Mytilus species under rope culture in Scotland: implications for management

Mussel (Mytilus spp.) production is one of the most economically important aspects of global aquaculture and, in Scotland, production has increased from 262 t in 1986 to 4,219 t in 2006. Until recently, mussel production in Scotland was considered to be based exclusively on the blue mussel Mytilus edulis, the native species. In Europe, production is known to consist of M. edulis and M. galloprovincialis, while a third less profitable species M. trossulus occurs in the Baltic Sea, where it is unexploited. In Scotland, the sympatric occurrence of M. edulis, M. trossulus, M. galloprovincialis and their hybrids in cultivation in Loch Etive has recently been reported, with significant losses in production attributed to the presence of M. trossulus. Samples of mussels were taken at three depths from 10 rope-farm sites in Loch Etive. The distribution of the Mytilus species and their hybrids in Loch Etive was investigated using the Me 15/16 nuclear DNA locus for species and hybrid identification. All three species and their hybrids were detected and the data were in agreement with the Hardy–Weinberg model suggesting panmixia. Frequencies of M. galloprovincialis and its hybrids were very low. Overall, M. trossulus (37%) was found to be more common than M. edulis (30%) and 23% of the sampled mussels were M. trossulus × M. edulis hybrids. Species distribution did not correlate with year of settlement nor with salinities taken at the time of sampling. There were significant differences in species distribution due to site location and depth, with M. edulis being more frequent at 8 m depth and M. trossulus more common at shallower depths. These differences might be exploitable in management strategies for continuing production, for example to decrease the prevalence of M. trossulus at shellfish farms in favour of the more desirable M. edulis.     

Performance of Mussels,Mytilus edulis,Mytilus trossulus,and Their Hybrids in Cultivation at Three Scottish Lochs

Mussel aquaculture is a growing industry in Scotland, dominated by the production of Mytilus edulis. However, Mytilus trossulus, Mytilus galloprovincialis, and their hybrids with M. edulis have recently been detected in Scotland. At some farms, M. trossulus have been associated with significant losses in production because of presenting low meat yields and fragile shells. In the present study, M. edulis, M. trossulus, and their hybrids were examined from three cultivation areas in order to investigate shell and meat characteristics and advise on future management of mixed‐species areas in Scotland. Results suggest that M. edulis is likely to consistently outperform M. trossulus and hybrids in mixed‐species areas, but that M. trossulus meat yields from different areas can vary considerably. The ability to classify mussels into two (M. edulis and non‐edulis) groups suggests that separation of M. trossulus genotypes could potentially be achieved by grading the harvested mussels. In the most affected areas, however, the fallowing of cultivation sites and importation of unispecific M. edulis spat appear as an alternative. Management of this problem in Scotland is likely to involve the implementation of area‐specific measures, and the establishment of clear guidance on good practice aiming at preventing further distribution of M. trossulus.     

Comparative growth of cultured blue mussels, Mytilus edulis, M. trossulus and their hybrids, in naturally occurring mixed-species stocks

In Atlantic Canada, both Mytilus edulis and M. trossulus occur naturally in sympatric populations and readily hybridize forming genotypically complex assemblages that vary widely in relative species composition throughout the region. We monitored growth in single year‐class cohorts of rope‐cultured Mytilus spp. during a 14‐month period from sleeving of seed to harvest at three commercial mussel farms in Notre Dame Bay, Newfoundland, Canada. Electrophoretic variation at the mannose phosphate isomerase (Mpi) enzyme locus was used to categorize the two species (M. edulis and M. trossulus) and their hybrids. Seed populations varied significantly among sites in relative proportions of each species and hybrids. Growth in shell length, wet weight and shell weight varied significantly with Mpi genotype at all three farm sites. Growth in flesh weight varied significantly with Mpi genotype at two of the three sites. Growth rates for all weight variables were consistently significantly greater in M. edulis than in sympatric M. trossulus. Growth in shell length was significantly different between sympatric M. edulis and M. trossulus at only one farm site. Growth rates of hybrids were typically intermediate between sympatric M. edulis and M. trossulus, but were often statistically similar to one or other of M. edulis or M. trossulus. We conclude that the production of rope‐cultured blue mussels in Atlantic Canada is significantly influenced by genotype‐dependent growth variation in naturally occurring mixed‐species stocks. Our results also suggest that farm site production may be increased and intrasite variability reduced at farm sites currently using indigenous mixed‐species stocks by substitution and growout of transplanted seed from unispecific M. edulis stocks.     

