Polychaete Pygospio Elegans Research Articles

Diversity, Distribution, and Development of Hyperparasitic Microsporidia in Gregarines within One Super-Host.

Metchnikovellids (Microsporidia: Metchnikovellida) are poorly studied hyperparasitic microsporidia that live in gregarines inhabiting the intestines of marine invertebrates, mostly polychaetes. Our recent studies showed that diversity of metchnikovellids might be significantly higher than previously thought, even within a single host. Four species of metchnikovellids were found in the gregarines inhabiting the gut of the polychaete Pygospio elegans from littoral populations of the White and Barents Seas: the eugregarine Polyrhabdina pygospionis is the host for Metchnikovella incurvata and M. spiralis, while the archigregarine Selenidium pygospionis is the host for M. dogieli and M. dobrovolskiji. The most common species in the White Sea is M. incurvata, while M. dobrovolskiji prevails in the Barents Sea. Gregarines within a single worm could be infected with different metchnikovellid species. However, co-infection of one and the same gregarine with several species of metchnikovellids has never been observed. The difference in prevalence and intensity of metchnikovellid invasion apparently depends on the features of the life cycle and on the development strategies of individual species.

Coinfection patterns of two marine apicomplexans are not associated with genetic diversity of their polychaete host.

Coinfections of two or more parasites within one host are more of a rule than an exception in nature. Interactions between coinfecting parasites can greatly affect their abundance and prevalence. Characteristics of the host, such as genetic diversity, can also affect the infection dynamics of coinfecting parasites. Here, we investigate for the first time the association of coinfection patterns of two marine apicomplexans, Rhytidocystis sp. and Selenidium pygospionis, with the genetic diversity of their host, the polychaete Pygospio elegans, from natural populations. Host genetic diversity was determined with seven microsatellite loci and summarized as allelic richness, inbreeding coefficient, and individual heterozygosity. We detected nonsignificant correlations between infection loads and both individual host heterozygosity and population genetic diversity. Prevalence and infection load of Rhytidocystis sp. were higher than those of S.pygospionis, and both varied spatially. Coinfections were common, and almost all hosts infected by S.pygospionis were also infected by Rhytidocystis sp. Rhytidocystis sp. infection load was significantly higher in dual infections. Our results suggest that factors other than host genetic diversity might be more important in marine apicomplexan infection patterns and experimental approaches would be needed to further determine how interactions between the apicomplexans and their host influence infection.

Eelgrass meadows harbor more macrofaunal species but bare sediments can be as functionally diverse

Seagrass meadows are important ecosystem engineers, providing habitat for a range of marine organisms which sustain many ecosystem functions. Due to global loss of seagrass, conservation and restoration incentives are rapidly increasing. However, it is not well understood how environmental setting affects the structural and functional diversity of macrofaunal communities in eelgrass meadows (Zostera marina), and to what extent their importance as macrofaunal habitat is affected by abiotic variables across spatial scales. In the present study, we explored macrofaunal communities in eelgrass meadows and adjacent bare habitats across contrasting environments in Southern Sweden. We assessed taxonomic and functional diversity in concert and evaluated environmental properties as explanatory variables for the observed differences. Our results showed that eelgrass meadows supported a significantly higher taxonomic richness and abundance compared to adjacent bare sediments whereas differences in structural and functional diversity was mainly a function of site and highly influenced by dominance of the polychaete Pygospio elegans. Interestingly, functional redundancy was higher in brackish compared to marine sites, illustrating the effect of generalist species that cover a wide range of functional traits. Abiotic sediment variables did not explain the variation in structural or functional diversity, but were largely overshadowed by the effect of eelgrass and its belowground biomass. From a restoration perspective, our results imply that the positive effects of eelgrass restoration are similar across environments and that the presence of eelgrass is the most important factor in structuring benthic communities, but the effects on functional diversity and redundancy need further attention.

Molecular phylogeny and new light microscopic data of Metchnikovella spiralis (Microsporidia: Metchnikovellidae), a hyperparasite of eugregarine Polyrhabdina sp. from the polychaete Pygospio elegans.

