Marenzelleria viridis (Polychaeta: Spionidae): a review of its reproduction

The features of the reproductive biology such as morphology of gametes, larval morphology, larval development and development of gametes of Marenzelleria viridis Type II from the Baltic were summarized. Further reproductive features of Baltic Sea populations are given and the purpose of the review is to account for the successful immigration of M. viridis into the oligohaline areas of the Baltic considered against a background of such variables as population density and structure, salinity, temperature, food availability, oxygen and sulphide levels. Gametogenesis started in spring. Fecundity of animals depended on salinity, temperature, age and size of worms. Mature oocytes contained large cortical alveoli not yet known for polychaetes. Animals spawned in autumn in all years of investigation. The pelagic larvae of M. viridis Type II were found mainly from September to November. Larval development depended on water temperature and lasted about 4 to 12 weeks. Successful larval development from egg to juvenile was not possible below salinities of 5, but colonization of oligohaline regions took place by larvae with more than 4 setigers or by swimming juveniles. Reproductive features of M. viridis Type II from the Baltic were compared and discussed with the results of M. viridis Type I populations from the North Sea and North America. The two M. viridis types reproduced at different time, M. viridis Type I reproduced in spring and M. viridis Type II in autumn. Both types showed also differences in larval development, gametal development and sex ratio of mature worms.     

Marenzelleria cf. viridis (Polychaeta: Spionidae) – ecophysiological adaptations to a life in the coastal waters of the Baltic Sea

Since its introduction into the Baltic Sea about ten years ago, the polychaete species Marenzelleria cf. viridis has spread rapidly. Meanwhile, this spionid settled in remarkable numbers predominantly in the coastal waters which provide some of the more variable and unstable habitats in terms of abiotic conditions. In the present paper, some of the underlying biochemical and physiological processes were experimentally examined which enables this polychaete to deal with several kinds of environmental stress such as variations in salinities, low oxygen concentrations and occurrence of hydrogen sulphide. The results obtained reveal that in the process of acclimation to different salinities free amino acids are involved, mainly alanine and glycine. The content of these amino acids were increased in worms acclimated to a higher salinity. When exposed to low ambient oxygen concentrations (severe hypoxia) M. cf. viridis started to produce energy anaerobically via pathways known from several other marine organisms. The presence of hydrogen sulphide in higher concentrations (1 mmol l^-1) results in a more rapid production of succinate (indicator for anaerobic energy production via the succinate-propionate pathway) as compared to hypoxia alone, indicating a higher energy demand. Nevertheless, when exposed to enhanced ambient hydrogen sulphide concentrations (3 mmol l^-1), M. cf. viridis is less affected by this toxic compound than other marine species, such as the related species Marenzelleria cf. wireni (North Sea). The field data indicate that the metabolic response obtained under experimental conditions is also important in the worm's natural habitat. Although the extent of anaerobic metabolites produced as well as the amount of thiosulphate (main sulphide detoxification product) was lower in the field as compared to the experiments, M. cf. viridis very likely has to face hypoxia and hydrogen sulphide in higher concentrations in its natural habitat.     

Marenzelleria cf. wireni (Polychaeta: Spionidae) from the Tay estuary. Metabolic response to severe hypoxia and hydrogen sulphide

The appearance of the spionid polychaete Marenzelleria spp. in the Tay estuary (Scotland) was first reported in 1984. Since estuaries are known as environments where abiotic conditions fluctuate widely, a laboratory study was conducted aimed to elucidate how this introduced species deals with low oxygen concentrations and exposure to hydrogen sulphide. Experimental evidence reveals that during severe hypoxia Marenzelleria cf. wireni switched to anaerobic pathways known from other marine invertebrates. Under the influence of sulphide (1 mmol l^-1) accumulation of anaerobic end products is more pronounced. It is assumed that in the presence of sulphide, due to its inhibition of aerobic respiration, the worms have to switch to anaerobic energy production directly after the onset of hypoxia. Seasonal differences in the metabolic reaction were found. It was shown that in specimens that were filled with gametes (winter), accumulation of end products of anaerobic metabolism was more pronounced than in immature worms (summer). The amount of succinate, an indicator for anaerobic energy production for instance, was nearly twice as high in the winter specimens. Since the amount of thiosulphate (a typical sulphide detoxification product) in the tissues is relatively low, it is suggested that Marenzelleria cf. wireni also accumulates other detoxification products. Sulphite, however, also known as a detoxification product, was found only in traces. Field measurements provide evidence that the potential of Marenzelleria cf. wireni to survive low oxygen and high sulphide concentrations is clearly higher than that normally needed in the Tay estuary.     