Survey of mussel (Mytilus) species at Scottish shellfish farms

Mytilus trossulus and its hybrids with the native species Mytilus edulis and also Mytilus galloprovincialis have been recently detected in both farmed and natural populations of mussels in Scotland, adding complexity to the known M. edulis × M. galloprovincialis hybrid zone in Europe. Mytilus trossulus was first found in Scotland at aquaculture sites, where it was associated with significant losses in production and reductions in profitability. Mytilus trossulus and its hybrids have been identified at higher frequencies in farm stock than in natural intertidal populations, raising concerns about the potential wider impact of M. trossulus genotypes at mussel aquaculture sites in Scotland. The present study clarifies the composition of Mytilus species in cultivation at Scottish mussel aquaculture farms, by screening samples from 41 mussel aquaculture sites using a high-throughput real-time PCR methodology. Mytilus edulis alleles were detected at all sites. Mytilus galloprovincialis alleles were detected from samples collected at 39 sites, and M. trossulus alleles were found at five sites. Overall, M. trossulus genotypes were restricted in their geographical distribution. It is suggested that, in order to support the sustainability of the Scottish shellfish cultivation industry, the economic impact arising from the presence of M. trossulus could be limited through good management practice.     

A biogeography‐based management for Mytilus chilensis: The genetic hodgepodge of Los Lagos versus the pristine hybrid zone of the Magellanic ecotone

1. This study was intended to identify mussel species from the Magellanic ecotone, quantifying interspecific hybridization within Mytilus and depicting the genetic architecture of Mytilus chilensis in its South Pacific range.
2. The analysis comprises the sub‐Antarctic Magallanes Province as a rich ecotone of climates, ecosystems and admixed faunas embedded among the biogeographic regions of the Pacific, the Atlantic, and Antarctica.
3. Highly conserved molecular sequences within species were used to identify species, and polymorphic microsatellites were used to calculate the genetic architecture of M. chilensis. The absence of the invasive species Mytilus galloprovincialis from the M. chilensis range clarifies previous doubts on its expansion southward from the Arauco Gulf. The ubiquitous presence of the typical Glu‐5′‐Me‐15/16 PAP allele of Mytilus trossulus in the Northern Hemisphere might come from hull biofouling, but rather it seems to be an ancient polymorphism conserved in M. chilensis as occurs in blue mussels from other regions of the Southern Hemisphere. There is a very limited connectivity (F_ST = 0.167) between two latitudinal gene pools of M. chilensis that are highly divergent in composition, architecture, and ecological relevance.
4. Fifty years of aquaculture enhancement in Los Lagos explains its high diversity and genetic heterogeneity among patches, so its mussel management should seek a balance between exploitation and environmental sustainability.
5. The Magellanic ecotone bears a pristine M. chilensis × Mytilus edulis platensis hybrid zone around the Southern Cone, larger (450 km) than previously thought. Such a hybrid zone permeates one of the last remaining wilderness areas in the world (Cape Horn Biosphere Reserve) and is a natural laboratory for addressing introgression, hybridization, and evolution of Mytilus spp. genomes in their last southern frontier.

     

Feeding physiology of the Argentine mussel <Emphasis Type="Italic">Mytilus edulis platensis</Emphasis> (d’Orbigny, 1846): does it feed faster in suspended culture systems?

