Digestive selection underlies differential utilization of phytoplankton and sedimentary organics by infaunal bivalves: Experiments with cockles (Cerastoderma edule) using cross-labelled mixed diets

Impact of Dietary Lipid to Carbohydrate Ratio on Elemental Stoichiometric Relationships in Growth Phenotypes of Ruditapes Decussatus

Understanding the mechanisms of nutrient regulation in bivalves is crucial for optimizing their growth under varying dietary conditions. In the present work, juveniles of the carpet shell clam (Ruditapes decussatus) from the same cohort were size-segregated to obtain fast and slow growing phenotypes. These clams were then conditioned to diets presenting a range of lipid/carbohydrate proportions but similar carbon:nitrogen (C:N) ratios. Subsequently, experiments were conducted to determine elemental (C and N) balances in order to achieve the following aims: (a) To identify strategies of homeostatic nutrient regulation in relation to either endogenous (growth phenotype) or dietary factors and (b) to quantify the extent to which stoichiometric adjustments (at both pre- and postabsorptive levels) are accomplished throughout the successive components of elemental balances. The elemental balances of both C and N exhibited higher values under the lipid-rich diets, indicating the presence of nutritional limitations in juvenile clams fed on low lipid/carbohydrate proportion, resulting from a greater digestive imbalance of lipids in diets of low digestibility coupled to limited dietary lipid income. These nutritional limitations were more effectively managed by the fast-growing phenotype, pointing to the importance of enhanced energetic status in sustaining homeostatic nutrient regulation. The stoichiometric coupling between consumed diets and the biosynthetic requirements of growing tissues relied on postabsorptive rather than preabsorptive mechanisms, although notable discrepancies in this regard were observed between conditioning diets.

Food-web comparisons between two shallow vegetated habitat types in the Baltic Sea

Coastal vegetated habitats maintain highly diverse communities, where the contribution of macrophyte production is significant for macroinvertebrate primary consumers. In the brackish-waters of the Baltic Sea, the taxonomical diversity of different macrophytes includes both marine and limnic species. To study the basal food-web differences of two key vegetated habitat types, either dominated by a perennial brown macroalgae (Fucus vesiculosus) or by angiosperm plants, ¹³C and ¹⁵N compositions of different primary producers and macroinvertebrate consumers were examined, and their diets were estimated by Bayesian mixing models. Carbon isotope diversity of primary producers was high especially in the hard-bottom Fucus-dominated habitats, which was also reflected in a larger consumer isotope niche. However, consumer isotope niche among sites was similar within the same habitat type. Our models indicated that the perennial macrophyte dietary median contribution was about 25% for deposit feeders and omnivores in both habitat types, while epigrazers preferred filamentous algae (30-60%). The niche positions of the abundant clams L. balthica, M. arenaria and C. glaucum differed between the two habitats, but they showed only small (<10% units) differences in their macrophyte dietary contributions. The isotopic compositions of the dominating primary producer assemblage reflected significantly in the isotope niche structure of the associated primary consumers.

Effects of suspended mineral coal dust on the energetic physiology of the Caribbean scallop Argopecten nucleus (Born, 1778)

The effects of increasing concentrations of suspended mineral coal dust on the energetic physiology of the Caribbean scallop Argopecten nucleus were studied, at a concentration range that is environmentally relevant and representative of areas proximate to coal loading and shipping ports. Adult hatchery-produced animals were exposed to different concentrations of coal dust, i.e. 0, 2, 9 and 40 mg L^-1. At increasing concentrations of coal dust, the rates of filtration and pseudofeces production increased, while the rates of ingestion and absorption remained constant. The rates of oxygen consumption and ammonium excretion decreased, as well as the absorption efficiency and the scope for growth. Suspended coal dust particles, at concentrations higher than or equal to 2 mg L^-1, were ingested preferentially over microalgae by A. nucleus, causing reductions in its absorption capability, metabolism and in the amount of energy for growth and reproduction, thus generating physiological stress.

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.

qPCR analysis of bivalve larvae feeding preferences when grazing on mixed microalgal diets

Characterization of the feeding preferences of bivalve larvae would help improving the bivalve aquaculture and hatchery by providing appropriate microalgal diets. However, inaccurate and laborious identification and counting of microalgal species have challenged the selective feeding of bivalves. In the present study, we developed a highly specific and sensitive assay using quantitative polymerase chain reaction (qPCR) to assess the selective feeding of bivalve larvae based on species-specific primers targeting to microalgal 18S rDNA sequences. The assay exhibited good specificity. The detection limits of the qPCR assay were 769, 71, 781 and 21 18S rDNA copies for Chaetoceros calcitrans, Isochrysis galbana, Platymonas helgolandica and Nannochloropsis oculata, respectively. Using such assay, we found that C. calcitrans and I. galbana were preferentially ingested, whereas N. oculata was preferentially rejected in biodeposits of four bivalve species, Tegillarca gransa, Cyclina sinensis, Scapharca subcrenata and Sinonovacula constricta. Furthermore, our growth experiments revealed that C. calcitrans and I. galbana could significantly promote the shell growth, whereas feeding of N. oculata resulted in poorer growth of four bivalve species. These data indicated that qPCR might be useful in screening of efficient and reliable microalgal species for each bivalve species, leading to improved bivalve aquaculture and hatchery.