Feeding biology of the pelagic larvae of Marenzelleria cf. viridis (Polychaeta: Spionidae) from the Baltic Sea

Phytoplankton < 20 µm was a principal dietary component of the larvae of Marenzelleria cf. viridis. Maximum ingested particle size increased as animal size increased, reaching a maximum diameter of 80 µm for larvae with 6 to 10 setigers. The larvae started ingesting particulate matter at the 1-setiger stage and were able selectively to ingest phytoplankton and polystyrene particles of various sizes. Larvae in the 6 to 10-setiger size group did not differ from those in the 11 to 17-setiger size group in respect of size selectivity for polystyrene particles. The gut passage time for Chlorella vulgaris was 20 min. The ingestion rate was limited by food concentrations even at concentrations much higher than those encountered in the natural biotope, saturation being reached at a concentration of 28.5 times 10⁶ cells ml^-1 (117.7 mg C l^-1. The low maximum filtration rate of only 1.19 µl ind.^-1 h^-1 indicates that the filtering capacity of the larvae is low. The larvae are still capable of food uptake at 1 °C. Further experiments demonstrated that larval growth and survival were strongly dependent on both food concentration and quality. Larval growth was food-limited under biotope conditions of the Darss–Zingst Boddens and even more so under Baltic Sea conditions. The results indicate that Marenzelleria cf. viridis is a species adapted to eutrophic conditions prevailing in brackish waters.     

Factors controlling the distribution of Marenzelleria cf. viridis, Pygospio elegans and Streblospio shrubsoli (Polychaeta: Spionidae) in the southern Baltic Sea, with special attention for the response to an event of hypoxia

The large-scale distribution patterns of the estuarine spionid polychaetes Marenzelleria cf. viridis, Pygospio elegans, and Streblospio shrubsoli were studied in the Pomeranian Bay (southern Baltic Sea) at 20 stations in April 1993 and their relationship with hydrographic factors and sediment composition investigated. The densities of M. cf. viridis and S. shrubsoli decreased rapidly with increasing distance offshore. A corresponding decrease in phytoplankton concentration offshore is suggested as the main cause for the observed spatial patterns. P. elegans was more evenly distributed. At 10 selected stations further samples were collected between April 1993 and July 1996 to study the response of abundance and biomass of the three polychaetes to changes in environmental conditions and to investigate interspecific interactions within the macrofauna. Salinity, pelagic food supply, and sediment parameters did not change. At three of these stations redox potential of the sediment and macrofauna composition were affected by an unusual hypoxic/anoxic event in July/August 1994. All three spionid species were able to survive moderate seasonal hypoxic conditions, but died after exposure to severe anoxia. Special attention was focused on the re-establishment of the spionids. The rate with which M. cf. viridis and P. elegans re-colonised defaunated stations varied between a few weeks and two years. These differences in rate of re-establishment were attributed to the distance from undisturbed recruitment areas, and to the severity of the oxygen deficiency. Increasing densities of P. elegans and S. shrubsoli after the hypoxic event coincided with a reduced abundance of the bivalve Mya arenaria, suggesting a negative interspecific interaction.     

Bibliography on the genus Marenzelleria and its geographical distribution, principal topics and nomenclature

A bibliography is given for the genus Marenzelleria. All together, 236 publications were found dealing with M. viridis, M. wireni and M. jonesi and their synonyms. The contents of the publications are briefly reviewed in tabular form identifying the nomenclature used, the geographical distribution and the topic of the paper in each case.     

