Occurrence and detrimental effects of the bivalve-inhabiting hydroid <Emphasis Type="Italic">Eutima japonica</Emphasis> on juveniles of the Japanese scallop <Emphasis Type="Italic">Mizuhopecten yessoensis </Emphasis>in Funka Bay,Japan: relationship to juvenile massive mortality in 2003

In November 2003, we first observed prevalent occurrence of a hydroid, Eutima japonica, on soft body tissues of age zero Japanese scallop (Mizuhopecten yessoensis) juveniles cultured in large areas of Funka Bay, Hokkaido. The occurrence coincided with massive death of juvenile scallops. A major objective was to clarify ecological relationships between the symbionts, and to infer the relationship between symbiosis and the massive mortality. To do this, we investigated distributions of association rates of hydroids with juvenile scallops at 15–34 sites over 3 years (2003–2005), with age one adult scallops at 24 sites in 2003, and with mussels at 13 sites in 2004. We studied seasonal changes in association rates with juvenile scallops, and numbers of polyps per juvenile scallop at three sites from November 2003 to June 2004. We also quantified the hydroid impacts on juvenile scallop shell length growth and triglyceride accumulation in the digestive gland. The association rate of E. japonica polyps with juvenile scallops was high in large areas of Funka Bay in 2003, and overlapped the distribution of mussels bearing polyps. Association rates with age one adult scallops were very low in November 2003, even at the sites where polyps were very common on juvenile scallops. Levels of hydroid occurrence in juvenile scallops varies by year. We found that hydroids presence in juvenile scallops declined drastically in 2004 and 2005. The association rates with juvenile scallops, and numbers of polyps per juvenile scallop declined during winter, until they disappeared completely in the following June. Since polyps were rare in adult scallops, we believe that infection of juvenile scallops was probably initiated from the planulae produced by medusae released from polyps growing on Mytilus spp., especially M. galloprovincialis. Subsequently, the inhabitation spread intraspecifically and interspecifically. In juvenile scallops, inhabitation of polyps reduced shell length growth by 43%, and triglyceride accumulation in digestive glands by 24–47%. Inhabitation of E. japonica on juvenile scallop is best regarded as parasitism, rather than inquilinism or commensalism. Occurrence of polyps was probably not a direct lethal factor for juvenile scallops, because there were some sites where association rates were high, but mortalities were low. Massive mortalities in 2003 may have resulted from simultaneous impacts of heavy polyp load and stresses caused by the way in which the animals were handled (transferred from cages for pre-intermediate culture to cages for intermediate culture), because the massive mortality occurred within a month of the transfer. The presence of polyps in juvenile scallops does not affect the quality of the product in Funka Bay, because market size scallops are hydroid-free.     

Assessment of the effect of environmental pollution in a marina on caged mussels using chemical and genotoxic analysis

An integrated approach using the contamination levels and DNA damage in mussels (Mytilus galloprovincialis) was applied in order to assess the chemical contamination in a marina (Eastern coastline of Aegean Sea). Mussels, which were harvested from a reference site (Foca), were transplanted into a marina situated along the coast of Izmir Bay. The transplanted mussels were collected at the 14th, 30th and 60th day of the experimental period. Polycyclic aromatic hydrocarbon (PAH) levels (27–51?ng?g^?1?wet?weight) detected in the mussels were similar to the levels detected in other coastal areas of the Mediterranean Sea. The marina’s sediment was found to be contaminated with PAHs (∑PAH?=?25?µg?g^?1) of pyrolytic origin and may become a source of pollution and a threat to the marine environment. In order to assess the DNA damage, the haemolymph and gill cells of the mussels were used for the comet analysis and considered as an indicator of exposure to genotoxic chemicals including 16 PAH compounds and metals. The highest levels of DNA damage expressed as %Tail-DNA (%T-DNA) were observed at the end of the experiment (21.5% T-DNA). The correlation analyses conducted between 2-, 3-, 4-ring PAHs in mussels and %T-DNA in haemolymph and gill cells showed a significant positive correlation. This investigation confirmed that transplanted mussel can be a useful tool to determine PAH contamination in marinas.     

Roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis>) as an important predator on blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) populations in a brackish water environment,the northern Baltic Sea

Suspension-feeding bivalves are organisms of major functional importance in several aquatic environments around the world. They are also important food items for many fish and benthivorous seabirds. It has commonly been thought that predation pressure on blue mussel (Mytilus edulis) populations is negligible in the Baltic Sea, owing to the scarcity of major invertebrate predators such as starfish and crabs. It has recently been shown, however, that the blue mussel is the main food item for roach (Rutilus rutilus) in the archipelago areas of the western Gulf of Finland, where this freshwater fish species has become increasingly abundant, mainly due to increased eutrophication. To quantify the influence of roach predation on blue mussel populations we measured the standing biomass and size structure of the local blue mussel population and used a bioenergetic model to estimate mussel consumption by individual roach during two consecutive summers, 1997 and 1998. The results of the model were combined with existing data on roach abundance, giving annual consumption estimates of 75–105 kg blue mussel dry weight ha^–1 in the study area, approximately two-thirds of these consumed mussels being >10 mm. This corresponds to approximately one-third of the standing population of mussels >10 mm in the area. Our results suggest that the predation effects of vertebrates on Baltic blue mussel populations are not insignificant, as previously believed. Predation by roach and other predators may have an important structuring effect on unstable blue mussel communities within the Gulf of Finland, where the species lives at the edge of its range.Communicated by M. Kühl, Helsingør     

Positive effects of the introduced green alga, Codium fragile ssp. tomentosoides, on recruitment and survival of mussels

The green macroalga, Codium fragile ssp. tomentosoides, is an important component of sheltered low-shore assemblages on breakwaters along sandy shores in the northern Adriatic Sea. Macroscopic thalli of C. fragile are not perennial, but develop from propagules and/or undifferentiated forms in early spring, when the dominant native space-occupier, the mussel Mytilus galloprovincialis, recruits. By mid-summer, rapid growth of C. fragile leads to the formation of a dense canopy. We investigated the effects of juvenile and adult thalli of C. fragile on recruitment, survival and growth of mussels. Two experiments tested the hypotheses: (1) that recruitment of mussels is greater within patches of juvenile thalli (primordia) of C. fragile than on adjacent bare surfaces; (2) that the presence of a canopy of C. fragile affects the survival and growth of mussel recruits. The number of recruits of mussels was significantly larger within clumps of primordia of C. fragile than on bare surfaces. The removal of the canopy of C. fragile affected negatively the density of mussels after 2 months from the start of the experiment, but there were no effects on the mean size of individuals, nor on the size–frequency distribution. The same trend persisted after 4 months from the start of the experiment. These results show that re-colonisation of space by mussels is enhanced by C. fragile. Given that mussels, in turn, have the potential to reduce recruitment rates of C. fragile, quick recovery of mussel beds after disturbances could be crucial for controlling the abundance of this alga on breakwaters. Results also suggest that the effects of introduced species on native assemblages can be explained only through studies encompassing different life-stages of interacting organisms.     

Detrimental effects of the isopod, <Emphasis Type="Italic">Edotia doellojuradoi,</Emphasis> on gill morphology and host condition of the mussel, <Emphasis Type="Italic">Mytilus edulis platensis</Emphasis>

There are many reported associations between mussels and other invertebrates, such as pea crabs, polychaetes, turbellarians and copepods, which live in their mantle cavities. The boundary between commensalism and parasitism is often indistinguishable because of insufficient knowledge or because the interaction is variable. Preliminary evidence led to a closer examination of the relationship between the mussel, Mytilus edulis platensis, and an isopod, Edotia doellojuradoi, previously described as commensalism. Monthly intertidal samples of mussels were taken from September 2004 to August 2005 at Caleta Cordova Norte (45°43′S, 67°22′W) in southern Argentina and assessed for the prevalence and abundance levels of isopods. Mussels with and without isopods were measured, examined for evidence of gill damage and their condition (soft tissue dry weight) was determined. The overall isopod prevalence in mussels was 57.9% and infestation varied with mussel length, with maximum occurence at 30.2 mm (medium-sized mussels). Experimental evidence indicated that the position of the isopod inside the mussel depended on the feeding activity of the mussel. Female isopods were observed grasping the ventral food groove of the gill demibranchs and feeding on the mucous food strand produced by the mussel. Juveniles and males were observed clustered together on the dorsal side of the single female in each occupied mussel, suggesting extended maternal care. Gill damage was observed in 58.2% of mussels at the Argentine site and was significantly associated with isopod occurrence. Categorical regression analysis showed that the most important factor associated with the degree of gill lesions was the number of male and juvenile isopods per mussel, followed by the length of female isopods and the developmental stage of juveniles. Conversely, the degree of gill damage decreased with increasing mussel length. Overall, E. doellojuradoi had a significant effect on mussel condition throughout the year, with low flesh weight in mussels with isopods, except during the austral summer and early autumn. In contrast to previous studies, which concluded that the isopod was a commensal, the present study clearly demonstrates that E. doellojuradoi is a parasite of M. edulis platensis. Other symbiotic interactions formerly classified as commensal might not be innocuous on further investigation, especially if samples are taken at multiple sites and at different times of the year.     

