Food intake of Macoma balthica (mollusca) in relation to seasonal changes in its potential food on a tidal flat in the Dutch Wadden Sea

Molecular diet analysis enables detection of diatom and cyanobacteria DNA in the gut of Macoma balthica

Detritivores are essential to nutrient cycling, but are often neglected in trophic networks, due to difficulties with determining their diet. DNA analysis of gut contents shows promise of trophic link discrimination, but many unknown factors limit its usefulness. For example, DNA can be rapidly broken down, especially by digestion processes, and DNA provides only a snapshot of the gut contents at a specific time. Few studies have been performed on the length of time that prey DNA can be detected in consumer guts, and none so far using benthic detritivores. Eutrophication, along with climate change, is altering the phytoplankton communities in aquatic ecosystems, on which benthic detritivores in aphotic soft sediments depend. Nutrient-poor cyanobacteria blooms are increasing in frequency, duration, and magnitude in many water bodies, while nutrient-rich diatom spring blooms are shrinking in duration and magnitude, creating potential changes in diet of benthic detritivores. We performed an experiment to identify the taxonomy and quantify the abundance of phytoplankton DNA fragments on bivalve gut contents, and how long these fragments can be detected after consumption in the Baltic Sea clam Macoma balthica. Two common species of phytoplankton (the cyanobacteria Nodularia spumigena or the diatom Skeletonema marinoi) were fed to M. balthica from two regions (from the northern and southern Stockholm archipelago). After removing the food source, M. balthica gut contents were sampled every 24 hours for seven days to determine the number of 23S rRNA phytoplankton DNA copies and when the phytoplankton DNA could no longer be detected by quantitative PCR. We found no differences in diatom 18S rRNA gene fragments of the clams by region, but the southern clams showed significantly more cyanobacteria 16S rRNA gene fragments in their guts than the northern clams. Interestingly, the cyanobacteria and diatom DNA fragments were still detectable by qPCR in the guts of M. balthica one week after removal from its food source. However, DNA metabarcoding of the 23S rRNA phytoplankton gene found in the clam guts showed that added food (i.e. N. spumigena and S. marinoi) did not make up a majority of the detected diet. Our results suggest that these detritivorous clams therefore do not react as quickly as previously thought to fresh organic matter inputs, with other phytoplankton than large diatoms and cyanobacteria constituting the majority of their diet. This experiment demonstrates the viability of using molecular methods to determine feeding of detritivores, but further studies investigating how prey DNA signals can change over time in benthic detritivores will be needed before this method can be widely applicable to both models of ecological functions and conservation policy.

Seasonal variation in the diet of estuarine bivalves

Estuarine food webs are generally considered to be supported by marine pelagic and benthic primary producers and by the import of dead organic matter from the open sea. Although estuaries receive considerable amounts of freshwater phytoplankton and organic compounds from adjacent rivers, the potential contribution of these living and dead matter to estuarine food webs is often assumed to be negligible and, therefore, not examined. Based on stable isotope analyses, we report the importance of freshwater suspended particulate organic matter (FW-SPOM) for fuelling estuarine food webs in comparison to estuarine SPOM and microphytobenthos. This previously neglected food source contributed 50–60% (annual average) of food intake of suspension-feeding bivalves such as cockles (Cerastoderma edule), mussels (Mytilus edulis) and Pacific oysters (Magallana gigas) at the Balgzand tidal flats, an estuarine site in the western Wadden Sea (12–32 psu). For these species, this proportion was particularly high in autumn during strong run-off of SPOM-rich freshwater, whilst estuarine SPOM (20%-25%) and microphytobenthos (15%-30%) were relatively important in summer when the freshwater run-off was very low. These findings have implications for our understanding of the trophic interactions within coastal food webs and for freshwater management of estuarine ecosystems.

Bioturbation activity of three macrofaunal species and the presence of meiofauna affect the abundance and composition of benthic bacterial communities