Metchnikovellids are a deep-branching group of microsporidia, parasites of gregarines inhabiting the alimentary tract of polychaetes and some other invertebrates. The diversity and phylogeny of these hyperparasites remain poorly studied. Modern descriptions and molecular data are still lacking for many species. The results of a light microscopy study and molecular data for Metchnikovella spiralis Sokolova et al., 2014, a hyperparasite of the eugregarine Polyrhabdina sp., isolated from the polychaete Pygospio elegans, were obtained. The original description of M. spiralis was based primarily on the analysis of stained preparations and transmission electron microscopy images. Here, the species description was complemented with the results of in vivo observations and phylogenetic analysis based on the SSU rRNA gene. It was shown that in this species, free sporogony precedes sac-bound sporogony, as it occurs in the life cycle of most other metchnikovellids. Spore sacs are entwined with spirally wound cords, and possess only one polar plug. Phylogenetic analyses did not group M. spiralis with M. incurvata, another metchnikovellid from the same gregarine species, but placed it as a sister branch to Amphiacantha. The paraphyletic nature of the genus Metchnikovella was discussed. The taxonomic summary for M. spiralis was emended.

Evolutionary relationships of Metchnikovella dogieli Paskerova et al., 2016 (Microsporidia: Metchnikovellidae) revealed by multigene phylogenetic analysis.

The species Metchnikovella dogieli (Paskerova et al. Protistology 10:148-157, 2016) belongs to one of the early diverging microsporidian groups, the metchnikovellids (Microsporidia: Metchnikovellidae). In relation to typical ('core') microsporidia, this group is considered primitive. The spores of metchnikovellids have no classical polar sac-anchoring disk complex, no coiled polar tube, no posterior vacuole, and no polaroplast. Instead, they possess a short thick manubrium that expands into a manubrial cistern. These organisms are hyperparasites; they infect gregarines that parasitise marine invertebrates. M. dogieli is a parasite of the archigregarine Selenidium pygospionis (Paskerova et al. Protist 169:826-852, 2018), which parasitises the polychaete Pygospio elegans. This species was discovered in samples collected in the silt littoral zone at the coast of the White Sea, North-West Russia, and was described based on light microscopy. No molecular data are available for this species, and the publicly accessible genomic data for metchnikovellids are limited to two species: M. incurvata Caullery & Mesnil, 1914 and Amphiamblys sp. WSBS2006. In the present study, we applied single-cell genomics methods with whole-genome amplification to perform next-generation sequencing of M. dogieli genomic DNA. We performed a phylogenetic analysis based on the SSU rRNA gene and reconstructed a multigene phylogeny using a concatenated alignment that included 46 conserved single-copy protein domains. The analyses recovered a fully supported clade of metchnikovellids as a basal group to the core microsporidia. Two members of the genus Metchnikovella did not form a clade in our tree. This may indicate that this genus is paraphyletic and requires revision.

Acute and chronic response to a change in salinity of the euryhaline polychaete Pygospio elegans (Claparède)

Estuaries are thought to be very harsh environments because their physico-chemical parameters, such as salinity, temperature and oxygen, can fluctuate substantially. The distribution of species living in an estuary is largely determined by how well they can cope with such fluctuations. The spionid polychaete Pygospio elegans is common in boreal estuarine habitats that show strong salinity fluctuations, living in habitats ranging from fully marine salinities to brackish environments with salinities as low as 5. In this study we investigated the abilities of P. elegans to cope with an acute as well as a long-term change in salinity. Specimens originating from a salinity of about 15 were exposed to salinity 15 as control and 5 and 30 as low and high salinity treatments. In acute exposure experiments, we measured body volume and tissue water content as well as gene expression of seven genes related to volume- and osmoregulation within 4 h of exposure. In a long-term experiment, we monitored survival, growth and reproduction as well as potential changes in DNA methylation within 6 weeks after a gradual salinity change. In response to abrupt exposure to a hyposmotic medium (salinity 5) we observed increased body volume that could not be fully restored, increased mortality, and no clear change in gene expression. Hence, P. elegans might be a weak cell-volume regulator that cannot cope well with sudden drops in salinity. In contrast, a gradual, long-term change in salinity seemed to be less stressful, although worms at salinity 5 showed slightly increased mortality and reduced or delayed maturity. Overall, there were fewer detrimental effects of exposure to a higher salinity. While the tested salinities fall within the tolerance range of P. elegans from this population, decrease in salinity seems to be a stronger stress.

Seasonal genetic variation associated with population dynamics of a poecilogonous polychaete worm.