Spatio-temporal variation of hydroids and polychaetes associated with Cystoseira amentacea (Fucales: Phaeophyceae)

The temporal and spatial distribution of hydroids (sessile fauna) and polychaetes (vagile) on the brown alga Cystoseira amentacea (Fucales: Phaeophyceae) have been studied in the mid-littoral zone of the Apulian coast (Italy). Samplings were carried out in February and July 1997, at 1.5 m depth, at three sites (Gargano, Costa Merlata and Otranto), about 200 km apart from each other. Three replicates per site were collected for polychaetes, and another three, for hydroids. Samples yielded 142,426 polyps belonging to 20 hydroid species and 3,088 polychaete specimens belonging to 58 species. Seasonal variations in abundance and number of species were evident in hydroids. The only species probably exclusive of Cystoseira (the hydroid Clytia viridicans) was present only in the summer. Polychaetes showed marked seasonal differences only at the Gargano site. Multivariate techniques were used to compare, in both periods, assemblages within and among sites. Analysis of similarity testing revealed that, for both polychaetes and hydroids, assemblage structures significantly differed among sites and between seasons. Thus, the null hypothesis that the distribution in time and space of the epifauna on the same algal species is homogeneous over a wide geographical scale has been rejected, in spite of the homogeneity of the substrate. In both groups, quantitative differences of a few species seemed to be more important than qualitative ones in determining the spatial separation of sites, the species pool remaining rather consistent. A small-scale variability among replicates was also detected, especially at Gargano, for hydroids. Since the epifauna can allow better comparisons among sites than the actual distribution of the algal species, both sessile and vagile epiphytes of Cystoseira can be considered valid tools for evaluating environmental changes on coastal hard-bottom communities.     

Health risks of heavy metals in the mediterranean mussels as seafood

In recent years, mussels have already become commercially important seafood species worldwide. Mussels accumulate a wide range of metals in their soft tissue. Thus, the determination of accumulated concentrations of heavy metals, such as Pb, Cd, Hg, and As, in mussels is essential because of their usage as seafood and the potential adverse effects of their consumption on human health. In this review, these issues are presented and discussed using the Mediterranean mussel Mytilus galloprovincialis as an example. M. galloprovincialis is very efficient at converting low value victuals into high quality animal protein. The production of M. galloprovincialis in Mediterranean countries has been increasing rapidly, but Spain is still the largest producer of mussels. Only China has a larger production of these mussels than Spain. M. galloprovincialis is a filter feeding animal and accumulates a wide range of metals from their environment. The metal concentrations in the soft tissue of M. galloprovincialis are indicators of marine ecosystems contamination. In the same time, the bioaccumulation of heavy metals remains an issue concerning the consumption of mussels. Thereby, the Cd, Pb, Hg, and As concentration measurement in mussel soft tissue as a seafood have become significant. A review of literature data revealed large variations in the Cd, Pb, Hg, and As concentrations in M. galloprovincialis from their endemic areas, e.g., Mediterranean, Adriatic, and Black Sea, and the concentrations of these toxic metals were generally in the following order: As > Pb > Cd > Hg. The guidelines on heavy metals for seafood safety set by different countries and associations are reviewed. Comparison of the published data with European legislation showed that the levels of the heavy metals generally did not exceed the existing limits in all the mussels analyzed, excluding mussels from hot spots, such as lagoons and harbors, in the Mediterranean, Adriatic, and Black Sea.     

