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.     

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.     

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.

     

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.     

Gametogenic asynchrony of mussels Mytilus in a mixed-species area: Implications for management

Marine mussels Mytilus edulis, Mytilus galloprovincialis, Mytilus trossulus and their hybrids have recently been reported to occur sympatrically in both wild and farmed populations in Scotland. The presence of M. trossulus has led to significant economic losses at some aquaculture sites. Interest in understanding the reproductive cycles of these mussels in a mixed-species aquaculture area, and the potential to use this information to favour cultivation of the preferred species M. edulis, has motivated a seasonal study of the maturation stages of M. edulis, M. trossulus and M. trossulus × M. edulis hybrids, the most frequent genotypes at one site in Loch Etive, Scotland. To investigate larval abundance of these genotypes, plankton samples were collected simultaneously at the same site over the year and analysed using a real-time PCR assay for the identification of Mytilus species-specific alleles. The main spawning events of M. trossulus and M. trossulus × M. edulis hybrids occurred slightly later, but also more frequently and over a longer period than those of M. edulis. M. trossulus alleles were detected in plankton samples throughout the year, always in larger numbers than M. edulis alleles, except during February. Overall, results suggest that the settlement of M. trossulus and/or M. trossulus × M. edulis hybrids may occur during most of the year. This implies that management of the timing of the deployment of settlement ropes to significantly favour the settlement and production of M. edulis is unlikely to be successful. The real-time PCR methodology has provided valuable information on the temporal patterns of abundance of alleles of each species at the planktotrophic stage of development, and demonstrated the potential to identify and quantify Mytilus larvae in plankton samples.     

Comparison of the physiological energetics between Mytilus chilensis,Mytilus galloprovincialis and their hybrids,under laboratory conditions

Laboratory‐produced juvenile individuals of the species Mytilus chilensis, M. galloprovincialis and their hybrids were subjected to physiological measurements under an experimental diet of Isochrysis galbana (30 × 10⁶ cells L^?1), 13°C temperature and a salinity of 30 psu. Pure species individuals showed a higher clearance rate (CR). Mytilus chilensis had a CR of 1.13 L h^?1, while M. galloprovincialis registered only 0.78 L h^?1. Also, pure taxa registered higher values (above 70%) of absorption efficiency when compared with hybrid individuals. Ammonia excretion in M. chilensis and M. galloprovincialis was 1.5% and 0.4%, respectively, while hybrids registered significantly lower values. Under these experimental conditions, M. chilensis registered the highest scope for growth (P < 0.05), compared with M. galloprovincialis and their hybrids. However, the net growth efficiency index (K ₂) in hybridization type I (♀Mg × ♂Mc) was higher (P < 0.05) than other experimental groups. The invasive mytilid M. galloprovincialis showed values that are very similar to those obtained with the hybridization I group (♀Mg × ♂Mc). Finally, we discuss that water temperature is an important factor in the biogeographic separation of both species and the potential effects that the settlement of the invasive species may have for Chilean mussel production.     

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.     

The genetic structure of Mytilus chilensis (Hupé 1854) populations along the Chilean coast based on RAPDs analysis

The Chilean blue mussel Mytilus chilensis is an important commercial species. However, little has been published on the population genetics of this species, despite the need to implement management and conservation policies. Randomly amplified polymorphic DNA‐polymerase chain reaction analysis was used to estimate genetic variation within and between eight natural populations along the whole range of its Chilean natural distribution (ca. 1900 km from Arauco (VIII Region) to Punta Arenas (XII Region)). The values of Nei's unbiased genetic distance, D (0.030–0.107), among populations were small, despite the large geographic separation. A mantel test using 50 000 randomizations showed evidence for a significant correlation (r=0.74, P<0.05) between genetic and geographic (coastal) distance. Punta Arenas population was the most genetically differentiated from the others, although the scale of differentiation was not large (D=0.076–0.107). The levels of gene flow (Nm=1.55) found in this study prevent differentiation among populations by genetic drift. This is the result of the long‐lived planktotrophic larvae of M. chilensis, which provides this species with considerable dispersal ability throughout its range, which is favoured by the ocean currents along the Chilean coast. A restricted larval dispersal towards the north due to the Cape Horn Current derived from the West Wind Drift could be the cause of the higher genetic differentiation of Punta Arenas population from the northern populations. For management purposes of the M. chilensis fishery, the results provide no evidence for discrete stocks, with the possible exception of the Punta Arenas population. The present study provides the baseline data in order to continue further characterization of these mussel populations, considering the great increase in aquaculture of this species.     

