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Carbon use efficiency (CUE), the proportion of carbon (C) consumed by microbes that is converted into biomass, is an important parameter for soil C models with explicit microbial controls. While often considered as a single parameter, CUE is an emergent property of multiple microbial processes, including assimilation efficiency, biomass-specific respiration, enzyme production, and respiratory costs of enzyme production. These processes occur over variable time scales and imply different fates for C, and the same emergent CUE value can result when C is allocated in fundamentally different ways (e.g. a high investment in enzyme production vs. a high assimilation cost). We developed a model that represents the individual processes underlying emergent CUE to test how shifts in microbial allocation alter equilibrium soil C pool sizes. We found that an increase in emergent CUE that results from a change in assimilation efficiency, biomass specific respiration, or respiration costs from enzyme production causes soil organic C (SOC) to decline, while the same change in emergent CUE resulting from a change in enzyme production causes SOC to increase. We also used the model to test the sensitivity of CUE from isotopic C tracer estimates to changes in microbial allocation processes. We found that these estimates do not account for the same microbial processes represented by emergent CUE in models. We propose that considering microbial processes explicitly rather than representing CUE as a single parameter can improve data-model integration. In addition, modeling microbial processes explicitly will account for a wider range of possible outcomes from shifts in microbial C allocation, such as when increased SOC results from increasing CUE.

Abstract Many mangrove crab species drag freshly fallen leaf litter into their burrows and store it there for some time prior to consumption. Potential explanations for this behavior include (a) avoidance of competition for a scarce resource, (b) prevention of removal of a scarce resource upon tidal outwelling, or (c) processing of an initially unpalatable food source, be it physically through leaching, through the activity of the sediment microbiota, or driven by the activity of the crab itself. To test the latter, we study the effect of burying mangrove leaf litter on two relevant physicochemical litter characteristics and on its digestibility to crabs in a laboratory experiment, using artificial substratum with low microbial activity. Freshly fallen leaves of two common mangrove species, Bruguiera gymnorhiza and Ceriops tagal, were left for two weeks either inside burrows used by Neosamartium asiaticum (Sesarmidae), or on top of the substratum, before they were offered to the crabs as food. Leaf toughness and total phenolic content differed significantly between mangrove species. Upon two weeks of decay, phenolic content of both leaf litter species and toughness of B. gymnorhiza​ changed significantly. However, neither litter characteristcs nor assimilation efficiency differed between treatments. We conclude that storing mangrove leaf litter in crab burrows in a microbe‐poor environment does not affect litter digestibility. Hence, crabs themselves do not contribute to litter processing during storage inside the burrow. If it is litter processing, rather than the avoidance of competition or litter removal by tides, that renders the storage of litter inside the burrow advantageous, it will be microbial activity that drives this process. Further studies should explicitly focus on processing through the activity of the sediment microbiota.

In this study, we investigate the broader impacts of microplastics on ecosystem health and ecological risks, specifically through altered assimilation efficiency of aquatic organisms, an individual-level effect that has garnered significant attention recently. The Ecopath with Ecosim model is employed to assess these impacts, focusing on Baiyangdian Lake, China. We examine three scenarios: no microplastics effects (EPScenario 1) and current and assumed high microplastics concentrations effects (EPScenario 2 and EPScenario 3). Results indicate that considering the adverse effects of microplastics on aquatic organisms' assimilation efficiency worsens Baiyangdian Lake's ecosystem health. "Bad" memberships are 0.459, 0.506 and 0.555 for EPScenarios 1–3, respectively. Ecosystem maturity is severely impacted, reducing it to less than half its original level due to decreased total respiration of consumers and increased total system throughput in the entire ecosystem. Furthermore, there are over 3% reductions in ecosystem function due to the decrease in the relative ascendency and over 18% reductions in food web structure caused by the decrease in Finn's cycling index and Finn's mean path length. A critical aspect of this research is the relationship between assimilation efficiency and microplastics dynamics in the food web. Diminished assimilation efficiency results in lower microplastics concentrations in detritus but increased levels in water. This shift significantly affects trophic transfer, reducing microplastics concentrations in organisms that primarily feed on detritus. Conversely, the accumulation of microplastics on plant surfaces in water elevates the concentrations in organisms that feed predominantly on plants. These dynamics are crucial for understanding the ecological risks associated with microplastics, which vary significantly based on the primary food sources of aquatic organisms. The risk escalates in food chains dependent on detritus and diminishes in those reliant on plant-based food chains. This insight is also vital in assessing human health risks related to dietary intake of microplastics from aquatic food sources, highlighting increased risks for consumers of herbivorous and filter-feeding fish. Overall, this research offers critical insights into the complex effects of microplastics on freshwater ecosystems, emphasizing assimilation efficiency's role in managing ecological and health implications. These findings have significant practical implications for ecosystem management, providing a basis for strategies to mitigate microplastic impacts on aquatic organisms and ecosystem health. Additionally, understanding these effects can inform public health initiatives by highlighting risks from consuming contaminated aquatic organisms. Effective management and policy measures can be also developed to reduce microplastic pollution, protecting both ecosystems and public health. • Evaluating microplastics (MPs) effect on ecosystem health and risk through decreasing assimilation efficiency. • MPs degrade ecosystem maturity significantly, harming function and food web structure. • MPs alter food chain trophic transfer patterns and vary ecological risks based on prey primary food sources. • Human dietary habits influence exposure risk to MPs. [ABSTRACT FROM AUTHOR]