Distribution of a marginal population of <Emphasis Type="Italic">Mytilus edulis</Emphasis>: responses to biotic and abiotic processes at different spatial scales

Physical and biological processes interact to produce pattern in nature. Pattern is scale dependent as processes generating pattern are heterogeneous in time and space. We tested some causes of variation in abundance and distribution of three marginal populations of sublittoral blue mussels, Mytilus edulis, in the non-tidal northeastern Baltic Sea. We studied the role of substrate inclination, perennial algae and siltation along local wave exposure gradients on mussel distribution over a regional salinity gradient. We found marked differences on regional scales (p < 0.001) with lower densities and biomasses of mussels with declining salinity. Along local gradients, mussel densities increased with increasing exposure (p < 0.001) and declining slope and sedimentation (p < 0.01). Site specifically, densities of blue mussels and the perennial red algae, Furcellaria lumbricalis, were positively related, results supported by a colonisation experiment. Also, young post-recruits showed significant relations to adult biomass, wave exposure, algal biomass, bottom slope and sediment cover. Findings showed that the relative importance of the determinants affecting blue mussels at the edge of their range vary with scale and are affected by the density and size structure of mussel populations. The study provides an indication of the types of factors that may be invoked as causes of spatial variation in marginal blue mussel populations and reinforces the need to consider multiple aspects when distributional patterns are assessed.     

Toxic metal health risk by mussel consumption

Seafood is a major dietary food worldwide. However, seafood consumption by humans can induce health risk because seafood may be contaminated by various pollutants. The mussel Mytilus galloprovincialis is widely distributed in the coastal waters of Montenegro, SouthEast Adriatic Sea. Here, Zn, Fe, Cu, Ni, Cd, Pb, As, and Hg contents in M. galloprovincialis from ten sites were analyzed to investigate health risks associated with the consumption of wild and cultivated mussels. Since there is a lack of data on the mussel consumption rate in Montenegro, the amount of mussels that can be ingested weekly over a lifetime with no risk of negative health effects was calculated using provisional tolerable weekly intakes (PTWI). We found that Cd concentrations were the limiting factor for mussels as a food. The weekly consumptions of 0.64–1.2 kg of fresh wild and 0.84–1.2 kg of fresh cultivated mussel would be sufficient to reach the PTWI_Cd, which may result in a risky weekly intake of Cd for long-term exposure. Moreover, weekly intake of 125 g mussels was used to calculate the dietary intake of trace elements by mussel consumption and compared with the prescribed PTWIs. Here, we found that there is no risk for human health for all investigated elements. In this case, the highest Cd level obtained in wild and in cultivated mussels represents 19.8 and 14.9% of the PTWI_Cd, respectively. This is the first study in Montenegro giving an assessment of the health risk from trace elements via the consumption of wild and cultivated M. galloprovincialis.     

Horizontal,vertical and seasonal distributions of epiphytic hydrozoa on the algaHalimeda tuna in the Northwestern Mediterranean Sea

The thalli ofHalimeda tuna (Ellis & Solander) grow horizontally from rocky surfaces and present an upper and lower surface which can be colonized by epiphytes. Thalli ofH. tuna were collected twice a month from March 1985 to March 1986 at Tossa de Mar (northeast Spain). Thirty five species of hydroid were recorded living onH. tuna thalli; of these, the 15 most abundant species were classified into three groups on the basis of their horizontal zonation on the thalli: those most abundant on the proximal, medial or distal articles. Those hydroids common on the proximal (oldest) articles were relatively large and were present throughout the year, those common on the medial articles were smaller and often occurred in dense monospecific patches, while those common on the distal (youngest) articles were present for only short periods and were the most opportunistic hydroids found onH. tuna. We hypothesize that this zonation reflects interspecific niche-selection, enabling successful competition for space with other hydroids, algae and bryozonas. Three hydroid species were more abundant on the upper surfaces of the algae than on the lower, probably because of gravitationally enhanced settlement of planula larvae coupled with (unusually) limited hydrorhizal growth on the underside. Three opportunistic species were more common on the lower surfaces of the thalli, possibly reflecting the greater availability of substratum in the shade, where epiphytic algae were less abundant. Many hydroids declined in abundance in summer because of the death of old thalli, the growth of new thalli and of apical articles on existing thalli, and possibly because of interspecific competition with epiphytic algae. The death of old thalli and the growth of new thalli reduced the abundance of epiphytic algae; this may have facilitated the increase in abundance of hydroids recorded in the autumn.     

