R.H. Bogaards

Spatial and seasonal differences in the PCB content of the mussel Mytilus edulis

Seasonal and spatial variation in the concentration of PCBs, fats (non-polar lipids) and total lipids and the condition of the mussel Mytilus edulis were assessed in three differing water bodies of the Dutch delta area. Highest concentrations of PCBs in the mussel were found in the Westerschelde estuary, with much lower concentrations in the Oosterschelde and the brackish lake Grevelingenmeer. Spatial differences were strongly related to salinity; lower concentrations were found at the more saline stations, pointing to freshwater inputs as being the origin of the PCBs in mussels. The PCB concentration in mussels, on the basis of dry or total weight, in general increased during summer, autumn and winter and decreased strongly during spring. The strong decrease is related to the spawning of gametes. Seasonal changes in the PCB concentration on the basis of the dry weight were not related to changes in the fat content or the condition of the animals. Only PCBs on a fat basis were negatively related to fat content, indicating a dilution of PCBs during seasonal fat accumulation and concentration of PCBs during fat utilization, in such a way that the total PCB concentration in the animal remains the same. It seems that, besides reproduction, equilibrium partitioning is the most probable mechanism that determines the PCB content of mussels.

Uniform variation in genetic traits of a marine bivalve related to starvation, pollution and geographic clines

Consistent patterns of genetic variation in the marine bivalve Macoma balthica (L.) were found after exposure to low levels of copper, starvation, and along geographic clines. The geographic clines were related to temperature and salinity. Genetic differences were primarily found in the LAP (Leucine aminopeptidase) locus; under stress the frequency of one specific allele, and thereby the heterozygosity, decreased strongly. The degree of changes depended on age and condition of the animals; adult animals showing stronger changes than juveniles. Low concentrations of copper yielded stronger genetic changes than high concentrations. It is concluded that genetic diversity and stress-sensitivity of species change predictably along a gradient in response to environmental harshness. Animals near the southern limit of their range were more sensitive to additional stress.

The glycogen content in stressed marine bivalves: The initial absence of a decrease

Abstract 1. 1. Changes in the glycogen content, condition, stomach content and acetic acid concentration of mussels Mytilus edulis and cockles Cerastoderma edule were followed during periods of up to 14 days of exposure (to air) at temperatures of 5 and 20°C. 2. 2. In animals with a high glycogen content the glycogen is not used during the first 3 to 7 days, at high and low temperature respectively. 3. 3. After this latent period the glycogen concentration often decreased, coinciding with a high mortality and an increase of the concentration of acetic acid. 4. 4. In cockles with a low glycogen content, and kept at a high temperature, glycogen can be used from the beginning of the stress period. 5. 5. Between species no clear differences were found. 6. 6. The stomach content decreased during exposure; however, the stomach content amounted to only 0.5 to 0.7% of the body weight, and is thought to be of minor importance as an energy source during the stress period. 7. 7. Especially at the higher temperatures glycogen finally is transformed into acetic acid. 8. 8. It is concluded that during exposure, the animals do not die because of a lack of energy reserves, but because of a high accumulation of acids.

A comparative study on the relation between copper and condition in marine bivalves and the relation with copper in the sediment

The relation between condition and copper concentration was assessed in three bivalve species (the Baltic clam Macoma balthica, the cockle Cerastoderma edule and the mussel Mytilus edulis) from several Dutch and French estuaries. In general, the copper concentration in the bivalves was negatively related to condition. The slope of this relation was strongest in the Baltic clam M. balthica, with an almost inversely proportional change of Cu concentration at a change of condition, indicating an almost constant Cu content (body burden) per individual, i.e. no elimination or accumulation of Cu. In the mussel M. edulis, the slope of this relation was absent or the weakest, with an almost uniform Cu concentration at a change of condition, indicating an almost proportional elimination (or accumulation) of Cu at a decrease (or increase) of the condition. Thus in Baltic clams the copper body burden is kept at an almost constant level, whereas in mussels the copper concentration is almost constant. In Baltic clams, but not in mussels and cockles, spatial differences were related to copper concentrations in the sediment. It is discussed that mussels maintain a homeostatic Cu concentration within a short time-period (days; copper highly exchangeable by complexation to metallothioneins), whereas the Cu concentration in Baltic clams is more slowly regulated (several weeks; fossilization of copper in mineral granules) and remains on an average in (partitioning) equilibrium with the Cu concentration in the sediment. Therefore, the Baltic clam is a better indicator of sediment copper pollution than the commonly used mussel. In addition, the observed differences between species were compared with differences in their life-strategies. The Cu concentration in the sediment is strongly related to the silt fraction (< 16 μm) of the sediment.