Poecilogonous species show variation in developmental mode, with larvae that differ both morphologically and ecologically. The spionid polychaete Pygospio elegans shows variation in developmental mode not only between populations, but also seasonally within populations. We investigated the consequences of this developmental polymorphism on the spatial and seasonal genetic structure of P. elegans at four sites in the Danish Isefjord‐Roskilde‐Fjord estuary at six time points, from March 2014 until February 2015. We found genetic differentiation between our sampling sites as well as seasonal differentiation at two of the sites. The seasonal genetic shift correlated with the appearance of new size cohorts in the populations. Additionally, we found that the genetic composition of reproductive individuals did not always reflect the genetic composition of the entire sample, indicating that variance in reproductive success among individuals is a likely explanation for the patterns of chaotic genetic patchiness observed during this and previous studies. The heterogeneous, unpredictable character of the estuary might maintain poecilogony in P. elegans as a bet‐hedging strategy in the Isefjord‐Roskilde‐Fjord complex in comparison with other sites where P. elegans are expected to be fixed to a certain mode of development.

METCHNIKOVELLA DOGIELI SP. N. (MICROSPORIDIA: METCHNIKOVELLIDA), A PARASITE OF ARCHIGREGARINES SELENIDIUM SP. FROM POLYCHAETES PYGOSPIO ELEGANS

Cysts and free spores of a metchnikovellid microsporidium were found in several specimens of an archigregarine Selenidium sp. isolated from polychaetes Pygospio elegans. Samples were collected at the littoral area of the Kandalaksha Bay of the White Sea in the year 2016. We examined this material with high-quality light optics in stained and live preparations. The structure of cysts and the host range suggest that this species belongs to the genus Metchnikovella Caullery et Mesnil, 1897. The length of the cysts varied from 9.5 to 34 .m (av. 23.8 .m); the width of the cysts was 4.8–9.2 µm (av. 8.2 µm). The number of cyst-bound spores varied from 7 to 18. Cyst-bound spores were oval or ovoid and arranged in two or three rows. The length of spores was 2.2–3.0 µm (av. 2.6 µm); the width was 1.4–2.9 µm (av. 1.7 µm). Free spores were similar to cyst-bound ones in shape and size. We summarized available data on the species of the genus Metchnikovella. The analysis of these data proved that the shape and size of the examined cysts and the host range of this parasite differ from those of any known species. We named the observed organism Metchnikovella dogieli n. sp.

Chaotic genetic patchiness and high relatedness of a poecilogonous polychaete in a heterogeneous estuarine landscape

The genetic structure of benthic marine invertebrates is often described as “chaotic” when genetic structure cannot be explained and barriers to dispersal and gene flow cannot be identified. Here, chaotic patterns of genetic structure for the polychaete Pygospio elegans (Claparede) sampled at 16 locations from the heterogeneous Isefjord–Roskilde Fjord estuary complex in Denmark were found. There was no isolation by distance, and the geography of the estuary complex did not seem to pose a barrier to dispersal and gene flow in this species. We investigated whether characteristics of the environment could be related to the genetic structure and possibly restrict gene flow in this species. Additionally, since P. elegans is poecilogonous, producing larvae with different pelagic developmental periods, we investigated whether observed developmental modes in the samples might clarify the genetic patterns. None of the tested factors explained the population genetic structure. However, a high degree of relatedness among individuals in almost all samples was found. Samples with a larger percentage of young individuals had more related individuals, suggesting that different cohorts could be comprised of individuals with different degrees of relatedness. Relatedness within a site could be increased by limited larval dispersal, collective dispersal of related larvae, sweepstakes reproductive success, or asexual reproduction, but distinguishing between these requires further study. Using a “seascape genetics” approach allowed us to investigate some of the numerous potential factors that could influence population genetic structure in a poecilogonous species.

Description of Metchnikovella spiralis sp. n. (Microsporidia: Metchnikovellidae), with notes on the ultrastructure of metchnikovellids.