Given concerns of increasing rates of species extinctions, the relationship between biodiversity and ecosystem functioning has become a major research focus over the past two decades. Many studies have shown that biodiversity per se (e.g. species richness) or species-specific traits may be good predictors of changes in ecosystem function. Although numerous studies on this subject have focused on terrestrial systems, few have evaluated benthic marine systems. We used the Limecola balthica community as a model to test whether the number or identity of three well-studied macrofaunal species influence the sediment bacterial compartment, which drives important biogeochemical processes and influence ecosystem functioning. We also investigated the poorly known role of meiofauna in the interactions between macrofauna and bacteria. Eight combinations of 0-3 species were maintained in microcosms for 34 days in the presence or absence of meiofauna. The abundance and composition of the bacterial community, defined by the relative percentage of cells with a high (HNA) vs low (LNA) nucleic acid content, were measured. Species identity of macrofauna was a better indicator of changes in the microbial compartment than was species richness per se. In particular, the gallery-diffuser behaviour of the polychaete Alitta virens likely induced strong changes in sediment physical and geochemical properties with a major impact on the bacterial compartment. Moreover, the presence of meiofauna modulated the influence of macrofauna on bacterial communities. This study provides evidence that species identity provides greater explanatory power than species richness to predict changes in the bacterial compartment. We propose that multi-compartment approaches to describe interactions amongst different size classes of organisms and their ecological roles should be further developed to improve our understanding of benthic ecosystem functioning.

Modeling of Bioaccumulation in Marine Benthic Invertebrates Using a Multispecies Experimental Approach

The causal links between species traits and bioaccumulation by marine invertebrates are poorly understood. We assessed these links by measuring and modeling polychlorinated biphenyl bioaccumulation by four marine benthic species. Uniformity of exposure was achieved by testing each species in the same aquarium, separated by enclosures, to ensure that the observed variability in bioaccumulation was due to species traits. The relative importance of chemical uptake from pore water or food (organic matter, OM) ingestion was manipulated by using artificial sediment with different OM contents. Biota sediment accumulation factors (BSAFs) ranged from 5 to 318, in the order Nereis virens < Arenicola marina ≈ Macoma balthica < Corophium volutator. Calibration of a kinetic model provided species-specific parameters that represented the key species traits, thus illustrating how models provide an opportunity to read across benthic species with different feeding strategies. Key traits included species-specific differentiation between (1) ingestion rates, (2) ingestion of suspended and settled OM, and (3) elimination rates. The high BSAF values and their concomitant variability across the species challenges approaches for exposure assessment based on pore water concentration analysis and equilibrium partition theory. We propose that combining multienclosure testing and modeling will substantially improve exposure assessment in sediment toxicity tests.

Heavy metals in Macoma balthica and extractable metals in sediments from the southern Baltic Sea

Concentrations of Zn, Cu, Ni, Fe, Mn, Co and Cd in soft tissues of Macoma balthica and their extractable forms in the adjacent sediments from 15 sampling sites in the southern Baltic Sea were determined by AAS. The samples were collected during cruises of M/S Hestia and R/V Baltica in July-August 2006. In order to estimate the degree of element bioaccumulation in the molluscs with respect to their extractable forms of the associated sediments, the bioaccumulation factor (BF) was calculated. ANOVA clearly indicates spatial differences between the concentrations of the metals, and factor analysis (FA) confirms that spatial variations exist between the concentrations of metals in M. balthica and their extractable forms in the adjacent sediments from the Gulf of Gdańsk and open Baltic waters. This differentiation can be explained by differences in environmental parameters between these two areas. M. balthica may be useful as a biomonitor of heavy-metal bioavailability and contamination in the Baltic Sea.

Abnormal features of Macoma balthica (Bivalvia) in the Baltic Sea: alerting symptoms of environmental adversity?

Recent studies of the Baltic clam Macoma balthica (L.) from the southern Baltic (the Gulf of Gdansk) have revealed striking morphological, histological and cytogenetic features. Strong deformation of the shell, including elongation of the posterior end and the appearance of an easily visible flexure in this part, has been recorded. The population contribution of the deformed blunt shelled ("irregular") clams ranged from 0% to 65% and tended to increase with depth. The morphologically "irregular" clams had higher accumulated tissue concentrations of trace metals (As, Ag, Cd, Pb, Cu and Zn), indicating a different metal handling ability. Adverse conditions in deeper water regions of the Gulf (e.g. hypoxia, hydrogen sulphide, elevated bioavailability of contaminants) have been suggested as inducers of the phenotypical changes (morphological deformation) in part of the population and, in parallel, of the specific physiological adaptations that result in higher metal accumulation in the "irregular" clams. Cytogenetic and histological analyses showed the presence of tumours in gill cells and digestive system of the affected clams, the prevalence of disseminated neoplasia ranging from 0% to 94% depending on the site. The disease was manifested by a modified karyotype (i.e. an abnormal number and morphology of chromosomes), a higher activity of nucleolar organizer regions (AgNORs), and tissue lesions (enlarged cells, actively proliferative with pleomorphic nuclei). Bottom sediments showed acute toxicity and have been proposed as a source of an initialising carcinogenic factor. However, none of the ecotoxicological studies provided was successful in the clear demonstration of a single (or multifactorial) agent that can account for the disseminated neoplasia.

