Morphological and physiological differences between two morphotypes of Zostera noltii Hornem. from the south-western Iberian Peninsula

The morphological and physiological differences between two morphotypes of Z. noltii Hornem. were studied in the intertidal meadows on the south-western Iberian Peninsula (Palmones river estuary and Ria Formosa). A small-leaved morphotype (SM) grows mainly at high intertidal sites, meadow edges or in recently deposited sandbanks, whereas a large-leaved morphotype (LM) generally thrives in well-structured beds or in deeper places. This study deals with the morphological, biochemical and physiological differences between these morphotypes as well as the ecological implications of the occurrence of different morphotypes in the same meadow. Shoot length, leaf width, rhizome internode length, roots per node, root length, leaf nutrient and pigment contents, and photosynthetic rates of both morphotypes were compared. The below-ground architecture (root and rhizome complex) of both morphotypes was more developed in sites characterized by higher hydrodynamics and/or a lower nitrogen content in sediments. Both morphotypes showed similar values for photosynthetic efficiency, dark respiration rate and compensation irradiance. On the other hand, the net photosynthetic capacity was much greater (5-fold) for the SM. This difference could explain the greater growth rate and faster leaf turnover rate of the SM compared with the LM. The occurrence of the SM in newly settled areas (and in the meadow edges) could be explained on the basis of its higher growth rate, which would allow a faster spreading of the meadow and/or better recovery after burial resulting from stormy weathers. Received: 20 December 1998 / Received in revised form: 21 April 1999 / Accepted: 23 April 1999     

Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea

During autumn migration (September to December), brent geese (Branta b. bernicla) and wigeon (Anas penelope) feed on the seagrass Zostera noltii in the nearshore, upper tidal zone leeward of the island of Sylt (eastern North Sea). To graze on leaves and shoots above the sediment and on rhizomes and roots below, these birds reworked the entire upper 1 cm layer of sediment eight times within this 3-month period. In addition, brent geese excavated pits 3–10 cm deep by trampling in order to feed on below-ground phytomass. About 12% of the seagrass beds became pitted to an average depth of 4.5 cm. Using net exclosures, it was estimated that birds removed 34 g dry weight m^–2 of above-ground and 28 g of below-ground phytomass. This corresponds to 45% of the phytomass in September. Of the overall loss of phytomass from September to December, 63% was caused by birds. Roughly half of the leaves fell off anyway until December and the other half were taken by the birds. Below the ground, phytomass remained almost constant where birds were excluded, while with birds phytomass of rhizomes and roots was halved. In spite of this strong effect, in the next vegetation period the blade density was lower at former exclosure sites compared to the ambient seagrass bed. The underlying process seems to be a self-inhibition of dense overwintering seagrass by mud accretion. Assuming our experimental results can be scaled up to the entire seagrass bed, we hypothesize that in the sheltered upper intertidal zone, seasonal erosion caused by herbivorous geese and ducks is necessary for the persistence of Z. noltii. Received: 7 January 1999 / Received in revised form: 23 August 1999 / Accepted: 25 August 1999     

The larval nephridia of the brackish-water polychaete,Nereis diversicolor

The larval nephridia of the brackish-water polychaete Nereis diversicolor are described for the first time, and have been studied to determine if their times of development and structural characteristics are consistent with a role in the osmotic regulation of the larva. As shown in serial paraffin sections and by interference-contrast optics, the nephridia of the three-setiger larva consist of a single pair of very large metanephridia, arising in the 3rd larval setiger, but with their elongated terminal ducts and coiled ciliated tubules pushed forward into the 2nd setiger; their open metanephrostomes and anterior anchoring filaments lie dorsal to the 2nd set of setae. In contrast, the definitive or juvenile metanephridia, arising in the 4th and subsequently formed setigerous segments, have short terminal ducts and coiled ciliated tubules confined to the segments on which their external nephropores open; their nephrostomes are ventrally located and open into the rear of the next anterior segment. These findings are in contrast to the claims of Edouard Meyer (1887), who described two pairs of closed protonephridia in the 2nd and 3rd larval setigers of Perinereis cultrifera. Although it is not excluded that the single larval pair of metanephridia of N. diversicolor may arise as protonephridia, Meyer's claim of two pairs of larval protonephridia was an observational error. The larval nephridia of the marine Platynereis dumerilii resemble in form, but are considerably smaller than, those of N. diversicolor. It is concluded that the hypertrophied pair of larval metanephridia of N. diversicolor is an evolutionary adaptation to existence in habitats of low and unpredictably varying salinity. Their development occurs irrespective of the prevailing salinity; hence, it must be genetically determined.     

