Evidence for a symbiosis between bacteria of the genus Rhodobacter and the marine sponge Halichondria panicea : harbor also for putatively toxic bacteria?

Halichondria panicea (Pallas) is a marine sponge, abundantly occurring in the Adriatic Sea, North Sea, and Baltic Sea. It was the aim of the present study to investigate if this sponge species harbors bacteria. Cross sections through H. panicea were taken and inspected by electron microscopy. The micrographs showed that this sponge species is colonized by bacteria in its mesohyl compartment. To identify the bacteria, polymerase chain reaction (PCR) analysis of the 16S rRNA gene segment, typical for bacteria, was performed. DNA was isolated from sponge material that had been collected near Rovinj (Adriatic Sea), Helgoland (North Sea), and Kiel (Baltic Sea) and was amplified with bacterial primers by PCR. The data gathered indicate that in all samples bacteria belonging to the genus Rhodobacter (Proteobacteria, subdivision α) are dominant, suggesting that these bacteria live in symbiotic relationship with the sponge. In addition, the results show that the different samples taken contain further bacterial species, some of them belonging to the same genus even though found in sponges from different locations. The possibility of the presence of toxic bacteria was supported by the finding that organic extracts prepared from sponge samples displayed toxicity, when analyzed in vitro using leukemia cells. Received: 7 March 1997 / Accepted: 2 October 1997     

The cockle <Emphasis Type="Italic">Cerastoderma edule</Emphasis> at Northeast Atlantic shores: genetic signatures of glacial refugia

The cockle Cerastoderma (Cardium) edule (L.) ranges from NW Africa to northern Scandinavia. Abundance in shallow coastal sediment is often high, and it attracts commercial harvest. In this study, a complex genetic pattern has been revealed by mitochondrial DNA in 383 individuals from 19 sampling sites. Parsimony network analysis of 79 haplotypes identified two dominant central haplotypes separated by low divergence. One is characteristic for a homogenous southwestern group of populations from Africa to the British Isles. The other is characteristic for a heterogeneous northern group with a deviant Arctic population. At the entrance of the Baltic Sea, a mixture zone of the dominant haplotypes was found. The estimated population expansion time for the northern haplotype group predates that of the southwestern one, suggesting northern glacial refugia and a subsequent southern expansion of C. edule populations.     

Are there two species of the polychaete genus Marenzelleria in Europe?

North Sea and Baltic Sea populations of the introduced polychaete Marenzelleria viridis (Verrill, 1873) reproduce at different times (spring and autumn, respectively). Enzyme separation by starch gel electrophoresis revealed major differences between specimens from the Baltic Sea and those from the North Sea (collected in 1992 and 1993) but a high degree of homogeneity among populations from the same sea. Three enzyme loci, glucose-6-phosphate isomerase (GPI-A, GPI-B) and malate dehydrogenase (MDH), were fixed to 100% by different alleles in the North and Baltic Sea populations, respectively. Different alleles are dominant for mitochondrial aspartate aminotransferase (mAAT) with allele frequencies of ca. 0.97 in all sampled populations from the North Sea and Baltic Sea, respectively, but heterozygotes were found in all populations. These genetic differences could be due to environmentally induced selection or genetically different origins of the populations, suggesting that populations of the genus Marenzelleria in the North and Baltic Seas may be two different species.     

A phenomenological approach shows a high coherence of warming patterns in dimictic aquatic systems across latitude

To predict the coherence in local responses to large-scale climatic forcing among aquatic systems, we developed a generalized approach to compare long-term data of dimictic water bodies based on phenomenologically defined hydrographic events. These climate-sensitive phases (inverse stratification, spring overturn, early thermal stratification, summer stagnation) were classified in a dual code (cold/warm) based on threshold temperatures. Accounting for a latitudinal gradient in seasonal timing of phases derived from gradients in cumulative irradiation (2.2?days per degree latitude), we found a high spatial and temporal coherence in warm–cold patterns for six lakes (84?%) and the Baltic Sea (78?%), even when using the same thresholds for all sites. Similarity to CW-codes for the North Sea still was up to 72?%. The approach allows prediction of phase-specific warming trends and resulting instantaneous or time-delayed ecological responses. Exemplarily, we show that warming during early thermal stratification controls food-web-mediated effects on key species during summer.     