Genetic traits in the bivalve Mytilus from Europe, with an emphasis on Arctic populations

Abstract Genetic and some ecophysiological traits of mussels collected in the European Arctic, up to their northeastern distribution limit in the Barents Sea, were studied and compared with traits of mussels from the Mediterranean, Atlantic and Baltic. The genetic traits of these populations were analysed by isoenzyme electrophoresis on seven loci in order to assess the Mytilus complex to which populations in the Arctic region belong. Ecophysiological variables, the weight-index and glycogen were analysed to assess the physiological fitness of the populations. Three distinct groups were recognised: (1) Mytilus (edulis) galloprovincialis in the Mediterranean and Spain, (2) M. (edulis) edulis along the Atlantic coast from the Netherlands northwards into Russia, and (3) the Baltic Mytilus (edulis) trossulus. The mussels from populations in the Russian Arctic all belong to the Atlantic Mytilus (edulis) edulis group. The genetic variability and ecophysiological measures indicated that the sub-Arctic White Sea mussel populations have a relatively lower performance capacity, whereas those in the Arctic at the edge of their northern distribution showed a surprisingly strong performance.

The respiratory performance and survival of the bivalve Macoma balthica (L.) at the southern limit of its distribution area: a translocation experiment

The hypothesis was tested that animals near their extreme Southern limits, due to high temperatures, have a high respiration rate, whereby they reach an extreme low weight-index and ultimately disappear. At estuarine stations the respiration rate of Macoma balthica (L.) (Baltic clam) did not show interseasonal changes, indicating seasonal acclimation, but within the season the respiration increased with increasing temperature, indicating the absence of short-term acclimation. In clams translocated from the Netherlands towards the Bidasoa estuary, 200 km South of their Southern distribution limit, their respiration rate was higher and weight-index lower than in specimens living in Dutch estuaries. Irrespective of an effect of the temperature, clams exposed in experiments to water from Bidasoa showed a higher respiration than clams exposed to water from the other stations. Moreover, at non-estuarine stations with a low food content, the clams showed reversed acclimation, i.e., the respiration rates in winter were much lower than summer rates, most probably a strategy to conserve energy by means of a depressed metabolism. A weight index of 5 mg DW/cm(3) and glycogen content of 2% DW are suggested as the minimal values below which the metabolic energy balance of Baltic clams becomes more negative and the clam population disappears. It was concluded that factors other than temperature influenced the respiration and weight-index of clams, and hence their presence or absence, e.g., food concentration, innate seasonal cycles, and possible pollutants in the water.

Relationship between PCB concentrations and reproduction in mussels Mytilus edulis

Abstract Changes in the concentrations of PCBs and fats were followed in mussels Mytilus edulis, transferred vice versa between a polluted estuary (Westerschelde) and a relatively low-pollution sea-arm (Oosterschelde). The concentrations in the mantle, which contains the gonads, were determined separately from the rest of the body. The fat content of the mantle increased 1 1 2 - to 2-fold during the observation period (autumn, winter) and remained constant in the rest of the body. Polychlorinated biphenyls (PCBs) in mussels transferred from the polluted to the less polluted area were not eliminated. In the mussels transferred to the polluted area the increase of the PCB content was 2- to 3-fold over a period of 3 months. PCBs in the mantle, and therefore in the gonads, can amount to more than 40% of the total PCB load in a mussel. It was concluded that during the spawning of gametes a substantial proportion of the PCBs may be shed.

Sensitivity to stress in the bivalve Macoma balthica from the most northern (Arctic) to the most southern (French) populations: low sensitivity in Arctic populations because of genetic adaptations?

The stress sensitivity, determined in copper exposure experiments and in survival in air tests, and the genetic structure, measured by means of isoenzyme electrophoresis, were assessed in populations of the Baltic clam Macoma balthica (L.) from its southern to its northern distribution limit, in order to test the hypotheses that near the distribution limit the clams would be more stress sensitive and would have a lower genetic variability. The populations in west and north Europe show a strong genetic resemblance. The populations in the sub-Arctic White Sea are genetically slightly different, and show a low stress sensitivity. The populations in the Arctic Pechora Sea are genetically very distant from the other populations, and show the lowest stress sensitivity. Near the southern distribution limit, in agreement with the hypotheses, genetic variability is low and stress sensitivity high. On the other hand, in contrast to expectation, near the northern distribution limit, in the populations of the Pechora Sea, the genetic variability was higher, thus not reduced, and the stress sensitivity was low compared to all other populations. Yet, it remains a question if such is due to gradual physiological acclimatization (and ongoing differential selection) or to genetic adaptation.

Physiological responses of Macoma balthica to copper pollution in the Baltic

Physiological and behavioral responses to Cu exposure were measured in the Baltic clam Macoma balthica from the Gulf of Gdansk, southern Baltic Sea. The burrowing activity, mortality rate, glycogen content, condition index and free amino acid (FAA) composition were analysed as indicators of stress in a series of field and laboratory studies. M. balthica exposed to Cu showed clear Cu-concentration related differences in burrowing activity and mortality rate, but no consistent differences in the condition index, glycogen content, and free amino acids. The clams from a less polluted area reacted more strongly and were more sensitive to additional stress as compared to organisms from a more polluted region. The effect of Cu on the ecophysiology of Baltic clams in the field was probably obscured by reproduction-related changes in the organism. The role of sediment as a potential source of Cu in the Baltic clam was discussed.

Sensitivity to stress of the estuarine bivalve Macoma balthica from areas between the Netherlands and its southern limits (Gironde)

Abstract Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the speciess southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.