results show that the southern African Mytilus more closely resembles M. galloprovincialis in anterior adductor, posterior adductor and hinge size. It also has a more ventrally located maximal shell width like the Mediterranean mussel. The electrophoretic examination of 23 proteins showed that many alleles diagnostic for M. galloprovincialis appeared in the southern African Mytilus. Nei's genetic distance between the southern African Mytilus and M. galloprovincialis was 0.010 ± 0.004 and the average genetic distance between these samples and M. edulis was 0.162 ± 0.077.The presence of M. galloprovincialis in southern Africa could be explained by biogeographic dispersal along the west coast of Africa during Pleistocene cooling, or by a recent inadvertant or intentional introduction by man. To test these two hypotheses, we examined ≈750 mussel shells collected from Koi-san middens and ≈350 shells from a raised-beach deposit all located on the west coast of Cape Province, South Africa. The ages of these deposits range from 1500 yr to 120000 yr and pre-date the arrival of Europeans to the Cape. This examination did not reveal any shells of Mytilus. We, therefore, conclude that the introduction of M. galloprovincialis to southern Africa was a recent event. We suggest that, since the first confirmed reference to Mytilus in southern Africa was made from a 1972 collection, the introduction occurred within the last two decades.     

Nature and role of newly described symbiotic associations between a sea anemone and gastropods at bathyal depths in the NW Atlantic

The hormathiid sea anemone Allantactis parasitica was found living as an epibiont on numerous species of gastropods at depths of 725-1100 m along the continental slope of eastern Canada. The proportion of bathyal gastropods hosting 1-6 sea anemones reached 72.5% in a single trawl. Although A. parasitica was occasionally found on other substrata (i.e. empty shells, pebbles), laboratory trials confirmed that they preferably associate with living gastropods. Settlement of planula larvae occurred significantly more often on the shells of live bathyal gastropods than on all other substrata present in the tanks. Juvenile sea anemones (∼ 1 mm diameter) readily moved from the mud or other inert substrata onto shells of burrowed bathyal gastropods. Conversely, larvae, juveniles and adults of A. parasitica never associated with any shallow-water gastropods when given the opportunity. Trials exposing predatory sea stars (Leptasterias polaris) from shallow and bathyal depths to bathyal gastropods (Buccinum undatum) with epibiotic A. parasitica, and to asymbiotic bathyal and shallow-water B. undatum, revealed adaptive behaviours in both prey and predator. Shallow-water gastropods (devoid of epibionts) reacted defensively to L. polaris, whereas bathyal gastropods relied mostly on their epibionts to protect them, thus falling prey to L. polaris when the epibionts were removed. L. polaris from bathyal depths typically ignored symbiotic gastropods, but they consistently preyed on asymbiotic ones, while L. polaris from shallow areas initially attempted to prey on all gastropods, but learned to avoid those harbouring sea anemones. Furthermore, living as epibionts afforded sea anemones a means to escape one of their own predators, the sea star Crossaster papposus. The mutualistic relationship between hormathiid sea anemones and bathyal gastropods from the NW Atlantic may have evolved in response to predation pressure.     

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.     

Feeding behavior and differential absorption of biochemical components by the infaunal bivalve Mulinia edulis and the epibenthic Mytilus chilensis in response to changes in food regimes

We measured changes in the feeding rate and food absorption efficiency of two suspension feeding bivalves, cross-trasplanted between habitats with special emphasis on their capacity for differential absorption of biochemical components from their food supply. Mulinia edulis were moved from the intertidal zone to the subtidal zone, and Mytilus chilensis from the subtidal to the intertidal zone for a period of 7 days, and then compared with animal that had not been transplanted. Experimentally prepared diets similar to those available in the two different environments were offered to the bivalves, and their rates of feeding and differential uptake of biochemical components were determined and statistically compared. The two species did not achieve complete acclimation of their feeding behaviour during the transplant period since the highest ingestion rates for biochemical components occurred under dietary conditions that reflected their habitats of origin. Absorption efficiency showed greater acclimation than the other physiological parameters measured, indicating the capacity of these species to modulate their enzymatic-digestive activity depending on food composition. We conclude that both Mytilus and Mulinia have a certain degree of physiological plasticity in their feeding behaviour and assimilatory balance of biochemical components, being greater in Mytilus. When both species encounter ambient food conditions characteristic of their normal habitats, they show maximum values of food absorption, while under conditions where their typical diets are exchanged (Mytilus in intertidal and Mulinia in subtidal), the energy absorbed declines in each, but in ways very different between the two species. Thus, Mytilus exposed to high concentrations of low quality seston reduced the energy absorbed by 31.7% compared to its normal habitat, while Mulinia exposed to low concentrations of high-quality food reduced their energy absorption by 64%.     

