Variable feeding behavior in three species ofMacoma (Bivalvia: Tellinacea) as a response to water flow and sediment transport

Many species of tellinacean bivalves mainly deposit-feed but are also known to vary their feeding behavior in response to predation and to the availability of suspended organic matter relative to that of sedimentary organic matter. This study showed that three species of the Pacific genusMacoma (Macoma nasuta Conrad, 1937,Macoma secta Conrad, 1837, andMacoma inquinata Deshayes, 1855, from the San Juan Islands, Washington, USA) varied their deposit-feeding behavior in response to water flow near the sediment-water interface and to sediment transport. In the summers of 1987, 1988 and 1989, water velocity was varied in a large racetrack flume, in order to change both water velocity and bottom sediment transport. In quiet water, the inhalant siphon was protruded far from the siphon hole. As water velocity increased, with little or no sediment transport, the deposit-feeding radius decreased. Qualitative observations suggested that this was related to the drag on the siphon. InM. secta, the distance of siphon protrusion was not related to body size. Under conditions of higher near-bottom water velocity, combined with bottom sediment transport, some individuals ceased to deposit-feed, while others sustained feeding by ingesting sediment within the siphon hole, at least 1 cm beneath the sediment-water interface. These results suggest that hydrodynamic conditions are a major determinant of feeding behavior, and previous explanations of variable feeding behavior as a response to predation may have to be adjusted to accommodate this expanded set of responses.     

Feeding current characteristics of three morphologically different bivalve suspension feeders, Crassostrea gigas, Mytilus edulis and Cerastoderma edule, in relation to food competition

Introduced Pacific oysters (Crassostrea gigas) have shown rapid expansion in the Oosterschelde estuary, while stocks of native bivalves declined slightly or remained stable. This indicates that they might have an advantage over native bivalve filter feeders. Hence, at the scale of individual bivalves, we studied whether this advantage occurs in optimizing food intake over native bivalves. We investigated feeding current characteristics, in which potential differences may ultimately lead to a differential food intake. We compared feeding currents of the invasive epibenthic non-siphonate Pacific oyster to those of two native bivalve suspension feeders: the epibenthic siphonate blue mussel Mytilus edulis and the endobenthic siphonate common cockle Cerastoderma edule. Inhalant flow fields were studied empirically using digital particle image velocimetry and particle tracking velocimetry. Exhalant jet speeds were modelled for a range of exhalant-aperture cross-sectional areas as determined in the laboratory and a range of filtration rates derived from literature. Significant differences were found in inhalant and exhalant current velocities and properties of the inhalant flow field (acceleration and distance of influence). At comparable body weight, inhalant current velocities were lower in C. gigas than in the other species. Modelled exhalant jets were higher in C. gigas, but oriented horizontally instead of vertically as in the other species. Despite these significant differences and apparent morphological differences between the three species, absolute differences in feeding current characteristics were small and are not expected to lead to significant differences in feeding efficiency.     

Filtration rate,using a new indirect technique,in thirteen species of suspension-feeding bivalves

A method for determining filtration rates in undisturbed suspension-feeding bivalves is described. Concentrations of particulate matter in the water collected in the inhalant (C _i) and exhalant (C _e) currents were estimated with an electronic particle counter. The clearance was calculated as , where Fl=flow rate through the tube collecting exhalant water. Only above critical levels of water flow (Fl) were clearances representative of filtration rates. At 10° to 13°C, the filtration rates (F, 1 h^-1) within one or two orders of magnitude of dry weight (w, g), in Cardium echinatum L., C. edule L., Mytilus edulis L., Modiolus modiolus (L.) and Arctica islandica (L.) followed the allometric equations: 4.22w ^0.62, 11.60w ^0.70, 7.45w ^0.66, 6.00w ^0.75 and 5.55w ^0.62, respectively. Five species of bivalves [Spisula subtruncata (da Costa), Hiatella striata (Fleuriau), Cultellus pellucidus (Pennant), Mya arenaria L. and Venerupis pullastra (Montagu)] filtered with the same rates as individuals of Cardium echinatum and A. islandica of equivalent soft weight. In Pecten furtivus and P. opercularis filtration rates were about twice the rates measured in individuals of Mytilus edulis of comparable body weight. The gill area in M. edulis increases with size at the same rate as the filtration rate.     