Colonization of Europe by two American genetic types or species of the genus Marenzelleria (Polychaeta: Spionidae). An electrophoretic analysis of allozymes

Allozyme electrophoresis was conducted in an attempt to identify the origin ofMarenzelleria sp. found in the North Sea and Baltic Sea. The analysis covered eight enzymes with ten loti from nine populations found on the North American Atlantic toast, these populations in the North Sea and five populations in the Baltic. The North Sea spionids correspond to the Type IMarenzelleria from North American coastal waters between Barnstable Harbor (Massachusetts) and Cape Henlopen (Delaware). Nei's genetic distance between these North American populations and those from the North Sea wasD = 0.010 to 0.020. TheMarenzelleria sp. found in the Baltic Sea very probably stems from North American populations of Type II found from the region of Chesapeake Bay (Trippe Bay) south to the Currituck Sound (North Carolina). The genetic distance between these North American populations and the Baltic populations isD = 0.000 to 0.001. The invaders appear to have lost little of their genetic variation while colonizing the North and Baltic Seas. Probably, both colonizing events tan be attributed to large numbers of individuals reaching Europe simultaneously on one or more occasions. In addition, aMarenzelleria Type III was found by electrophoresis among specimens from Currituck Sound (North Carolina), rohere it is sympatric withMarenzelleria Type II. Salinity is discussed as an important factor for the establishment ofMarenzelleria Type I in the North Sea and Type II in the Baltic Sea.     

Pelagic Sargassum community change over a 40-year period: temporal and spatial variability

Pelagic forms of the brown algae (Phaeophyceae) Sargassum spp. and their conspicuous rafts are defining characteristics of the Sargasso Sea in the western North Atlantic. Given rising temperatures and acidity in the surface ocean, we hypothesized that macrofauna associated with Sargassum in the Sargasso Sea have changed with respect to species composition, diversity, evenness, and sessile epibiota coverage since studies were conducted 40 years ago. Sargassum communities were sampled along a transect through the Sargasso Sea in 2011 and 2012 and compared to samples collected in the Sargasso Sea, Gulf Stream, and south of the subtropical convergence zone from 1966 to 1975. Mobile macrofauna communities exhibited changes in community structure and declines in diversity and evenness within a 6-month time period (August 2011–February 2012). Equivalent declines in diversity and evenness were recorded in the same region (Sargasso Sea, 25°–29°N) in 1972–1973. Recent community structures were unlike any documented historically, whether compared to sites of the same latitude range within the Sargasso Sea, or the broader historical dataset of sites ranging across the Sargasso Sea, Gulf Stream, and south of the subtropical convergence zone. Recent samples also recorded low coverage by sessile epibionts, both calcifying forms and hydroids. The diversity and species composition of macrofauna communities associated with Sargassum might be inherently unstable. While several biological and oceanographic factors might have contributed to these observations, including a decline in pH, increase in summer temperatures, and changes in the abundance and distribution of Sargassum seaweed in the area, it is not currently possible to attribute direct causal links.     

New indicator species in the Baltic zooplankton in 1972

The occurrence of zooplankton species, indicating a salinewater influx from the North Sea into the Baltic Sea in 1972, is described. Particular attention is paid to 8 species so far unknown in this region (Ectopleura dumortieri, Rosacea plicata, Tomopteris kefersteini, Aetideus armatus, Centropages typicus, Eucalanus elongatus, Metridia lucens, Electra pilosa). The appearance of these zooplankters in the Southern Baltic Sea is discussed in relation to the hhdrographic changes taking place in this area in 1972. Based on these data, assessments are made regarding the intensity of dynamic exchange processes in the Baltic and North Atlantic waters in 1972.     