The present paper reports results of a transmission electron microscopy study of a new metchikovellid microsporidium. It was isolated from gregarines Polyrhabdina sp. inhabiting guts of polychaetes Pygospio elegans sampled at the White Sea silt littoral zone. Free sporogony (FS) occurred in the life cycle of the microsporidium alongside sac-bound sporogony (BS). Free spores resided in a parasitophorous vacuole and were of typical metchnikovellidean structure, uninucleate and oblong. They measured on sections 2·0-3·2×1·3-1·9 μm. The life cycle included pre-sporogonial stages represented by dikaryotic cells and 4-nucleate cells with coupled nuclei. A multinucleate sporogonial plasmodium of FS split in numerous (>10) sporoblasts. In BS segregation of sporoblasts occurred within thick-walled cysts by internal budding. Spore sacs of this microsporidium, measuring on average 11·6×4·7 μm, were limited by a thick electron-dense wall, externally ornamented with spirally wound cords of dense material. These oval spore sacs contained eight barrel-shaped spores, comparable in size and ultrastructure to FS spores. Ultrastructure of both types of spores and intracellular development of the new microsporidium and Metchnikovella spp. were similar, suggesting they belong to the same genus. In this paper we describe a new species Metchnikovella spiralis and discuss morphology of metchnikovellids in the context of putative evolutionary history of Microsporidia.

Fine structure of Metchnikovella incurvata Caullery and Mesnil 1914 (microsporidia), a hyperparasite of gregarines Polyrhabdina sp. from the polychaete Pygospio elegans

Class Rudimicrosporea Sprague 1977, with its single family Metchnikovellidae, comprises hyperparasites of gregarines from the guts of marine invertebrates. Metchnikovellids remain poorly studied in spite of their significance to the evolutionary history of microsporidia; their ultrastructure and life cycles require further investigation. Here we present results of the light- and electron-microscopy study of Metchnikovella incurvata Caulleri and Mesnil 1914, isolated from lecudinid gregarines, parasitizing polychaetes Pygospio elegans in the White Sea littoral zone, and yet described only on the light-microscopic level. The life cycle of this microsporidium includes 2 sporogonies: free (FS) and sac-bound (SBS). In FS, sporonts develop into multinuclear cells (sporogonial plasmodia), which generate sporoblasts and free spores residing in direct contact with the host cytoplasm. Electron microscopy revealed their metchnikovellidean structure: a horseshoe-shaped nucleus, short manubrium perpendicular to the long axis of the spore, and a polar cap in a separate membrane container. Merogony was not observed. The earliest stages of SBS were chains of binucleate cells. They underwent a series of nuclear and cell divisions, produced extracellular envelopes, and split into boomerang-shaped spore sacs, containing up to 16 spores each. Ultrastructure and sizes of sac-bounded spores were similar to those of free-living ones. An amended diagnosis of M. incurvata is provided.

Temporal changes in intertidal macrofauna communities over eight decades: A result of eutrophication and climate change

The comparison of three different macrofauna studies from the 1930s, 1970s and 2009 conducted in the Jade Bay revealed pronounced temporal and functional changes in community structure over eight decades. We focus on species composition, abundance, community structure and spatial extent in the intertidal area of the Jade Bay (Wadden Sea Lower Saxony, southern North Sea). Species inventory increased from 65 taxa in the 1930s to 83 in the 1970s to 114 taxa in 2009 caused by cryptogenic species and immigrations by species of assumed non-native origin. On the mudflats, a strong increase of the oligochaete Tubificoides benedii was observed between the 1930s and 2009, while the abundance of the bivalve Scrobicularia plana and the amphipod Corophium volutator decreased. The mobile amphipod Bathyporeia sarsi and the tube-building polychaete Pygospio elegans, dominant species for sandflats and mixed sediments in the 1930s, had greatly decreased in numbers in the 1970s and were still low in abundance in 2009. Bivalve species standing stocks declined in 2009, compared to the 1930s and 1970s. In the 1970s, an increase in the capitellid polychaete Heteromastus filiformis abundance was noted for the whole intertidal, but declined strongly in 2009. The seagrass bed area decreased by about 50% from the 1930s to the 1970s, but increased threefold since the 1970s. Assuming that cold winters, eutrophication and pollution effects mainly contributed to these changes in community structure between the 1930s and 1970s, climate warming, decreasing nutrient loads and species introductions are more likely for the temporal changes in the macrofauna communities between the 1970s and 2009. In addition, it is suggested that in the Jade Bay a higher sediment accumulation rate outpaced the rate of rising sea level.