Temperature and emergence effects on the net photosynthesis of two Zostera noltii Hornem. morphotypes

Apparent photosynthetic rates (APS) of two Zostera noltii Hornem. morphotypes were measured in air and in water at different temperatures with a closed infra-red gas analysis system (IRGA).Hyperbolic functions accurately described the photosynthesis-CO₂ relationships when the leaves were exposed to air. The photosynthetic behaviour in water, on the contrary, could not be described by Michaelis type kinetics, due to the existence of a rapid transition from the initial slope to the saturation phase. Both morphotypes (narrow-leaved, NLM and large-leaved, LLM) showed higher APS rates in water than in air, although the highest APS rates, in air as well in water, were recorded for the NLM.Temperature had a significant influence on the photosynthetic parameters: APS_max (maximum photosynthetic rate) decreased (in air and in water) with increased temperature in both morphytypes; compensation points (CP) in air increased at high temperature, especially in the LLM. NLM specimens showed enhanced affinity (lower Km) with increasing temperature in air. On the contrary, Km values in water were not significantly affected by temperature.The results suggest that NLM specimens are better adapted than the LLM to occur exposed to air. The distributional pattern of the two morphotypes in the Palmones Estuary is discussed on the basis of their photosynthetic behaviour.     

Habitat suitability of the Wadden Sea for restoration of Zostera marina beds

A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea. Received: 26 April 1999 / Received in revised form: 15 October 1999 / Accepted: 16 October 1999     

Effects of pesticides on seagrass beds

In the German Wadden Sea there has been a remarkable decline in seagrass beds. It was the aim of this study to test whether herbicide contamination could be a reason for this. Concentrations of triazine herbicides such as atrazine, simazine and terbutylazine as well as phenylurea herbicides were measured in Wadden Sea sediments within or in the neighbourhood of seagrass meadows. Sediments were thus used as a marker for medium-term contamination of the Wadden Sea. The respective concentrations were examined in relation to the density and status of the seagrass meadows. Preliminary results show that there may be a connection between seagrass decline and herbicide contamination in the parts of the Wadden Sea sampled. A comparison with other contamination is also given. Received: 11 January 1999 / Received in revised form: 1 December 1999 / Accepted: 8 December 1999     

Policy plans and management measures to restore eelgrass (Zostera marina L.) in the Dutch Wadden Sea

The Dutch Wadden Sea has been changed dramatically over the last centuries by human activities like land reclamation and different forms of fishery. This has, amongst other things, led to changes in the number of biological communities. One of the changes was the near extinction of eelgrass (Zostera marina L.) in the Dutch Wadden Sea. The deterioration of the area led to policy plans in the late 1980s that aimed at restoring the original natural communities of which the eelgrass community was one. This paper presents a restoration strategy which contains a selection procedure for suitable transplantation sites. The selection procedure is based on factors such as sediment composition, exposure time, current velocity and wave action. These were combined in a GIS-based map integrating these factors. One important action in the restoration process is to increase the number of freshwater discharge points to meet the requirements of the brackish water community in general and the growing conditions for eelgrass in particular. Received: 27 October 1999 / Received in revised form: 3 February 2000 / Accepted: 3 February 2000     

Effects of Copper on the Burrowing Behavior of Estuarine and Coastal Invertebrates,the Polychaete Nereis diversicolor and the Bivalve Scrobicularia plana

Alterations of the burrowing behavior of two benthic invertebrates living in intertidal mudflats, the polychaete Nereis diversicolor and the bivalve Scrobicularia plana, were studied in individuals exposed to soluble copper. The design of the contamination procedure took into account the results assessing the influence of some natural factors potentially able to influence burrowing (artificial vs. natural sediment, lighting, size of the shell for bivalves). Animals were exposed for 4 d to concentrations ranging from 25 to 150 μ g Cu l^{? 1}. At the end of exposure, the burrowing kinetics in clean sediment were determined after 1 and 2 d, then the animals were frozen until acetylcholinesterase (AChE) activity determination. Even at the lowest tested concentrations, copper caused hypoactivity in organisms belonging to both species studied. Behavioral impairments were not related to AChE activity inhibition. Metabolical or physiological disturbances could be the cause of these impairments. Concentrations affecting burrowing behavior were below those responsible for lethality in these species.     

