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.     

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.     

Heterozygosity and growth in transplanted mussels

Growth comparisons were made involving mussels (Mytilus spp.) collected from five different localities in Britain in 1980–1981. Two of the localities, Mumbles, South Wales, and Bude, Southwest England, have pure populations of M. edulis and M. galloprovincialis, respectively. The other three, Whitsand Bay, Southwest England, Croyde Bay, Southwest England and Robin Hood's Bay, Northeast England, have hybrid populations with both M. edulis and M. galloprovincialis ancestry. To make growth comparisons, mussels from different populations were mixed in oyster nets and transplanted to three different localities. After periods of transplantation varying between several months and one year, growth was assessed by measuring increase in shell length or dry body weight. Starch-gel electrophoresis was used to assay variation in the transplanted mussels at three allozyme loci partially diagnostic for M. edulis and M. galloprovincialis. The results provide evidence of small growth differences between populations and between allozyme genotypes within populations. These differences accounted for no more than a few percent of the total variation in growth between mussels. Statistically significant results were obtained, but were frequently found not to be reproducible. There is no clear evidence of a growth difference between M. edulis and M. galloprovincialis. Allozyme heterozygotes appear to have growth rates intermediate between allozyme homozygotes; this study thus fails to provide evidence for overdominance with respect to growth rate.     

Interactive effects of wave exposure and tidal height on population structure of the mussel Perna perna

The influence of wave exposure and of tidal height on mussel (Perna perna Linnaeus) population structure (size, density, biomass and adult/juvenile correlations) was examined at 18 sites along the south coast of South Africa. Sites were classified as exposed or sheltered prior to sampling, without reference to the biota, on the basis of aspect, topography and wave regime. A single set of samples was collected from each site during three spring tide cycles. Adult mussels on these shores almost always attach directly to the rocks, and layering of mussels is virtually absent. Shore height always had a strong influence on population structure, but exposure had significant effects only lower on the shore, and almost exclusively on mussel sizes. Principal component analysis (PCA), based on size distribution data for each population, revealed a general upshore decrease in the modal size of the adult cohort. The effects of exposure on size distribution, however, varied with tidal height. PCA separated exposed zones, with larger mussels, from sheltered zones on the low-shore. Farther upshore the two shore types were increasingly confounded. The maximum size of mussels showed a similar pattern, with significant differences (ANOVA, p < 0.05) between exposed and sheltered sites only on the low- and mid-shores. Density was calculated from randomly placed quadrats (i.e. not necessarily from areas of 100% cover) and showed a different pattern. Adult (>15 mm) densities decreased up the shore, with low-, mid- and high-shore zones being significantly different from one another (ANOVA, p < 0.0001; followed by multiple range tests). However, exposure had no significant effect on density, nor was there a significant interaction with zone. Recruit (<15 mm) densities were positively correlated with adult (>15 mm) densities in all zones and for both exposure regimes ( p < 0.05 in all cases), but there was considerable variability and extremely low predictability in these relationships (r ² generally <0.2). Predictability tended to be greater towards the high-shore, where adults were more clumped. As with density, biomass was not affected by exposure, but decreased upshore as mean size and density decreased. A reduction in the influence of exposure farther upshore may be caused by greater emersion overriding the effects of exposure. The presence of free space within mussel beds and significant correlations between recruit and adult densities suggest that these mussel populations are recruit limited. Received: 7 January 2000 / Accepted: 6 July 2000     

Growth and reproductive rates of Littorina rudis from three contrasted shores in North Wales,UK