Polymorphism in Developmental Mode and Its Effect on Population Genetic Structure of a Spionid Polychaete, Pygospio elegans

Population genetic structure of sedentary marine species is expected to be shaped mainly by the dispersal ability of their larvae. Long-lived planktonic larvae can connect populations through migration and gene flow, whereas species with nondispersive benthic or direct-developing larvae are expected to have genetically differentiated populations. Poecilogonous species producing different larval types are ideal when studying the effect of developmental mode on population genetic structure and connectivity. In the spionid polychaete Pygospio elegans, different larval types have been observed between, and sometimes also within, populations. We used microsatellite markers to study population structure of European P. elegans from the Baltic Sea (BS) and North Sea (NS). We found that populations with planktonic larvae had higher genetic diversity than did populations with benthic larvae. However, this pattern may not be related to developmental mode, since in P. elegans, developmental mode may be associated with geography. Benthic larvae were more commonly seen in the brackish BS and planktonic larvae were predominant in the NS, although both larval types also are found from both areas. Significant isolation-by-distance (IBD) was found overall and within regions. Most of the pair-wise F(ST) comparisons among populations were significant, although some geographically close populations with planktonic larvae were found to be genetically similar. However, these results, together with the pattern of IBD, autocorrelation within populations, as well as high estimated local recruitment, suggest that dispersal is limited in populations with planktonic larvae as well as in those with benthic larvae. The decrease in salinity between the NS and BS causes a barrier to gene flow in many marine species. In P. elegans, low, but significant, differentiation was detected between the NS and BS (3.34% in AMOVA), but no clear transition zone was observed, indicating that larvae are not hampered by the change in salinity.

Macrobenthic recovery from hypoxia in an estuarine tidal mudflat

Macrobenthic recolonisation patterns after complete defaunation resulting from exper- imentally induced hypoxia were investigated in a polyhaline, estuarine mudflat. Based on simultane- ous sampling of biotic and environmental variables in replicated 16 m 2 control and defaunated plots, with a high resolution in time during 6 mo, the ecological interactions related to the macrobenthos reassembly were elucidated. Colonisation was predominantly determined by juvenile recruitment, and 3 successional stages were identified, each characterised by different species assemblages and environmental characteristics. During recovery, a shift in functional group dominance from mobile surface deposit feeders to tube-dwelling surface deposit feeders to biodestabilising taxa occurred, while their proportional dominance remained quite stable in the control plots throughout the experi- ment. Species colonisation patterns of later colonists revealed positive interactions with early colonis- ing opportunistic tube-building polychaetes Pygospio elegans, while later successional species (Het- eromastus filiformis, Macoma balthica) adversely affected the stable, favourable conditions created by the tube-building infauna. Transitions between different successional stages were related to recruitment of species, changes in environmental characteristics (oxygenation state of the sediment), direct and indirect ecological interactions (bio(de)stabilisation, exploitation competition for food). In general, our study suggests that macrobenthic reassembly after hypoxia is related to different types of interactions, all acting in a unique manner. Hence, macrobenthic successional dynamics in a tidal mudflat habitat should be considered as a dynamic process, related to resource availability, natural temporal variation, life history traits (e.g. opportunistic behaviour) and bio-engineering capacities of the colonising species.

Morphology of anterior regeneration in two spionid polychaete species: implications for feeding efficiency

Abstract. In marine soft‐sediment habitats, tissue loss by infuanal invertebrates can provide significant energetic input to higher trophic levels, have substantial impacts on individual behavior, growth, and fecundity, and resulting changes in bioturbation rates can secondarily affect community dynamics. The degree to which a community is affected by such sublethal predation depends in part on whether injured individuals can regenerate and on the speed at which they do so. Previously, we demonstrated differences in the rate of anterior segment and palp regeneration by the spionid polychaetes Pygospio elegans and Dipolydora quadrilobata. The current study examined the morphology of the anterior segment and palp regeneration in these species using scanning electron microscopy (SEM) and indirect immunohistochemistry with confocal laser scanning microscopy at 3, 6, 9, and 12 d post‐ablation. Antibodies for acetylated α‐tubulin and the neural tetrapeptide FMRFamide were used to label the regenerating nervous system. SEM revealed that the morphology of anterior tissue regeneration was similar for both species, but the ciliated food groove tended to form sooner on palps in P. elegans than on those of D. quadrilobata. In both species, palp regeneration and ciliated food groove formation were faster when only palps were removed. A shortened ciliated food groove is likely to reduce particle contact and transport efficiency in suspension and deposit feeding. Regenerating palp nerves were initially visible at 3 d following ablation of palps only, but at 6 d following ablation of five anterior segments. Following ablation of anterior segments, the regenerating nervous system was largely complete by 9 d, nuchal organs were innervated by 6 d, and processes of palp sensory cells were visible at 12 d. Contact chemoreception by sensory cells on the palps may be diminished during the early stages of regeneration, but chemoreception of waterborne cues via the nuchal organ should not.