高等植物大叶藻研究进展及其对海洋沉水生活的适应

综述和讨论了目前对海洋沉水植物大叶藻的研究进展 ,主要包括 :(1 )形态解剖结构特点 ,(2 )基本生理研究 ,(3 )耐盐机理 ,(4)生存限制因子 ,(5 )问题与展望。其中着重讨论了大叶藻与海洋沉水生活相适应的一些特点 ,特别是其对海水盐度的适应机理。     

Interactive effects of habitat selection, food supply and predation on recruitment of an estuarine fish

 Seagrass meadows are often important habitats for newly recruited juvenile fishes. Although substantial effort has gone into documenting patterns of association of fishes with attributes of seagrass beds, experimental investigations of why fish use seagrass habitats are rare. We performed two short-term manipulative field experiments to test (1) the effects of food supply on growth and densities of fish, and (2) effects of predation on the density and size distribution of fish recruits, and how this varies among habitat types. Experiments were conducted in Galveston Bay, Texas, and we focused on the common estuarine fish, pinfish Lagodon rhomboides. In the first experiment, replicate artifical seagrass and sand plots were either supplemented with food or left as controls. Recruitment of pinfish was significantly greater to seagrass than sand habitats; however, we detected no effect of food supplementation on the abundance of recruits in either habitat. Pinfish recruits in artifical seagrass grew at a significantly faster rate than those in sand habitats, and fish supplemented with food exhibited a greater growth rate than controls in both sand and artifical grass habitats. In our second experiment, we provided artificial seagrass and sand habitats with and without predator access. Predator access was manipulated with cages, and two-sided cages served as controls. Recruitment was significantly greater to the cage versus cage-control treatment, and this effect did not vary between habitats. In addition, the standard length of pinfish recruits was significantly larger in the predator access than in the predator exclusion treatment, suggesting size-selective predation on smaller settlers or density-dependent growth. Our results indicate that the impact of predation on pinfish recruits is equivalent in both sand and vegetated habitats, and thus differential predation does not explain the higher recruitment of pinfish to vegetated than to nonvegetated habitats. Since predators may disproportionately affect smaller fish, and a limited food resource appears to be more effectively utilized by fish in vegetated than in unvegetated habitats, we hypothesize that pinfish recruits may select vegetated habitats because high growth rates allow them to achieve a size that is relatively safe from predation more quickly. Received: 10 October 1996 / Accepted: 5 April 1997     

Seagrass (Zostera spp.) as food for brent geese (Branta bernicla): an overview

Brent geese (called brant in North America) are among the smallest and the most marine of all goose species, and they have very long migration routes between high Arctic breeding grounds and temperate wintering grounds. Like all other geese, brent geese are almost entirely herbivorous. Because of these ecological characteristics they have a high food demand and are strongly dependent on stopover sites to ”refuel” during the migration period. Three subspecies of brent geese are distributed around the Holarctic, forming seven populations with distinct migration routes. Most or all of these populations make heavy use of Zostera spp. during migratory stopovers on spring and/or autumn migration. Examples of Zostera stopover areas being used by large numbers of brent geese for several weeks each year are Izembek Lagoon (Alaska), lagoons in Baja California, the German/Danish Wadden Sea, the Golfe du Morbihan (France), British estuaries, and the White Sea (Western Russian Arctic). Brent geese feed on Zostera wherever they can, but they can only reach the plants at low tide or in shallow water. Changes in Zostera abundance affect brent goose distribution, and the ”wasting disease” affecting Atlantic Zostera stocks during the 1930s was at least partly responsible for a steep decline in brent goose population sizes on both sides of the Atlantic. While Zostera is of outstanding importance as food for brent geese, the impact of the geese on Zostera stocks seems to be less important – at many sites, the geese consume only a small amount of the available Zostera, or, if they consume more, the seagrass can regenerate fully until the following season. Received: 6 December 1998 / Received in revised form: 6 August 1999 / Accepted: 9 August 1999     

