Search

This article presents a set of Late Pleistocene marine mollusk radiocarbon (AMS) age estimates of 30–5014C kyr BP, whereas a MIS5 age (>75 ka) is indicated by quartz and feldspar OSL dating, biostratigraphy, U-Th dating, and age-depth relationships with sea level. These results indicate that the14C dates represent minimum ages. The age discrepancy suggests that the shells are contaminated by younger carbon following shell death. The enigmatic14C dates cannot be “solved” by removing part of the shell by stepwise dissolution. SEM analysis of the Late Pleistocene shells within a context of geologically younger (recent/modern, Holocene) and older (Pliocene) shells shows the presence of considerable amounts of an intracrystalline secondary carbonate precipitate. The presence of this precipitate is not visible using XRD since it is of the same (aragonitic) polymorph as the original shell carbonate. The combination of nanospherulitic-shaped carbonate crystals, typical cavities, and the presence of fatty acids leads to the conclusion that the secondary carbonate, and hence the addition of younger carbon, has a bacterial origin. As shell material was studied, this study recommends an assessment of possible bacterial imprints in other materials like bone collagen as well.

In this study we present the results of an intersex survey of Littorina littorea along the Belgian coast. Levels of female intersex and sterility were determined to assess TBT related adverse effects. In addition, we determined the levels of male penis shedding and trematode infestation and investigated the morphology of the shell. Significant differences were found for all these variables which clearly differentiated periwinkles from Zeebrugge (B2) from those at other locations. Intersex index (ISI) values were relatively low (i.e. 0.00–0.39), except at B2 where they ranged up to 3.52, the highest value ever reported in literature. Consequently, female reproductive impairment at B2 was severe. Indeed, up to 95% of female periwinkles were sterile at B2. In addition, 61% of the male periwinkles had shed their penis. Furthermore, no trematode infestation could be detected at B2 and specimens from this location had the largest and heaviest shells, which may be related to population demography and/or a different use of energy budgets.

Eight continuous corings in the west-central Netherlands show a 15 to 25 m thick stacked sequence of sandy to gravelly channel-belt deposits of the Rhine-Meuse system. This succession of fluvial sediments was deposited under net subsiding conditions in the southern part of the North Sea Basin and documents the response of the Rhine-Meuse river system to climate and sea-level change and to the glaciation history. On the basis of grain size characteristics, sedimentological structures, nature and extent of bounding surfaces and palaeo-ecological data, the sequence was subdivided into five fluvial units, an estuarine and an aeolian unit. Optical dating of 34 quartz samples showed that the units have intra Saalian to Weichselian ages (Marine Isotope Stages 8 to 2). Coarse-grained fluvial sediments primarily deposited under cold climatic conditions, with low vegetation cover and continuous permafrost. Finer-grained sediments generally deposited during more temperate climatic conditions with continuous vegetation cover and/or periods of sea-level highstand. Most of the sedimentary units are bounded by unconformities that represent erosion during periods of climate instability, sea-level fall and/or glacio-isostatic uplift.

The Scheldt estuary (The Netherlands) consists of two tributaries, of which the western part is heavily polluted (i.e. domestic and industrial pollutants, including heavy metals) and the eastern is relatively clean. Pollutants generally follow downstream decreasing concentration gradients, thus opposing a natural occurring salinity gradient, which decreases upstream. Hence, the Scheldt estuary forms an interesting setting to investigate the effects of both anthropogenic and natural kind.The population genetic structure of the periwinkle, Littorina littorea, has already been studied in this estuary using esterases and RAPD markers. Both studies revealed an estuary based genetic structuring. This genetic patterning is unexpected since L. littorea has a planktonic larval development and thus little genetic differentiation is expected. However, radular myoglobin studies have shown intraspecific variation in L. littorea on a macrogeographical scale. In this contribution, myoglobin profiles of the radular muscle were phenotypically analysed along the Scheldt estuary, using isoelectric focusing.A total of 395 periwinkles, descending from four eastern estuarine, five western estuarine and one marine sampling site were collected and morphometrically characterized after which their individual Mb profile was determined. Four Mb bands could be identified and the distribution of each of these bands differed significantly between the western and eastern Scheldt estuary. In addition, eight different Mb phenotypes were detected of which the distribution differed between both estuaries as well, yielding a significantly higher phenotypic variability in the Western Scheldt. A Dice similarity distance based multidimensional scaling minimum spanning tree (MDS-MST) analysis revealed no clear differentiation between both estuaries. Likewise the MDS-MST analysis failed to detect phenotypic structuring along the pollution-salinity gradient of the Western Scheldt sampling sites. However, the only marine sampling site was clearly separated from the nine remaining estuarine locations in the MDS-MST plot. These preliminary data suggest, in the presence of intense gene flow, (1) a high level of phenotypic Mb variability and (2) a marine versus estuarine phenotypic Mb structuring.