Trace Metal Studies on the Starfish Asterias rubens L. From the Western Baltic Sea

In 1984, 115 samples of the starfish Asterias rubens L. collected in the south-eastern part of Cadet Trench (Mecklenburg Bay/Western Baltic Sea) were analyzed for their contents of a few major (calcium, magnesium) and trace elements (cadmium, copper, iron, mercury, manganese, nickel, lead, selenium and zinc). Distinct differences were found between starfish from different stations, and these are attributed to the composition of the sediments acting as a substrate for their prey (mussels, snails). Except for cadmium, the concentrations of the elements studied all correlated negatively with the diameter and weight of the starfish.

Parallel analyses of starfish arms and the central discs of the same animals showed that iron, zinc, copper and mercury levels were 16 to 30% higher, and selenium, manganese, magnesium, calcium and lead were 4 to 9% higher in the arms. Cadmium concentrations were 20% higher in the central discs than in the arms.

Stock estimations (about 52,000 tons fresh weight) show that starfish play a significant role in the benthic ecosystem of the western Baltic Sea. They can consume up to 200,000 tons of mussels and clams (Mytilus edulis, Macoma baltica) a year and may therefore represent a notable factor in the trace metal balance of the region.     

The importance of methane and thiosulfate in the metabolism of the bacterial symbionts of two deep-sea mussels

Undescribed hydrocarbon-seep mussels were collected from the Louisiana Slope, Gulf of Mexico, during March 1986, and the ultrastructure of their gills was examined and compared to Bathymodiolus thermophilus, a mussel collected from the deep-sea hydrothermal vents on the Galápagos Rift in March 1985. These closely related mytilids both contain abundant symbiotic bacteria in their gills. However, the bacteria from the two species are distinctly different in both morphology and biochemistry, and are housed differently within the gills of the two mussels. The symbionts from the seep mussel are larger than the symbionts from B. thermophilus and, unlike the latter, contain stacked intracytoplasmic membranes. In the seep mussel three or fewer symbionts appear to be contained in each host-cell vacuole, while in B. thermophilus there are often more than twenty bacteria visible in a single section through a vacuole. The methanotrophic nature of the seep-mussel symbionts was confirmed in ¹⁴C-methane uptake experiments by the appearance of label in both CO₂ and acid-stable, non-volatile, organic compounds after a 3 h incubation of isolated gill tissue. Furthermore, methane consumption was correlated with methanol dehydrogenase activity in isolated gill tissue. Activity of ribulose-1,5-biphosphate (RuBP) carboxylase and ¹⁴CO₂ assimilation studies indicate the presence of either a second type of symbiont or contaminating bacteria on the gills of freshly captured seep mussels. A reevaluation of the nutrition of the symbionts in B. thermophilus indicates that while the major symbiont is not a methanotroph, its status as a sulfur-oxidizing chemoautotroph, as has been suggested previously, is far from proven.     

Investigation of the sources of aliphatic hydrocarbons in the mussel Mytilus edulis from north sea oil production platforms by Capillary glc and CGCMS†

Aliphatic hydrocarbons isolated from mussels collected over a 20 month period from three North Sea Forties field oil production platforms have been examined by capillary gas chromatography and gas chromatography‐mass spectrometry.

The biological hydrocarbons consist predominantly (300–700 μh g^‐1 lipid) of C₃₁ and C₃₃ n‐alkenes with 2, 3 and 4 double bonds probably derived from a dietary intake of Emiliania huxleyi, a microscopic coccolithophorid alga. In some of the mussels C₁₈, C₂₀ and C₂₂ n‐alkanes are present in unusually high abundance compared to their odd carbon number homologues. Possible reasons for this distribution are discussed.

The concentrations of fossil fuel hydrocarbons (mainly unresolved complex mixtures) in the mussels ranged from 330 to 5,298 μg g^‐1 of lipid. No significant increase in values was detected two months after the start of discharge of treated Forties production water from the Forties D platform.