Utilization of a fumigated sediment by two benthic deposit-feeders:Abra alba (Mollusca: Bivalvia) andEupolymnia nebulosa (Annelida: Polychaeta)

The present study tested the utilization of dead microbial biomass by two benthic deposit-feeders:Abra alba (Wood) (Mollusca: Bivalvia) andEupolymnia nebulosa (Montagu) (Annelida: Polychaeta). Clams were collected in the Canet lagoon during spring 1989. Worms were collected in the Port-Vendres harbour during spring 1989. The¹⁴C-labelled (glutamic acid, 24 h) sediment used during the study was sterilized with 1% chloroform, washed with sterile seawater, and dried (60°C; 48 h). This sterilisation procedure, called fumigation is the least harmful to the sediment (Novitsky 1986). Both clams and worms were incubated in the presence of the fumigated sediment for 5, 10, 20, and 50 h. At the end of each experiment we recorded the radioactivity in four compartments: (1) sediment, (2) dissolved organic matter (DOM), (3) CO₂, and (4) animals. The radioactivity of the sediment was subdivided into five fractions: (i) soluble in 2N HCl, (ii) soluble in hot 5% trichloroacetic acid (TCA), (iii) soluble in 1N NaOH, (iv) soluble in hot 6N HCl, (v) residual (after combustion in a Leco carbon analyser). In the first set of experiments, after 20 h of incubation, 5.4 and 4.7% of the total radioactivity was taken up by clams and worms, respectively. However, a model revealed that this uptake could have been correlated with the release of radiolabelled DOM (33% of total radioactivity during the first 5 h). In order to test this assumption, we used the same protocol with three additional washes of the fumigated sediment. This resulted in a significantly lower uptake by the clams (1.9% of the total radioactivity byt = 50 h), whereas the worms exhibited an uptake similar to that in the initial experiment (5.1% of total radioactivity byt = 50 h). These results underline the importance of considering interactions with DOM when applying radiotracer techniques to the study of benthic food chains. The average ingestion rates of fumigated sediment byA. alba andE. nebulosa were 5.2 10^–2 mg sediment dry wt mg^–1 clam h^–1 and 3.5 10^–2 mg sediment dry wt mg^–1 worm h^–1, respectively, which is comparable to previous data reported for other deposit-feeding bivalves and polychaetes feeding on natural sediment or detritus. The low radioactivity recorded for CO₂ together with the similarity of the changes in the partitioning of the radioactivity within the sediment between control experiments and experiments carried out in the presence of clams or worms suggest low assimilation efficiencies. Therefore, the present study supports the fact that dead microbial biomass does not constitute an important food source for benthic deposit-feeders.     

Accumulation of metals by marine picoplankton

Pumping rates in undisturbed suspension-feeding bivalves were measured with a thermistor microflowmeter. Pumping rates are related to the dry weight of Clinocardium nuttallii, Macoma nasuta, Mytilus californianus and Chlamys hastata as the following equations: P=0.85 W^0.84, P=0.23 W^0.855, P=2.216 W^0.715, and P=2.419 W^0.943 respectively. Pumping rate and gill area increase at the same rate in C. nuttallii, M. californianus and C. hastata. The pumping rate per unit gill area is highest for C. nuttallii (approximately 0.14 cm³/s/cm²), with intermediate values for M. californianus (0.08 cm³/s/cm²) and C. hastata (0.11 cm³/s/cm²); the lowest rate was found in the deposit-feeder M. nasuta (0.0057 to 0.0089 cm³/s/cm²), which also has the smallest gill to body weight. C. nuttallii has the relatively smallest gill of the suspension feeders and the highest pumping rate per unit gill area. The absolute pumping rates, however, were highest for the filibranchs M. californianus and C. hastata. Indications are given that these differences are the results of different fluid-mechanical/ecological strategies to minimize the cost-benefit ratio.Contribution No. 132 of the Marine Science and Maritime Studies Center, Northeastern University     

Influence of algal and suspended sediment concentrations on the feeding physiology of the hard clam Mercenaria mercenaria

