Next-generation phylogeography of the cockle Cerastoderma glaucum: Highly heterogeneous genetic differentiation in a lagoon species

AIM

Coastal lagoons form an intriguing example of fragmented marine habitats. Restricted gene flow among isolated populations of lagoon species may promote their genetic divergence and may thus provide a first step toward speciation. In the present study, the population genetic structure of the lagoon cockle Cerastoderma glaucum has been investigated to clarify the complex phylogeographic pattern found in previous studies, to localize major genetic breaks, and to discuss their origin and maintenance.

LOCATION

The Atlantic and Mediterranean coasts, including the Baltic, North Sea, and Black Sea.

METHODS

A total of 204 C. glaucum individuals from 14 populations were genotyped using restriction site-associated DNA sequencing (RADseq). The genetic diversity, divergence, and structure were analyzed using genome-wide single nucleotide polymorphisms (SNPs). Phylogenetic relationships were inferred under a coalescent model using svdquartets.

RESULTS

The RADseq approach allowed inferring phylogeographic relationships with an unprecedented resolution. Three deeply divergent lineages were identified within C. glaucum that are separated by many genetic barriers: one lineage in the Aegean-Black Sea region, one in the Ionian Sea, and the last one widely distributed from the Western Mediterranean to the Baltic Sea. The nested branching pattern displayed on the species tree largely agrees with the likely scenario of C. glaucum postglacial expansion from the Mediterranean to the Baltic Sea.

MAIN CONCLUSION

The genetic differentiations between geographically separated lagoons proved to be strong, highlighting the evolutionary influence of these naturally fragmented habitats. The postglacial expansion created complex patterns of spatial segregation of genetic diversity with allele frequency gradients in many outlier loci, but also discrepancies between the nuclear and mitochondrial genetic markers that probably arose from genetic surfing of mitochondrial variation.

Abnormalities in bivalve larvae from the Puck Bay (Gulf of Gdansk, southern Baltic Sea) as an indicator of environmental pollution

This study described the occurrence of abnormalities in bivalve larvae from the Puck Bay. Analyses of plankton samples collected in 2012-2013 showed that larval Mytilus trossulus, Mya arenaria, and Cerastoderma glaucum exhibited abnormalities that could indicate adverse environmental impacts. The deformities were mainly in shells, but missing soft tissue fragments and protruding vela were also noted. In addition to larval studies, we analyzed benthic postlarvae of Mytilus trossulus. Interestingly, grooves and notches at different locations of the prodissoconch, dissoconch, and shell margin were observed. Some of these deformations were reminiscent of the indentations found on the shell edge of larvae. Comparing the proportion of abnormal postlarvae to larvae with shell abnormalities suggested that the survival of larvae with shell abnormalities was low. Overall, our results suggested that the ratio of abnormal bivalve larvae could be used as an indicator of the biological effects of hazardous substances in the pelagic environment.

Geographic and seasonal patterns and limits on the adaptive response to temperature of European Mytilus spp. and Macoma balthica populations

Seasonal variations in seawater temperature require extensive metabolic acclimatization in cold-blooded organisms inhabiting the coastal waters of Europe. Given the energetic costs of acclimatization, differences in adaptive capacity to climatic conditions are to be expected among distinct populations of species that are distributed over a wide geographic range. We studied seasonal variations in the metabolic adjustments of two very common bivalve taxa at European scale. To this end we sampled 16 populations of Mytilus spp. and 10 Macoma balthica populations distributed from 39° to 69°N. The results from this large-scale comprehensive comparison demonstrated seasonal cycles in metabolic rates which were maximized during winter and springtime, and often reduced in the summer and autumn. Studying the sensitivity of metabolic rates to thermal variations, we found that a broad range of Q₁₀ values occurred under relatively cold conditions. As habitat temperatures increased the range of Q₁₀ narrowed, reaching a bottleneck in southern marginal populations during summer. For Mytilus spp., genetic-group-specific clines and limits on Q₁₀ values were observed at temperatures corresponding to the maximum climatic conditions these geographic populations presently experience. Such specific limitations indicate differential thermal adaptation among these divergent groups. They may explain currently observed migrations in mussel distributions and invasions. Our results provide a practical framework for the thermal ecophysiology of bivalves, the assessment of environmental changes due to climate change and its impact on (and consequences for) aquaculture.

