Gamete production,somatic growth and multiple-locus enzyme heterozygosity inMytilus edulis

Mussels,Mytilus edulis L., were collected from the lower shore in Stony Brook Harbor, New York, USA, in April 1984. The mussels were in spawning condition at the time of collection. Mantle weight was used as an index of fecundity and somatic tissue weight and growth rate were measured so that the relative production of gametes and somatic tissue with age could be calculated. For each individual mussel the genotypes at loci coding for five enzymes were determined. Somatic tissue production exceeds gamete production for the first two and a half years of life, but thereafter it declines and is exceeded by gamete production. There is a positive correlation between multiple-locus enzyme heterozygosity and fecundity in mussels which have grown beyond the size at which gamete production starts to exceed somatic production. There is no correlation between heterozygosity and fecundity in younger individuals. We conclude that the higher scope for growth of more heterozygous bivalves, described elsewhere, is translated into allocation of energy to somatic growth during early life and into gamete production in later life. Our observations perhaps explain why, in other studies, the correlation between growth rate and heterozygosity in bivalves is manifested most clearly when young individuals are examined.     

Shell shape variation in populations of Mytilus chilensis (Hupe 1854) from southern Chile: a geometric morphometric approach

The pattern of shell shape variation in populations of the mussel, Mytilus chilensis (Hupe 1854) from Southern Chile was analyzed as a function of sample origin (cultivated vs. wild) and latitude, using standard tools of geometric morphometrics for landmark data. Additionally, posterior adductor muscle index (PAMI), Freeman condition index and shell thickness were measured in each sample. Highly significant differences in shell shape components were found among mussel populations. These differences are related to the origin of samples (expansion of the posterior adductor muscle scar, elongation of the lateral ligament and of the ventral umbo position in non-cultivated samples) and to latitude (more elongated shells and more extended posterior adductor muscle scar in most southern samples when compared with the northernmost ones). PAMI and shell thickness were statistically higher in wild population, and Freeman condition index was higher in cultivated shells. It is suggested that in wild populations of M. chilensis, the mussels may face higher predator pressures and other environmental stress factors. Consequently, individuals may be using higher energy fraction to reinforce shells and to promote adductor muscle growth at the expense of somatic growth. In contrast, individuals found in calm aquaculture environments are relatively protected from predators and use most of their assimilated energy in somatic growth. In turn, this growth depends on changes that covariate with shell morphology.     

How to toughen up your mussels: using mussel shell morphological plasticity to reduce predation losses

Improvements in stocking strategy and management could increase the yield of mussels that are on-grown from harvested wild seed mussel resources and thereby enhance the sustainability of this shellfishery. A field experiment was undertaken to ascertain shell characteristics (compression strength and thickness) of seed mussels grown at different shore heights, whether these characteristics changed after a period of growth under identical conditions, and if these characteristics reduced predation losses by crabs and birds. Results indicated that mussels grown at higher shore levels attained shell characteristics beneficial to predation resistance and that these were maintained after a period of growth at a lower shore level. A novel management plan for mussel cultivation was formulated from the results of this study by manipulating shore position according to the attainment of these predator resistant shell attributes and the spatial distribution of the main natural mussel predators (crabs and birds). This technique was expanded to address the mussel cultivation problem of low natural seed settlement.     

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.     

Prevalence, patterns, and effects of shell damage on Geukensia demissa in South Carolina estuarine habitats