Data are presented on growth and reproductive rates of Littorina rudis Maton from 3 contrasted habitats: a sheltered saltmarsh, a moderately sheltered boulder shore and an exposed cliff face. Growth was fastest in L. rudis from the moderately sheltered boulder shore, followed closely by snails from the sheltered saltmarsh. Growth was much slower and the asymptotic size much less in L. rudis from the exposed cliff face, this being attributed to the reduced time available for foraging caused by longer and more erratic periods when the substratum dries out. Eggs extracted from the brood chambers of the saltmarsh L. rudis took about 72 d to hatch in the laboratory at 10°C. No seasonal trend was detected in the percentage of sexually mature snails, which fluctuated erratically round about 54 to 99% in each population. Similarly, the ratio of females: males fluctuated erratically about 1.0. The number of eggs and embryos held in the brood chamber peaked in May–June and birth rates (release of young from the brood chamber) peaked in July-August. Following a lull in August, the brood chamber contents were maintained at moderately high levels throughout the winter, whereas the birth rates declined to very low levels. These differences in the seasonal cycle of brood chamber contents and birth rates were probably caused by seasonal differences in embryological development rate and in the propensity of the young to leave the brood chamber. Fecundity increased with parental size and, because of this, the smaller, slower growing exposed-shore L. rudis had much lower potential fecundities than L. rudis from the other shores. However, the size-specific fecundity of the exposedshore L. rudis was about 1.7 times that of L. rudis on the more sheltered shores. This difference was apparently not attributable to smaller young or to a greater capacity of the brood chamber in the exposedshore L. rudis, and must therefore have been caused by higher rates of egg production. 18% of mature females from the boulder shore and 5% of those from the cliff face were oviparous, having a jelly gland in place of the brood chamber.     

Ecological energetics of the seaweed zone in a marine bay on the Atlantic coast of Canada. I. Zonation and biomass of seaweeds

An intertidal and underwater survey of the zonation of seaweed in St. Margaret's Bay, NovaScotia, Canada showed 8 major zones as one proceeded away from shore: (1) Fucus and Ascophyllum; (2) Chorda with filamentous browns; (3) Chondrus crispus; (4) Zostera marina; (5) Laminaria digitata with L. longicrusis; (6) Laminaria longicruris; (7) L. longicruris with Agarum cribrosum; (8) Agarum cribrosum with Ptilota serrata. Zostera occurred at the same level as O. crispus but replaced it in sheltered water. Ascophyllum was more abundant in sheltered water. L. digitata was confined to the more exposed, steeper shore. The average horizontal extent of the seaweed zone was 369 m, and the greatest depth of significant amounts of seaweed 20 to 30 m. Laminaria longicruris contributed 36% of the total biomass, and Laminaria spp. and Agarum together constituted 83%, while intertidal seaweeds contributed less than 10% of the biomass. The estimated average total biomass per m of shore line was 1,481 kg fresh weight, 326 kg dry weight, 98 kg carbon, or 980x10³ kcal. When averaged over the whole area of the bay, the corresponding figures were 1.38 kg/m² fresh weight, 0.30 kg/m² dry weight, 91 g/m² carbon or 912 kcal/m². Dry matter of Laminaria was 15 to 27% of fresh weight in blades, 10 to 12% in stipes. The dry matter content of blades was least in spring and highest in autumn, but carbon content and calorific value of dry matter showed little difference with species or season.Contribution to the International Biological Programme CCIBP 108.Bedford Institute Contribution BI 249.     

Annual reproductive cycle of the green sea urchin, Strongylocentrotus droebachiensis, in differing habitats in Nova Scotia, Canada

We monitored the reproductive cycle of Strongylocentrotus droebachiensis (OF Müller) between April 1993 and August 1995 in kelp beds, barren grounds and grazing fronts at both a wave-exposed and a sheltered site along the Atlantic coast of Nova Scotia. Gonad index and histological analyses showed that S. droebachiensis has an annual reproductive cycle that is synchronous across sites and habitats, and between females and males. Spawning occurs in March/April of each year but a small proportion of sea urchins in the study populations also spawned in fall 1995. During most of the year, sea urchins in kelp beds and grazing fronts have a higher gonad index than those in barren grounds. Gonad indices also tended to be higher at the wave-exposed than the sheltered site. Interannual variability in peak gonad index was significant in the barren grounds at the wave-exposed site and in the grazing front at the sheltered site. The gametogenic cycle is characterized by six stages based on the abundance of nutritive and germinal/gametic cells. Nutritive phagocytes are abundant after spawning and replaced by increasing numbers of germinal and gametic cells as the gametogenic cycle progresses. The temporal patterns of abundance of each cell type were similar among habitats indicating that the gonads were qualitatively similar despite large differences in gonadal mass. The quantity of gut contents (ratio of food volume to body volume) was similar among habitats, but the quality (percentage of organic material) tended to be higher in kelp beds and grazing fronts than in barren grounds suggesting that differences in gonad index of S. droebachiensis in different habitats are related to differences in diet. The high density of sea urchins in grazing fronts combined with their high fecundity suggests that they make the greatest contribution, per unit area, to the overall larval pool. Received: 20 May 1997 / Accepted: 21 January 1998     