Metal sources to the Baltic clam Macoma balthica (Mollusca: Bivalvia) in the southern Baltic Sea (the Gulf of Gdansk)

Metal concentrations of Cu, Fe, Mn, Ni, Pb and Zn in an infaunal facultative deposit-feeding bivalve, the Baltic clam Macoma balthica, in the Gulf of Gdansk (southern Baltic Sea) were assessed and compared to selected concentrations of metals in the environment. Between October 1996 and September 1997, dissolved and easy extractable (by 1M HCl) metal fractions of total suspended particulate matter (TPM) in the overlying water and of surficial sediments (<63 microm) were measured monthly at five sublittoral sites in the Gulf of Gdansk, and accumulated tissue metal concentrations in M. balthica were determined simultaneously. The study highlights the importance of sediment geochemistry as a factor modifying ambient trace metal bioavailabilities. Surficial sediments appeared to contribute most to the accumulation of Cu and Pb in M. balthica, reflecting the high metal availability in the Gulf. Assimilation of Cu from sediments is controlled by Mn components possibly through an inhibitory effect of Mn oxyhydroxides, while Pb accumulation from sediments depends on the organic content of the sediment. A dual metal uptake pathway, with a suspended particulate-bound fraction and surficial sediments, was apparent for Mn and Zn. Partitioning of Mn in sediments was related to the concentration of labile Fe, with increased levels of Fe tending to inhibit the accumulation of Mn by the clam. Tissue accumulated Zn might have been altered by the clam's internal regulation, making Zn tissue concentrations, to some degree, independent of its environmental level. The principal source of Ni accumulated by the clams exists in the soluble phase.

Survey for neoplasia in Macoma balthica from the Gulf of Gdansk by flow cytometry

Using flow cytometry, 234 Macoma balthica were examined during a survey to determine frequency of neoplasia in the Gulf of Gdansk (Poland). Clams were collected in 4 locations and DNA content in gill tissue cells was determined by flow cytometry using propidium iodide staining. Cell permeabilization was induced by osmotic shock. Prevalence of neoplasia ranged from 9.6 to 26.7% depending on location. DNA content in aneuploid cells was higher than in normal dividing cells. The fluorescence value for aneuploid cells corresponded to tetraploid/pentaploid cells. Three stages of neoplasia were defined, based on the percentage of aneuploid cells determined by flow cytometry. Histopathological and cytogenetic analyses were also carried out on the same clams for comparative study. Proportions of normal and affected clams detected using flow cytometry were similar to those identified using both methods. In the present study, no clear relationship was demonstrated between prevalence of neoplasia and pollutant detection in the different sampling sites.

Neoplasia detection in Macoma balthica from the Gulf of Gdansk: comparison of flow cytometry, histology and chromosome analysis

A flow cytometry protocol was applied for the detection of neoplasia in Macoma balthica L. from the Gulf of Gdansk (Baltic Sea, Poland). A simple method, based on an osmotic shock, was used to permeabilise gill cells. The cytometric pattern of normal clams consisted of 2 peaks, a major peak B and a smaller peak C. The cytometric pattern of affected clams consisted of 2 peaks named B' and C'. Two parameters were used to define the stages of abnormalities in M. balthica clams based on the percentage of cells in peaks B, C, B' and C' and on the ratio between the fluorescence value of peaks B, C, B' and C' in all individuals. Three stages of neoplasia were clearly distinguished by flow cytometry considering peak C'. Stage 1 was characterised by a major population of cells in peak B' and more than 10% of cells in the C' peak. Stage 2 consisted of a lower percentage of cells in peak B' and more than 25% of cells in peak C'. Stage 3 of the neoplasia was characterised by a further reduction in peak B' and more than 40% of cells in peak C'. Flow cytometry allowed for objective detection of neoplasia and provided a rapid method for measuring the DNA content of thousands of cells per individual. The accuracy of flow cytometry was assessed by comparing with standard histological techniques, used here as a reference technique for the detection of neoplasia, and with chromosome analysis. All individuals were analysed in parallel using the 3 techniques. The proportion of normal and affected individuals diagnosed using flow cytometry was comparable to the proportion determined by histology and chromosome analysis.