Enzyme activities and allozyme polymorphism in two genetic types (or sibling species) of the genus Marenzelleria (Polychaeta: Spionidae) in Europe

Two populations of Marenzelleria spp. from the North Sea and the Baltic Sea that had already been characterized by allozyme electrophoresis and found genetically different in 1994 were again examined in 1997. The genetic identity (I = 1.0) examined in 1994 and 1997 shows that the structures of the populations of genetic Type I in the North Sea (Ringkobing Fjord) and genetic Type II in the Baltic Sea (Darss–Zingst Bodden chain) are stable. Three diagnostic gene loci that were determined additionally (citrate synthase, one locus; alanine aminotransferase, two loci) support the hypothesis of the existence of two sibling species of the genus Marenzelleria in Europe. In addition, the activities of ten electrophoretically separated enzyme systems were measured in the worms immediately after removal from their habitat and after they had been kept in the laboratory for 2 months. Under both conditions four enzymes showed significant differences between genetic types. The higher activity of the alanine aminotransferase of the North Sea worms (Type I) is discussed in the context of the different preferred salinities of the two Marenzelleria sibling species. Received: 21 January 1999 / Accepted: 13 July 1999     

Latitudinal patterns of diversity and abundance in North Atlantic intertidal boulder-fields

In order to study taxon richness, biodiversity and abundance patterns in the North Atlantic from temperate latitudes through Arctic to high Arctic latitudes, we recorded the faunas (at ELWS level) colonizing 20 cobbles from three sites at each of seven boulder-field localities (south-west England, 50°N; Wales, 51°N; west Scotland, 56°N; Iceland, 64°N; Tromsø, 70°N; Svalbard, 77°N, 79°N). Inverse correlations were found between latitude and all measures of richness (species, orders, and phyla numbers) and biodiversity (S–W, P). However, these correlations were driven mostly by the consistently impoverished Arctic sites; an even cline of decreasing diversity from south to north did not exist. Multidimensional scaling revealed two communities, temperate–subarctic species-rich and high arctic species-poor. Evenness as measured by Pielou’s index was similar across all latitudes. Abundance data exhibited a similar trend to the species richness and diversity data with a significant negative correlation with latitude, but when Arctic data were excluded the correlation vanished.     

Benthic amphipods (Crustacea: Malacostraca) in Icelandic waters: diversity in relation to faunal patterns from shallow to intermediate deep Arctic and North Atlantic Oceans

Patterns in diversity, species replacement and species composition of gammaridean amphipods (Crustacea: Malacostraca) were studied in two areas on different sides of the Greenland–Iceland–Faeroe Ridge. One was sited south of the Ridge in the North Atlantic Ocean off southern Iceland and the other north of the Ridge in the Arctic Ocean (subarctic and arctic waters of the Iceland Sea) off northern Iceland. Samples were taken with a Rothlisberg–Pearcy epibenthic sled within the depth range of about 50 to 1200 m at a total of 28 stations. In all 21 150 individuals were found belonging to 156 species. Of these 102 were found in the northern area with 64 of the species restricted to this area. In all 94 species were found in the southern area with 52 of the species restricted to this area. This suggests a similar regional diversity, but a different faunal composition. The amphipods showed similar species replacement patterns with depth on both sides of the Ridge, while the diversity patterns differed between the areas. In the North Atlantic the number of species increased with depth, while in the Arctic there seemed to be no pattern at all. It is suggested that diversity patterns of amphipods on each side of the Ridge are shaped by different factors, and these may not be the same as those determining diversity patterns of other groups in the area. Salinity (as indicative of water masses) was found to be the most important environmental variable in explaining the species composition when both areas were considered, suggesting a number of contributing factors (adaptation to water masses, dispersal, etc.). Water temperature was the most important variable in explaining the variance in the species composition in the northern area, while depth was the most important variable in explaining the variance in the species composition of the southern area. Received: 9 March 1997 / Accepted: 10 December 1997     

Microsatellite analysis of plaice (Pleuronectes platessa L.) in the NE Atlantic: weak genetic structuring in a milieu of high gene flow