Shelled molluscs frequently exhibit a record of damage on exterior surfaces that can evidence past predation attempts and may affect survival and growth. In South Carolina populations of the ribbed marsh mussel, Geukensia demissa, >90% of the individuals and up to 60% of the total shell area are damaged. A trend toward greater amounts of damage occurred on mid-marsh compared to oyster reef mussels from the barrier beach side of inlets. Shell damage effects on survivorship and shell and tissue growth were assessed seasonally during multi- and single-season field experiments. Mussels from a common mid-marsh site were divided into size classes (~50 or 70 mm), treated to create two damage levels (undamaged and damaged), and replaced within mid-marsh exclusion cages to minimize additional shell damage. In both multi- and single-season experiments increased shell damage resulted in significantly greater mortality. Linear shell growth was unaffected by increased damage, but 50 mm mussels grew twice as fast. Shell mass increased 16–50% in the multi-season and single-season winter period, but decreased 7–12% during the single-season summer period. Tissue mass significantly decreased 31–43% in 50 mm damaged mussels, but increased by 33% for 70 mm mussels in both multi-season and the single-season winter period experiments. Shell damage did reduce tissue mass 43% in 70 mm single-season summer mussels. Experimental results indicate shell damage from a simulated increase in predation can affect negatively both survival and growth of marsh mussels. Seasonal timing of shell damage and initial mussel size also influenced the effects of sublethal predation on shell and tissue growth. The previously unrecognized importance of sublethal predation and the resultant significant negative effects of shell damage on survival and growth will affect the distribution and population dynamics of G. demissa in coastal marshes and will influence the overall contribution of ribbed mussels to estuarine ecosystems.     

Lethal and sub-lethal effects of phototrophic endoliths attacking the shell of the intertidal mussel Perna perna

Animals that bore into calcareous material can cause considerable damage to molluscan shells. In contrast, smaller microbial phototrophic endoliths have until recently been thought of as relatively benign. Phototrophic endoliths (primarily cyanobacteria) infest the shells of 50 to 80% of midshore populations of the mussel Perna perna (L.) in South Africa. This infestation causes clearly visible shell degradation, and we record here ecologically important lethal and sub-lethal effects (e.g. changes in growth and reproductive output) of the endoliths on their mussel hosts. Endolith infestation reduced the strength of shells significantly and also affected shell growth. In situ marking of shells, using the fluorochrome calcein, showed that infested and non-infested mussels increased in shell length at the same rate. However, the rate of increase in shell thickness (associated with shell repair) was significantly faster in infested than in uninfested individuals. This increase in the rate of shell thickening was not sufficient to compensate for rapid endolith-induced shell degradation and, around the site of adductor muscle attachment, infested shells were thinner than their uninfested counterparts. The shells of 18% of recently dead mussels had holes induced by endolith erosion. This effect was highly size dependent, and the proportion of mortality due to endoliths rose to almost 50% for the largest mussels. The re-routing of energy due to shell repair had important sub-lethal effects on the reproductive rates of mussels. During the reproductive period, mean dried flesh mass for large (>70 mm), non-infested P. perna was substantially higher than for infested individuals. This difference was almost entirely due to differences in gonad mass, which was approximately 100% higher for non-infested mussels. We conclude that, by attacking the shell, phototrophic endoliths reduce both the longevity and reproductive output of large mussels on the midshore. Received: 26 January 1999 / Accepted: 17 August 1999     

Scope for growth of Mytilus?galloprovincialis (Lmk., 1819) in oligotrophic coastal waters (Southern Tyrrhenian Sea, Italy)

The ‘scope for growth’ (SFG) tool was used to study the growth performance of cultivated populations of Mytilus galloprovincialis (Lmk., 1819) in an oligotrophic area of the Southern Mediterranean Sea. The study was carried out between 1993 and 1996 by using data from four seasonal oceanographic cruises and from growth experiments. Water samples were collected and analysed for total suspended matter (TSM), particulate organic carbon (POC) and nitrogen (PON), particulate lipids, proteins and carbohydrates and chloropigments. The sum of the carbon equivalents of carbohydrates, proteins and lipids is indicated as the total biopolymeric particulate organic carbon (BPC) and was converted into a unit of energy in order to calculate the SFG of a theoretical mussel of 5 cm length. In order to test the performance of mussel growth at two depths (5 and 15 m water depth), mussel body size [as ash free dry weight (AFDW)] and the actual concentrations of BPC were used to calculate the monthly SFG using the physiological energetic relationships suggested in the current literature. Data from the field cruises led us to characterise the study site as ultra-oligotrophic (annual average of chloropigment concentration approximately 0.5 μg L⁻¹). SFG calculations allowed us to identify a site where mussels grown successively were found to reach a commercial size in approximately 12 months. The good agreement obtained between energetic response and subsequent production response suggests that the available energy from particulate food could be fully available for organic production for maintaining “proportionate” growth trajectories, even in a ultra-oligotrophic system.     