Feeding physiology of the carnivorous gastropod Thais clavigera (Kuster): do they eat “soup”?

The ability to feed on suspended and dissolved organic nutrients may have been retained in predatory gastropods during evolution. The carnivorous muricid neogastropod Thais clavigera feeds on prey by boring through their shells, followed by extracellular digestion and suction of the nutrient-rich fluid of the prey's body tissues. This study reports on the effect of feeding on suspended and soluble organic nutrients (SSONs) on the survival, growth, and various physiological activities including scope for growth and glycogen stores of T. clavigera. Juvenile T. clavigera of similar shell length (23.8±1.7 mm) were either starved, fed with mussel Septifer virgatus, fed with SSONs from homogenized mussel flesh (S. virgatus), or fed with both mussels and SSONs, and kept in artificial seawater (salinity: 30‰) for 50 days. Ingestion of SSONs by the animals was significant. Feeding with the “soup” (i.e., SSONs) reduced tissue wastage and improved condition index of the snails. T. clavigera fed in this manner were intermediates between the starved and the mussel-fed groups in terms of mortality, growth, food consumption, respiration, scope for growth, and glycogen content measurements. Furthermore, T. clavigera fed with both mussels and SSONs exhibited an identical energy requirement and similar values of various physiological measurements as that of those fed solely on mussel flesh. Feeding of SSONs contributed >10% of the overall energy requirement when both SSONs and mussel prey were available. The results indicate that energy from suspended and dissolved organic nutrients can contribute to the maximization of energy input in T. clavigera, which may favor better survivorship and thus lifetime fitness.     

Evidence of a sharper decrease in a non-indigenous mussel Mytilus galloprovincialis than in indigenous bivalves from 1978 to 2006 on Japanese rocky shores

In various studies encompassing large spatiotemporal scales, the densities of non-indigenous species and indigenous species in the same habitat increased and decreased, respectively, prior to reaching an equilibrium. We examined whether this is true for a non-indigenous mussel Mytilus galloprovincialis, introduced into Japan before 1934, and the sympatric indigenous bivalves: Septifer virgatus, Septifer bilocularis, Hormomya mutabilis, Saccostrea kegaki, Saccostrea mordax and Barbatia (Savignyarca) virescens. Seven quadrat surveys during 1978–2006 on 19 rocky shores (26.6–41.4°N) showed unexpected results. Density of M. galloprovincialis decreased on 89% of the shores where this mussel had previously been found. In contrast, densities of the indigenous bivalves decreased on lower percentages (17–71%) of shores where the bivalves had been found. Shore-to-shore mean density of M. galloprovincialis decreased from the 1970s (27.0–88.7 m⁻²) to the 2000s (0.0–1.1 m⁻²), more sharply than any of the indigenous bivalves (from 4.6–9.1 m⁻² to 0.7–1.5 m⁻² for S. bilocularis, which showed the most marked reduction). The conspicuous decrease for M. galloprovincialis suggests that some factors on the studied shores have become disadvantageous especially for this non-indigenous mussel from the earlier to the later surveys.     

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.     

Brown shrimp (Crangon crangon, L.) functional response to density of different sized juvenile bivalves Macoma balthica (L.)

Variability in infaunal bivalve abundance in the Wadden Sea is largely determined by recruitment variability. Post-settlement, but pre-recruitment bivalve mortality is high and related to the occurrence of their most abundant predator, the brown shrimp Crangon crangon. To investigate if the mortality patterns of newly settled bivalves can be explained by the foraging behavior of brown shrimp, we carried out experiments on shrimp functional response to three size classes of juveniles of the Baltic Tellin Macoma balthica. The functional response curves for all three prey sizes (0.62 mm, 0.73 mm, and 0.85 mm) were the hyperbolic Holling's type II. The attack rate was highest for the smallest prey size (a = 0.31, medium and large prey a = 0.22); the handling time was longest for the largest prey size (T_h = 29 s, small and medium prey T_h = 15 s). Thus, a large body size is advantageous for the bivalves over the whole density range. Knowledge of individual foraging behavior is needed to model predation mortality of bivalves. The consumption rates in the experiment were theoretically high enough to account for M. balthica mortality in the field.     