Mortality through ontogeny of soft-bottom marine invertebrates with planktonic larvae

The present survey covers one spawning season of marine benthic invertebrates in a large geographical area, the inner Danish waters, and includes a wide range of habitats with steep salinity and nutrient load gradients. The loss ratios of soft-bottom marine invertebrates from one development stage to the next is calculated based on average abundances of pelagic larvae, benthic post-larvae and adults of Bivalvia, Gastropoda, Polychaeta and Echinodermata, with planktonic development. This gives a rough estimate of the larval and post-larval mortality. Loss ratios between post-larvae stage and adult stage (post-larval mortality) varies from 3:1 to 7:1 (71.2–84.9%) and loss ratios between larvae and post-larvae (larval mortality) and between larvae and adult, ranging from 7:1 to 42:1 (85.2–97.6%) and from 45:1 to 210:1 (97.8–99.5%), respectively. The results show a remarkable unity in loss ratios (mortality) between the mollusc taxa (Bivalvia and Gastropoda) at the phylum/class level. This similarity in loss ratios among the mollusc taxa exhibiting the same developmental pathways suggests that the mortality is governed by the same biotic and abiotic factors. Larval mortality is estimated to range from 0.10 d − 1 to 0.32 d − 1 for Bivalvia and ranging from 0.09 d − 1 to 0.23 d − 1 for Polychaeta. The species loss ratios combined with specific knowledge of the reproduction cycles give estimated loss ratios (mortality) between the post-larvae and the adult stage of 25:1 and 14:1 for the bivalves Abra spp. and Mysella bidentata. For the polychaete Pygospio elegans the loss ratio (larval mortality) between the larvae and the post-larval stage is 154:1 and between the post-larvae and the adult stage 41:1. For Pholoe inornata the loss ratio between post-larvae and adults is 7:1. The present results confirm that the larval stage, metamorphosis and settlement are the critical phase in terms of mortality in the life cycle for Bivalvia. Assuming steady state based on actual measurements of pelagic larval densities an estimated input to the water column of pelagic bivalve larvae is ranging from 10,930 to 17,157 larvae m − 2 d − 1 and for Polychaeta between 2544 and 3994 larvae m − 2 d − 1 . These estimates seem to correspond to the reproductive capacity of the observed adult densities using life-table values from the literature. The potential settlement of post-larvae is 43 post-larvae m − 2 d − 1 for Bivalvia and 56 post-larvae m − 2 d − 1 for Polychaeta. The adult turnover time for Bivalvia is estimated to be 1.5 years and 2.1 years for Polychaeta. This exemplifies that species with short generation times may dominate in very dynamic transitional zones with a high frequency of catastrophic events like the frequent incidents of hypoxia in the inner Danish waters.

Population structure and reproductive biology of Pygospio elegans (Polychaeta: Spionidae) on an intertidal sandflat, Firth of Forth, Scotland

Abstract. The population structure and reproductive biology of a population of the spionid polychaete Pygospio elegans was studied at Drum Sands, Firth of Forth, Scotland, between March 1997 and February 1998.P. elegans was numerically dominant at the site and exhibited a population density increase through the sampling period. The population showed a seasonal pattern with two acute periods of recruitment during the year, one in spring and one in winter. Planktotrophic larvae were the only larval type produced throughout the sampling period. Larvae were released at the 3–5 setiger stage and settled at a size of 18–22 setigers. The spring recruits grew at a rate of 23 setigers per month, reaching sexual maturation size during September of the same year. The adult sex ratio was female‐biased throughout the study period (mean = 2.99:1 ± 0.51). When female:ripe male sex ratios were highest (summer and fall), the proportion of females brooding larvae decreased suggesting that sexual reproduction is seasonal. Asexual reproduction was not observed in the population of P. elegans on Drum Sands. These findings suggest that a high degree of variability is a trait of the reproductive biology of P. elegans..