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     

Genotype-dependent fecundity and temporal variation of spawning in hybrid mussel (Mytilus) populations

Mussel samples were collected at 4 to 6 wk intervals throughout 1987 from two hybridMytilus edulis/M. galloprovincialis populations, at Croyde Bay and Whitsand Bay, in southwest England. These were analyzed at two polymorphic loci which are diagnostic for allozyme differences which typifyM. edulis andM. galloprovincialis. Dry mantle weight as a function of shell length was determined for all individuals of each sample. Size-frequency data for the two populations was obtained in September 1987 and March 1988. For all genotypes at both sites, fecundity was a function of shell length, and in both populations the frequency ofM. galloprovincialis alleles was positively correlated with shell length. At both sites, allozyme genotype explained a significant amount of variation in mantle weight either when assessed as a main effect or when assessed as an interaction with shell length or time of collection. At Croyde,M. galloprovincialis mussels had greater estimated fecundity per unit length than theM. edulis mussels. Differences in the timing of spawning activity between theM. edulis and theM. galloprovincialis mussels were inferred, and these differences might act to reduce the amount of interbreeding at Croyde. At Whitsand, a reduced level of variability in the timing of spawning activity and fecundity between the genotypes was observed and explained by a higher degree of genetic mixing. Because theM. galloprovincialis mussels had (1) a greater estimated fecundity at any length, and (2) a greater mean length than theM. edulis mussels, the mean genotypic annual fecundity perM. galloprovincialis mussel was 2.8 times greater than an individualM. edulis mussel at Croyde, and 2.2 times greater than an individualM. edulis mussel at Whitsand. This evidence thatM. galloprovincialis mussels have an advantage in fecundity, and thus perhaps in fertility, taken together with the evidence thatM. galloprovincialis also has a higher viability, indicates directional selection in favour of theM. galloprovincialis phenotype. Because of the observed temporal stability of the population it seems likely that this selection is counterbalanced by a massive imigration ofM. edulis spat from neighbouring populations.     

Influence of predation by a crab on the distribution of the size-groups of three intertidal gastropods in South Australia

Two South Australian rocky intertidal platforms were sampled in April 1981 and December 1982 (Marino Rocks) and in December 1982 (Lady Bay), respectively. Three snail species, Nerita atramentosa Reeve, 1855, Bembicium nanum (Lamarck, 1822) and Austrocochlea concamerata (Wood, 1828), exhibited shore-level size-gradients, with smaller individuals occupying the higher intertidal levels. The reef crab (Ozius truncatus M.-Edwards, 1834), whose distribution overlaps that of the gastropods, is an important predator of these gastropods. The shore-level distribution of shell sizes can be explained by differential selection against smaller individuals by reef crabs. The decreased density of small snails at lower intertidal levels may be the result of either their consumption by reef crabs and/or their movement to higher shore levels where crab predation is less intense.     

Induced defensive responses by the bivalve Mytilus edulis to predators with different attack modes

Predators such as crabs, whelks, and sea stars attack their bivalve prey in different ways, and predator-induced defenses are an important means of protection. The degree to which induced defenses are specific to different predators, however, remains largely unknown. In laboratory experiments (June to August 1998), we raised mussels (Mytilus edulis L.) in the presence of a drilling predator [the whelk Nucella lapillus (L.)] or a crushing predator [the crab Carcinus maenas (L.)] to determine whether induced changes in prey shell thickness, size, or shape occurred and whether changes were predator-specific. Over a 2 month period, juvenile mussels were exposed to waterborne cues from actively feeding crabs or whelks. Mussels produced thicker shell lips in response to both predators relative to control mussels raised in their absence, and the difference was significantly greater in response to whelks than to crabs. Mussels exposed to whelks showed significantly smaller increases in shell length and width and total wet weight than did mussels exposed to crabs. Thus, there may be a trade-off between shell thickness and linear shell growth and a potential delay in attaining a size refuge from predation. Received: 4 August 1999 / Accepted: 31 January 2000     