Short-term laboratory feeding experiments were conducted to determine the response of the hard clam Mercenaria mercenaria (L.) (32 mm in mean shell length) to increasing sediment concentrations. Clams were fed mixed suspensions of Pseudoisochrysis paradoxa (50 and 150 cells l^-1) and bottom sediments (0 to 44 mg l^-1). Algal ingestion rate deelined with increasing sediment loads. This resulted primarily from a reduction in clearance rate, which declined by 0.08 l h^-1 g^-1 (1.3%) for every 1 mg l^-1 increase in sediment load. This reduction was of similar magnitude for juvenile (13 mm) clams. At the algal concentrations tested, pseudofaeces production was intermittent and inconspicuous below about 10 mg silt l^-1. Loss of algae in pseudofaeces increased with increasing sediment loads; however, even at the highest silt and algal concentrations, clams lost a maximum of only 18% of the algae cleared from suspension. Thus, pseudofaeces production is not expected to cause significant loss of algal food at the sediment concentrations normally encountered in the natural environment ( ca 40 mg silt l^-1). Absorption rate of total organic matter remained constant, at least up to silt concentrations of 20 mg l^-1. Experiments using dual ⁵¹Cr:¹⁴C-formaldehyde-labelled sediment indicated that clams were able to counteract the dilution of algae by absorbing a considerable fraction (21 to 22%) of detrital sedimentary organics. Absorption efficiency of pure P. paradoxa ranged from 82% at 50 cells l^-1 to 58% at 150 cells l^-1. Integration of physiological rate measurements suggests that at moderate to high algal concentrations (300 g Cl^-1), growth improvement by the addition of silt, documented in mussels, surf clams and oysters, is unlikely to occur in M. mercenaria. It is suggested that a suspension-feeding bivalve's success in maximizing its energy gain in a turbid environment depends on the combination of two features: a high selection efficiency and a high rate of pseudofaeces production. It is proposed that species which regulate ingestion primarily by producing pseudofaeces are better adapted to cope with high suspended sediment loads than species such as M. mercenaria, which control ingestion mainly by reducing clearance rate.Contribution No. 451 from the Marine Sciences Research Center, State University of New York at Stony Brook, USA     

Application of Laser Doppler Spectroscopy (LDS) in determining swimming velocities of motile phytoplankton

Pumping rates in Mytilus edulis L. were measured by means of a constant-level-tank method, in which hydrostatic pressure differences between inhalant and exhalant water levels were recorded by means of a laser beam reflected from a tethered mirror floating on the water surface. Hydrostatic pressure gradients were determined to ±0.05 mm H₂O or better. The developed technique of directly measuring pumping rates in mussels is not subject to the artefacts of other methods. The pumping rates measured in M. edulis were substantially higher than those previously determined by means of direct techniques, but similar to the maximum filtration rates, as obtained by means of two indirect techniques, i.e. about 50 ml min^-1 for a 0.15 g dry weight mussel. Positive hydrostatic pressures drastically affected water pumping. The pumping rate decreased linearly with increasing hydrostatic pressures towards a maximum pump pressure between 3 and 5 mm H₂O. Negative pressures only affected the pumping rate slightly or insignificantly, except when the mussels were exposed to rapidly increasing negative pressures. Under this condition a shunt was presumably established between the inner demibranchs, allowing water to bypass the gills.     

Gill pump characteristics of the soft clamMya arenaria

Pumping rates in the soft clamMya arenaria, collected in June 1987 in the Great Belt, Denmark, were determined as rates at which clams cleared suspensions of algae,Dunaliella marina. The frictional resistance of the siphons to water flow was estimated by studying the effect of excision of the siphons at the base on the rate of water pumping. The frictional resistance was also calculated from the Poiseuille equation. Excision of the siphons had no measurable effect on the pumping rate, and calculations indicated pressure losses ranging between 0.3 to 1.2 mm H₂O. This is consistent with the conception that the capacity of filter feeding bivalves to process large amounts of water depends upon a low resistance to water flow in the siphons.     