Distribution of Zoobenthos in the Bottom Sediment of the Near-Shore Zone of the Baltic Sea in Nida–Klaipėda Stretch

Distribution of sediment in the sea bottom of Nida–Klaipėda water area is determined by peculiarities of the bottom relief, hydrodynamic conditions, geological structure of shallow and nearshore zones. Sediment reflects the main features of sedimentation and biocenoses that are related to this process. While studying regularities of the distribution of the biocenoses of the sea ground fauna, abundance of animals and amount of biomass, a direct connection can be noticed with the granulometric composition of the bottom sediment and the depth. Coarser sediment of the bottom make optimum conditions for the development of zoobenthos. With the sediment getting finer and the depth decreasing, the biomass of some zoobenthic organisms also decreases (e.g. Macoma baltica), but the abundance of polychaetes (Pygospio elegans) notably increases. In the water area of the ground dumping, zoobenthic populations resistant to dumping of grounds are forming. Intensive bottom sedimentation with greater amounts of substrata m...

Evidence for poecilogony in Pygospio elegans (Polychaeta:Spionidae)

The spionid polychaete Pygospio elegans displays more than one developmental mode. Larvae may develop directly, ingesting nurse eggs while brooded in capsules within the parental tube, or they may hatch early to feed in the plankton before settling. Asexual reproduction by architomic fragmentation also occurs. Geographically separated populations of P. elegans often display different life histories. Such a variable life history within a single species may be interpreted either as evidence of sibling speciation or of reproductive flexibility (poecilogony). Four populations from the English Channel were found to demonstrate differing life histories and were examined for morphological and genetic variability to determine whether P. elegans is in fact a cryptic species complex. Significant but minor inter-population polymorphisms were found in the distribution of branchiae and the extent of spoonlike hooded hooks. These externally polymorphic characters did not vary with relation to life history, and variation fell within the reported range for this species. Cellulose acetate electrophoresis was used to examine 10 allozyme loci, 5 of which were polymorphic. Overall, observed heterozygosity (H o = 0.161) was lower than that expected under Hardy-Weinberg equilibrium (H e = 0.228). Significant heterozygote deficiencies, detected at the Est' and Xdh * loci in all populations (except Xdh * at Ryde Sand, Isle of Wight, UK), are discussed. F-statistics were used to examine patterns of genetic structuring among both separate and pooled populations. F ST values at all polymorphic loci indicated a significant level of genetic differentiation between populations, most probably related to isolation by geographic distance. No direct relationship between life history and genetic structure could be detected. Overall genetic identity among the 4 populations was high (I=0.977 to 0.992). Overall, populations displaying larval brooding did not appear to be reproductively isolated from populations displaying a fully planktonic larval mode. Present data support the hypothesis that P. elegans is poecilogonous.

Drifting algae and zoobenthos — Effects on settling and community structure

Shallow (5 to 10 m) sandy bottoms in the Baltic Sea are important areas for zoobenthic production. The infaunal communities are generally governed by the hydrographical conditions are transport of the sediment through wind effects. With increasing eutrophication in the Baltic Sea, drifting mats of annual algae ( Cladophora, Stictyosiphon, Polysiphonia, Rhodemela, Sphacelaria, Pilayella, Furcellaria, Ceramium, etc) have become increasingly common, adding to the structuring and regulating factors for the infauna. In 1990 and 91, a field-study (SCUBA diving; zoobenthos and algae sampling) was carried out in the Åland archipelogo, in thennorthern and their structuring effect on the zoobenthos. Algal biomass increased from 150 ± 19 g DW·m −2 in 1990 to 832±60 g DW·m −2 in 1991, having no effect on oxygen saturation in 1990, but showing signs of reduced oxygen saturation in 1991. Organic content of the sediment remained stable (0.60 to 0.74%) during the entire study period. The zoobenthic community showed significant responses to the drifting algae at population level and in terms of community structure (by 1991: significantly reduced species number; low similarity values (40 to 65%) between bare sand and under the algae). The main species affected were the dominating bivalve Macoma balthica, the polychaetes Pygospio elegans and Manayunkia aestuarina, and the amphipod Corophium volutator. The settlement of M. balthica spat was significantly reduced by the algae (>70% in 1990/91), and no individuals of the dominating polychaetes were recorded under the mat. C. volutator, however, benefited from the algae, and greatly increased in numbers. The results clearly demonstrate the types of physical effects drift-algae will have no sandy-bottom benthos, and show that significant changes in the communities over large areas can be expected with increasing eutrophication.