Marenzelleria cf. viridis and the sulphide regime

To find out how the polychaete Marenzelleria cf. viridis could spread successfully into the habitat of the Darss-Zingst Bodden Chain, one important environmental factor for sediment dwelling animals was examined: hydrogen sulphide. To investigate the stress of this environmental factor, hydrogen sulphide was continuously examined in the pore water of the sediment and burrows of M. cf. viridis. Metabolic activity was recorded by direct and indirect calorimetry. Depending on water temperature, organic matter content of the sediment and salinity, the sulphide concentration in the pore water varied between 1.5 and 4.2 mmol l^-1 being high during summer and in winter when the sediment and overlying water was ice covered. In microcosm experiments water of M. cf. viridis-burrows showed variations in sulphide between 145 and 210 µmol l^-1 but pore water concentration was much higher (6.5 mmol l^-1). In the presence of oxygen animals exhibited an accelerated metabolic rate which was met by a fully aerobic metabolism at Po₂ of 20 to 7.5 kPa and sulphide concentration of 215–245 µmol l^-1. When oxygen is absent the heat production was only slightly elevated (103%) when compared to the anoxic control. The elevated heat production of the animals during sulphide exposure and oxygen may be due to detoxification processes. In this case thiosulphate is formed probably via mitochondrial oxidation and therefore may account for additional ATP-gain.     

Dispersal and development ofMarenzelleria spp. (Polychaeta, Spionidae) populations in NW Europe and The Netherlands

The North American spionid polychaeteMarenzelleria cf.wireni was first recorded in the North Sea by Scotland in 1982.Marenzelleria cf.viridis was first found in the Baltic Sea in 1985. Tentative routes of dispersal since then are presented in this paper. In the Netherlands, a biological monitoring programme has revealed populations ofM. cf.wireni in the Ems estuary, Wadden Sea, and in the SW Netherlands. In the Dollard (Ems estuary) a large population has developed (2000–3000 individuals m^?2; 8–16 g ash-free dry weight m^?2). Since the introduction, the macrozoobenthic community has changed from being dominated (by biomass) by bivalves, to domination by polychaetes. Recently, a similar population started to develop at Balgzand (western Dutch Wadden Sea).     

Where did Marenzelleria spp. (Polychaeta: Spionidae) in Europe come from?

Abundant populations of Marenzelleria spp. were reported for the first time in the North Sea during the late 1970s and then in the Baltic Sea in 1985. Genetic analysis by means of allozyme electrophoresis and sequencing of a segment of mitochondrial 16srDNA showed that two different genetic types or sibling species of Marenzelleria were present in Europe. Marenzelleria Type I is found only in the North Sea, whereas Type II has been found in both the North Sea and the Baltic Sea. The North Sea animals (Type I) correspond to Type I specimens found in North American coastal waters between Nova Scotia and Cape Henlopen (Delaware), while Marenzelleria Type II from the Baltic Sea correspond to Marenzelleria type II animals from the Arctic (Tuktoyaktuk Harbor, Northwest Territories, Canada), New Hampshire and coastal waters between Chesapeake Bay southward to the Ogeechee River (Georgia). Human mediated introduction (by shipping) and natural range expansion are discussed as the possible causes of these virtually simultaneous invasions by two sibling species. Marenzelleria Type I appears to colonize habitats with a higher salinity and/or in which salinities tend to fluctuate considerably. The osmolality of the coelomic fluid after acclimation to various salinities between 0.25 and 18 is the same for both sibling species/genetic types. Although the two types do not differ in respect of hyperosmoregulation (<10), differences may exist in their cell volume regulation or its time course in the almost isoosmotic range at salinities >10.     