Particle size-selection of two deposit feeders: the amphipod Corophium volutator and the prosobranch Hydrobia ulvae

The feeding biology of the deposit-feeding amphipod Corophium volutator is compared to that of the coexisting, deposit-feeding prosobranch Hydrobia ulvae. Regarding ingestion of particles, both forms show size selection which alone can explain their coexistence. Particle size-selection also explains some qualitative differences in the composition of the food of the two forms; thus, diatoms play a relatively larger role in the diet of H. ulvae than in the diet of c. volutator, whereas bacteria are probably relatively more important for the latter. Results of experiments with feeding of C-14 labelled microorganisms are in accordance with the findings on particle size-distribution of the gut contents, and show that (1) C. volutator can only utilize bacteria adsorbed to particles within the size range 4 to 63 (this is why the presence of clay and silt particles in the sediment are necessary for efficient feeding of this amphipod); (2) C. volutator can utilize bacteria suspended in the water pumped through its burrow for respiration if silt and clay particles are present in the sediment. (3) H. ulvae can utilize large particles, and also browses on surfaces, and some evidence is brought forward that it also utilizes mucus for trapping microorganisms. The coexistence of deposit-feeding animals is discussed. It is concluded that the number of coexisting, closely related species is usually small, and that their resource partitioning is probably mainly based on particle-size selectivity. In the case of unrelated forms (e.g. H. ulvae and C. volutator, a number of behavioural, physiological and morphological differences, and also the widespread ability of deposit feeders to utilize alternative feeding mechanisms may also lead to resource partitioning. Thus, there are often several niche dimensions related to feeding allowing a certain diversity of coexisting deposit feeders.     

Stable isotope study of sedimentary carbon utilization by Capitella spp.: effects of two carbon sources and geochemical conditions during their diagenesis

Two experiments were conducted to determine the effect of the type of sediment organic matter and geochemical conditions during diagenesis on the stable carbon isotope ratio ¹³C of the deposit-feeding polychaete worms Capitella spp. Laboratory experiments showed ¹³C values of-13.5 to-13.9%. for worms grown on fresh and aged kelp, Macrocystis pyrifera. Field experiments on worms in the Santa Barbara Channel, California (USA), in 1986 revealed more negative ¹³C values (to-19%.) when oil and kelp were added to sediments. The more negative values suggest the incorporation of oil carbon into worm tissues, supporting previous indications of the importance of hydrocarbons in food webs near oil seeps.     

Comparative morphology and cytology of siphons and siphonal sensory organs in selected bivalve molluscs

 The form, anatomy and cytology of the water-passages in siphon-possessing and siphon-lacking species of a selected group of bivalve molluscs from the Red Sea (Callista florida, Circe currogata, C. crocea, Lucinia dentifera, Dosinia histrio, Pitar hebraea, Tridacna maxima, Pteria aegyptica), and from the Mediterranean Sea (Spondylus spinosus, Pinctada radiata, Pinna nobilis, Donax trunculus, D. semistriatus, Mactra stultorum, Tapes decussatus, Petricola lithophaga, Brachidontes pharaonis) were studied by means of light and electron microscopy. In the mytiloids, ostreoids and ptrioids studied, the water-passages are covered by a ciliated epithelium and the few tentacles around their openings are identical to those found on the mantle edges. Contrary to this, in the veneroids studied, the siphons are covered by a microvillar epithelium and their tentacles, especially of the inhalant siphon, reveal characteristic branching. The so-called Type I, Type II and Type III ciliated sensory organs on and within the siphons are similar to those observed in other bivalves, differing from each other in the dimension, number and form of cilia. In the studied mytiloid species only Type I and Type II organs were found. The form and structure of the siphons and the distribution of sensors on them can be used as markers for ecological as well as phylogenetic considerations. Received: 28 January 2000 / Accepted: 28 May 2000     

Patterns of micro- and meiofaunal abundance in marine sediments,measured with the adenosine triphosphate assay