Scope for growth of <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> (Lmk., 1819) in oligotrophic coastal waters (Southern Tyrrhenian Sea,Italy)

The ‘scope for growth’ (SFG) tool was used to study the growth performance of cultivated populations of Mytilus galloprovincialis (Lmk., 1819) in an oligotrophic area of the Southern Mediterranean Sea. The study was carried out between 1993 and 1996 by using data from four seasonal oceanographic cruises and from growth experiments. Water samples were collected and analysed for total suspended matter (TSM), particulate organic carbon (POC) and nitrogen (PON), particulate lipids, proteins and carbohydrates and chloropigments. The sum of the carbon equivalents of carbohydrates, proteins and lipids is indicated as the total biopolymeric particulate organic carbon (BPC) and was converted into a unit of energy in order to calculate the SFG of a theoretical mussel of 5 cm length. In order to test the performance of mussel growth at two depths (5 and 15 m water depth), mussel body size [as ash free dry weight (AFDW)] and the actual concentrations of BPC were used to calculate the monthly SFG using the physiological energetic relationships suggested in the current literature. Data from the field cruises led us to characterise the study site as ultra-oligotrophic (annual average of chloropigment concentration approximately 0.5 μg L⁻¹). SFG calculations allowed us to identify a site where mussels grown successively were found to reach a commercial size in approximately 12 months. The good agreement obtained between energetic response and subsequent production response suggests that the available energy from particulate food could be fully available for organic production for maintaining “proportionate” growth trajectories, even in a ultra-oligotrophic system.     

Age,growth and population structure of <Emphasis Type="Italic">Modiolus barbatus</Emphasis> from the Adriatic

Age, growth and population structure of Modiolus barbatus from Mali Ston Bay, Croatia were determined using modal size (age) classes in length frequency distributions, annual pallial line scars on the inner shell surface, internal annual growth lines in shell sections of the middle nacreous layer and Calcein marked and transplanted mussels. The length frequency distributions indicated that M. barbatus attain a length of ∼40 mm in 5–6 years indicating that a large proportion of the population in Mali Ston Bay is <5 years old. Some mussels of ∼60 mm were predicted to be 14 years old using the Von Bertalanffy growth (VBG) equation. Up to the first 6 pallial line scars were visible in young (<6 years) mussels but in older shells the first scars became obscured by nacre deposition as the mussel increased in length and age. The age of the older shells (>6 years) was determined from the middle nacreous lines in shell section, which formed annually in winter between February and March; the wider dark increments forming during summer (June to September). The oldest mussel, determined from the middle nacreous lines, was >12 years, with the majority of mussels aged between 3 and 6 years of age. The ages of mussels ascertained using the growth lines were not dissimilar to the ages predicted from the length frequency distributions. Age at length curves produced using modal size class data were not different from the data obtained using the pallial scar rings and internal growth lines. Taken together these data suggest that M. barbatus attains a length of 40 and 50 mm within 5 and 8 years, respectively. Eighty one percent of individual M. barbatus injected with a Calcein seawater solution (300 mg Calcein l⁻¹), into their mantle cavity successfully deposited a fluorescent line, which was visible in suitably prepared shell sections under ultra violet light. Incorporation of Calcein into the mussel shells was seasonally variable with the lowest frequency of incorporation in mussels marked in February and recovered in May. Seasonal shell growth was observed with significantly higher growth rates in mussels marked in May and removed in August (ANCOVA, F _3,149 = 23.11, P < 0.001). Mussels (∼18 to 22 mm) marked in May and recovered in August displayed maximal growth rates of >2.5 mm month⁻¹ compared with a mean mussel growth rate of 1.2 ± 0.6 mm month⁻¹. At other times of the year mussel shell growth ranged from immeasurable to 1.48 mm month⁻¹.     