Multiple Invasions – A Polychaete Genus Enters the Baltic Sea

Since 1985, the nonindigenous polychaete species Marenzelleria neglecta has been found in the Baltic Sea. The species, which was introduced by ship ballast water, spreads rapidly and dominates in many habitats today. Using three gene segments of the mitochondrial DNA (16S rDNA, Cytochrom oxidase I, Cytochrom b), we investigated four populations of the western and northern Baltic Sea in a preliminary survey and compared them with four other populations from the North Sea, the Baltic Sea and from the Arctic. First, we could demonstrate the applicability of the markers to discriminate the species with certainty. Second, with M. viridis and M. arctia, we could detect two more species of the same genus, which have recently been introduced into the Baltic Sea. One of these, M. arctia, was hitherto known as an exclusive arctic member of the genus. The impact of these two recently invaded Marenzelleria species onto the autochthonous fauna needs to be evaluated in the future. The Baltic Sea as a ‘natural aquarium’ now offers the possibility to investigate sibling species simultaneously. However, correct identification and denomination of Marenzelleria species are indispensable prerequisites for all future studies. Molecular markers allow the exact identification of all Marenzelleria species and must be used whenever a classical taxonomic identification is uncertain.     

Distribution and life cycle of the North American Spionid polychaeteMarenzelleria viridis (Verrill, 1873) in the Ems estuary

In 1983 the first specimens of the North American spionid polychaeteMarenzelleria viridis were found along the European mainland shore in the Ems estuary. Since then, this polychaete has spread over several estuaries around the North Sea and the Baltic. In the inner part of the Ems estuary juveniles were predominantly present in muddy sediments high in the intertidal zone; in more sandy sediments at higher salinities juveniles and adults co-occured. Detailed information was obtained at a muddy and at a sandy station. Gametes were present in the coelomic fluid from November through March. In May new recruits were found in the sediment samples, reaching densities of over 10⁵ M⁻² at the muddy station, andc. 2000 m⁻² at the sandy station. During summer, densities decreased at the muddy station, coinciding with a density increase at the sandy station, suggesting migration of juveniles from a nursery to the adult habitat. Cage experiments showed that the decrease of juveniles at the muddy station could be attributed to migration and not to mortality due to predation. At the sandy habitat palps and anterior parts ofM. viridis made up 4–11% of the stomach content of juvenile plaice (Pleuronectes platessa). In juvenile flounder (Platichthys flesus) only in Aprilc. 10% of the stomach content consisted ofM. viridis. During 1983–1990 increasing densities ofM. viridis at the sandy habitat coincided with a reduced abundance ofNereis diversicolor, however, this inverse relationship was not found to be statistically significant. Density fluctuations ofM. viridis andCorophium volutator showed a significant positive relationship, the cause of which is not yet understood.     

Cryptic species in marine polychaete and their independent introduction from North America to Europe

The vast body of ballast water carried across oceans by freight ships represents a major source for the introduction of foreign species into marine ecosystems. The worm Marenzelleria viridis, originally found only in North America, appeared in estuaries of the North Sea in 1979 and 6 years later also in the Baltic, where it has developed into a major faunal element. Two competing hypotheses are discussed here: either both populations owe their presence to a single introductory event in the North Sea, or each population originated from a separate introduction. Our phylogeographic analysis of Baltic, North Sea and American Marenzelleria, based on mitochondrial 16S rDNA sequences (326- bp segment) of 98 individuals from 17 localities on the North American, North Sea, and Baltic coasts not only favors the two-event hypothesis, but also separates the locations of origin for the introductions. Eighteen mitochondrial genotypes were identified altogether. In agreement with allozyme data, three lineages were identified: genotypes assigned to the same lineage differed from each other by up to 5 point mutations, and those assigned to different lineages differed by up to 17. The existence of three morphologically indistinguishable, and thus cryptic, species is therefore suggested. The individuals from the Baltic Sea probably originated from the Atlantic coast of the United States between Chesapeake Bay and Georgia, and the North Sea populations may stem from the U.S. coast region north of Chesapeake Bay to Nova Scotia. Despite their similar morphologies, the two European Marenzelleria species may differ ecologically with respect to their preference for habitat salinity. Assuming that transport via ballast water occurs quite frequently, we hypothesize that both European cryptic species of Marenzelleria may originally have been introduced to both the North Sea and the Baltic Sea but that neither of them was able to proliferate in both water bodies owing to their differential physiological performances at high and low salinities.      