Adenosine triphosphate (ATP) measurements are used to determine vertical and seasonal distributions of microorganisms and meiofauna in sediments from a 14 m-deep mud bottom in central Long Island Sound on 12 sampling dates from April 1975 to October 1976. Below the topmost 1 cm of sediment, ATP measurements can be useful in estimating and comparing standing stocks of microorganisms and meiofauna. In the top 1 cm, however, large quantities of newly settled bivalves (Yoldia limatula, Nucula annulata, and Mulinia lateralis) and juvenile polychaetes (Owenia fusiformis) in summer and fall months account for total ATP concentrations. The ATP content of individual meiofauna ranges from 1.97 ng individual copepod nauplius^-1 to 190.7±60 ng individual M. lateralis ^-1. In general, the total ATP content of individual polychaetes and bivalves is much higher than that of individuals of other groups. However, on a mg ATP per g wet or dry tissue basis, the ATP content of micro- and meiofaunal taxa are not significantly different. In addition to providing a means for comparing micro- and meiofaunal standing stocks, ATP measurements permit examination of the relative contribution of different meiofauna to the total living biomass of meiofauna in sediments. Total sediment ATP concentrations are greatest in the top 1 cm at all seasons, and decrease with increasing depth in the sediment. Annual concentrations in the topmost centimeter average 4.22 g g dry sediment^-1 and range seasonally from 1.09 to 7.64 g g dry sediment^-1. At a depth of 10 cm, values average 0.16 and range from 0.019 to 0.35 g g dry sediment^-1. High ATP concentrations in surface sediment reflect high concentrations of microorganisms and meiofauna at the sediment-water interface. The top 2 cm of sediment contain 71% of all meiofauna, with 41% occurring in the topmost cm. In general, densities are lowest in the winter and highest during the spring and summer, averaging 490 individuals 10 cm^-2, and varying from 87 to 1366 individuals 10 cm^-2. Because of wide variation in recruitment patterns of the benthos in Long Island Sound, the extremes of the range in meiofaunal densities can be observed in the same month in two different years. In order to monitor and compare standing stocks of organisms less than 1 mm in size in sediments, the ATP assay can save hours of processing time compared with alternate methods such as direct counts.     

Relationship between filtration activity and food availability in the Mediterranean mussel <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis>

The filtration activity of the Mediterranean mussel, Mytilus galloprovincialis, was assessed under different concentrations and compositions of seston by using a new automated image acquisition and analysis system. This approach allowed for frequent and simultaneous measurements of valve gape and exhalant siphon area. Filtration rates were measured through clearance measurements whereas pumping rates were measured using hot-film probes. The average filtration rate (17.5 l g h⁻¹ DW⁻¹ for a 0.36 g DW mussel) recorded during the present study was higher than those available for Mytilus edulis when standardized to flesh dry weight but almost equivalent (17.5 l h⁻¹ g DW⁻¹ for a 53 mm shell length mussel) to those rates when standardized to shell length. Immediately after the addition of algal cells (Isochrysis galbana; 4.5 μm in size), valve gape, exhalant siphon area and filtration rate increased quickly as mussels reached their maximum filtration activity. These three parameters then gradually decreased until complete closure of the shell. The algal cell concentration inducing this transition was close to 800 cells ml⁻¹ and 0.5 μg Chl a l⁻¹. When algal concentration was maintained above this threshold by successive algal additions, both valve gape and exhalant siphon area remained maximal. Temporal changes in the exhalant siphon area were continuous as opposed to those of valve gape. Therefore, despite the significant correlation between these two parameters, valves and siphon were sometimes dissociated due to a reduction of the area or even a closure of the exhalant siphon while the valves remained open. The velocity of exhaled water tended to be constant irrespective of exhalant siphon area and thus pumping rates were a linear function of exhalant siphon area. Consequently, reductions in exhalant siphon area and pumping rate were almost similar in M. galloprovincialis. Our results thus clearly support the hypothesis that exhalant siphon area constitutes a better proxy of pumping rate than valve gape as already suggested for Mytilus edulis. Finally, the high filtration rates measured during the present study together with the high concentrations of inorganic matter (> 40 mg DW l⁻¹) requested to alter those rates suggest that the studied mussels were well adapted to oligotrophic waters featuring strong hydrodynamism and frequent sediment resuspension events.     