To determine the pollutant hydrocarbon sources, the sterane and terpane distributions of hydrocarbon fractions isolated from the mussels and from a number of fossil fuels, including Forties crude oil, were examined by mass fragmentography. The steranes in the mussels from the Forties C and D platforms contained higher proportions of regular 14α(H), 17α(H) components than Forties oil in which the steranes were mainly 13ß(H), 17α(H) diasteranes. In addition, the terpane distributions of the mussel fractions differed markedly from that of Forties crude oil, particularly in the relative abundance of diterpanes to triterpanes, which was higher in the mussel fractions than in the oil, and in the absence of 17α(H), 18α(H), 21ß(H)‐18, 30‐bisnorhopane, a known component of Forties crude. Furthermore, the ratio of C₂₉ to C₃₀ triterpanes was greater than unity in the mussels but much less than unity in the Forties oil. These data preclude Forties or other similar North Sea crudes as the major source of pollutant aliphatic hydrocarbons in the mussels. The relative concentrations and distributions of steranes and triterpanes suggest that the most likely source of pollutants is a Middle Eastern based oil derived either from rig activities, or from background pollution in the North Sea. The low concentration of ≥ C₂₇ steranes in the more polluted mussels (e.g. Forties B; UCM > 200 ppm dry wt.) suggests that gas oils used on the platforms may be the major source of petrogenic hydrocarbons in these samples.     

Allometry and growth of the green-lipped mussel Perna canaliculus in New Zealand

Length is the most precise (and the most practical) linear measurement for predicting total weight (r>0.98 at P=0.001) in the green-lipped mussel Perna canaliculus Gmelin. The allometry varies with the environmental conditions under which the mussels grow, resulting in morphologically distinct forms of raft- and shore-grown mussels. Mussels grown intertidally are wider, less high and heavier than mussels of similar length grown in suspension. Increase in length and total weight of P. canaliculus grown in suspended cultivation was recorded at 8 experimental sites around New Zealand, during 1973–1975. Comparisons are drawn with growth on an intertidal mussel bed, where length increase was less than half that in the same period in suspension. The growth rate of mussels transferred from intertidal to suspended conditions depends on the size at transfer. Close similarity in growth rate occurred at the majority of sites in spite of a direct correlation between water temperature and length increment and substantial, differences in temperature between sites. Reasons for the uniformity are suggested. Average values for growth at sites over the northern half of New Zealand were 73 mm length (32.5 g weight) after 12 months, 113 mm (110 g) after 2 years. Growth continued throughout the year, highest growth rates corresponding to highest water temperatures. Variation due to depth was not significant. Larger mussels grew more slowly. P. canaliculus can be grown in suspended cultivation in New Zealand at a rate comparable to that in other commercial mussel-farming areas.     

Effect of a tripeptide on the aggregational behaviour of the blue mussel<Emphasis Type="Italic"> Mytilus edulis</Emphasis>

Aggregation of blue mussels (Mytilus edulis) is stimulated by environmental chemical stimuli. Experiments carried out in a basin with a one-way current showed that individual mussels were attracted to upstream mussel concentrations and moved actively in their direction. The involvement of a tripeptide in this migration was implicated by experiments demonstrating that individual mussels were effectively attracted and moved actively towards a source of glycine–glycine–arginine at concentrations of 0.56–3.78×10^–10 M. A distinct seasonal difference in the extent of movement towards mussel concentrations was found. From the beginning of autumn, movement decreases linearly towards zero movement in winter.     

Mussels flexing their muscles: a new method for quantifying bivalve behaviour

We employed a novel technique to quantify how blue mussels Mytilus edulis react to predation risk in their environment by quantifying mussel gape using a Hall sensor attached to one shell valve reacting to a magnet attached to the other. Change in gape angle per second (CHIGA) versus gape angle plots resulted in a distribution with a boundary, which defined the maximum CHIGA of a mussel at all gape angles. CHIGA boundary plots for all individual mussels were similar in form. However, the CHIGA boundary increased in extent with mussel length (maximum CHIGA for mussel valve closures for mussels 2.98 and 79.6 mm long were −1.5 and −11°s⁻¹, respectively), showing that larger mussels opened and closed most rapidly. Mussel extract added to the seawater, a factor believed to signal predation, caused mussels to close significantly faster than otherwise (P < 0.001). This approach for assessing how mussels react to their environment indicates that mussel response to predation is graded and complex and may well indicate animal-based assessments of the trade-off between effective feeding and the likelihood of predation.     

