Ontogeny of osmoregulation in the European sea bass Dicentrarchus labrax L.

 Under natural conditions, sea bass eggs hatch in the open sea and young-of-the-year sea bass are found close to estuaries and enter brackish and even freshwater lagoons in the Mediterranean and the eastern Atlantic. The ontogeny of osmoregulation in the European sea bass, Dicentrarchus labrax (Linnaeus, 1758), was studied in different developmental stages, from hatching to large juveniles, exposed to a range of salinities at 17 °C. The experiments were carried out from March to August 1998 in southern France. The type of hyper-hypo-osmotic regulation did not change during development. All stages hyper-regulated at low salinity (under 10–11‰) and hypo-regulated at higher salinities. The acquisition of the full ability to hypo-regulate occurred in four steps. Osmoregulatory capacity was size- and age-dependent and reached its maximum for fish 17–26 mm long, 63–86 days after hatching. The iso-osmotic salinity was 10.2–11.6‰. Our results suggest that early development of osmoregulatory ability, and thus of salinity tolerance in sea bass, may provide an advantageous flexibility for the timing of migration towards low-salinity habitats. Received: 22 May 2000 / Accepted: 18 December 2000     

Spermatozoan ultrastructure and spermatogenesis in aplousobranch ascidians,with some phylogenetic considerations

Spermatozoa and some stages of spermatogenesis were studied for four species of aplousobranch ascidians. Spermatozoa of Clavelina lepadiformis (Müller) (family Clavelinidae) are plesiomorphous in that they have apical acrosomal vesicles and a moderately elongated, cylindrical nucleus. The elongated mitochondrion is twisted ea. 11/2 times around the nucleus. In their ultrastructural morphology, C. lepadiformis sperm conform in some respects to the least-derived ascidian sperm, those of the phlebobranch ascidian Ciona intestinalis L.; however, the sperm of Clavelina lepadiformis have two apomorphies not shared with those of Ciona sp.: (1) the mitochondrion of Clavelina lepadiformis is long and spiralled along the entire nucleus rather than being comparatively compact and not at all helical; (2) the mitochondrial cristae are elongated parallel to the long axis of the nucleus, whereas in Ciona sp. sperm the cristae are unmodified. In Distaplia sp., Aplidium sp. and Synoicum pulmonaria (Ellis and Solander) the spermatozoa are more derived and consist of a proximal cylindrical and a distal corkscrew-like part. The mitochondrion in Distaplia sp. and Aplidium sp. contains electron-dense material and extends in a long thread around the nucleus. In S. pulmonaria the mitochondrion surrounds the anterior part of the nucleus in mature spermatozoa, and an elongated, dense structure displaying fine striation is enclosed in the mitochondrion in late spermatids. The sperm ultrastructural morphology observed in this study is consistent with the majority view that clavelinids are closest to the ancestral ascidian but is also consistent with other conclusions, particularly that the Cionidae are closest to the stem ascidian.     

Potential impact of a toxic dinoflagellate (Alexandrium excavatum) bloom on survival of fish and crustacean larvae

During the last 20 yr the western half of the Dutch Wadden Sea has undergone significant eutrophication: concentrations of P and N compounds and planktonic algae have roughly doubled, as has primary production. Though oxygen levels are often low in summer, anoxic areas are small and rare due to strong tidal mixing. During the 1970 to 1990 period, macrozoobenthos was sampled annually at 15 stations at Balgzand, a 50-km² tidal-flat area in the westernmost part of the Wadden Sea. Not only did the estimates of total numbers, biomass, and production double during these two decades, but significant changes in the composition of the benthic community were observed, too: (1) the numerical proportion of polychaetes increased at the expense of molluscs and crustaceans, (2) the overall mean weight per individual of the macrozoobenthos decreased (numbers of individuals of small-sized species increased more rapidly than those of large-sized species), and (3) though absolute numbers and biomass of all feeding types increased, the share of carnivores declined and that of deposit feeders increased; the proportion of suspension feeders showed little change. This study refers to true macrobenthos only (1-mm sieve) and further excludes two taxa (Corophium spp. andHydrobia ulvae) which occasionally exercised an undue influence on numbers. Mass mortalities caused by low oxygen concentrations were of a small-scale nature only. Total number of species fluctuated without a clear trend. As a consequence of the increasing numerical densities, trends in species numbers were slightly increasing when expressed per unit of area and slightly decreasing when estimated per 100 individuals (by rarefaction).     