Trace metals in the brown mussel Perna perna from the coastal waters off Yemen (Gulf of Aden): how concentrations are affected by weight, sex, and seasonal cycle

The effects of seasonal cycle, sex of individuals, and changes of soft tissues weight on accumulated trace metal concentrations (Cd, Cu, Fe, Mn, Pb, Zn) were examined in the brown mussel Perna perna collected monthly from a natural rocky habitat in the coastal waters off Yemen, the Gulf of Aden, for a period of ten months. Basic hydrological parameters were recorded simultaneously. All metals analyzed displayed seasonal fluctuations with different temporal patterns and variable amplitudes. Similar seasonal cycles were observed for Cu, Mn, and Pb with an increase in accumulated concentration during the rainy period (NE monsoon), and a decrease thereafter. The concentrations of Cu, Mn, and partially Pb appeared to be related to environmental changes, the concentration of Pb possibly also being related to changes in body weight. Accumulated concentrations of Cu and Mn thus seem to reflect actual metal bioavailability in the ecosystem quite efficiently. The tissue levels of Fe and Cd changed inversely to fluctuations in body weight with additional variation due to monsoon-related environmental changes. The behaviors of Fe and Cd are therefore driven by seasonally changing body weight with a considerable contribution of external factors including fluctuations in hydrological conditions and metal exposure. The Zn concentrations tended to increase gradually throughout most of the year regardless of its concentration in the environment. Zinc is considered to be mainly regulated by physiological mechanisms in the mussel, making its accumulated metal concentration independent to some degree of environmental levels. Significant differences in trace metal concentrations between sexes (in favour of females) might have resulted from more intense formation of reproductive tissues and metal accumulation in sexual products of females during the prespawning and spawning periods.

Argyrophilic nucleolar organizer regions (AgNORs) in interphases and metaphases of normal and neoplastic gill cells of Macoma balthica (Bivalvia: Tellinidae) from the Gulf of Gdansk, Baltic Sea

Chromosome analysis of gill cells of different populations of Macoma balthica (L.) from the Bay of Gdansk (Baltic Sea) revealed 2 clam categories, 1 with neoplastic features and 1 without. Silver-staining was performed on interphase and metaphase cells of both categories. The mean argyrophilic nucleolar organizer region (AgNOR) count per abnormal interphase cell was significantly higher than in normal interphase cells. Normal silver-stained metaphases had 3 nucleolar organizer region (NOR) chromosome phenotypes. The location of the NORs in the most frequent phenotype (55.6% in 54 metaphases scored) was interstitial on the largest metacentric chromosome pair, Pair No. 1. Abnormal silver-stained metaphases had a higher number of active NOR sites. Different phenotypes were observed (frequency greater than 10% for 67 metaphases scored); 2 were similar to those in normal metaphases and 5 were ectopic. The higher activity of AgNORs observed in abnormal cells confirmed the diagnosis of malignant neoplasia.