Molecular identification key based on PCR/RFLP for three polychaete sibling species of the genus <Emphasis Type="Italic">Marenzelleria</Emphasis>, and the species’ current distribution in the Baltic Sea

Studies of Marenzelleria species were often hampered by identification uncertainties when using morphological characters only. A newly developed PCR/RFLP protocol allows a more efficient discrimination of the three species Marenzelleria viridis, Marenzelleria neglecta and Marenzelleria arctia currently known for the Baltic Sea. The protocol is based on PCR amplification of two mitochondrial DNA gene segments (16S, COI) followed by digestion with restriction enzymes. As it is faster and cheaper than PCR/sequencing protocols used so far, the protocol is recommended for large-scale analyses. The markers allow an undoubted determination of species irrespective of life stage or condition of the worms in the samples. The protocol was validated on about 950 specimens sampled at more than 30 sites of the Baltic and the North Sea, and on specimens from populations of the North American east coast. Besides this test we used mitochondrial DNA sequences (16S, COI, Cytb) and starch gel electrophoresis to further investigate the distribution of the three Marenzelleria species in the Baltic Sea. The results show that M. viridis (formerly genetic type I or M. cf. wireni) occurred in the Öresund area, in the south western as well as in the eastern Baltic Sea, where it is found sympatric with M. neglecta. Allozyme electrophoresis indicated an introduction by range expansion from the North Sea. The second species, M. arctia, was only found in the northern Baltic Sea, where it sometimes occurred sympatric with M. neglecta or M. viridis. For Baltic M. arctia, the most probable way of introduction is by ship ballast water from the European Arctic. There is an urgent need for a new genetic analysis of all Marenzelleria populations of the Baltic Sea to unravel the current distribution of the three species.     

An environmental gradient dominates ecological and genetic differentiation of marine invertebrates between the North and Baltic Sea

Environmental gradients have emerged as important barriers to structuring populations and species distributions. We set out to test whether the strong salinity gradient from the marine North Sea to the brackish Baltic Sea in northern Europe represents an ecological and genetic break, and to identify life history traits that correlate with the strength of this break. We accumulated mitochondrial cytochrome oxidase subunit 1 sequence data, and data on the distribution, salinity tolerance, and life history for 28 species belonging to the Cnidaria, Crustacea, Echinodermata, Mollusca, Polychaeta, and Gastrotricha. We included seven non‐native species covering a broad range of times since introduction, in order to gain insight into the pace of adaptation and differentiation. We calculated measures of genetic diversity and differentiation across the environmental gradient, coalescent times, and migration rates between North and Baltic Sea populations, and analyzed correlations between genetic and life history data. The majority of investigated species is either genetically differentiated and/or adapted to the lower salinity conditions of the Baltic Sea. Species exhibiting population structure have a range of patterns of genetic diversity in comparison with the North Sea, from lower in the Baltic Sea to higher in the Baltic Sea, or equally diverse in North and Baltic Sea. Two of the non‐native species showed signs of genetic differentiation, their times since introduction to the Baltic Sea being about 80 and >700 years, respectively. Our results indicate that the transition from North Sea to Baltic Sea represents a genetic and ecological break: The diversity of genetic patterns points toward independent trajectories in the Baltic compared with the North Sea, and ecological differences with regard to salinity tolerance are common. The North Sea–Baltic Sea region provides a unique setting to study evolutionary adaptation during colonization processes at different stages by jointly considering native and non‐native species.     