The feeding behavior of Mytilus edulis platensis, one of the most important aquaculture resources on the East Coast of southern South America, was studied by analyzing clearance rate (CR) and ingestion rate (IR) to test the hypothesis that cultivated mussels can attain higher clearance and ingestion rates than their wild counterparts. A number of morphometric relationships between cultivated and wild mussels were also compared. Gill surface (GS) growth relative to length (L) is isometric in M. e. platensis, with no significant differences between wild and cultivated mussels. At low food concentrations (<15 Chaetoceros gracilis cells μl⁻¹), the CR is maximum and similar in both cultivated and wild mussels, decreasing when the concentration of experimental food surpasses a threshold level. This concentration threshold is higher in cultivated mussels than in wild ones. While culture conditions do not affect either GS growth or potential CR, they do affect CR regulation patterns in response to fluctuations in food concentration, allowing the attainment of higher maximum IR.     

Effect of light on oxygen consumption and ammonia excretion in Haliotis discus discus, H. gigantea, H. madaka and their hybrids

Oxygen consumption and ammonia excretion rates of three abalone species, Haliotis discus discus, H. gigantea, H. madaka and their hybrids were measured at 20 °C by incubating for 3 h under dark and light (N = 9–13 for each species and hybrid). Animals were fasted before and during the experiment and measurements were made first under dark followed by light on the next day. The rates increased with the increase in body weight and were higher under light than dark. On average, H. discus discus had higher oxygen consumption (DD; dark = 0.039, light = 0.04 ml/g/h) than H. gigantea (G; D = 0.033, L = 0.036) and H. madaka (M; D = 0.034, L = 0.035); the hybrids had varied patterns with respect to their parental species [DD × M (D = 0.032, L = 0.038); M × DD (D = 0.03, L = 0.038); G × DD (D = 0.035, L = 0.04) and DD × G (D = 0.03, L = 0.034), mother first]. M (0.261, 0.298 μmol/g/h) had the highest ammonia excretion rate while G (0.162; 0.264) and DD (0.229; 0.232) had the lowest under dark and light, respectively. The hybrids had varied patterns in comparison with their parents (DD × M = 0.247, 0.32;M × DD= 0.177, 0.28; DD × G = 0.249, 0.364 and G × DD = 0.116, 0.155). The O/N ratios under both conditions in all species and hybrids indicated that they had carbohydrate dominated metabolism. Results demonstrated physiological variability among the species and hybrids indicating necessity of different strategies for their management and aquaculture.     

PCR-based nuclear and mtDNA markers and shell morphology as an approach to study the taxonomic status of the Chilean blue mussel,Mytilus chilensis (Bivalvia)

Most recent authors have called the Chilean blue mussel (formerly Mytilus chilensis) Mytilus edulis. Mussels from four nominal species (Mytilus edulis, M. trossulus, M. galloprovincialis and M. chilensis) were collected from the eastern coast of Newfoundland, Canada, New Zealand and the southern coast of Chile. Canonical discriminant analysis of selected shell morphometrical characters showed that although there is some overlap in canonical variates, the three nominal species, M. edulis, M. trossulus and M. chilensis can be morphologically divided into three distinct groups. The two nuclear-DNA markers (ITS and Glu-5) and the mtDNA marker studied indicate that the nuclear and mitochondrial genomes of M. edulis and M. trossulus are different. M. edulis, M. galloprovincialis and M. chilensis share similar restriction fragment patterns for the mtDNA and for the ITS nuclear-DNA marker, while the Glu-5 polymerase chain reaction (PCR) assay showed that the M. edulis and M. chilensis genotypes disagree, producing species-specific banding patterns that can be used as a diagnostic marker between the two nominal species. The molecular and morphometric data reported here give more evidence to support the view of other authors that the Chilean blue mussel should be included as M. edulis subspecies: Mytilus edulis chilensis.     

Marteiliosis in mussels: a rare disease?