Freshwater Mussel,Lamellidens Marginalis (Lamarck) (Mollusca: Bivalvia: Unionidae) As an Indicator of River Pollution

Abstract

Analyses were made of heavy metals, manganese, nickel, copper, zinc and lead in water samples and soft body, shell and different tissues (gills, digestive glands, mantle and viscera) of the Unionid mussel, Lamellidens marginalis collected from two tributaries of the Cauvery river. Water samples from Station I contained higher concentrations of the metals than those from Station II. the concentration of metals in water at both stations were in the descending order: Mn > Zn > Pb > Ni > Cu. However, the concentrations of metals in the soft body were in the descending order: Zn > Mn > Pb > Ni > Cu at both stations in all size groups of mussels tested. the concentration of zinc maintained a linear relationship with the size of the mussels, but manganese showed a reverse trend. Small size (4-5 cm) mussels accumulated more manganese (105.5 μg.g^?1 dry wt.) than larger ones (7-8 cm; 6.5 μg.g^?1 dry wt.). Both young and old is accumulate the same level of lead, copper and nickel in the soft body. the order of concentrations of metals (Mn, Pb, Zn, Ni and Cu) in the shell of mussels from both stations coincided with the order of concentrations of background water except for lead. the accumulation of lead was higher in shell (30.4-36.2 μg.g^?1 dry wt.) than in soft body (6.4-12.0 μg.g^?1 dry wt.). the pattern of concentration of metals in the various tissues reveal that the digestive glands have greater ability than other tissues to concentrate most metals under study. the concentration factors for soft body, shell and different tissues are presented. the advantages in using the common mussel for biomonitoring of contaminants in water is also discussed.     

Testing the effects of wave exposure,site, and behavior on intertidal mussel body temperatures: applications and limits of temperature logger design

The rocky intertidal mussel Mytilus californianus is exposed to potentially damaging thermal conditions during low tide. However, because the temperatures of ectothermic organisms are driven by multiple climatic factors, we do not fully understand what the body temperatures of intertidal invertebrates are under field conditions, or how thermal stress varies between intertidal sites. We designed a temperature logger that thermally matches (similar size, color, morphology and thermal inertia) living mussels of the species M. californianus, and tested its ability to provide realistic measurements of body temperature in the field. As part of these tests, we examined the propensity of M. californianus to gape, a behavior in which the mussel opens its shell valves, and which may permit evaporative cooling. Because our instruments were unable to mimic this behavior, we tested the degree to which gaping contributes to animal cooling by exposing M. californianus to a range of climatic conditions while recording body temperatures, gaping behavior and water loss. Results indicated no significant influence of gaping on body temperature. Tests comparing temperatures of loggers to those of real mussels under laboratory and field conditions showed that thermally matched loggers recorded temperatures within a few degrees of living animals and that unmodified loggers regularly incurred errors of up to 14°C. We then deployed a series of thermally matched loggers at two sites in central Oregon (Boiler Bay and Strawberry Hill) previously hypothesized to display site-specific differences in aerial temperature, and at various wave-exposure regimes within each site. Significant differences were demonstrated between sites using a subset of temperature metrics in a multivariate analysis. Yearly peaks in maximum temperature, average daily maximum temperature, and degree hours were useful in discriminating between sites. In 2001, wave-exposed sites at Strawberry Hill displayed fewer degree–hours than wave-protected sites, but an equivalent or greater maximum temperature. In 2002 both of these metrics were significantly lower in wave-exposed sites. Boiler Bay and Strawberry Hill differed in thermal regime, but not in a way that would indicate one was hotter than the other.Communicated by P.W. Sammarco, Chauvin     

Spatial variability of length-specific production in shell,somatic tissue and sexual products of Macoma balthica in the Lower St. Lawrence Estuary