Zonation of intertidal macrobenthos in the estuaries of Schelde and Ems

Based on data, collected in 1980–1990, the intertidal benthic macrofauna of the Schelde and Ems estuaries was compared. The spatial occurrence of the benthic macrofauna along the salinity gradient, including the freshwater tidal area was emphasized. Both estuaries appeared to have a very similar species composition, especially at genus level. The higher number of species observed in the Schelde estuary was probably due to a greater habitat diversity. In both estuaries species diversity decreased with distance upstream. The total density did not vary along the estuarine gradient, whereas biomass is highest in the polyhaline zone.In both estuaries distinct intertidal benthic communities were observed along the salinity gradient: a marine community in the polyhaline zone, a brackish community in the mesohaline zone, and a third community in the oligohaline and freshwater tidal zones of the estuary. These three communities were very similar between both estuaries. Their main characteristics were discussed together with the occurrence and distribution of the dominant species.For the Schelde estuary and to a lesser extent also for the Ems estuary, there was evidence that anthropogenic stress had a negative effect on the intertidal macrobenthic communities of the oligohaline/freshwater tidal zone. Only Oligochaeta were dominating, whereas the very euryhaline and/or true limnetic species were missing. In the mesohaline zone, the Schelde estuary was dominated by large numbers of short-living, opportunistic species, whereas in the Ems estuary relatively more stable macrobenthic communities were observed. A comparison with some other European estuaries showed in general similar trends as those observed for the Schelde and Ems estuaries.     

Diversity and abundance of “Pelagibacterales” (SAR11) in the Baltic Sea salinity gradient

The candidate order “Pelagibacterales” (SAR11) is one of the most abundant bacterial orders in ocean surface waters and, periodically, in freshwater lakes. The presence of several stable phylogenetic lineages comprising “Pelagibacterales” correlates with the physico-chemical parameters in aquatic environments. A previous amplicon sequencing study covering the bacterial community in the salinity gradient of the Baltic Sea suggested that pelagibacteral subclade SAR11-I was replaced by SAR11-IIIa in the mesohaline region of the Baltic Sea. In this current study, we investigated the cellular abundances of “Pelagibacterales” subclades along the Baltic Sea salinity gradient using catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). The results obtained with a newly designed probe, which exclusively detected SAR11-IIIa, were compared to CARD-FISH abundances of the marine SAR11-I/II subclade and the freshwater lineage SAR11-IIIb (LD12). The results showed that SAR11-IIIa was abundant in oligohaline–mesohaline conditions (salinities 2.7–13.3), with maximal abundances at a salinity of 7 (up to 35% of total Bacteria, quantified with a universal bacterial probe EUB). As expected, SAR11-I/II was abundant (27% of EUB) in the marine parts of the Baltic Sea, whereas counts of the freshwater lineage SAR11-IIIb were below the detection limit at all stations. The shift from SAR11-IIIa to SAR11-I/II was confirmed in the vertical salinity gradient in the deeper basins of the Baltic Sea. These findings were consistent with an overlapping but defined distribution of SAR11-I/II and SAR11-IIIa in the salinity gradient of the Baltic Sea and suggested the adaptation of SAR11-IIIa for growth and survival in mesohaline conditions.     

Population dynamics, growth and production of the neozoon Marenzelleria cf. viridis (Verrill, 1873) (Polychaeta: Spionidae) in a coastal water of the southern Baltic Sea

The propagation of an immigrant from North America, viz. the spionid Marenzelleria cf. viridis in the Darss-Zingst Bodden (DZB) (southern Baltic Sea), was studied at three stations from March 1992 to December 1995. Highest mean abundances (over 28 000 ind. m^-2) and wet weights (400 g m^-2) were recorded at station 2 in 1994. The spionid also reached its highest dominances in terms of biomass (40 to 90%) at this station, which was selected for the population dynamics, growth and secondary production studies. The spionid has a life span of about 3 years, and many individuals achieved sexual maturity after one year. Their growth curve is steepest during the first year of life, during which the animals grow to a length of about 180 segments. However, growth depression was observed during the ripening of the gametes in April, May and June. Secondary production was in the region of 55 to 85 g AFDW m^-2 y^-1. Productivity (P/B) varied considerably from generation to generation, ranging between 0 and 4.8 with an average between 1.2 and 1.6.     