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.     

Narrow valve gaping in the invasive mussel Limnoperna securis: implications for competition with the indigenous mussel Mytilus galloprovincialis in NW Spain

The black pygmy mussel Limnoperna securis (Lamarck 1819) is endemic to the brackish waters of New Zealand and Australia, but over the past decade, it has successfully invaded the inner Galician Rias of NW Spain. There is growing concern that L. securis will expand its range to the outer zones of the Rias, where it would pose a threat to the intensive raft culture of the indigenous mussel Mytilus galloprovincialis (Lamarck 1819). In this paper, we compare the valve-opening behaviour of the two mytilids under simulated raft conditions, i.e. full-strength seawater (35 g l^?1) and a low current flow regime (2–5 cm s^?1). Modes of valve-opening amplitudes that were most frequently observed in both species were in the range of 60–90 %, indicating a tendency towards full valve openness. Both species displayed circadian periodicity (τ = 24 h): maximal gaping was generally observed during periods of darkness and minimum gaping during daylight hours. The only prominent difference in behaviour between the two species was related to the degree of valve opening. The maximum recorded gape angle was 8.2° (SE = 0.9) for L. securis versus 14.8° (SE = 1.4) for M. galloprovincialis. This difference may place L. securis at a competitive disadvantage on substrates, where the two species coexist, such as over rocky shores or potentially mussel culture ropes.     

Heritability of body size in the Chilean blue mussel (Mytilus chilensis Hupé 1854): effects of environment and ageing

Selection, either natural or artificial, induces an inter‐generational response only when the trait exhibits additive genetic variation. An index of additive genetic variation is narrow‐sense heritability (h²): the ratio between additive genetic variance and phenotypic variance. Here, we present narrow‐sense heritabilities for mass and size in the Chilean blue mussel Mytilus chilensis at different ages and in three different populations. We found that h² changed with age and that maternal‐non‐additive variance was high, generating relatively low estimates of h² in most cases. Our results, together with previous studies, suggest that genotype‐by‐environment interaction does not appear to play a significant role. However, we found that non‐additive and common environmental variance are important determinants of phenotypic morphological variation in body size in M. chilensis. Further studies are needed to establish the precise contributions of these sources of variation, to assess the real potential for response to selection.     

Effect of temperature on survival,growth and development of Mytilus galloprovincialis larvae

Mussel aquaculture is widely prevalent worldwide, but generally relies on natural seed collection, which does not always meet the needs of the producers. Thus, development of mussel hatcheries is of economic interest in some parts of the world, such as Europe; it provides opportunities not only on annual reliability of seed but also on genetic improvements. To broaden knowledge on mussel larval physiology, we carried out temperature treatments (17, 20 and 24 °C) on Mytilus galloprovincialis larvae under laboratory conditions. The trials ended when 30% of the larval population was in the post‐larval stage. The temperature coefficient Q₁₀ indicated a strong relationship between temperature and increase in growth from 17 to 20 °C, but not between 20 and 24 °C. Exposure of M. galloprovincialis larvae to 17 °C resulted in poor growth, low survival and a delayed development and was considered to be inadequate for M. galloprovincialis larval culture. Rearing the larvae at 20 or 24 °C produced better growth, higher survival rates and faster metamorphosis as compared with 17 °C. The temperature region within 20 and 24 °C was suggested as adequate for the mussel M. galloprovincialis larval culture, and implications of these results on the development of commercial hatcheries were discussed.     

<Emphasis Type="Italic"> Xenostrobus securis </Emphasis>(Lamarck, 1819) (Mollusca: Bivalvia): first report of an introduced species in Galician waters

The presence of the non-indigenous species, the black-pygmy mussel Xenostrobus securis, is reported here for the first time in an intense shellfish farming area off Galicia (NW Spain). Very high concentrations of this mytilid bivalve have colonized estuarine waters located at the inner part of the Ria de Vigo. The invasive role of X. securis is discussed in the context of the wide ecological tolerance of the species and the recent finding of settlements of this species on numerous colonies of the economically-important blue mussel Mytilus galloprovincialis. The mode of introduction of the black-pygmy mussel is also discussed in relation to human management activities.     