The population genetic structure of the periwinkle Littorina littorea was analysed using random amplified polymorphic DNA (RAPD). Three primers, coding for six putative polymorphic loci were surveyed to infer the genetic structure of seven populations located along the heavily polluted Western (i.e. in order of decreasing pollution load W1, W2, W3 and R1) and the relatively clean Eastern Scheldt (E1, E2 and E3) estuary (The Netherlands). A genetic distance based UPGMA (Unweighted pair group method with arithmetic mean) dendrogram revealed an estuary-related structuring, as Eastern and Western Scheldt sites formed two separate clusters. The Western Scheldt cluster was, however, much more heterogeneous, with three RAPD loci revealing a significant genetic heterogeneity compared to none when the Eastern Scheldt sites were compared. Overall mean heterozygosity levels were high, but did not reveal a difference between the estuaries. The current data (1) confirm the patterns of variation previously observed with electrophoretic analyses of esterases and (2) strongly support that these patterns of variation have a genetic basis, in the presence of intense gene flow. In addition, it is suggested that selection, rather than bottleneck effects, induced by the less favourable living conditions at W1, W2 and W3 are responsible for the genetic patterning.

The Scheldt estuary receives large amounts of pollutants from mainly freshwater sources. This results in rather strong decreasing pollutant gradients from the freshwater to the seawater end of the estuary. Although the situation has improved considerably over the last decade the gradients are still clearly present. Metal and organic pollution loads are usually expressed in terms of the total concentrations of the pollutants present in the exposure phases (i.e. dissolved, suspended, sediment). However, in many cases there is no clear relationship between the concentrations of the contaminants in the environment and the accumulation and effects in biological systems. This is related to the fact that both uptake and effects strongly depend on the biological availability of the pollutants and on the physiological organisation and condition of the organisms being exposed. The situation is especially complex in estuarine systems such as the Scheldt, which are characterised by relatively strong salinity and pollution gradients. The development of functional models for the effects of pollution on estuarine organisms requires the coupling of exposure, to accumulation and effects data taking into account changes in abiotic and biotic conditions. Using both field and laboratory approaches the effect of exposure conditions on the accumulation and toxicity of heavy metals and some classes of organic microcontaminants are studied using sentinel organisms such as the blue mussel, Mytilus edulis. The results are used to evaluate the pollution status of the Scheldt estuary, determine the relative importance of different exposure routes and set exposure treshold values based on exposure-effects relationships.

We used allozyme electrophoresis to compare the genetic variation of an introduced and strongly isolated population of the edible clam Tapes decussatus in the Azores (Lagoa de Santo Cristo, Sao Jorge) with populations from the main range of the species (Ria and Thau). Observed and expected heterozygosity values, number of polymorphic loci, and mean number of alleles per locus in the main-range populations fall within the limits reported for T. decussatus and other Venerid clams. In contrast to previous studies on Venerid clams, we observed no heterozygote deficiencies. In the introduced Lagoa population, we observed a strong reduction of allelic diversity and expected heterozygosities and an effective population size of only 5.30. The Lagoa population is only slightly differentiated from populations from the species' main range and may thus be of low "biological value." Exploitation of T. decussatus could therefore be allowed to continue but must follow strict collection guidelines, especially given that only 15% of the area is suitable for exploitation. Otherwise, a unique component of the Azorean fauna that also serves as a fishery resource may be lost.

The mollusc fauna of the tidal marshes along the river Scheldt was investigated. To this end, 31 marshes were sampled along a salinity gradient from Berlare (Belgium; freshwater) to Vlissingen (the Netherlands; marine). Our faunistic survey yielded eight new localities for the caenogastropod Assiminea grayana, of which seven in the Belgian part. In the marsh of Doel ("Prosperhaven") we discovered the first inland population of the pulmonate mollusc Myosotella myosotis, a species which in Belgium was hitherto only known from three coastal localities. Finally, additional records of Pisidium subtruncatum for the Scheldt estuary are reported.