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.     

Physiological ecology of a mussel with methanotrophic endosymbionts at three hydrocarbon seep sites in the Gulf of Mexico

In situ growth rates were determined, using two, 1-yr mark/recapture experiments, conducted between September 1991 and July 1993, for an undescribed mytilid, Seep Mytilid Ia, at three hydrocarbon seep sites in the Gulf of Mexico. The sites are located at depths of 540 to 730m, approximately 27°45N; 91°30W, and are separated by distances of 6 to 18 miles. These seep mytilids harbor methanotrophic endosymbionts and use methane as both a carbon and energy source. The mussel habitats were chemically characterized by analysis of water samples taken from precisely located microenvironments over, among and below the mussels, using small-volume, interstitial water samplers and the Johnson Sea Link submersible. Substantial differences were found in habital conditions, growth rates, and population structure for the mussels at the three sites examined. The growth rate of these seep mytilids reflects the methane concentration in their immediate habitat. Mussels at sites with abundant methane had growth rates that were comparable to shallow water mytilids at similar temperatures (5 to 8°C) with increases in shell length up to 17 mm yr^–1 documented for smaller mussels (<40 mm shell length). In conjunction with measurements of growth rates, three condition indices (glycogen content, tissue water content, and the ratio of ash-free dry weight to shell volume) were used to determine the relationship between the condition of the mussels, their growth rates, and their habitat chemistry. The three condition indices were correlated with growth rate and were often significantly different between mussels in different samples.     

Reproductive isolation and reproductive output in two sympatric mussel species (Mytilus edulis, M. trossulus) and their hybrids from Newfoundland

The mussels Mytilus edulis L. and M. trossulus Gould are found sympatrically in most areas of Newfoundland, with a low frequency of hybrids. To assess the potential for reproductive isolation, we sampled mussels from three sites in an eastern Newfoundland Bay from May–October 1996 to determine if there were differences in the reproductive cycles of the two species and their natural hybrids. In mussels with sheil lengths of 38–42 mm, males and females with mature gametes were dominant in June for M. edulis and hybrids, while M. trossulus showed a lower frequency of individuals with mature gametes. M. trossulus and hybrids spawned over a prolonged period (from late spring to early autumn) compared with most M. edulis individuals that spawned over a period of 2–3 weeks in July. This asynchrony in spawning activity between the two species may partially explain the low frequency of hybrids found in previous studies of these mussel populations. Female and male hybrids between M. edulis and M. trossulus showed normal gonad development, ripening and spawning, providing an opportunity for the introgression of genes between the two species. M. trossulus had a higher reproductive output than M. edulis of similar shell length, while hybrids showed intermediate values of reproductive output. M. trossulus females produced smaller eggs than either M. edulis or hybrids. Differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland. Published online: 13 August 2002     

Modelling plant resource allocation and growth partitioning in response to environmental heterogeneity

Plant morphological adjustment in response to spatial resource heterogeneity is an important factor that determines the outcomes of plant–plant and plant–environment interactions. In this study, a dynamic model of resource allocation and growth partitioning at the whole-plant level is presented. The aim is to suggest a mechanism by which plants are capable of modifying their resource allocation to favour the growth of their growing parts sited in resource-rich patches. In this model, an individual plant is treated as a population of relatively independent subunits competing for internal resources. The growth decision of individual shoot and root subunits depends on their local endogenous nutrient status. The allocation of nutrients to different shoot parts and root parts is determined by the structure of the vascular networks. No specific partitioning functions and driving coefficients are introduced in the model to coordinate resource allocation and growth partitioning at the whole-plant level. Vascular tissues acquire resources from the nutrient flow passing through them to grow and maintain their activities. The simulation results show that, based on simple rules of nutrient supply, transport and utilization, plants are able to integrate activities at the whole-plant level to allocate proportionally more growth to their growing parts in the most favourable positions.     