Rearing experiences of the polychaete Sabella spallanzanii in the Gulf of Taranto (Mediterranean Sea,Italy)

In order to utilize the biomass of the polychaete Sabella spallanzanii Gmelin as a dietary supplement for fish nourishment, we conducted some experiences of rearing this worm in the Mediterranean Sea. A preliminary study on the growth of this species on suspended plastic nets was conducted in two sites of the Apulian coast with different trophic features. Data for the first 12 months show that S. spallanzanii reared in eutrophic conditions grows producing a biomass higher than the natural population. The acquired experience was afterward tested in a pilot plant located in the Mar Piccolo of Taranto, a eutrophic area where polychaetes were co-cultured with Mytilus galloprovincialis within a long-line farm. The slight mortality recorded during the first 2 months and quite negligible after 4 months, the high increase in volume and biomass of the reared polychaetes, especially at high density (70 individuals for each net), demonstrated the advantage of rearing S. spallanzanii in the pilot plant with low cost of production and high profit. If the acquired technology is applied to a medium farm containing up to 400 sectors, the biomass produced indeed might reach about 1.36 tons/year. In conclusion, on the basis of our results, we demonstrated the possibility to obtain high-value worm biomass coupled to mussel biomass in a completely non-fed culturing system without increasing, but presumably reducing the environmental impact due to mussel farming.     

Feeding and growth in captivity of the octopus Enteroctopus megalocyathus Gould, 1852

The effect of diet on the growth and survival in captivity of the octopus Enteroctopus megalocyathus Gould, 1852 was researched to establish culture potential. This is one of two commercially important octopus species that inhabit the Chilean coast. A positive energy budget, averaging 678 J day^?1 g of dry weight (dw), was recorded in specimens maintained in tanks and fed ad libitum on a diet of three species of crustacean decapods. On the other hand, the energy budget of octopus specimens fed on the mussel Mytilus chilensis (Hupé) was negative, averaging ?250.90 J day^?1 g dw^?1. Ingestion rates varied between diets (t=46.45; d.f.=18; P<0.001), but no variations were recorded in energy loss resulting from metabolism (t=0.67; d.f.=16; P>0.05) or ammonia excretion (t=1.39; d.f.=16; P>0.05). An average instantaneous growth rate (IGR) of between 1.96% and 0.49% was recorded in specimens fed on crustaceans, depending on the size of the octopus and the period of time. The IGR in specimens fed on mussels was ?0.32±0.05. The IGR of specimens fed on crustaceans and maintained in suspended systems was 1.36±0.9%. The mean mortality with crustacean and mussel diets was 11.07±5.5% and 74±1% respectively. The results indicate that E. megalocyathus can survive and grow in captivity, both in tanks and in suspended systems, when fed on a diet of crustaceans.     

Experimental harvesting of Mytilus galloprovincialis: Can an alien mussel support a small-scale fishery?

In an effort to stimulate new fisheries and address historic imbalances in access to fishing rights, there has been a recent focus on the development of small-scale fisheries in South Africa. To assess the biological viability of a fishery for the alien mussel Mytilus galloprovincialis, an experimental fishery operated by two impoverished coastal communities was initiated. Harvesting took place on a rotational basis at three sites, nested within four locations. At each of these 12 sites, 5 treatments were undertaken to span a spectrum of harvesting intensities (F = 0, 0.3, 0.6, 0.9 and a once-off total removal). A dynamic biomass-based fisheries model was developed to predict changes in exploited populations over time. Monthly maximum sustainable yield (MSY) estimates peaked at 1560 kg per 100 m of shore in March–April and September–October, but dropped by two orders of magnitude to 15 kg per 100 m of shore during the remainder of the year. The two peaks in MSY corresponded to the peak spawning periods of M. galloprovincialis along the South African west coast. Consequently, harvesting will only be viable if focused within two seasons spanning the peaks in MSY. Under these conditions, a range of harvesting intensities between F = 0.1 and 0.3 would permit long-term biological sustainability of a fishery. If implemented, this would represent the first instance of a marine invasive species being utilised to achieve socio-economic goals in South Africa.     