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.     

Effects of experimental harvesting on recruitment of an alien mussel Mytilus galloprovincialis

Successful fisheries management is underpinned by an understanding of the processes that underlie the population dynamics of exploited stocks. This study considered the effects of experimental harvesting on recruitment of Mytilus galloprovincialis along the west coast of South Africa, where harvesting of this alien species is being contemplated. In particular, the role of settlement habitat availability in the form of adult mussels was analysed. To track the effects of a spectrum of harvesting intensities, five treatments were implemented: F = 0 (i.e. a control), F = 0.3, F = 0.6, F = 0.9 and F = 1. At these harvesting intensities 0%, 30%, 60%, 90% or 100% of mussel biomass was removed respectively at the outset of the experiment. A significant negative exponential relationship (p < 0.01) was found between M. galloprovincialis recruit density and harvesting intensity, with intensities greater than F = 0.3 dramatically reducing recruitment. This pattern was recorded throughout the intertidal zone and remained temporally constant over 2 years. Significant positive linear relationships (p < 0.01) between recruit density and adult mussel biomass or density indicate a strong correlation between availability of settlement habitat and recruitment. It is likely that the high recruit density recorded at low harvesting intensities (2000-20 000 per 0.01 m²) exceeds the level required for population maintenance. However, if settlement habitat is eliminated or significantly reduced, as was achieved by F = 0.3 or above, recruitment may become limiting. Thus, to protect stock replenishment, harvesting of M. galloprovincialis in this region should take place at intensities less than F = 0.3.     

Chemically induced predator avoidance behaviour in the burrowing bivalve Macoma balthica

The responses of the burrowing bivalves Macoma balthica and Cerastoderma edule to chemical cues emitted by feeding shore crabs Carcinus maenas were investigated. M. balthica held in the laboratory and exposed to chemical signals in effluent water discharging from tanks containing C. maenas fed 20 M. balthica day^− 1 reacted by increasing their burial depths from approximately 30 mm to depths of > 60 mm, over a period of several days. When the signal was removed the bivalves gradually returned to their original depth over 5 days. C. edule similarly exposed to effluent from crabs feeding on conspecifics showed no response. In an attempt to identify the signal inducing this burrowing response, M. balthica were exposed to a variety of chemical signals. Crabs fed M. balthica elicited the strongest response, followed by crabs fed C. edule. There were also small responses to effluent from crabs fed on fish, crabs previously fed on M. balthica and to crab faeces, but no responses to starved crabs, crushed M. balthica, or controls. We conclude that increased burrowing depth of M. balthica is induced by some as yet unidentified chemical cue produced by feeding crabs and is strongest when the crabs were fed on M. balthica. Unexpectedly, neither the presence of crabs themselves, nor of damaged conspecifics, was effective in eliciting a burrowing response. The mortality rates of M. balthica and C. edule selected by crabs when burrowed at normal depths and after exposure to effluent from feeding crabs were different. Crabs selected 1.5 times more C. edule than M. balthica when both species were burrowed at their normal depths, but 15 times more after the tanks had been exposed to effluent from feeding crabs for 5 days. The burrowing response of M. balthica thus appears to reduce mortality significantly by displacing predation pressure on to the more accessible C. edule.     

Predatory behavior and preference of a successful invader, the mud crab Dyspanopeus sayi (Panopeidae), on its bivalve prey