Among 1280 cultured and wild adult Mediterranean mussels, Mytilus galloprovincialis, collected over a 1 ‐ year surveillance period from the Slovene Adriatic Sea, 0.3% were histologically positive for the presence of Marteilia spp. The infection was concentrated in winter. Employing the molecular methods of PCR, cloning, DNA restriction and sequencing, only Marteilia refringens type M was detected in all the infected mussels. Although all life‐cycle stages of M. refringens severely infected digestive glands, only sporadic disruption of epithelial cells of digestive tubules and focal destruction of digestive tubules were observed in the infected mussels. This was the first detection of M. refringens in M. galloprovincialis from the Slovene Adriatic Sea with the lowest prevalence reported to date. In addition, our results highlight the need for sequencing to complement the established PCR‐RFLP analysis for correct parasite typing.     

Sperm concentration in the mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> L.: a spectrophotometric measurement protocol

Quantifying spermatozoa concentration in milt samples is very important for aquaculture purposes, particularly when equal numbers of gametes are needed for experimental purposes, and different methods can be used. Haemocytometer and Coulter counting are the most precise methods to count spermatozoa, but they are very difficult to use when large numbers of samples are involved. A standard curve of spectrophotometric absorbance (A) against sperm concentration ([S]) was used herein instead, and an equation for estimating spermatozoa concentration for the mussel Mytilus edulis L. was obtained. The best wavelength to measure mussel spermatozoa was 320 nm, with a precision two or three times higher than at other wavelengths. The equation for spermatozoa concentration was A = 8.592 × 10⁻⁵ [S] + 0.0190 (r ² = 0.9979). Recommendations for choosing the best wavelength are discussed.     

Effects of off‐bottom shellfish aquaculture on winter habitat use by molluscivorous sea ducks

• 1. Shellfish farming is an expanding segment of marine aquaculture, but environmental effects of this industry are only beginning to be considered.
• 2. The interaction between off‐bottom, suspended oyster farming and wintering sea ducks in coastal British Columbia was studied. Specifically, the habitat use of surf scoters (Melanitta perspicillata) and Barrow's goldeneyes (Bucephala islandica), the most abundant sea duck species in the study area, was evaluated in relation to natural environmental attributes and shellfish aquaculture.
• 3. The extent of shellfish farming was the best‐supported habitat variable explaining variation in surf scoter densities, and the only habitat attribute from the considered set that was a strong predictor of Barrow's goldeneye densities. In both cases, the findings indicated strong positive relationships between densities of sea ducks and shellfish aquaculture operations. These relationships are presumably the result of large numbers of wild mussels (Mytilus trossulus) that settle and grow on aquaculture structures and are preferred prey of these sea ducks.
• 4. Previous work has shown that aquaculture structures provide good conditions for recruiting and growing mussels, including refuge from invertebrate predators, which in turn provides higher densities of higher quality prey for sea ducks than available in intertidal areas. This offers a rare example in which introduction of an industry leads to positive effects on wildlife populations, which is particularly important given persistent declines in numbers of many sea ducks.

Copyright © 2008 John Wiley & Sons, Ltd.     

Pacific oysters,Crassostrea gigas,established in Scotland

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Physical and biochemical properties of effluent leaving an onshore Atlantic cod (Gadus morhua,Linnaeus 1758; Gadiformes: Gadidae) aquaculture facility and potential use in integrated multi‐trophic aquaculture

The physical and biochemical properties of Atlantic cod (Gadus morhua) wastes were analysed, and the waste remediation potential of blue mussels (Mytilus edulis) was assessed. Waste generated daily by Atlantic cod represented 24.9% of the cod feed added to the system. Particle distributions determined using a Coulter Multisizer and image analysis revealed that the majority of the particles in terms of numbers occupy the smaller size ranges; however, larger particles occupy a larger proportion of the volume. Effluent was composed of particles <70 μm (36%), 70–500 μm (31%) and particles >500 μm (33%) by weight. The amount of dissolved carbon and nitrogen associated with the effluent represented 3.1% and 3.7%, respectively, of the total feed added to the system daily. Particles <70 μm had significantly less organic matter, lipids and fatty acids and were expected to be ingested more by mussels than larger particles. The major lipid classes present in effluent were free fatty acids, triacylglycerols, phospholipids, acetone mobile polar lipids and sterol. Cod effluent contained two essential fatty acids DHA and EPA, a diatom marker (16:1ω7), as well as two zooplankton markers 22:1ω11 and 20:1ω9, which accumulated in mussels and may serve as markers for aquaculture wastes. Although only 36% of the effluent was of a size suitable for mussel ingestion, this size fraction has the greatest potential to spread to surrounding areas. These particulates may be useful as an alternate food source when natural seston is low.     