Arsenic exists universally in freshwater and marine environments, threatening the survival of aquatic organisms and human health. To elucidate arsenic bioaccumulation and biotransformation processes in aquatic organisms, this review evaluates the dissolved uptake, dietary assimilation, biotransformation, and elimination of arsenic in aquatic organisms and discusses the major factors influencing these processes. Environmental factors such as phosphorus concentration, pH, salinity, and dissolved organic matter influence arsenic absorption from aquatic systems, whereas ingestion rate, gut passage time, and gut environment affect the assimilation of arsenic from foodstuffs. Arsenic bioaccumulation and biotransformation mechanisms differ depending on specific arsenic species and the involved aquatic organism. Although some enzymes engaged in arsenic biotransformation are known, deciphering the complicated synthesis and degradation pathway of arsenobetaine remains a challenge. The elimination of arsenic involves many processes, such as fecal excretion, renal elimination, molting, and reproductive processes. This review facilitates our understanding of the environmental behavior and biological fate of arsenic and contributes to regulation of the environmental risk posed by arsenic pollution.

The Sábalo, Prochilodus lineatus (Valenciennes, 1837), is one of 270 fish species in the Rio Paraná system, and it comprises >50% of the fish biomass. Its diet is flocculant benthic biofilm comprised of algae, bacteria and non‐living organic matter: a food resource apparently of little value to other fishes. Digestion and assimilation of key nutrients from its biofilm diet by P. lineatus were described and quantified in an attempt to discover how this species is so successful. The fish begin a feeding period with empty digestive tracts, accumulate food during the feeding period and then void the gut content at the end of the feeding period. Early in a feeding period, sand accumulates in the gizzard‐like pyloric stomach where it serves as both a grinding medium and a sieve. After sufficient sand has been acquired, food particles passed from the pyloric stomach to the intestine are dramatically reduced in size to <20 μ maximum dimension, whereas larger particles including mineral matter and plant fibres are retained. Total ash, hydrolysis‐resistant‐ash and hydrolysis‐resistant‐organic‐matter were tested as reference materials against which to measure assimilation and hydrolysis‐resistant‐organic matter best met the assumptions of the technique. Comparison of the first food ingested to food ingested later in the feeding period shows that grinding of food and selective retention of larger particles results in a three‐fold increase in the assimilation of ash‐free‐dry‐mass (to 56%) and hydrolysis‐labile‐organic‐matter (to 67%), and a six‐fold increase in the assimilation of amino acids (AAs; to 74%). When food quality is assessed in terms of g AA assimilated · kJ−1 energy assimilated, the quality of food ingested by P. lineatus ranges from a maintenance level of 5 to 12 mg AA · kJ−1, a level expected to produce near maximum growth. Thus, the processing of food in the pyloric stomach is integral to the success of P. lineatus in establishing large populations on a diet of flocculent benthic biofilm. [ABSTRACT FROM AUTHOR]