Many species of marine fish are typified by large population sizes, strong migratory behavior, high fecundity, and pelagic eggs and larvae that are passively transported by ocean currents, all features that tend to increase gene flow, and hence reduce genetic partitioning, among localized populations. The plaice, Pleuronectes platessa, is a commercially important demersal species that exhibits all of these characteristics. We analyzed genetic variation at eight microsatellite loci in samples of spawning adults (N = 348) from the coasts of Ireland, Iceland, and, for the first time, from the Baltic Sea. Significant differentiation was observed between Iceland and Irish and Baltic Sea samples. However, there were no genetic differences between Irish and Baltic Sea samples, which contrast with the significant differentiation reported between Baltic Sea and North Sea/Atlantic populations of other flatfish species. To increase the data set, we carried out a cross-calibration exercise, allowing us to perform a joint analysis of data with an earlier study on adult and juvenile plaice (N = 480) collected over a broad geographic range, using six microsatellite loci in common to the two studies. Significant differentiation was observed between fish collected at the northern (Iceland, Faeroes, Norway) and southern (Bay of Biscay) parts of the species range. In contrast, the results showed little evidence of genetic structuring over much of the continental shelf of Europe. We believe that bathymetric and hydrographic barriers are the major factors shaping genetic structure, while lack of structure over much of the European continental shelf may be explained by a combination of past historical events, population structure, and dynamics of the species.     

Variability of epifauna and temperature in the northern North Sea

Benthic epifauna in three areas of the northern North Sea was studied from 1999 to 2007 to investigate the effect of temperature changes on community structure and species abundance and biomass. Abundance and/or biomass of 16 epifauna species was significantly correlated with temperature anomalies of the mean sea surface temperature (SST) from 1971 to 2000. The response of species to SST changes was different in the study areas depending on species life history and, most likely on food supply, which in turn is strongly influenced by the timing and duration of primary production and regional hydrographical conditions (e.g. stratification). Also, changes in community structure were obvious in the three areas between 2002 and 2003 coinciding with high temperature anomalies and SST. On the other hand, these changes were mainly caused by the variability in abundance of dominant species and altogether no clear trends in community structure were found. In contrast to epifauna communities in the shallow southern North Sea temperature changes in the northern North Sea affected only single epifauna species until now.     

Genetic variability and level of differentiation in North Sea and Baltic Sea populations of the green alga<Emphasis Type="Italic"> Cladophora rupestris</Emphasis>

Cladophora rupestris is a perennial filamentous macroalga belonging to the Chlorophyta. It is widely distributed on both sides of the northern Atlantic Ocean and penetrates into the brackish Baltic Sea down to ca. 4 psu salinity. In this paper we present evidence for genetic differentiation of a Baltic form of this marine alga. We assessed genetic structure within and among 11 populations ranging along a salinity gradient from the Norwegian coast to the northern Baltic Sea proper. Samples of 328 individuals were studied using starch-gel protein electrophoresis to evaluate genetic variability and interpopulation differentiation based on allozymes. Of 11 loci examined, only one was polymorphic. For this locus, encoding superoxide dismutase (SOD-3), a total of seven alleles were distinguished. We found two genetically differentiated groups of populations of C. rupestris, one Baltic Sea group and one North Sea group, with a distinct border in the southern Kattegat near the entrance to the Baltic Sea. The genetic differentiation for SOD-3, expressed as pairwise F_ST values between the populations, was generally higher within the Baltic Sea group (0.10-0.43) than within the North Sea group (0.05-0.10); in the latter group also fewer pairs of populations differed significantly. Pairs of populations from different groups had the highest F_ST values (0.20-0.60). Hierarchical analysis of variance showed that 29.6% of the total variation in the SOD-3 locus was explained by variation between the two groups, while only 4.2% was explained by variation among the populations within the groups. The remaining variation (66.2%) was found within the populations.     