Survival and growth of local and transplanted blue mussels (Mytilus trossulus, Lamark)

Consumer demand for fish and shellfish has led to an increase in aquatic species movement for aquaculture purposes. One potential drawback to the successful transplantation of animals for aquaculture is unpredictable performance due to local adaptation effects. This study used a common environment experiment to examine the potential for local adaptation in Mytilus trossulus (Lamark) on the east coast of Vancouver Island, British Columbia (BC). Newly settled mussels were collected from two sites, Chemainus and Quadra Island (approximately 150 km apart), reared in cages at the Quadra collection site, and mortality and shell length were monitored. Mussels were cage‐reared for 384 days with an overall survival of 59% (Quadra) and 39% (Chemainus). The Quadra mussels were initially smaller (5.07 mm) than the Chemainus mussels (5.90 mm), but the two were similar at the end of the experiment (23.5 and 23.6 mm). Transplanted mussels had a significantly higher mortality over the course of the experiment, primarily due to a severe episode in the early summer, and had significantly lower relative growth rates at three of 11 measurement dates. Overall, the local mussels performed better than the transplanted mussels. This study demonstrates the potential for local adaptation effects between populations of mussels separated by only a relatively small geographic distance.     

DNA fragmentation in blue mussel (Mytilus edulis) sperm: aquaculture and fisheries implications

The objective of this study was to assess sperm DNA longevity in blue mussels (Mytilus edulis) using a dynamic assessment of sperm DNA fragmentation (SDF) after sperm activation. Mature blue mussels (n = 57) in Vigo (Galicia, Spain) were obtained, specifically rope farmed blue mussels (n = 38) and wild blue mussels (n = 19). After the sperm collection, a subsample was assessed for SDF (0 h), while the rest of the sample was incubated for 6, 9, 12, 24 and 48 h at 15°C, assessing each time point using the Sperm‐Halomax kit (Halotech DNA, Madrid, Spain). The Kaplan–Meier estimator, log‐rank (Mantel–Cox) test and Mann–Whitney U‐test were used for statistical analyses (spss v. 16.0), α = 0.05. The rate of SDF (r‐SDF) between rope farmed and wild blue mussels over 0–6 h incubation was not significantly different (P = 0.278), but was for 6–24 h (P = 0.004). Differences in r‐SDF were observed when comparing the means between the two groups (P < 0.0001). Individual differences in r‐SDF existed among the rope farmed (P < 0.0001) and wild blue mussels (P < 0.0001). Wild blue mussels presented a higher DNA longevity than the farmed blue mussels. Selection of blue mussel males with a low level of sperm DNA damage and greater sperm DNA longevity may result in better fertilization and seed production.     

A comparison of growth and biochemical composition of <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> (Lmk.) and <Emphasis Type="Italic">Mytilus edulis</Emphasis> (L.) on the West coast of Cotentin,Normandy, France 1999–2000

The blue mussel, Mytilus edulis has been reared along the Normandy coast line since the 1960s. The gonadal cycle of this mussel species shows a sharp decrease in meat quality during the winter period after spawning. This decline in meat quality is so severe that sales have to be suspended from December to July. Another species of mussel, Mytilus galloprovincialis, which is reared in the lagoons along the French shore of the Mediterranean Sea has a different spawning cycle. An experiment was undertaken to study the meat quality of M. Galloprovincialis throughout the year after the spat were transferred from the Mediterranean Sea to the Normandy coast. This species showed an immediate adaptation to the gonadal cycle of M. edulis. Despite suggestions from researchers, no interest was expressed to consider such transfers of M. galloprovincialis in the future.     