Many mangrove crab species drag freshly fallen leaf litter into their burrows and store it there for some time prior to consumption. Potential explanations for this behavior include (a) avoidance of competition for a scarce resource, (b) prevention of removal of a scarce resource upon tidal outwelling, or (c) processing of an initially unpalatable food source, be it physically through leaching, through the activity of the sediment microbiota, or driven by the activity of the crab itself. To test the latter, we study the effect of burying mangrove leaf litter on two relevant physicochemical litter characteristics and on its digestibility to crabs in a laboratory experiment, using artificial substratum with low microbial activity. Freshly fallen leaves of two common mangrove species, Bruguiera gymnorhiza and Ceriops tagal, were left for two weeks either inside burrows used by Neosamartium asiaticum (Sesarmidae), or on top of the substratum, before they were offered to the crabs as food. Leaf toughness and total phenolic content differed significantly between mangrove species. Upon two weeks of decay, phenolic content of both leaf litter species and toughness of B. gymnorhiza​ changed significantly. However, neither litter characteristcs nor assimilation efficiency differed between treatments. We conclude that storing mangrove leaf litter in crab burrows in a microbe‐poor environment does not affect litter digestibility. Hence, crabs themselves do not contribute to litter processing during storage inside the burrow. If it is litter processing, rather than the avoidance of competition or litter removal by tides, that renders the storage of litter inside the burrow advantageous, it will be microbial activity that drives this process. Further studies should explicitly focus on processing through the activity of the sediment microbiota. [ABSTRACT FROM AUTHOR]

As solitary filter feeder, Styela clava shows superior water purification capacity in the integrated multi-trophic aquaculture (IMTA) model consisting of S. clava, microalgae, and sea cucumber, which should be an ideal system to reduce organic and inorganic pollution in the water. It is of great significance to study the physiological energetics of S. clava for the application of this IMTA system. The physiological energetics of S. clava was determined in a simulated natural culture system to provide information for comparing to the energetic data derived from feeding ecological studies. The ascidians were grouped into five cohorts based on body size in terms of their wet weight of 1.5-2.0 g, 2.5-3.5 g, 5.0-6.5 g, 8.0-9.5 g, and 11.0-14.0 g, respectively. Clearance rate (CR), ingestion rate (IR), and assimilation efficiency (AE) of ascidian were estimated using flow-through feeding chambers. The results showed that the CR and IR increased exponentially with the ascidians' body size at different temperatures. Body size showed little effect on AE at the same water temperature. However, significant differences were observed between different temperature regions in the range of 12-28°C regardless of body size. S. clava performed far better in feeding behavior at 20°C than that at higher or lower temperatures. Within the range of diet concentration (as particulate organic matter, POM) from 2.11 to 6.11 mg POM/L, the CR and IR increased exponentially with POM. This study provides basic data for the application of ascidian in the IMTA system. [ABSTRACT FROM AUTHOR]

Suspended particulate matter (SPM) carries a major fraction of metals in turbid coastal waters, markedly influencing metal bioaccumulation and posing risks to marine life. However, its effects are often overlooked in current water quality criteria for metals, primarily due to challenges in quantifying SPM's contribution. This contribution depends on the SPM concentration, metal distribution coefficients ( K ), and the bioavailability of SPM-bound metals (assimilation efficiency, AE), which can collectively be integrated as a modifying factor (MF). Accordingly, we developed a new stable isotope method to measure metal AE by individual organisms from SPM, employing the widely distributed filter-feeding clam d ), and the bioavailability of SPM-bound metals (assimilation efficiency, AE), which can collectively be integrated as a modifying factor (MF). Accordingly, we developed a new stable isotope method to measure metal AE by individual organisms from SPM, employing the widely distributed filter-feeding clam Ruditapes philippinarum as a representative species. Assessing SPM from 23 coastal sites in China, we found average AEs of 42% for Zn, 26% for Cd, 20% for Cu, 8% for Ni, and 6% for Pb. Moreover, using stable isotope methods, we determined metal K d of SPM from these sites, which can be well predicted by the total organic carbon and iron content ( R 2 = 0.977). We calculated MFs using a Monte Carlo method. The calculated MFs are in the range 9.9-43 for Pb, 8.5-37 for Zn, 2.9-9.7 for Cu, 1.4-2.7 for Ni, and 1.1-1.6 for Cd, suggesting that dissolved-metal-based criteria values should be divided by MFs to provide adequate protection to aquatic life. This study provides foundational guidelines to refine water quality criteria in turbid waters and protect coastal ecosystems.