Allozymic implications of the propagation of eelgrass Zostera japonica within a river system

We examined the allozymic variation of Zostera japonica, a seagrass in brackish-water zones, for plant samples collected from the Ohashi River (four localities including nine sampling sites within 7.5km) and Lake Nakaumi (one location), to which it is connected, in Japan to study plant propagation. Among five enzymes detected, phosphoglucoisomerase (PGI) showed three alleles (b, c, d) and five genotypes (bb, cc, dd, bd, cd), but the other four enzymes were monomorphic in 185 plant samples. PGI genotype variation was found not only in large meadows but also in small (<1 m) vegetation patches, indicating genet coexistence within a small spatial scale. The observed genotype frequencies in large meadows did not differ significantly from Hardy—Weinberg equilibrium. However, no plant sample included genotype bc. These results suggest that new genet recruitment by mating among genets has occurred but that recruitment may not be very frequent. The allele frequencies differed significantly, not only between distant (1.4 km) meadows but also within short distances (20–40 m) within a meadow. Plants carrying allele c were collected from the upper but not from the lower part of the Ohashi River. These results suggest successful dispersal by water currents is restricted.     

Research on polychaete annelid osmoregulatory peptide(s) by immunocytochemical and physiological approaches. Computer reconstruction of the brain and evidence for a role of angiotensin-like molecules in Nereis (Hediste) diversicolor OF Müller

Summary— Immunohistochemical and physiological studies were carried out on Nereis (Hediste) diversicolor OF Müller in order to obtain evidence concerning the neuroendocrine control of polychaete osmoregulation. The occurrence in this animal of peptides immunologically related to mammalian angiotensin II and I (AII and AI) and oxytocin (OT) was demonstrated in the brain and the ventral nerve cord (VNC) perikarya and nerve fibres as well as in a few peripheral structures (peripheral nerves, epithelial cells, nuchal organ, intestine and nephridia). The exact localization of immunoreactive cells was achieved by serial sections of brain and ventral nerve cord followed by a three-dimensional reconstruction of brain ganglionic nuclei using the CATIA (‘Conception Assistée Tridimensionnelle Inter Active’) Dassault system program. Injections of polyclonal antisera against AII or OT provoked a partial inhibition of the increase in body weight in Nereis exposed to hypo-osmotic medium. The effect of a-AII seemed more pronounced than that of a-OT. In a subsequent test, injections of synthetic AII and AII-amide (peptide recently isolated from an achaete (Salzet et al (1995) J Biol Chem 270, 1575–1582) enhanced the increase in body weight and, therefore, strenghthened the hypothesis of the neuroendocrine control of Nereis osmoregulation. The antidiuretic effect of both synthetic peptides in this study was indicative of the exact role of Nereis endogenous melecule(s). AII was less potent than its amidated form. If AI-like can easily be struck off the list of putative endogenous osmoregulaory factors, the role of OT-like substance in Nereis osmoregulation, which is partially demonstrated in this study, needs to be clarified by further physiological experiments using injection of synthetic peptide(s) or endogenous substance(s). All these results are discussed and compared to those recently obtained in an achaete annelid (Salzet et al (1993) Brain Res 631, 247–255; Salzet et al (1993) Brain Res 601, 173–184; Salzet et al (1995) J Biol Chem 270, 1575–1582.     

Expression and evolution studies of ets genes in a primitive coelomate,the polychaete annelid,Hediste (Nereis) diversicolor

The Ets family includes numerous proteins with a highly conserved DNA-binding domain of 85 amino acids named the ETS domain. Phylogenetic analyses from ETS domains revealed that this family could be divided into 13 groups, among them are ETS and ERG. The ets genes are present in the Metazoan kingdom and we have previously characterized the Nd ets and Nd erg genes in the polychaete annelid Hediste diversicolor. Here, we isolated a fragment encoding the ETS domain from Nd Ets, by genomic library screening. By Northern blot analysis, we showed that this gene was transcribed as one major mRNA of 2.6 kb and one minor mRNA of 3.2 kb. By in situ hybridization, we observed that Nd ets was expressed in the intestine and oocytes and that Nd erg was expressed in cellular clumps present in the coelomic cavity, in an area of proliferating cells situated between the last metamere and the pygidium. Finally, we showed that Nd erg shared the expression pattern of Nd ets in oocytes. Molecular modeling studies have revealed that the spatial structure of ETS domain of Nd Ets and Nd Erg was conserved, in comparison to the murine Ets-1 and human Fli-1 proteins, respectively.     