Liquid-chromatographic hybrid triple–quadrupole linear-ion-trap MS/MS analysis of fatty-acid esters of dinophysistoxin-1 in bivalves and toxic dinoflagellates in Japan

Fatty-acid esters of dinophysistoxin-1 (DTX1) in scallops Patinopecten yessoensis, mussels Mytilus coruscus, and toxic dinoflagellate Dinophysis species, collected from Japanese seawater, were analyzed by liquid-chromatography mass spectrometry (LC–MS). Precursor ion monitoring, multiple reaction monitoring for 18 fatty-acid esters of DTX1, and full-scan MS/MS spectra obtained with a hybrid triple–quadrupole linear-ion-trap mass spectrometer showed that 14:0, 16:0, and 16:1 esters were the most abundant 7-O-acyl-DTX1 analogues in bivalves. Fatty-acids esters formed by conjugations at hydroxyl positions other than the 7-position of DTX1 were not detected in the bivalves. DTX1 and okadaic acid-16:0 fatty-acid esters have been reported as the most abundant ester in bivalves in several previous studies; however, we found that 7-O-16:1-DTX1 was the most abundant ester in some mussels in which 16:1 was more dominant than 16:0 in the free fatty-acid profile. Comparison between 7-O-acyl-DTX1 and free fatty-acid profiles in the same bivalve samples suggests that polyunsaturated fatty acids are selectively excluded in enzymatic acylation of DTX1. No 7-O-acyl-DTX1 was detected in any single-cell isolates of D. fortii, D. acuminata, D. mitra, D. norvegica, D. tripos, D. infundibulus, and D. rotundata.     

Protective socking material for cultivated mussels: a potential non-disruptive deterrent to reduce losses to diving ducks

Predation of cultivated mussels by diving ducks can threaten the viability of mussel farms. Conventional scaring tactics have had limited success at deterring ducks from feeding on cultivated mussels, because of rapid habituation and 24-h feeding cycles of some waterfowl species. We tested a socking material containing a biodegradable protective layer against predation by diving ducks on experimental blue mussel (Mytilus edulis) aquaculture sites in Prince Edward Island, Canada. We also assessed the effect of this new socking material on mussel growth and survival. Results showed that the protective socking has the potential to reduce losses to ducks, but did not perform well for all mussel seed sizes used by the industry. The protective layer also did not biodegrade fast enough, trapping a portion of mussels inside the sock, affecting growth and survival of these mussels. Improvements are needed to make this solution effective for all mussel sizes used by the industry and to make its production more cost effective.     

Gonad development, larval settlement and growth of Mytilus edulis L. in a suspended population in Hvalfjördur, south-west Iceland

Annual cycles of gamete development in Mytilus edulis L. in south-west Iceland were investigated during 1986 to 1987. Histological preparations of the gonads showed that all individuals were fully mature in the beginning of June in both years and one spawning period was recorded each year at a sea temperature of 10-12^oC. The main spawning took place from the middle of July to the middle of August, and from the middle of June to the end of July in 1986 and 1987, respectively. Redevelopment of the gonads after spawning was observed in February when both food availability, measured as chlorophyll a concentration, and temperature were low. Rapid gonad maturation took place during the spring phytoplankton bloom in March-April. Larval settlement was heavy on the artificial collectors used. The length of the larval period was estimated as being 4-5 weeks and peak settlement occurred in the middle of September. The size of the early plantigrades settling on the collectors during the study period indicated direct settlement of mussel larvae from the plankton onto the collectors. One year after settlement the spat averaged 24.6 mm ± 6.0 (SD) shell length and two years after settlement a market size of 50.8 mm ± 5.7 (SD) was reached. Growth was correlated with food availability, measured as chlorophyll a concentration. The growth season lasted from March to October with the greatest shell growth in late summer. The results showed that the growth of M. edulis was markedly increased by suspending the animals in a more favourable environment than that found on the natural mussel beds.     

Settlement of Ostrea edulis is determined by the availability of hard substrata rather than by its nature: Implications for stock recovery and restoration of the European oyster

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