An ecosystem model for estimating potential shellfish culture production in sheltered coastal waters

A generic ecosystem model has been developed for estimating the potential production of shellfish culture and the effect of that cultivation on the pelagic ecosystem in sheltered coastal waters. The model describes the dynamics of a simple food web, nutrient cycling and growth of shellfish. The design of the model is closely tied to the temporal and spatial scales that are important in determining the sustainable production level for a particular embayment. The pelagic ecosystem, mussel energetics, population dynamics and hydrodynamics are coupled to allow fully dynamic predictions of the effect of the shellfish density. When applied to Beatrix Bay, an intensive culture embayment in the Pelorus Sound of New Zealand, the model successfully captured main features of the observed system behaviour. The hydrodynamic regime of the bay controls mussel growth and production. Although high fluxes of water into the bay suppress nutrient and carbon cycling signals in the system, the model simulations demonstrated that the mussel cultivation can have considerable effects on the ecosystem of the bay including food depletion and nutrient cycling. One of the most obvious effects is nutrient enhancement through mussel excretion at low cultivation densities, which promotes primary production particularly during the N-limitation period in summer. The sensitivity analysis identified uncertainty in some parameters and indicated areas for which experimental studies could lead to model improvement. The modelling exercise has established a primary predictive tool for managing mussel aquaculture of a coastal embayment to estimate relationships between the stock level and the growth rate of mussels, and the potentially achievable harvest and stocking density.     

Relationship between body zinc concentration and allometric growth measurements in the mussel Mytilus edulis

The frequency distribution of shell lengths in the mussels Mytilus edulis (L.) taken from a site in the Tyne Estuary (UK) in summer, 1980, typically displayed a bimodal shape, with a small peak in the 8 to 20 mm size range and a much broader peak in the 22 to 50 mm region. A collection of mussels was made consisting of two groups, one from each of the above size peaks. The group of larger mussels had a significantly higher mean zinc concentration than the group of smaller mussels, mainly because a few large individuals had very high zinc concentrations. The ratios of shell width:height, width:length and length:height were also significantly higher in larger mussels. However, the ratio of flesh dry wt:shell dry wt (flesh condition) was lower in larger individuals. The whole soft tissue zinc concentration was positively correlated with width:height and width:length, but negatively correlated with flesh wt:shell wt. It was concluded that allometric ratios may provide an attractive alternative to simple size characteristics as a basis for trace metal determinations, particularly in a comparison of metal levels in mussel populations from widely differing habitats where absolute size is a poor indicator of age or growth rate.Contribution No. 1221 of the Center for Estuarine and Environmental Studies of the University of Maryland     

A new bathymodioline mussel symbiosis at the Juan de Fuca hydrothermal vents

Until recently, the only major hydrothermal vent biogeographic province not known to include bathymodioline mussels was the spreading centers of the northeast Pacific, but deep-sea dives using DSV Alvin on the Endeavor segment of the Juan de Fuca Ridge (47°56N 129°06W; ∼2,200 m depth) in August 1999 yielded the only recorded bathymodioline mytilids from these northeastern Pacific vents. One specimen in good condition was evaluated for its relatedness to other deep-sea bathymodioline mussels and for the occurrence of chemoautotrophic and/or methanotrophic symbionts in the gills. Phylogenetic analyses of the host cytochrome oxidase I gene show this mussel shares evolutionary alliances with hydrothermal vent and cold seep mussels from the genus Bathymodiolus, and is distinct from other known species of deep-sea bathymodiolines, suggesting this mussel is a newly discovered species. Ultrastructural analyses of gill tissue revealed the presence of coccoid bacteria that lacked the intracellular membranes observed in methanotrophic symbionts. The bacteria may be extracellular but poor condition of the fixed tissue complicated conclusions regarding symbiont location. A single gamma-proteobacterial 16S rRNA sequence was amplified from gill tissue and directly sequenced from gill tissue. This sequence clusters with other mussel chemoautotrophic symbiont 16S rRNA sequences, which suggests a chemoautotrophic, rather than methanotrophic, symbiosis in this mussel. Stable carbon (δ¹³C = −26.6%) and nitrogen (δ¹⁵N = +5.19%) isotope ratios were also consistent with those reported for other chemoautotroph-mussel symbioses. Despite the apparent rarity of these mussels at the Juan de Fuca vent sites, this finding extends the range of the bathymodioline mussels to all hydrothermal vent biogeographic provinces studied to date.     