Free amino acids in the clam Macoma balthica L. (Bivalvia, Mollusca) from brackish waters of the southern Baltic Sea

Fourteen acidic and neutral free amino acids (FAA) were investigated in soft tissue of Macoma balthica from different depth zones of the Gulf of Gdansk (Baltic Sea) over a full seasonal cycle. The dry weight of the bivalves and physico-chemical parameters of overlying bottom water and surface sediments were measured simultaneously at each site. In the brackish waters of the Baltic, the main pool of FAA is composed of Ala, Gln, Arg, Gly and Orn which represent approximately 80% of the total. Compared to the full saline environments, the composition of FAA in the clams from the Baltic differs substantially. The differences can be attributed to the lower salinity of the Baltic. In the Baltic, Gly appears to play a most important role in regulating intracellular osmolarity in the clams, a function performed primarily by Tau in Atlantic and North Sea populations. Spatio-temporal variations of the FAA are affected by biotic and environmental parameters; their respective influence differs with the amino acids. The concentration of Arg depends on its uptake from the external medium. However, its level might be temporarily modified by stress-induced metabolic transformation (e.g. hydrolysis to Orn) caused by changes in the ambient environment. The concentration of Ala increases with depth, probably because of physiological adaptations of the animal to diminishing oxygen concentration through anaerobic glucose catabolism. Biosynthesis of Ala, similarly to Gln, in the shallower zone is generally related to the physiological state of an organism. The concentration of Gly is most likely regulated by internal mechanisms driven by gonadal development and reproduction.

The relationship between metal concentrations and phenotypes in the Baltic clam Macoma balthica (L.) from the Gulf of Gdansk, southern Baltic

Soft tissue accumulated concentrations of nine metals (V, As, Se, Ag, Cd, Pb, Mn, Cu and Zn) were studied in two main phenotypes (1) according to external shell colour (white and pink), and (2) according to shell shape (shell with a rounded posterior end--"regular" and with an elongate posterior end and a notable flexure--"irregular") of the Baltic clam Macoma balthica from southern Baltic Sea off Poland. No differences in metal concentrations were observed between colour-based phenotypes. By contrast, "irregular" clams exhibited generally higher concentrations of all elements in their tissues than "regular" bivalves. This finding provides the first reference on a potential linkage of shell deformation with tissue metal concentrations within one entire population of clams living in the same habitat. Different ability of metal handling in the shape-based phenotypes is presumably related to different physiological capacity of the bivalves induced by the selection effect of specific environmental conditions. It is suggested that unfavourable conditions in deep waters of the Gulf of Gdansk (e.g. hypoxia/anoxia, hydrogen sulphide, elevated bioavailability of metals) induces, in a certain part of the population, morphological deformation of shell (thereby leading to irregular shape) and in parallel physiological adaptations which result in greater sensitivity to trace metals of "irregular" clams. This hypothesis however, requires further investigation with special focus on genetic divergences between phenotypes because till now we cannot exclude the co-occurrence of two types (semi-species) of clams in the Gulf: an Atlantic type and a Baltic type. Genetic analysis with a use of DALP technique revealed strong intrapopulational polymorphism but no fingerprints or intraspecific polymorphism characterising any of the phenotypes considered (both colour- and shape-based). Since eight polymorphic loci were clearly identified further studies of population genetic structure hold optimistic promise.

Distribution of dissolved and labile particulate trace metals in the overlying bottom water in the Vistula River plume (southern Baltic Sea)