The purpose of this research is to determined the effect of insecticide profenofos to the rate of consumption and assimilation efficiency of cow- dung by earthworms Pontoscolex corethrurus Fr. Mull. This research used Completely Randomized Design with 6 treatments and 5 repetition. Treatments that was given to sample are the insecticide profenofos with concentration of 0 % (control); 0.1%; 0.2 %; 0.3 %; 0.4 %; 0.5 %. Data analysis was using Varians Analysis. If there was real difference then data analysis continued with The Duncan Test on level of confidence of 95%. The results of this research show us that several concentration of insecticide carbaryl have the real effect to the average of consumption rate and the efficiency of absorption. The lowest average of consumption rate on earthworm Pontoscolex corethrurus Fr.Mull is on concentration of 0.5 % (0.23 ± 0.02 mg/g day) and the highest average of consumption 26 rate on earthworm Pontoscolex corethrurus Fr. Mull is on concentration of 0% (control) (2.53 ± 0.05 mg/g day). The lowest average of approach digestyon earthworm Pontoscolex corethrurus is on concentration of 0 % (control) (40.78 28 ± 2.56 %) and the highest average of digestion earth worm Pontoscolex corethrurus Fr.Mull is on concentration of 0.5 % 29 (70.76 ± 3.67 %).

Juvenile growth rate is a key life‐history trait with a major effect on various fitness components. Duration of the growing season changes with latitude, often resulting in latitudinal variations in growth rate. While many studies have shown a latitudinal cline of growth rate, most have used linear measurements of growth rate without considering the nonlinear nature of growth trajectories. Furthermore, elucidation of the proximate mechanisms causing increased growth rates would provide a fundamental understanding of the evolutionary process of the latitudinal cline of growth rates, which are largely unknown.We used common garden experiments to examine the latitudinal variation of growth rate. We analysed the larval growth curve of 14 populations of the univoltine Japanese rhinoceros beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae) found along a 2,000‐km latitudinal gradient. To clarify the mechanisms responsible for the latitudinal cline of growth rate, we compared food consumption and growth efficiency (i.e. the capacity to convert food into biomass) using three populations at high, middle and low latitudes.There was a strong positive correlation between growth rate and latitude and a strong negative correlation between age at the inflexion point of the growth curve and latitude, indicating that larvae from northern populations grew more rapidly and reached the inflexion point earlier than those from southern populations.The food intake increased linearly towards higher latitude. The growth efficiency of larvae from middle latitude was greater than that of larvae from low latitude, but there was no difference in the growth efficiency between high‐ and middle‐latitude populations.The increased growth rate at higher latitude is probably a local adaptation to complete development and growth during the short growth season. The rapid growth rate at higher latitudes could be explained by both the increased food consumption and the growth efficiency. However, the growth efficiency is probably maximized in the middle‐latitude population, suggesting that some physiological costs prevent further increase in growth efficiency at higher latitudes. This study provides compelling evidence for latitudinal cline of juvenile growth rate and disentangles the contributions of food intake and growth efficiency to the cline. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Fish consumption is considered to be a major human exposure route for mercury (Hg), but assessing the actual Hg bioavailability from consumed fish is challenging. In this study, we conducted both in vivo bioavailability (using a mouse model) and in vitro bioaccessibility (using various gastrointestinal digestion methods) assessments of Hg from consumed fish. Lyophilized fish muscles which already absorbed Hg through natural incorporation were introduced to mice by active feeding. Assimilation efficiency (AE) was measured as a short-term kinetic parameter, while a 7-day accumulation of Hg in mice blood, liver and kidney was determined. The AEs of Hg in mice ranged between 82 and 96% and showed a positive relationship with MeHg in fish independent of the fish species. For long-term bioavailability tests in which the Hg retention in organs was measured after a 7-day exposure, most Hg was found to be accumulated in liver and kidney, resulting in a strong correlation between Hg dosage and accumulation in mice organs. The long-term absolute bioavailability of mice was comparable between the liver and kidney, but much lower in the blood. The calculated absolute total Hg bioavailability ranged between 38% and 99% and decreased as the Hg dosage increased. Results of bioaccessibility tests varied considerably among different methods, illustrating that there were limitations for the in vitro bioaccessibility assay to predict the digestive dynamics of Hg in mammalian gastrointestinal tract. Our study strongly demonstrated the expediency of direct determination of Hg bioavailability, but more bioaccessibility assessments should be explored and optimized as an alternative to traditional animal experimentation. Unlabelled Image • Dietary assimilation of Hg in the mice was high and positively related to MeHg in fish. • Hg accumulation in mice organs was strongly correlated with Hg dosage. • Absolute total Hg bioavailability decreased as the Hg dosage increased. • Bioaccessibility tests varied considerably among different methods • Direct determination of Hg bioavailability is preferred to bioaccessibility measurement. [ABSTRACT FROM AUTHOR]