Behavioural responses of the marine bivalve Macomona liliana exposed to copper- and chlordane-dosed sediments

Sublethal behavioural responses including avoidance and burial rate were compared with 10 d acute morbidity and mortality in bioassays using juvenile Macomona liliana 1 to 3 mm long. The bivalves showed significant movement away from all copper-dosed sediments, with maximal movement after 96 h at 25 mg Cu kg (dry wt)^-1. Increasing copper concentrations slowed the rate of burial, and above 15 mg Cu kg (dry wt)^-1, most shellfish failed to bury after 90 min. After 10 d exposure, morbidity (defined as inability to rebury) occurred at 15 mg Cu kg (dry wt)^-1 and mortality at 30 mg Cu kg (dry wt)^-1. In avoidance trials, chlordane had a significant effect on overall movement from 20 g kg (dry wt)^-1, but increasing concentrations reduced movement away from dosed sediment. Chlordance appeared to have no effect on burial rate. Survival was affected by 10 d exposure to 400 g chlordane kg (dry wt)^-1, with increased morbidity and death. The relative sensitivities of the responses shown by M. liliana to copper were avoidance > burial/morbidity > mortality, with a 6-fold difference in the response threshold between avoidance and mortality. With chlordance, an avoidance response was detected at a concentration 20-fold lower than that causing morbidity. Behavioural responses of M. liliana have the potential to provide a sensitive bioassay.     

Transfer of brevetoxins to a tellinid bivalve by suspension- and deposit-feeding and its implications for clay mitigation of <Emphasis Type="Italic">Karenia brevis</Emphasis> blooms

Blooms of the brevetoxin-producing Karenia brevis in the Gulf of Mexico cause massive fish kills, food poisoning and adverse respiratory effects in humans. Sedimentation of toxic cells following inert clay application could reduce toxin incorporation by commercially important suspension-feeding bivalves and thus prevent direct public health impacts, but could potentially lead to brevetoxin (PbTx) accumulation by benthic deposit-feeders. The goal of this study was therefore to compare suspension- and deposit-feeding as pathways for brevetoxins. We investigated: (1) the effect of toxic K. brevis on both feeding modes using a facultative deposit-suspension feeding tellinid bivalve, the clam Macoma balthica, as a model species and (2) the relative effectiveness of brevetoxin transfer via suspension- and deposit-feeding over 24-h exposure. Sedimentation of K. brevis was achieved by treatment with 0.25 g phosphatic clay l⁻¹ and brevetoxin concentrations were measured by ELISA. Karenia brevis reduced both suspension- and deposit-feeding activity. This study demonstrates that brevetoxins can be rapidly accumulated by a surface deposit-feeding bivalve from sedimented K. brevis cells and that comparable toxin levels can be attained by both suspension- and deposit-feeding modes [1.2–1.6 μg PbTx (g tissue wet weight)⁻¹]. Deposit-feeding clams generally do not pose a direct threat to humans but may provide a pathway for brevetoxin food web transfer.     

Uptake of bacterial extracellular polymer by a deposit-feeding holothurian (Isostichopus badionotus)

A laboratory experiment shows that exopolymer (=mucopolysaccharide) produced by the estuarine marine bacterium Pseudomonas atlantica can be a source of nutrition for the deposit-feeding holothurian Isostichopus badionotus (Selenka). Holothurians fed sediment prepared with ¹⁴C-labeled exopolymer incorporated radioactive carbon into their rete mirabile.     

Adaptations of subtropical Venus clams to predation and desiccation: endurance of<Emphasis Type="Italic"> Gafrarium tumidum</Emphasis> and avoidance of<Emphasis Type="Italic"> Ruditapes variegatus</Emphasis>

Intertidal endobenthic bivalves are often dislodged from sediments by hydrodynamic forces. As a result, they encounter the dangers of predation and desiccation, which are generally harsh near the sediment surface. To cope with such dangers, the bivalves possibly possess: (1) a strong body to endure predation and desiccation stress, (2) quick mobility to avoid the stresses, or (3) a high growth rate for attaining a size refuge. The present study examined which of these modes are adopted by the subtropical cobbled-shore Venus clams Gafrarium tumidum (Röding, 1798) and Ruditapes variegatus (Sowerby, 1852), revealing the following interspecific differences. (1) G. tumidum survived better than R. variegatus did in harsh experimental conditions, namely: the experimental cages exposed to predation and desiccation on a cobbled shore; a laboratory aquarium with a predatory crab Scylla serrata; and ovens with high temperatures (27°C and 34°C). (2) R. variegatus was more mobile than G. tumidum was, digging into the sediment on a cobbled shore more rapidly at both high and low tides. (3) The two species with shell lengths 20–30 mm showed similar growth rates (median: –0.2 to 44.5 m day^–1) in seasonal mark–recapture surveys over 2 years. Overall, to cope with the dangers of predation and desiccation G. tumidum appears to have a strong body, while R. variegatus displays rapid mobility, and neither species seems to attain a size refuge through rapid growth. Such species-specific modes are discussed in relation to the interspecific differences found in shell morphology.Communicated by T. Ikeda, Hakodate     