Overlying bottom water samples were collected in the Vistula River plume, southern Baltic Sea, (Poland) and analysed for dissolved and labile particulate (1 M HCl extractable) Cu, Pb, Zn, Mn, Fe and Ni, hydrological parameters being measured simultaneously. Particulate organic matter (POM), chlorophyll a and dissolved oxygen are key factors governing the chemical behaviour of the measured metal fractions. For the dissolved Cu, Pb, Zn, Fe and Ni two maxima, in the shallow and in the deeper part of the river plume, were found. In the shallow zone desorption from seaward fluxing metal-rich riverine particles account for markedly increased metal concentrations, as confirmed also by high particulate metal contents. For Pb, atmospheric inputs were also considered to have contributed to the elevated concentrations of dissolved Pb adjacent to the river mouth. In the deep zone desorption from detrital and/or resuspended particles by aerobic decomposition of organic material may be the main mechanism responsible for enrichment of particle-reactive metals (Cu, Pb, Zn) in the overyling bottom waters. The increased concentrations of dissolved Fe may have been due to reductive dissolution of Fe oxyhydroxides within the deep sediments by which dissolved Ni was released to the water. The distribution of Mn was related to dissolved oxygen concentrations, indicating that Mn is released to the water column under oxygen reduced conditions. However, Mn transfer to the dissolved phase from anoxic sediments in deeper part of the Vistula plume was hardly evidenced suggesting that benthic flux of Mn occurs under more severe reductive regime than is consistent with mobilization of Fe. Behaviour of Mn in a shallower part has been presumably affected by release from porewaters and by oxidization into less soluble species resulting in seasonal removal of this metal (e.g. in April) from the dissolved phase. The particulate fractions represented from about 6% (Ni) and 33% (Mn, Zn, Cu) to 80% (Fe) and 89% (Pb) of the total (labile particulate plus dissolved) concentrations. The affinity of the metals for particulate matter decreased in the following order: Pb > Fe > Zn > or = > Cu > Mn > Ni. Significant relationships between particulate Pb-Zn-Cu reflected the affinity of these metals for organic matter, and the significant relationship between Ni-Fe reflected the adsorption of Ni onto Fe-Mn oxyhydroxides. A comparison of metal concentrations with data from other similar areas revealed that the river plume is somewhat contaminated with Cu, Pb and Zn which is in agreement with previous findings on anthropogenic origin of these metals in the Polish zone of southern Baltic Sea.

Distribution of mercury and other trace metals in the cockle Cerastoderma glaucum from the Mediterranean Lagoon Etang de Thau

The concentrations of Ag, Cd, Co, Cr, Cu, Ni, Pb, Zn, Mn, and Fe in the soft tissue of the cockle Cerastoderma glaucum from six sites along the Mediterranean Lagoon Etang de Thau were determined by AAS; Hg was determined by the cold-vapor technique. Significant spatial variations in metal concentrations within the lagoon were recorded. Elevated levels of tissue Pb, Cr, and Hg in the Etang des Eaux Blanches, an area adjacent to the harbor and highly industrialized region, are likely of anthropogenic origin. Seasonally dependent variations in the tissue concentrations of trace elements at Crique de l'Angle were observed. The highest levels of Cr and Pb in the tissue were identified in March 1992, of Hg, Co, and Fe in period August 1990-January 1991, of Mn in August 1992, and Ag in January 1991. The gills contained the maximum amounts of Co and Ni, the mantle the most Fe and Hg, and the digestive gland (hepatopancreas) had most Cu.

[Karyologic study of gill neoplasia in Macoma balthica (Mollusca, Bivalvia)]

This paper reports the first karyological study of a gill neoplasia in the bivalve Macoma balthica from the Bay of Gdansk (Poland). Chromosomes were studied with an air-drying technique from gill tissue. Out of 47 specimens studied, 34 showed normal cells and a variable number of mitotic metaphases with a normal diploid chromosome number of 2n = 38, 6 had hypertrophied nuclei and a high number of mitoses with 70 to 98 chromosomes, and 7 specimens showed intermediate features. The karyotype of normal metaphases included 11 metacentric, 2 submetacentric and 6 subtelocentric chromosome pairs. The karyotype of abnormal metaphases, i.e. with a high number of chromosomes, revealed chromosomal aberrations inferring neoplastic disorders such as: different number of metacentric, submetacentric, subtelocentric and telocentric chromosome pairs than in the normal karyotype, increase of chromosome pairs especially in the small and medium-sized chromosomes, irregular monosomy and occurrence of microchromosomes. According to neoplasias recorded in bivalves species, the prevalence observed in this neoplasia is relatively high. As Macoma balthica inhabits the polluted Bay of Gdansk, the effects of environmental parameters should be elucidated.