Predator-inducible changes in blue mussels from the predator-free Baltic Sea

Blue mussels, Mytilus spp., have inhabited the brackish Baltic Sea, an environment lacking predatory crabs and starfish, for several thousand years. In this paper we examined whether Baltic Mytilus that were transplanted to the North Sea showed predator-inducible plasticity like their "marine" conspecifics. Our experiments showed that native North Sea Mytilus changed their morphology when exposed to waterborne scents from shore-crabs and starfish. These predators induced different kinds of changes, with emphasis on shell thickness and adductor muscle size, respectively. Baltic Mytilus responded in a similar way to crab scents, whereas starfish scents had a relatively weak effect on the morphology. Crab and starfish scent induced strengthening of the byssal attachment in North Sea Mytilus, with crabs providing more stimulation than starfish. Baltic mussels also improved the byssal attachment when exposed to either of the predators, but the attachment strength, as well as the response to crabs, were relatively weaker than that of North Sea mussels. We conclude that inducible plasticity still is present in Baltic Mytilus, despite their recent evolution in a predator-free environment. There is probably no strong selective pressure against inducible plasticity, but it could also be maintained in the population by gene flow from Mytilus in the adjoining North Sea. The question whether Baltic Mytilus are M. edulis or M. trossulus may also be relevant for the present results.     

Genetic differentiation in Hediste diversicolor (Polychaeta: Nereididae) for the North Sea and the Baltic Sea

Genetic differentiation between North Sea and Baltic Sea Hediste diversicolor (O.F. Müller, 1776) (Polychaeta: Nereididae) populations was studied by allozyme electrophoresis on starch gel. Thirteen loci were analyzed in eight populations. The level of genetic variation was very low (mean H _o = 0.000 to 0.015). Differentiation between H. diversicolor populations is quite high (F _ST = 0.892) and reflected by three enzyme loci (MDH-I*, MDH-II*, IDH-I*). The reduced gene flow (N _m<1) may be explained by the limited dispersal capacity of the species. Regardless of whether found in the North Sea or Baltic Sea, there appear to be two different genetic types which are parapatric or sympatric in some places. The two types hybridize at three localities, but no signs of hybridization have been found at one (Tallinn). Received: 26 June 1997 / Accepted: 10 September 1997     

Polymorphism of mannose phosphate isomerase in North Sea and Baltic Sea populations of the amphipods Gammarus zaddachi and G. salinus

Genetic variation at the mannosephosphate isomerase (MPI) locus was examined in the euryhaline sibling species Gammarus zaddachi Sexton and G. salinus Spooner. Both crustacean amphipods share identical enzyme mobilities, following electrophoresis on vertical starch gels. The MPI locus turned out to be highly polymorphic; it is encoded by 6 alleles in G. zaddachi and 7 alleles in G. salinus. Geographic variation of allelic diversity was studied in samples from 9 G. zaddachi and 10 G. salinus populations, primarily obtained from Baltic Sea and North Sea sites. Patterns of inter- and intraspecific heterogeneity are described. Differences in allelic composition exist between Baltic and North Sea samples of G. salinus. In G. zaddachi, levels of polymorphism are higher in North Sea populations than in those from Baltic Sea areas. The significance of these findings is discussed in the light of previous biochemical genetic investigations on the population structure of the two amphipods considered.     

Soil microarthropods (Acari,Collembola) from beach and dune: characteristics and ecosystem context

Soil microarthropods (Acari, Collembola) were analyzed along transects from shore to brown dune in two sandy coastal sites of the North Sea (Jutland, Denmark, and Spiekeroog, East Frisian island, Germany). Predatory mites (Acari: Gamasina) and Collembola were determined to species. The Jutland and the Spiekeroog sites each yielded 22 Gamasina species, 10 of which are common to both areas. Collembola were identified from the Spiekeroog samples only (also 22 species). Largest mite abundances were found in Jutland in primary dunes (419 tsd. ind./m²) and on Spiekeroog in old brown dunes (314 tsd.ind./m²). Compared to the mites, Collembola abundances are low, with maxima of 77 tsd.ind./m² in Jutland (yellow dune) and only 18 tsd. ind./m² on Spiekeroog (old vellow dune). The communities of predatory mites (Gamasina) and Collembola along transects from shore inland to brown dune are quite specific. High similarities are found for the predatory mites between the specific dune sites of the two North Sea areas, particularly for yellow dune. Additionally, preliminary data are presented for the southern Baltic Coast. The findigns are discussed in a broader context. Arguments focus on conservation and biogenic dune stabilization.