Predator-prey relationships between the panopeid crab, Dyspanopeus sayi, and the mytilid, Musculista senhousia, were investigated. Through laboratory experiments, prey-handling behavior, prey size selection, predator foraging behavior and preferences for two types of prey (M. senhousia and the Manila clam Ruditapes philippinarum) were assessed. Handling time differed significantly with respect to the three prey sizes offered (small: 15.0-20.0 mm shell length, SL; medium: 20.1-25.0 mm SL; and large: 25.1-30.0 mm SL); mud crabs were more efficient in predating medium-small than large prey. Although differences in prey profitability were not evident, D. sayi exhibited a marked reluctance to feed on larger-sized prey whilst smaller, more easily predated mussels were available. Size selection may be the result of a mechanical process in which encountered prey are attacked but rejected if they remain unbroken after a certain number of opening attempts. D. sayi exhibited inverse density-dependent foraging. A significant higher mortality of prey was evident at low prey density. Thus, at low predator density, the D. sayi-M. senhousia interaction was a destabilizing type II functional response. Interference responses affected the magnitude of predation intensity by D. sayi on M. senhousia, since as the density of foraging crabs increased, their foraging success fell. At high density (4 crabs tank⁻¹), crabs engaged in a high amount of agonistic activity when encountering a conspecific specimen, greatly diminished prey mortality. Finally, presenting two types of prey, Manila clam juveniles were poorly predated by mud crabs, which focused their predation mostly on M. senhousia. It is hypothesized that, when more accessible prey is available, mud crabs will have a minimal predatory impact on commercial R. philippinarum juvenile stocks.     

Feeding activity of scallops and mussels measured simultaneously in the field: Repeated measures sampling and implications for modelling

Many studies on the biology of bivalves have focused on identifying the importance of various environmental factors in regulating feeding behaviour in order to predict ingestion and growth responses in a variety of habitats. When taking into consideration the diversity of studies undertaken and the variety of methodologies employed, it is perhaps not surprising that there have been inconsistencies in both interspecific and intraspecific comparisons. In order to compare and contrast the way in which feeding activities of a mussel species (Mytilus edulis) and a scallop species (Placopecten magellanicus) respond to environmental factors and seston characteristics, we exposed both species simultaneously to a fluctuating assemblage of natural particles found in their local environment. Similar sizes of mussels (40-73 mm) and scallops (40-88 mm) were held in flow through chambers at two different sites in Atlantic Canada on four separate occasions, during which environmental conditions, including seston characteristics, were monitored and individual clearance rates were measured. Because the same individuals were measured over time, repeated-measures statistical procedures were used to analyze the data. Results indicate that weight standardized clearance rates of mussels were significantly higher than those of scallops only at the highest concentration tested (12.8 mg l^− 1), which translates into almost 3 times more material being filtered. Scallops appeared to respond to fluctuations in environment parameters and suspended food particles more consistently than mussels. These data suggest that the feeding behaviour of these two species is mediated differently by the same environmental parameters. Our study also demonstrates the importance of selecting appropriate statistical tests (e.g., repeated measures) for the analyses of data obtained from repeated sampling of the same individuals over time, and the need to consider species-specific feeding responses when developing models that predict the influence of bivalve feeding on ecosystem function.     

First observations of predation by New Zealand Greenshell mussels (Perna canaliculus) on zooplankton

Observations made overseas of predation by blue mussels and zebra mussels on mesozooplankton (>200 μm) have raised concern within New Zealand that the Greenshell mussel, Perna canaliculus, which is cultured in large tonnages throughout hundreds of marine farms within the New Zealand coastal zone, could exert ecologically detrimental effects by preying on zooplankton. We conducted experiments at Clova Bay, Pelorus Sound in May 2002 to determine the rates that P. canaliculus ingests prey, up to and including the mesozooplankton size range. Single mussels from farms were incubated with seawater enriched with zooplankton (>60 μm) in gently circulated 15-l pails. Depletion of chlorophyll-a (chl-a), ciliate microzooplankton, and nauplii, copepodites, and adults of copepods was determined over 5 h, relative to controls with no mussels. Two experiments were made over consecutive days. Gut contents of these experimental mussels, and of mussels examined soon after collection from a farm, were described.Gut contents of experimental and of freshly collected mussels (standard shell length ∼90 mm) had numerous copepod parts, whole copepods and larval bivalves present. Experimental mussels cleared chl-a and ciliates from 59- to 137-l individual⁻¹ day⁻¹, respectively, averaged across the two experiments. Faster ciliate than chl-a clearance was probably caused by the high proportion (56%) of phytoplankton below the retention size for P. canaliculus (ca. 5 μm) and by faster ciliate grazing in controls than treatments. The average clearance rates of adult, copepodite, and naupliar copepod stages by mussels were 20, 31, and 49 l individual⁻¹ day⁻¹, respectively. The clearance rates of each copepod stage were not significantly different between the two experiments. Clearance of nauplii was significantly greater than of adults and copepodites, while adult and copepodite clearance rates were nearly significantly different. The mean lengths of the adult, copepodite, and naupliar copepods were 430, 265, and 165 μm, respectively. The decreasing clearance rates with increasing size and development of prey (from ciliates, through naupliar, copepodite to adult copepods), suggested that prey escape ability, related to body size and/or morphology, affected capture rates. Mussel faecal samples indicated complete digestion of the gut contents. Pseudofaecal samples showed very low rejection rates of mesozooplankton by mussels. The results are considered in context of current biophysical modelling studies of impacts of large mussel farms in New Zealand. Designs of future experiments to improve accuracy of estimates of mesozooplankton clearance rates by P. canaliculus are considered.     