Genetic population structure of turbot (Scophthalmus maximus L.) supports the presence of multiple hybrid zones for marine fishes in the transition zone between the Baltic Sea and the North Sea

Genetic population structure of turbot (Scophthalmus maximus L.) in the Northeast Atlantic was investigated using eight highly variable microsatellite loci. In total 706 individuals from eight locations with temporal replicates were assayed, covering an area from the French Bay of Biscay to the Aaland archipelago in the Baltic Sea. In contrast to previous genetic studies of turbot, we found significant genetic differentiation among samples with a maximum pairwise FST of 0.032. Limited or no genetic differentiation was found among samples within the Atlantic/North Sea area and within the Baltic Sea, suggesting high gene flow among populations in these areas. In contrast, there was a sharp cline in genetic differentiation going from the low saline Baltic Sea to the high saline North Sea. The data were explained best by two divergent populations connected by a hybrid zone; however, a mechanical mixing model could not be ruled out. A significant part of the genetic variance could be ascribed to variation among years within locality. Nevertheless, the population structure was relatively stable over time, suggesting that the observed pattern of genetic differentiation is biologically significant. This study suggests that hybrid zones are a common phenomenon for marine fishes in the transition area between the North Sea and the Baltic Sea and highlights the importance of using interspecific comparisons for inferring population structure in high gene flow species such as most marine fishes.     

GENETIC VARIATION IN APHANIZOMENON (CYANOBACTERIA) COLONIES FROM THE BALTIC SEA AND NORTH AMERICA

Aphanizomenon Morren is an important member of the cyanobacterial community in the Baltic Sea, but studies of this genus have been hampered by the difficulty of growing it in laboratory culture. PCR amplification of DNA from colonies picked directly from water samples has circumvented this problem and made it possible to carry out an analysis of genetic diversity within the Baltic Sea and in two small North American lakes separated by just a few kilometers. The nucleotide sequence of the phycocyanin intergenic spacer and partial flanking coding regions of cpcB and cpcA was determined for 32 colonies of Aphanizomenon , 26 from the Baltic Sea, and 6 from the North American lakes. No variation was detected among the 26 Baltic Sea colonies, but two alleles, differing at 19 nucleotide positions (5.4%), were found in the North American lake colonies. Surprisingly, the two North American types were less closely related to each other than to the Baltic Sea genotype. The Baltic Sea Aphanizomenon is clearly distinct from A. flos-aquae at both the cpcB–cpcA and 16S rDNA loci, which lends phylogenetic support to their tentative separation based on ultrastructural analysis. We conclude that although there is significant genetic diversity in the genus Aphanizomenon , the population in the Baltic Sea is, in contrast to the Nodularia population, genetically homogeneous.     

General Patterns in Invasion Ecology Tested in the Dutch Wadden Sea: The Case of a Brackish-Marine Polychaetous Worm

The success of invasive aquatic species is determined by a variety of attributes such as wide environmental tolerance, high genetic variability, short generation time, early sexual maturity, high reproductive capacity, and a broad diet. Usually, introduced species, after some time lag since inoculation, show an exponential population increase and expansion. Maintenance of the immigrant species at a high population level will be dependent on interspecific competition with native species and availability of habitat and food. Eventually, the immigrant population may decline, for instance due to increased predation pressure, parasite infestation or loss of genetic vigour. These characteristic patterns in invasive species are reviewed for the case of the North American spionid polychaete Marenzelleria cf. wireni in the Dutch Wadden Sea. This species was first recorded in estuaries and coastal waters of the European continent in the Ems estuary (eastern Dutch Wadden Sea) in 1983. In the western part of the Dutch Wadden Sea the first specimens were found in 1989. The Ems estuary population showed the typical lag-phase, explosive increase, stabilisation, and eventual decline. In the western part of the Dutch Wadden Sea the latter two phases have not yet developed. The strong development and stabilisation of the population in the Ems estuary may have been caused by the availability of a yet not utilised food source. The species' final decline remains largely unexplained.