Accumulation of nodularin-like compounds from the cyanobacterium Nodularia spumigena and changes in acetylcholinesterase activity in the clam Macoma balthica during short-term laboratory exposure

In this laboratory study the effects of the cyanobacterium Nodularia spumigena (strain AV1) that produces hepatotoxic nodularin (NODLN), non-toxic Nodularia sphaerocarpa (strain UP16f) and purified NODLN on the infaunal clam Macoma balthica from the Baltic Sea were examined. N. sphaerocarpa (2.4 and 12.5 mg dw l(-1)), N. spumigena (4 and 20 mg dw l(-1), intracellular NODLN content ca. 4 and 20 microg l(-1)) and purified NODLN (10 and 50 microg l(-1)) were applied in experimental tanks at 24 h intervals for 96 h. Water samples were taken during the experiment for the measurement of soluble NODLN concentrations. The concentrations of total hepatotoxins in the soft tissues were analysed with enzyme-linked immunosorbent assay (ELISA) and NODLN with high-performance liquid chromatography/diode array detection (HPLC/DAD). Acetylcholinesterase (AChE) activity was measured from the foot tissue samples taken at 0, 24 and 96 h. In the water phase, 60-100% of NODLN in the pure-toxin treatments and all the NODLN in N. spumigena treatments appeared as a yet unidentified form with NODLN-like spectral characteristics. The compound was present in similar quantities also in the non-toxic N. sphaerocarpa treatments. In the toxic N. spumigena treatments the tissue concentration of hepatotoxic NODLN-like compounds (measured with ELISA) increased from the control levels of 0.16 to 16.6 microg g(-1) dw (24 h), reaching 30.3 microg g(-1) dw at 96 h. However, <5% of the toxin detected by ELISA could be shown to be NODLN in HPLC/DAD analysis. Matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF/MS) analyses revealed no NODLN-glutathione (GSH) conjugates in the tissues of M. balthica. Combining the responses in the AChE activity recorded after 24 and 96 h exposure, increases by 46% (N. spumigena) and 36% (soluble NODLN) compared with the control activity were observed in the low-level toxic exposures. Oppositely, decreases by 19% (N. spumigena) and 27% (soluble NODLN) of control activity were observed in the high-level exposures. Only the differences between the increased and decreased AChE activities were statistically significant, with individuals exposed to high levels expressing approximately 55% of the activity of those exposed to low concentrations. The results show that M. balthica readily ingests toxic N. spumigena and that accumulation of peptides takes place rapidly, which has potential food chain effects through toxin enrichment. However, it appears that M. balthica is at least partly able to metabolise NODLN. In addition to hepatotoxicity, NODLN seems to induce concentration-dependent neurotoxic effects; this must be taken into consideration when applying AChE activity as a biomarker of specific anthropogenic contamination (e.g. organophosphate and carbamate pesticides).

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.

Bioaccumulation of Cd, Se, and Zn in an estuarine oyster (Crassostrea rivularis) and a coastal oyster (Saccostrea glomerata)

We compared the bioaccumulation of Cd, Se, and Zn in an estuarine oyster (Crassostrea rivularis) and a coastal oyster (Saccostrea glomerata) from both the dissolved and particulate phases. A bioenergetic-based kinetic model was used to predict the exposure pathways of metals in the oysters. The assimilation efficiencies (AEs) of metals, determined for four species of phytoplankton and natural sediment, were in the range of 30-75% for Cd, 25-75% for Se, and 30-80% for Zn, respectively, in the two oysters. There was little difference in metal AE between the two oysters and among the three metals on the same particle type. Metal uptake rate constant, determined from the log-log relationship between metal influx rate and metal concentration in the dissolved phase, was higher for Cd and Zn in the estuarine oyster C. rivularis than in the coastal oyster S. glomerata. The uptake rate constants quantified for Cd, Se, and Zn were the highest among different bivalve species studied so far. The calculated metal absorption efficiency from the dissolved phase was similar to those found in other bivalve species. The measured efflux rate constant in the estuarine oyster C. rivularis was in the range of 0.01-0.03 day(-1) for Cd, Se, and Zn, and were comparable to those found in mussels and clams. In contrast, the efflux rate constant of Cd and Zn in the coastal oyster S. glomerata was 0.003-0.004 day(-1), which may be responsible for the high Cd and Zn concentrations in the oysters. The bioenergetic-based kinetic model predicts that under most circumstances likely experienced by oysters in nature, the majority of Se and Zn in the two oysters stem from uptake from the dietary phase. Dissolved uptake contributes significantly to metal accumulation only when the metal concentration factor (CF) in the particles is in the lower portion of the range typically found in nature. Sensitivity analysis indicates that the metal CF is a critical parameter in determining the exposure pathways of metals in the oysters. Our study demonstrated that estuarine and coastal oysters differed in their strategies in accumulating a high metal concentration in their tissues.