Effects of environmental factors on radiocadmium uptake by four species of marine bivalves

Temperature, salinity, bottom-sediment type, and zinc concentration all influenced Cd uptake by 4 marine bivalves (Mya arenaria, Mytilus edulis, Mulinia lateralis and Nucula proxima) in short-term static assay systems using ¹⁰⁹Cd as a tracer. The experimental system consisted of aquaria containing 20 l of seawater maintained under controlled light and temperature conditions. The water contained either 5 or 20 g/l Cd and tracer. Distribution and kinetics of the metal were monitored in the water column and organisms. The results demonstrate that Cd uptake rates differed widely among the organisms tested. An increase in temperature increased Cd uptake rate by all test organisms. A decrease in salinity increased Cd uptake by all organisms tested. The presence of bottom sediment depresses Cd accumulation in some benthic animals. Zinc in concentrations of 0.5 mg/l substantially decreased Cd uptake by Mytilus edulis and Mulinia lateralis. It is suggested that all important species and environmental variables be considered when studying heavy-metal uptake by marine organisms or when establishing water-quality criteria.     

Selectivity in feeding by the deposit-feeding bivalve Macoma nasuta

Macoma nasuta Conrad is primarily a deposit-feeding bivalve sucking the top millimeter of the sediment surface. Growth experiments show that surface sediment supports growth better than detritus falling from the water column. Gut clearance time is between 1 and 9 h (12°C). Fecal pellets are ejected in a regular rhythm. However, the total amount of feces per unit time shows considerable individual variation. Due to sorting in the mantle cavity, about 97% (dry weight) of the surface material is ejected again as pseudofeces. Selectivity by the bivalve is estimated by comparison of particle size and organic composition of sandy and muddy sediments and compared with feces produced by clams fed these sediments. Fecal pellets are in all cases richer in organic components than the sediment, indicating a high degree of selectivity. Ingestion and digestion of small animals (meiofauna) occur, but many of the ingested specimens survive. It is not possible to estimate the assimilation of organic matter by simple difference between the ingested sediment and the ejected feces. The difficulties in calculating energy budgets which arise from selective feeding and associated bacteria are discussed.     

Calorimetric studies on the energy metabolism of an infaunal bivalve,Abra tenuis,under normoxia,hypoxia and anoxia

Direct calorimetry was employed to measure the energy metabolism of infaunal bivalves, Abra tenuis, collected from a tidal lagoon in the Fleet, southern England, in June 1989, at various oxygen partial pressures. A significant anaerobic component (i.e., 20% of total metabolic rate) was detected under normoxia, presumably brought about by the intermittent ventilatory activity of this bivalve under these conditions. Under hypoxia (2.3 to 10 kPa, or 11 to 48% of full air saturation), however, the energy metabolism was maintained fully aerobic; the measured heat equivalent of oxygen uptake was not significantly different from the theoretical ranges for fully aerobic catabolism. Under anoxia, the rate of heat dissipation was reduced to 5–6% of the normoxic rate of heat dissipation. This conserves energy expenditure and would thus increase resistance of A. tenuis to anoxia or emersion. Physiological compensation by A. tenuis under conditions of declining oxygen tension involved a marked increase in ventilation rate. Comparison between fed and starved individuals indicated that costly physiological processes, such as digestion, absorption and growth declined at 10 and 5 kPa and were arrested at P_{O 2} (oxygen partial pressure) levels below 2.3 kPa. The present study provides evidence that there are no major differences between the metabolic responses of epifaunal suspension-feeding (eg. Mytilus edulis) and infaunal deposit-feeding (eg. A. tenuis) bivalves when exposed to environmental hypoxic stress.