Subcellular localisation and identification of95mTc- and241Am-binding ligands in the hepatopancreas of the lobsterHomarus gammarus

Following uptake of technetium-95m and americium-241 by lobsters (Homarus gammarus) from food, the subcellular distribution and binding in the major organ of accumulation, the digestive gland, were analysed.^95mTc was predominantly bound to soluble proteins; approximately 40% was bound to two proteins displaying the same elution characteristics as Cdthioneins as indicated by gel permeation and ion-exchange chromatography, indicating reduction and sequestration by metallothionein in vivo. The identity of a third anionic low molecular weight Tc-binding component was not established. No evidence was found for binding of Tc to haemocyanin, ferritin or transferrin. Particulate Tc [probably TcO(OH)₂] was bound to plasma membranes and was also present in lysosomes, indicating reduction of pertechnetate and pinocytosis in the digestive gland. In contrast,²⁴¹Am was mainly sequestered by the lysosomal/vacuolar system, indicative of uptake by pinocytosis. Approximately 2 to 3% of the Am was bound to ferritin and 20% was bound to unidentified ligands of 10 and 20 kdaltons mol. wt. There was no evidence for sequestration of Am by metallothionein. These studies prove that waste radionuclides discharged by the nuclear industry are metabolically active and enter the normal metal metabolism and detoxication pathways of a marine shellfish.     

Heavy metals and organic carbon in sediments from the Tuy River basin,Venezuela

The Tuy River basin, located in north-central Venezuela with an annual average temperature of 27°C and precipitation of 140 cm, was selected to conduct a geochemical study of bottom sediments, with the object of establishing the natural and human influences in the abundance and distribution of Fe, Mn, Cr, Co, Cu, Ni, Pb, Zn and organic carbon. The basin is lithologically divided into two sub-basins, north and south. The north sub-basin drains a iow-grade metasedimentary terrain with a population density of 800 persons km^–2 and approximateiy 600 industrial sites, while the south sub-basin in underlain by metavolcanic and ultramafic rocks, with a population density of less than 10 persons km^–2.Stream bottom sediment samples (150) were collected during the years of 1979–1986 in 16 unpolluted sites and 13 polluted sites. The sediments were air dried at room temperature and sieved through a 120 stainless steel mesh (125 m). Samples of grain size smaller than 125 m were analysed, the heavy metals being determined by atomic absorption spectrometry and the organic carbon (Corg) by dry combustion.The higher concentrations of heavy metals and organic carbon found in the pristine areas were in the south sub-basin, especially in those areas with higher annual precipitation and tropical forest. This indicated that the metavolcanic and ultramafic rocks yield higher concentrations of heavy metals than the metasedimentary rocks. It was also noted that the higher concentrations of Cr and Ni are associated with the ultramafic rocks. The results obtained from the sediment samples collected in the polluted sites showed that the elements Pb, Zn and Corg are enriched up to 4 times as a result of ail the human activities taking place in the basin. Organic carbon is an excellent indicator of domestic wastewater, Pb and Zn are good indicators of the automotive traffic and industrial effluents. The concentrations of each heavy metal did not show any significant correlation with grain size fractions; however, the concentration of organic carbon did show a negative correlation with grain size. The lithological, climatic and vegetation influence in the abundance of heavy metals and organic carbon in stream sediments clearly indicates the necessity of establishing background levels for the area under study when carrying out studies in environmental geochemistry.     

Ethologie alimentaire d'invertébrés benthiques filtreurs (ascidies). I. Dispositif expérimental. Taux de filtration et de digestion chez Phallusia mammillata

In a study on ascidian feeding ethology, rates of filtration and digestion of the unicellular algae Monchrysis lutheri by Phallusia mammillata (Cuvier, 1815) have been determined experimentally. All results were obtained under constant conditions of temperature, salinity, pH, light, and food, using 5 individuals in 9 experiments of 24 h each. The method of Winter (1969) has been modified by using an automatic system which enables constant food concentrations (±5%) to be maintained. Controls are made by a fluorometer. The rate of filtration is deduced from the quantity of culture food added each hour; this rate varies; a well defined rhythm is not observed, but alternating maxima and minima. The values obtained for, specimens of medium size (10 to 12 cm) vary from 3856 to 4730 ml/h and per gramme organic dry weight, the average being 4380 ml. The quantity of food actually converted by each ascidian, once it has passed the alimentary canal, is calculated from the amount of proteins present in food and faeces. Digestion rate varies from 4.65 to 5.75 mg of albumin equivalent/24 h and per gramme organic dry weight (mean 5.30 mg). The percentage food conversion (filtrated amount=100%) is high: 86 to 93%, with a mean of 90%.     