Variations in food intake of Pecten maximus (L.) from the Bay of Brest (France): influence of environmental factors and phytoplankton species composition

Previous studies carried out in the bay of Brest on daily shell growth of Pecten maximus have demonstrated that temperature is a major control on daily shell growth in contrast to food supply. However, repeated events of slow growth have been observed during diatom and dinoflagellate blooms. The aim of this study was to determine how fluctuations in environmental parameters influence P. maximus food intake and daily shell growth rate. In 1995, P. maximus food intake and growth were highest when Cerataulina pelagica (diatom) blooms occurred and lowest during Gymnodinium cf. nagasakiense (dinoflagellate) blooms. During blooms of other diatom species, P. maximus food intake and growth were high when the algal concentration did not exceed a critical threshold, dependent upon the dominant species and sedimentation rate of diatoms. These results demonstrate that the morphological and physiological features of phytoplankton bloom species strongly affect benthic microphytophagy, a component of benthic-pelagic coupling.

Differences in the benthic-pelagic particle flux (biodeposition and sediment erosion) at intertidal sites with and without clam (Ruditapes philippinarum) cultivation in eastern China

An annular flume or flux system (AFS) was deployed to measure the biodeposition and sediment resuspension processes at four stations in the mid-shore and low-shore zones along both natural and Manila clam (Ruditapes philippinarum) farming transects in Xuejiadao intertidal area located in Jiaozhou Bay, eastern China. The results showed that there was a significant correlation between biodeposition rates and the density (r=0.984, P<0.05) and biomass (r=0.977, P<0.05) of the suspension feeding Manila clam. The effect of biodeposition at the farming transect was more intensive than at the natural transect where the biodeposition effect was lower than that resulting from natural sedimentation. In contrast, the biodeposition rate at the culture site with the high density of Manila clams was four times the sedimentation rate. Following the addition of Manila clams to increase their density in natural sediments, there was a marked increase in both clearance and biodeposition rates at all four sites. At the site with the highest natural bivalve biomass, the biodeposition rate increased 1.5-fold, while at the site with the least bivalve biomass, it increased by 40 times compared with the biodeposition rate by natural biota. The mean clearance rate of Manila clam was 0.90+/-0.34 l h(-1) ind.(-1) and the biodeposition rate was calculated to be 0.06+/-0.01 g h(-1) ind.(-1). The critical erosion velocities of intertidal sediment at Xuejiadao were from 17.4 to 20.4 cm s(-1). Relationships describing suspended particulate matter (SPM) vs. current velocity were analysed by linear regression following log transformation of the SPM. Statistical analysis of the slopes of the regression lines revealed that there were distinct differences between the low-shore and mid-shore (P<0.05) at both transects, respectively. There was also distinct difference between the two mid-shore stations (P<0.05) possibly due to differences in the densities of bioturbators (e.g. Macoma incongrua etc.), however, there was no significant difference between the two low-shore stations (P>0.05). Furthermore, it was found that the site differences in sediment erodibility were not significantly correlated with measured physical properties of sediments and biota factors such as total macrofauna biomass, total abundance and macrofauna densities, Chl-a and Ph-a (P>0.05). However, there was a significant correlation between sediment erodability and both median grain size (P<0.01) and the density of the bioturbator M. incongrua (P<0.05). There was also evidence of a slight increase in sediment stability after 3 h of air exposure, but the effect was not overcome following the addition of Manila clams. It is hypothesised that the lower stability of sediments at the mid-shore level was probably associated with higher densities of bioturbators and with disturbance by shrimp farming near the high-shore.

20-isosterols from the marine bivalve Macoma balthica

A fraction of 20-isosterols has been isolated from the Baltic Sea bivalve Macoma balthica. The main component of this fraction, (20S)-cholest-5-en-3 beta-ol, has been characterized by 300 MHz nuclear magnetic resonance spectroscopy. 20-Isosterols in M balthica probably originate from sea-bottom sediments, an important component of a diet of the animal studied.