A field study on the spatial variability of production and some demographic parameters was conducted in 1988 in ten populations of Macoma balthica located on the north and south shores along the entire length (230 km) of the Lower St. Lawrence Estuary (LSLE, Canada) at the same intertidal level. Standard-length estimates (10 mm) of shell and somatic tissue production for the period of May to November were highly variable between stations and greater on the north shore than on the south shore. Standard-length estimates of the gamete production for the period of July to November were also highly variable between stations but there was no variation between shores. The inter-population variability of the standard-length estimates of production in shell, somatic tissue and sexual products was as large as the intrapopulation variability between both the upper and lower tidal levels measured in previous studies. There were no significant linear relationships between standard-length estimates of production and biotic (density) or abiotic (temperature, chlorophyll-a in top sediment, mean phi, water salinity) factors, but we observed some significant quadratic relationships between standard-length estimates of production and mean sediment-surface temperature during the growing season. The standard-length estimates of production were lower at the coldest and warmest stations than at the more temperature stations. There was also a significant negative linear relationship between mean sediment-surface temperature during the growing season and the grain size structure of sediment, indicating that the sediment texture, indirectly, largely influenced the inter-station and the inter-shore variability of production in shell, somatic tissue and sexual products of M. balthica in the LSLE.     

High intertidal community organization on a rocky headland in Maine,USA

A mosaic patchwork of the barnacle Balanus balanoides L., the mussel Mytilus edulis L., and the alga Fucus vesiculosus L. was found in the transitional region between the mid and high intertidal zones on a rocky headland on Mount Desert Island, Maine, USA. The development of the mosaic was observed by following recruitment and survival of B. balanoides in denuded patches located at the same tidal level along a 60 m stretch of shore. Barnacle recruitment was least under canopies of F. vesiculosus and greatest in open areas kept moist at low tide by surf. Barnacle survival after settlement was least under the F. vesiculosus canopy due to the whiplash effect of the algal fronds in the surf and greatest in open areas free from competition from mussels. In open areas, early mortality was correlated with settlement density. In areas of dense settlement (60 spat cm^-2) up to 90% mortality resulted within 5 months from crowding associated with growth. In older individuals crowding produced hummocks of elongated, weakly attached barnacles which were more prone to removal by surf than uncrowded barnacles. Mussels exerted competitive dominance over barnacles for space and the presence of mussel beds prevented further barnacle recruitment. Mussels suffered extensive mortality during winter storms when surf removed dense mats of weakly attached mussels. The patchy distribution of mussels and barnacles results from irregular rock substrata producing numerous environmental patches with respect to wave exposure and drainage at low tide, and from densitydependent mortality of both mussels and barnacles which creates patches of new colonizable space within each environmental patch.     

Calanoid copepod,cladoceran, and rotifer eggs in sea-bottom sediments of northern Californian coastal waters: Identification,occurrence and hatching

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels,Mytilus edulis L. and the Mediterranean musselM. galloprovincialis Lmk. This paper presents an in-depth analysis, carried out between October 1984 and December 1986, of genetic variability at two partially diagnostic loci,Odh andEst-D, in two exposed-shore populations ofMytilus spp. in the west of Ireland. Significant differences at theOdh locus were observed in the genetic composition of adult mussels from different tidal levels. These differences were repeatable whether one was analysing replicate samples at a single point in time, samples collected at different points in time, i.e., in different years, or samples collected from different shores. Mussels recruiting to artificial substrates set out for a period of one month at different tidal levels at one of these sites were also observed to be genetically different; mussels higher up the shore exhibited higher frequencies of those alleles characteristically at high frequency inM. galloprovincialis for both theOdh andEst-D loci. Hence, the genetic differences observed in adult mussels are much more exaggerated in juveniles and are already apparent within the first month of benthic life. Possible reasons for the observed microgeographic differentiation are discussed. It is concluded that the observed genetic differences between mussels at different tidal levels arise either in the pelagic/attachment stage or very shortly after settlement.     

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.     