Modelling the depletion and conservation of forestry resources: effects of population and pollution

 In this paper, a mathematical model is proposed to study the depletion of resources in a forest habitat due to the increase of both population and pollution. It is shown that if the rate of pollutant emission into the environment is either population dependent, constant, or periodic, the equilibrium biomass density of the resource settles down to a lower equilibrium than its original carrying capacity, the magnitude of which decreases as the equilibrium levels of the density of population and the concentration of pollutant increase. However, in the case of an instantaneous spill of pollutant into the environment, the equilibrium biomass density decreases with the increase of the equilibrium density of population only. It is found that if the population density and the emission rate of pollutant increase without control, the forestry resource may become extinct. A conservation model is also proposed, the analysis of which shows that the resource biomass can be maintained at a desired level by conserving the forestry resource and by controlling the growth of population and the emission rate of pollutant in the habitat. Received 1 June 1993; received in revised form 1 January 1997     

Transport of carbon dioxide and ammonium in bioturbated (Nereis diversicolor) coastal, marine sediments

Two identical experiments with sieved and homogenized sandy and muddy sediment were conducted to determine transport enhancement of porewater solutes (TCO₂ and NH₄ ⁺) by the presence of the polychaeteNereis diversicolor (1000–1500 m^–2). Flux measurements showed thatN. diversicolor enhanced the release of CO₂ and NH₄ ⁺ 1.5–5 times. Accordingly, porewater concentrations of these compounds were reduced considerably in the bioturbated zone of both types of sediments. Two different diagenetic models, effective (eddy) diffusion and nonlocal exchange, were used to describe solute profiles in the bioturbated sediments. In permeable sandy sediments advective porewater movements may occur more readily than in more cohesive muddy sediments. The effective diffusion model (with D_e=1.6–2.0 cm² d^–1) provided an excellent fit to the measured concentrations of both solutes below the bioturbated zone in permeable sandy sediment, whereas this model overestimated the concentration in the bioturbated zone. However, in the less permeable muddy sediment the effective diffusion model overestimated the NH₄ ⁺ profile considerably at all depths. The nonlocal exchange model (with=0.17–0.29 d^–1), on the other hand, provided an excellent fit in the less permeable muddy sediment, suggesting that solute profiles here were controlled by molecular diffusion, even in the presence of burrow irrigation. For the permeable sediment, the nonlocal exchange model (with=0.14 d^–1) underestimated the measured NH₄ ⁺ profile. Accordingly, linear slopes from plots of porewater TCO₂ as a function of porewater NH₄ ⁺ revealed that eddy diffusion (or advective porewater movements) was important in the bioturbated zone of this sediment type. However, combined with the generally more realistic shape of profiles derived by the nonlocal exchange, these evidences suggest that both eddy and molecular diffusion must operate in the bioturbated zone of permeable sediments.     

The mapping of phytochrome genes and photomorphogenic mutants of tomato

The map positions of five previously described phytochrome genes have been determined in tomato (Lycopersicon esculentum Mill.) The position of the yg-2 gene on chromosome 12 has been confirmed and the classical map revised. The position of the phytochrome A (phy A)-deficient fri mutants has been refined by revising the classical map of chromosome 10. The position of the PhyA gene is indistinguishable from that of the fri locus. The putative phyB1-deficient tri mutants were mapped by classical and RFLP analysis to chromosome 1. The PhyB1 gene, as predicted, was located at the same position. Several mutants with the high pigment (hp) phenotype, which exaggerates phytochrome responses, have been reported. Allelism tests confirmed that the hp-2 mutant is not allelic to other previously described hp (proposed here to be called hp-1) mutants and a second stronger hp-2 allele (hp-2 ^j ) was identified. The hp-2 gene was mapped to the classical, as well as the RFLP, map of chromosome 1. Received: 24 May 1996 / Accepted: 14 June 1996