Effects of mussel aquaculture on the nitrogen cycle and benthic communities in Kenepuru Sound,Marlborough Sounds,New Zealand

Nitrogen pools and transformations and benthic communities at a Perna canaliculus farm and a nearby reference site without direct influence of marine farming in Kenepuru Sound, New Zealand, were compared on four dates between September 1982 and May 1983. The organic nitrogen pool in the top 12 cm sediment was 7.4 to 10.8 mol m^-2 at the mussel farm and 6.1 to 8.9 mol m^-2 at the reference site. The nitrate and nitrite pools were similar in both sediments, but the ammonium pool in the mussel farm sediment was about twice as high as in the reference sediment. In January, the sediment ammonium concentrations ranged from 418 nmol cm^-3 (surface) to 149 nmol cm^-3 (12 cm depth) at the mussel farm and from 86 to 112 nmol cm^-3 at the reference site. The molar C:N ratio of the sediment organic matter was 6.2 to 7.2 at the mussel farm and 7.9 to 10.0 at the reference site. The molar N:P ratio of the sediment organic matter was 4.3 to 7.2 and 3.3 to 6.1 at mussel farm and reference site, respectively. The total nitrogen mineralisation rate in the top 12 cm sediment ranged from 21.7 to 37.1 mmol m^-2 d^-1 at the mussel farm and from 8.5 to 25.0 mmol m^-2 d^-1 at the reference site. Ammonium excretion by mussels was about 4.7% (January) and 7.4% (May) of the combined nitrogen mineralisation by mussels and sediment. The sediment-denitrification rate was 0.7 to 6.1 mmol m^-2 d^-1 at the mussel farm and 0.1 to 0.9 mmol m^-2 d^-1 at the reference site. In January, 76 and 93% of the nitrate reduced in the sediments were denitrified at the mussel farm and reference site, respectively. The denitrification rate on the mussel lines (determined on detritus-covered mussels) was twice the mussel farm sediment-denitrification rate and 10 times the reference sediment-denitrification rate. Total denitrification at the mussel farm was 21% higher than at the reference site. The loss of nitrogen through mussel harvest and denitrification was 68% higher at the mussel farm. The surface layers of both sediments contained about 75 mg m^-2 chlorophyll a. Sediment phaeophytin levels were 52 mg m^-2 at the reference site and 137 mg m^-2 at the mussel farm. While the benthic infauna of the mussel-farm sediment consisted only of polychaete worms, the reference sediment contained also bivalve molluscs, brittle stars and crustaceans.     