Blue mussels, Mytilus edulis, at the edge of the range: population structure, growth and biomass along a salinity gradient in the north-eastern Baltic Sea

The blue mussel, Mytilus edulis, is the most conspicuous animal species in the northern Baltic rocky sublittoral. In the studied area the species lives at the margin of its salinity tolerance. Although dwarfed by the low-salinity conditions, blue mussels in the northern Baltic are very abundant and have a decisive role in the benthic and pelagic ecosystems. We studied abundance, size distribution, biomass and growth rate of blue mussels along a 270 km salinity gradient in the northern Baltic Sea. Samples (n=317, 1998-1999) from moderately exposed and exposed rocky shores at seven study areas were collected in the southern Archipelago Sea in the west and into the central Gulf of Finland in the east, where the species is becoming increasingly rare. The results show a marked decline in mean mussel size and biomass from the saline west to the less saline east. The growth rate also decreased with lower ambient salinity. However, abundance of small mussels was considerably higher in the central and eastern parts of the study area. Vertically, the highest biomass was recorded at intermediate depths (5 and 8 m), being lower at both shallower (3 m) and deeper bottoms (12 m). It is concluded that salinity is the foremost factor determining size structure and growth rate among populations within the area. The results suggest that predation further influences the population structure of blue mussels living at the edge of their range in the central Gulf of Finland ultimately set by their salinity tolerance.     

Population structure of Brachidontes pharaonis (P. Fisher, 1870) (Bivalvia, Mytilidae) in the Mediterranean Sea, and evolution of a novel mtDNA polymorphism

Brachidontes pharaonis (Fisher P, 1870) is an Indo-Pacific mussel that has colonized the Mediterranean Sea via the Suez Canal. Mussels may have migrated by natural dispersal of pelagic larvae, or they may have been transported on the hulls of ships, or in ballast water, or by some combination of these. Mitochondrial COI sequences (618 bp) from 101 mussels from six localities in the central and eastern Mediterranean Sea and from one site in the Red Sea were used to describe population structure. Analysis of molecular variance indicated that frequencies differed among populations, and that 92% of the variation resided within populations. The majority of haplotypes were private alleles. No simple pattern of longitudinal variation was detected for haplotype frequencies, haplotype diversity, or nucleotide diversity. A variety of tree-building algorithms (NJ, MP, ME) gave consistent results, showing two sister groups. One clade had a leucine (L-form) and the other one a methionine (M-form) at the 30th amino acid. These sympatric forms were detected within all localities and they had similar frequencies in males and females. The M- and L-form were separated by a 7.3% sequence divergence, indicating that this is an ancient polymorphism. Closely related species of mussels had exclusively the L-form, so we infer that the M-form evolved from the L-form lineage of B. pharaonis, probably in the northern portion of the Red Sea. Neutrality tests detected an excess of rare mutations, a pattern of variation expected in expanding populations. Patterns of variation detected by Tajima’s D suggest that some form of balancing selection has maintained this polymorphism.     

Genetic structure of hybrid mussel populations in the west of Ireland: two hypotheses revisited

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels, the blue mussel, Mytilus edulis, and the Mediterranean mussel, M. galloprovincialis. Results from an Irish study in the 1980s, using partially diagnostic allozyme markers, indicated that mussels higher up the shore were more galloprovincialis-like than those lower down. In this study we set out to test two hypotheses: (a) recruits arriving on the shore are composed of genetically distinct cohorts that settle preferentially at different levels on the shore, and maintain genetic distinctiveness into adulthood; (b) recruits are genetically homogeneous, but once settled they diverge genetically over time, due to within-habitat site specific-selection. The diagnostic Me 15/16 DNA marker was used to analyse the genetic composition of newly-settled spat recruiting to artificial substrates, which were placed at two-week intervals from May–October 2002, on the mid- and low shore areas of two exposed sites in Galway Bay. Adult mussels were also collected on each sampling date. Results did not support the preferential settlement hypothesis, i.e., the genetic composition of primary settlers (≤ 500 μm) was similar between tidal heights and shores. Neither was there evidence of post settlement selective mortality, as adults were genetically similar to settling spat. In spat and adults the frequency of the M. galloprovincialis allele was high (0.56–0.80), due to high frequencies of M. galloprovincialis (> 37%) and hybrid (> 33%) genotypes, and correspondingly low frequencies of the M. edulis genotype (< 11%). Adult mussels from a nearby sheltered estuarine site, while significantly different to exposed shore mussels, still had low frequencies of the M. edulis genotype (< 17%), indicating no apparent advantage for the genotype in this environment. There are indications that the genetic composition of mussels may be changing on the Atlantic coasts of Ireland.