Spatial variability in growth and reproduction of the Pacific oyster Crassostrea gigas (Thunberg, 1793) along the west European coast

The Pacific oyster Crassostrea gigas was introduced in Europe for commercial purposes in the mid 1960s. It was initially thought that low winter temperatures would restrain this species' reproduction and settlement; however, its present distribution in areas where no introduction has taken place suggests that natural invasion and expansion has occurred. Along the European coast, wild populations of Pacific oysters are already found from northern Germany to southern Portugal. Whether C. gigas will continue to further expand through northern waters will depend on its physiological performance. In this study, the performance of wild oyster populations has been studied in terms of growth and reproduction at three stations: La Rochelle (France; 46°N), Yerseke (Oosterschelde estuary, The Netherlands, 51°N), and Texel (Wadden Sea estuary, The Netherlands, 53°N). The French population had the lowest somatic-shell mass ratio and an increase in maximum shell length, somatic and gonadal mass was observed from France to the Netherlands. In addition, mean oocyte diameter decreased significantly from south to north. The combination of increasing gonadal mass and decreasing oocyte volume suggests an increasing reproductive output in terms of egg numbers from France to The Netherlands. Differences in temperature between locations will at least be partly responsible for the observed patterns; however, other environmental factors (such as food availability, predation pressure, sediment type and/or seston concentration) cannot be excluded. Since smaller eggs (oocytes) are thought to have a longer development time, the environmental conditions along the Dutch coast may result in increased larval dispersal and possibly in further population expansion.     

Growth and reproduction of the bivalve Spisula subtruncata (da Costa) in Dutch coastal waters

The bivalve Spisula subtruncata is usually abundant in shallow coastal waters along the Dutch coast. However, its biomass has been decreasing since 1995. In order to assess whether reproductive failure may be the cause of the observed decline over the last decades, the energy investment in reproduction of a population of S. subtruncata from central Dutch coastal waters was studied. The population studied consisted of individuals of up to four years old. Shell length reached maximum values of around 32 mm and individual total body, somatic and gonadal ash-free dry mass reached maximum values of about 278 mg AFDM, 252 mg AFDM and 76 mg AFDM, respectively. A clear seasonal cycle in somatic and gonadal mass was observed. Somatic and gonadal mass indices increased in early spring and reached maximum values during summer, followed by a decrease to minimum values at the beginning of the following year. Spawning was in June–July and settlement of spat seems to have occurred in July–August. Mean oocyte diameter was 57.43 ± 0.03 μm, corresponding to a volume of 98972 μm³. These results suggested that reproductive failure was not the cause of the current population decline. Most likely, unsuccessful settlement of spat and/or severe predation during the first months of life were responsible for the observed patterns.     

Differential reproductive strategies of two bivalves in the Dutch Wadden Sea

Cerastoderma edule and Mya arenaria are two common bivalve species in European waters. Longevity and maximum size are much greater in the latter species. Because comparison of species life-history strategies states that a long life span (i.e. high annual survival) generally goes with lower fecundity, we hypothesise that reproductive output would be lower in M. arenaria than in C. edule. In the present paper, we studied the reproductive strategies of these two species in an intertidal and a subtidal area of the western Dutch Wadden Sea, by following seasonal changes in absolute and relative weights of somatic and gonadal tissues in these bivalves. Starting of spawning was similar in the two species, around May, except for intertidal M. arenaria, which initiated spawning in August. Individual energy investment in reproduction was similar for the two species but, unlike M. arenaria, C. edule spawned completely, releasing all energy of gonadal mass in the form of gametes. Mya arenaria used the gonad not only for reproduction but also for storage. In the intertidal area, we found a trade-off between longevity and reproduction, i.e. maximum reproductive output (expressed as a proportion of body mass) was higher in C. edule than in M. arenaria. However, since body size is larger and life span longer in M. arenaria than in C. edule, mean lifetime reproductive output per individual must be higher in the first than in the latter. Based on the differences in reproductive strategies of these two species, we hypothesise that the negative effects of warming climate on bivalve population dynamics in the Wadden Sea will be stronger in C. edule than in M. arenaria.     

Altimetry Data Assimilation Into a Numerical Model of Ocean Dynamics in the South Atlantic

Oceanology - The data assimilation (DA) of satellite observations of the ocean level from the Archiving Validating and Interpolating Satellite Observations (AVISO) into the IWM model (G.I. Marchuk...     

The urban geolinguistics of Cape Town

In the changing South Africa of the nineties language planning becomes a top priority on the agenda of constitutional reform. It appears however, that some misplaced enthusiasm is generated by language planners regarding the relatively well defined language regions on a national level. It should be kept in mind that approximately two-thirds of the South African population presently resides in high density urban areas. Contrary to the relatively clear-cut regionalization implied by the rural language patterns, a different form of regional demarcation and language planning is necessary in large metropolitan areas where the population concentrations are the result of a multilingual migration process. Cape Town is a suitable experimental ground to focus the methodological and conceptual shift from a national to an urban resolution level in language mapping. The aim of this paper is to address the following issues: The generalconceptual niche of cities in geolinguistics; the spatial identification ofpresent language distribution in Cape Town; the time-space pattern oflanguage change in Cape Town; the socialprofile of language speakers in Cape Town; and the consequences of the findings forlanguage planning in Cape Town and South Africa. Although Cape Town is linguistically probably one of the least diverse cities in South Africa, three language groups (Africaans, English and Xhosa) are clearly apparent within the limited surface area of the urban space. Their respective distribution patterns are relatively segregated within specific neighbourhoods of the city.     

Synoptic interpretation of seismic reflection and refraction data

Summary. The structure of the upper lithosphere beneath southern Germany, northern Switzerland and west-central Utah (U.S.A.) has been investigated in detail by various geophysical methods. A synoptic interpretation of travel time and amplitude data obtained in seismic refraction and wide-angle reflection surveys, combined with near-normal incidence reflection observations, now permits the elucidation of the fine structure in a more quantitative and unified manner. With this scheme it is possible to unambiguously identify low-velocity zones and to deduce velocity gradients if reliable amplitude information is included in the inversion process.     

Progress report on rhenish massif seismic experiment

This paper presents some data and results from a seismic refraction experiment, completed mainly in 1979 in the Rhenish Massif, Federal Republic of Germany and extending through Luxembourg and Belgium into the Paris Basin in France.Velocity-depth functions have been derived for each record section independently, based on the assumption that velocity varies only with depth: these models are being improved upon by time-term and ray-tracing methods capable of handling laterally varying velocity structures and by calculating synthetic seismograms.The P_g phase which is observed very clearly on all record sections represents a refracted wave, with velocity generally > 6 km/s, from depths below 1.5–5.5 km. Along the 600 km long main profile one intracrustal reflection can usually be recognized, while from the three shorter crossing profiles in the massif two intracrustal reflectors can always be seen. Beneath much of the main profile the crust-mantle boundary is either a first order discontinuity or thin (< 1 km) transition zone at ~30 km depth. However, beneath the Ardennes and West Eifel there is a 6–12 km thick transition zone before a velocity of 8.1 km/s is reached at ~36 km depth. Beneath the crossing profiles, there is generally a transition zone < 3 km thick between crust and mantle. In some cases, there can be recognized at the top of the mantle a thin high velocity layer which is underlain by a low velocity layer which, in turn, is underlain by a reflector 4–11 km below the crust-mantle boundary.