Regeneration of lost siphon tissues in the tellinacean bivalve Nuttallia olivacea

The inhalant siphon of the tellinacean bivalve Nuttallia olivacea is an important prey item for juvenile stone flounder Platichthys bicoloratus in estuaries in Japan. We examined quantitative siphon regeneration of N. olivacea in rearing experiments of siphon-removed bivalves (> 30 mm shell length) both in the laboratory and in their natural habitat. Under laboratory conditions, siphon-removed bivalves regenerated lost tissues quantitatively at 15 and 25 °C 1 mo after siphon removal, although regeneration was incomplete. A 3-mo caging experiment in the field showed that great regeneration occurred in siphon-removed bivalves. However, the siphon weight of removed bivalves was significantly smaller than that of non-amputated bivalves, suggesting the incomplete regeneration. In a 1-mo caging experiment, bivalves that had approximately 15% of their siphons amputated were selected at some intervals to illustrate the quantitative regeneration process. Estimated daily siphon production was remarkably high only a few days after amputation. It decreased greatly thereafter, but regeneration was not completed within 30 d. These results indicate that bivalves regenerate siphons rapidly just after losing siphon tissues and then regeneration is slowed down before it is completed.     

Assessing the potential benthic-pelagic coupling in episodic blue mussel (Mytilus edulis) settlement events within eelgrass (Zostera marina) communities

Coastal marine seagrass ecosystems are important nursery grounds for commercially and recreationally important species, and they serve as key settlement and recruitment sites for other species. We investigated several years (2001-2003) where episodic settlement events of blue mussels (Mytilus edulis) occurred in Barnegat Bay, NJ, USA. Population assessment indicated that blue mussels settled in eelgrass beds (Zostera marina) in late spring with peak densities exceeding 170,000 m⁻². Based on calculated filtration rates of M. edulis, we determined that for at least 53 days in 2001, the density and size distribution of M. edulis were sufficient to filter the water column volume in excess of twice a day, with maximum calculated filtration rates exceeding 8 m³ water m⁻² day⁻¹. While the settlement event in 2001 was very localized, in 2003, the settlement event was considerably more widespread throughout the bay, with maximum settling densities exceeding 175,000 individuals m⁻². Associated with these high densities, maximum calculated filtration rates exceeded 15 m³ water m⁻² day⁻¹. This filtration potential may have impeded the localized development of a brown-tide (Aureococcus anophagefferens) bloom in 2001, which occurred in other regions of the bay, but the widespread settlement event seen in 2003 may have impeded the development of any brown-tide blooms in Barnegat Bay during that summer. The decline in mussel densities throughout the summer may be a result of elevated water temperatures in this back bay, but at one site, the high settlement of M. edulis was followed by a substantial migration (>40 individuals m⁻²) of small sea stars (Asterias forbesii). In 2001, A. forbesii was a significant factor in reducing M. edulis density by the end of the summer at the Barnegat Inlet site and a community level assessment showed significant positive correlations between mussel aggregations and sea star densities (r=0.68-0.73, P<0.001). At this same site in 2003, the sea stars were again present in high densities (26 m⁻²) and were a potential mechanism for mussel decline. In other regions of the bay, sea star densities are very low, but numerous other predatory species exist, including blue crabs (Callinectes sapidus), green crabs (Carcinus maenus), spider crabs (Libinia spp.), and several Xanthid crabs. Given the high mussel densities seen in this study and the considerable predation by sea stars and other benthic predators, the benthic-pelagic coupling which these mussels provide in this system contributes to the high secondary production in these grass beds.