Regulation of worker reproduction in bumblebees (Bombus terrestris): workers eavesdrop on a queen signal

In the annual bumblebee Bombus terrestris, the onset of queen-worker conflict over male production is seasonally and socially constrained. Workers will do better if they start to reproduce (the so-called competition phase) only after ascertaining that larvae are committed to gyne development but before the season ends because they gain more by rearing sister-gynes than their own sons. Here, we tested two nonmutually exclusive hypotheses as to what triggers the onset of worker reproduction: Workers can directly monitor larval development and/or workers eavesdrop on the queen signal that directs gyne development. Exposing workers to gyne larvae through a double mesh did not advance the competition phase compared to control colonies. However, when workers, but not the queen, were allowed contact with gyne larvae, both the competition phase and gyne production were advanced. Thus, while larvae do not emit a volatile pheromone that discloses their developmental route, the physical contact of workers with such larvae triggers early competition phase. However, workers exclusively exposed to worker larvae (colonies prevented from producing gyne larvae) started to reproduce at the same time as control colonies. Replacing the resident queen with an older queen (from gyne-rearing colonies) advanced the competition phase, irrespective of worker age. The results are consistent with the hypothesis that workers eavesdrop on the queen pheromones. This is adaptive because it allows workers a broader time-window for reproduction and thus to gain fitness from rearing both sister-gynes and sons before the season ends without affecting colony development.     

Differences in sterol composition of clams (<Emphasis Type="Italic">Ruditapes decussatus</Emphasis>) from three rías in Galicia (NW Spain)

The sterol composition of three different populations of Ruditapes decussatus from three localities close to each other, but where different environment conditions prevail, was investigated over a period of 14 months. Sterols of adult clams were isolated by thin layer chromatography and identified by gas chromatography/mass spectrometry. In all samples, the major sterol component was cholesterol (>40% of total sterols); other sterols identified were 24-norcholesta-5,22-dienol, 22-cis-dehydrocholesterol, 22-trans-dehydrocholesterol, brassicasterol, 24-methylene-cholesterol, campesterol, stigmasterol, -sitosterol and isofucosterol. At each locality, changes in sterol levels followed specific and different patterns, which remained constant for the period studied and allowed the origin of the clams to be distinguished. Stepwise discriminant analysis, based on the percentage amounts of the sterols at each locality, indicated brassicasterol, 22-cis-dehydrocholesterol, cholesterol, 24-norcholestadienol and -sitosterol as discriminant variables that distinguish clam populations. Correct identification of the clam origin was achieved in 100% of the samples, demonstrating that sterols can justifiably be used as molecular biomarkers for determining the origin of this bivalve species.Communicated by O. Kinne, Oldendorf/Luhe     

Ant gardens: interspecific recognition in parabiotic ant species

 In French Guiana, parabiotic societies (natural mixed colonies) are frequently found in ant gardens. Crematogaster limata parabiotica (Myrmicinae), often associated with Camponotus femoratus (Formicinae), was found for the first time in parabiosis with ponerine ants: Pachycondyla goeldii and Odontomachus mayi. A detailed study of the relationships between Cr. l. parabiotica and O. mayi showed that each species is aggressive towards allospecific or conspecific individuals belonging to another colony, but tolerates allospecific individuals from the multi-species society. Studies of cuticular substances of the four ant species were made using gas chromatography. The results showed that each species, living alone or in parabiosis, possesses a specific chemical profile. Thus, the ants are able to recognise nestmate and non-nestmate individuals of the associated species, even though their cuticular profiles are different. The hypothesis that the nestmate allospecific profile is learned is suggested to explain this pattern of recognition. Received: 5 June 1996 / Accepted after revision: 17 October 1996     

Phaeocystis blooms and eutrophication of the continental coastal zones of the North Sea

It is suggested that novel nuisance algal blooms can result from major shifts in N/P or NH₄ ⁺/NO₃ ^- ratios. Inland hydraulic engineering caused a shift from P-limitation (before 1977) towards N-limitation (after 1977) in the Marsdiep area (Dutch coastal waters). Following this shift the colonial flagellate Phaeocystis sp. became more abundant and started to bloom during the nutrient-controlled period (later spring to autumn). Competition experiments showed that the N/P ratio can influence the species composition of marine phytoplankton. In addition, the natural distribution of some species like Rhodomonas sp. and Emiliania huxleyi may be affected by the frequency of nutrient pulses in the system. Phaeocystis was a poor competitor under P-limitation and a good competitor under N-limitation. Colony formation was absent under P- and NH₄ ⁺-limitation. Colonies were formed under NO₃ ^--limitation. These preliminary results suggest that colony-forming Phaeocystic blooms may be restricted (besides light-controlled environments) to those N-controlled environments where nitrate is consumed by Phaeocystis. The distribution of Phaeocystis along the European continental coast is evaluated on the basis of its ability to compete for nutrients and to form colonies when nitrate is the major N-source.