Spatfall and growth of the mussel,Mytilus edulis edulis,in hanging culture in the Westfjord area (68°5′ N), Norway

The growth of a population of Mytilus edulis edulis collected as spat at Hamarøy, Westfjord was followed for three growth seasons. The growth rate was similar to that at Trondheim (580 km south) but markedly dissimilar from that at Tromsø (200 km north). This suggests that there is a long coastal region of equal or similar growth for mussels, running from Trondheim about 600 km northwards to Westfjord.     

Culture of the blue mussel Mytilus edulis (Linnaeus, 1758) fed both phytoplankton and zooplankton: a microcosm experiment

With a 10‐week microcosm experiment, we demonstrated that the mussel Mytilus edulis could feed and grow upon zooplankton, phytoplankton and mixture of them. The group supplied with the mixture showed the highest shell growth rate, egestion rate and largest size of faecal pellets. Individuals feeding on seawater (the control group) had the lowest growth rate, egestion rate and smallest size of faecal pellets. Egestion rates and faecal sizes of all the groups decreased with experimental time. Therefore, the mussel M. edulis could derive energy from many kinds of diet particles. Most of these particles within the water column may play an important role in bivalve nutrition, feeding and aquaculture. A mixed diet of phytoplankton and zooplankton yielded better growth performance and metabolism than diets of each fed separately.     

The effect of high air and water temperature on juvenile Mytilus edulis in Prince Edward Island,Canada

Mussel aquaculture on Prince Edward Island (PEI), Canada, is an important but relatively new industry. Although seed manipulation using hatcheries for mussel culture occurs on the west coast of North America, seed supply on the east coast of Canada, including Prince Edward Island (PEI), is based solely on wild collection. Two techniques for culling seed (<10 mm) were studied in this experiment to look at the effect on productivity, measured as size, growth and survival. The separate effects of air exposure or high water temperature treatments on a sample of Mytilus edulis spat were examined in the lab and in the field. The in vitro treatments resulted in a ∼50% mortality from an air exposure of 11 h at 27 °C and ∼75% mortality from a 6-h exposure to 33 °C water. Survivors of each treatment (n=1152) were measured, along with controls (n=2304) and randomly placed in compartmentalized (condo) cages. Cages were deployed on a mussel farm in each of three bays on Prince Edward Island, Canada. Size, growth and survival were monitored over a 10-month period. After the initial treatment, survivors of the air exposure treatment were significantly (p<0.01) larger than the control. Survivors of the high water temperature treatment were smaller than the control (p<0.01). Results from the field study showed that the treatments had a significant effect on size, growth and survival. These results suggest that relatively simple husbandry practices of weeding out weaker mussel seed can affect productivity. Given the limited availability for lease expansion in PEI bays, new husbandry practices are an important avenue to investigate, in order for the mussel industry to maximize production capability.     

Observations on the increase of wild gilthead seabream,Sparus aurata abundance,in the eastern Adriatic Sea: problems and opportunities

The recent increase of the local population of gilthead seabream, Sparus aurata, in three areas along the southeastern Adriatic Sea: Malostonski Bay (Croatia and Bosnia and Herzegovina), Neretva Estuary (Croatia) and Boka Kotorska Bay (Montenegro) and its adverse effects on shellfish culture by preying on Mediterranean mussels, Mytilus galloprovincialis, and the European flat oyster, Ostrea edulis, are studied. The results from the analysis of the existing information show that the main reason for the recent increase is the escapes from local fish farm which enrich the local population constantly with new gilthead sea bream. The existence of practically endless food in the area of the shellfish farms allows the concentration of the population in the region instead of its dispersion along the Adriatic coast. Moreover, ecological analysis indicates that the gilthead seabream is facing a very low competition from other local species which enhances its capacity to further populate the region. While the impact on the ecosystem is not yet known, the socio-economic impact of the increase of the gilthead seabream population is evident today. Many shellfish farms are closing today in the region since the damages may reach over 90 % of the production.