Growth of juvenile Mercenaria mercenaria and the effect of resuspended bottom sediments

The influence of silt on growth of juvenile hard clams Mercenaria mercenaria (L.) (9 mm in mean shell length) was investigated in the laboratory using mixed suspensions of algae (50x10⁶ Pseudoisochrysis paradoxa cells l^-1) and fine-grained bottom sediments (0 to 44 mg l^-1). Growth rates, expressed as percent increase in ash-free dry tissue weight, were not significantly affected by sediment concentrations up to 25 mg l^-1. Significant reduction in growth (by 16% relative to controls fed only algae), and condition of clams, occurred at 44 mg silt l^-1. The results of the 3-week growth experiment agree well with predictions made in an earlier study by integrating results of shortterm physiological measurements. Growth rates obtained with experimental algal-silt diets at 21°C (2.6 to 3.3% increase in dry tissue weight d^-1) were comparable to those determined at ambient concentrations of Great South Bay particulates at 20°C (0.9 to 4.0% d^-1). Levels of particulate inorganic matter in seawater from Great South Bay, New York, exhibited pronounced daily changes, and ranged from 6 to 126 mg dry weight l^-1. Growth enhancement by the addition of silt to an algal diet, reported in mussels, surf clams and oysters, was not found in M. mercenaria. It is suggested that these three species are better suited than hard clams for culturing efforts in inshore turbid waters above uncompacted, muddy bottoms.Contribution No. 452 from the Marine Sciences Research Center, State University of New York at Stony Brook, USA     

Die Miesmuschel Mytilus edulis als Indikator für die Bleikonzentration in Weserästuar und in der Deutschen Bucht

Common mussels were collected at 13 stations of the Weser Estuary and the German Bight in 3 size catagories: 14 to 16 mm, 21 to 23 mm, 35 to 40 mm shell length. Equal numbers of the 3 groups from each station were analyzed separately for lead concentration by flameless atomic-absorption spectrophotometry. The lead concentration of the soft parts decreases from 6.4 μg/g dry weight 15 km northwest of Bremerhaven to 1.9 μg/g at Helgoland. At the same station, the concentration in small musels is significantly higher than in larger mussels. The lead concentration is exceptionally high in the kidney, quite high in the intestine (with the digestive glands) and in the adductor muscle, and rather low in the foot, gills, and mantle with gonads. Unpublished results of my laboratory experiments reveal that mussels directly reflect the actual lead concentration of their environment. The observed gradient in lead contents of M. edulis in the Weser Estuary can thus be explained by the dilution of the highly lead-polluted river water by the sea water of the German Bight. With this physiological property, M. elulis is, therefore, highly suitable as an indicator organism for lead, and possibly other heavy metal pollutants.     

Wave exposure effects on population structure and recruitment in the mussel <Emphasis Type="Italic">Perna perna</Emphasis> suggest regulation primarily through availability of recruits and food,not space

Recruitment and population structure of Perna perna in low shore mussel beds were investigated over 15 months at six sites along the south coast of South Africa. Initial, subjective classification of sites as wave exposed or wave sheltered (three of each) was confirmed using the dissolution of cement blocks to measure average water flux and dynamometers for maximum wave force. Recruitment occurred throughout the year, but recruit (1–5 mm) densities were significantly higher from January to April 1996 on both shore types. Recruit densities were positively correlated with adult (>15 mm) densities for both shore types (P < 0.05) but the correlations were extremely weak (r ² < 0.06 in each case). In areas with 100% cover, adult size (mean and maximum lengths) was greater on exposed sites, but density showed the reverse and was negatively correlated with maximum wave strength (r = −0.84). Despite differences in adult densities and sizes, biomass, which is a product of the two, showed no significant difference between the two shore types (ANOVA P > 0.05). Thus wave exposure dramatically affects density, recruitment and mussel size, but not recruitment timing or biomass where there is 100% cover, and mediates a three-way interaction among food supply, larval supply and intraspecific competition for space. In contrast to shores with saturation recruitment, mussel biomass here appears to be limited by recruit supply and constraints of food, especially on sheltered shores, while density is regulated through intraspecific competition for space primarily on exposed shores and at small spatial scales.