Response of mussel Brachidontes variabilis to chlorination

Brachidontes variabilis is a common fouling mussel species in cooling water systems of tropical coastal power stations. However, there are hardly any data available on the response of B. variabilis to chlorine, a commonly used antifouling biocide. Therefore, lethal and sublethal responses of this mussel to chlorine are of considerable interest to the industry. The response of mussels in terms of mortality pattern (LT₅₀ and LT₁₀₀) and physiological activities (oxygen consumption, filtration rate, foot activity and byssus thread production) in different size groups (with shell lengths of 7–24 mm) of B. variabilis was studied in the laboratory under different chlorine concentrations (0.25, 0.50, 0.75 and 1.00 mg l^?1 for sublethal responses and 1, 2, 3 and 5 mg l^?1 for mortality). The results showed that the exposure time for 100% mortality of mussels decreased significantly with increasing chlorine concentration. However, mussel size was not a determinant of its chlorine tolerance: all size groups tested (with shell lengths of 7–24 mm) took comparable exposure times to reach 100% mortality at a given chlorine concentration (1–5 mg l^?1). All size groups of B. variabilis showed a progressive reduction in physiological activities such as oxygen consumption, filtration rate, foot activity and byssus thread production, when chlorine residuals were increased from 0 to 1 mg l^?1. The data generated in the present work are compared with similar data available for other tropical fouling mussel species to see how far relative chlorine toxicity could have influenced the relative distribution of the mussels inside the seawater intake tunnel of a power station at Kalpakkam in India. It is shown that in this insufficiently chlorinated system, the relative distribution of Brachidontes striatulus, B. variabilis and Modiolus philippinarum reflects the relative tolerance of the species to chlorine.     

Hydrodynamic stress and habitat partitioning between indigenous (Perna perna) and invasive (Mytilus galloprovincialis) mussels: constraints of an evolutionary strategy

The ability of a mussel to withstand wave-generated hydrodynamic stress depends mainly on its byssal attachment strength. This study investigated causes and consequences of different attachment strengths of the two dominant mussels species on the South African south coast, the invasive Mytilus galloprovincialis and the indigenous Perna perna, which dominate the upper and the lower areas of the lower balanoid zone, respectively and co-exist in the middle area. Attachment strength of P. perna was significantly higher than that of M. galloprovincialis. Likewise solitary mussels were more strongly attached than mussels living within mussel beds (bed mussels), and in both cases this can be explained by more and thicker byssal threads. Having a wider shell, M. galloprovincialis is also subjected to higher hydrodynamic loads than P. perna. Attachment strength of both species increased from higher to lower shore, in response to a gradient of stronger wave action. The morphological features of the invasive species and its higher mortality rates during winter storms help to explain the exclusion of M. galloprovincialis from the low shore. The results are discussed in the context of the evolutionary strategy of the alien mussel, which directs most of its energy to fast growth and high reproductive output, apparently at the cost of reduced attachment strength. This raises the prediction that its invasive impact will be more pronounced at sites subject to strong but not extreme wave action.     

Effects of chronic copper exposure on the green mussel Perna viridis

The effects of chronic copper exposure on growth and physiological responses of the green mussel Perna viridis were investigated by exposing the mussels to 50 μg l⁻¹ Cu for 3 mo at 17 and 25 °C. These temperatures represent, respectively, the winter and summer seawater temperatures in Hong Kong. Differences in the level of response between mussels exposed for 3 mo to 50 μg Cu l⁻¹ generally increased with duration of exposure. The tissue concentration of copper had increased by 280 and 450% after 3 mo exposure at 17 and 25 °C, and growth performances were reduced, with the 25 °C sets suffering from larger negative impact of copper in most responses. The inhibitory effects of copper on production of the various body components generally followed the order linear shell growth (greatest) > tissue production > byssus production > shell production. There were also decreases in the condition index (43 and 35% reductions at 17 and 25 °C), clearance rates (10.3 and 18.5%), faeces production (11 and 16.3%), assimilation efficiency (6.8 and 9.2%) and oxygen consumption rate (12.8 and 24.8%). In contrast, the organic content of the faeces (9.2 and 13.2% increases at 17 and 25 °C) and rate of ammonia excretion (21 and 28.6%), increased upon chronic copper exposure. Many of the responses (e.g. changes in tissue copper content, body dry wt, shell organic content, clearance rate and oxygen consumption rate) exhibited fluctuating levels of impact during prolonged copper exposure, while others (e.g. faecal production rate, assimilation efficiency, tissue organic content) demonstrated steady decreasing trends with increasing exposure time. Received: 17 September 1999