Much is known about the genetic variance in certain components of metabolism, most notably resting and maximum metabolic rate. This is in stark contrast to the lack of information on genetic variance in the metabolic rate of individuals that feed and express routine activity, and how this rate correlates with other components of the energy budget or life history traits. Here we quantify genetic variance in metabolic rate (MR) under such conditions, as well as food consumption, juvenile somatic growth rate and age at maturation under ad lib food availability in a set of 10 clones of Daphnia magna from a natural population. Broad sense evolvabilities (0.16–0.56%) were on the same order of magnitude as those typically observed for physiological and life history traits, and suggest that all these traits have the potential to evolve within this population. We did not find support for the previously hypothesized positive genetic correlation between metabolic rate and growth rate. Rather, the patterns of genetic correlations suggest that genetic variance in food consumption is the single most influential trait shaping somatic growth rate, but that additional variance in growth can be explained by considering the joint effect of consumption and MR. The genetic variance in consumption and MR also translated into genetic variance in age at maturation, creating a direct link between these energy budget components and a life history trait with strong fitness effects. Moreover, a weak positive correlation between MR and food consumption suggests the presence of substantial amounts of independent genetic control of these traits, consistent with results obtained using genomic approaches. [ABSTRACT FROM AUTHOR]

Herbivorous ectothermic vertebrates are more diverse and abundant at lower latitudes. While thermal constraints may drive this pattern, its underlying cause remains unclear. We hypothesized that this constraint stems from an inability to meet the elevated phosphorus demands of bony vertebrates feeding on P‐poor plant material at cooler temperatures because low gross growth efficiency at warmer temperatures facilitates higher P ingestion rates. We predicted that dietary carbon:phosphorus (C:P) should exceed the threshold elemental ratio between carbon and P‐limited growth (TERC:P) for herbivores feeding at cooler temperatures, thereby limiting the range of herbivorous ectothermic vertebrates facing P‐limited growth.We tested this hypothesis using the Andean suckermouth catfishes Astroblepus and Chaetostoma. Astroblepus are invertivores that inhabit relatively cool, high‐elevation streams while Chaetostoma are grazers that inhabit relatively warm, low‐elevation streams. We calculated TERC:P for each genus across its elevational range and compared these values to measured values of food quality over an elevational gradient in the Andes. We also broadly summarized measurements of TERC:P across diverse groups of fishes.Supporting our hypothesis, we found that dietary C:P was predicted to exceed the TERC:P for the grazer Chaetostoma near the highest elevation where this genus has been recorded. Conversely, the TERC:P for the invertivore Astroblepus was consistently higher than that of Chaetostoma and thus its dietary C:P never approached the TERC:P. We found that, among all fishes, omnivores had higher average TERC:P than invertivores, and TERC:P did not vary with temperature.Our results suggest that, at least for Andean suckermouth catfishes, cool temperatures constrain herbivory at higher elevations. Increased gross growth efficiency at cooler temperatures evidently restricts the ability of P‐limited consumers to meet P demand. However, our survey of fish TERC:P estimates suggests that some fishes are able to circumvent this constraint through behavioural and life‐history adaptations that reduce P demand or increase P use efficiency.The physiological trade‐offs underlying these functional shifts reveal that geographic dietary patterns can be predicted by stoichiometric theory, but variation in food quality and consumer traits that reduce P demand and/or increase P efficiency can create exceptions to these patterns. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Abstract Inter-population variations in growth rate can result from independent or interactive effects of genetic and environmental factors, and be induced by some physiological differences as well. Toad-headed lizards (Phrynocephalus vlangalii) from a higher-elevation population were shown to have a higher growth rate than those from a lower-elevation population. The physiological basis of growth rate variation in this species is not well understood. Here, we investigated the feeding performance and resting metabolic rate (RMR) of lower- and higher-elevation individuals at different test ambient temperatures to evaluate the role of differences in energy intake, assimilation efficiency and metabolic expenditure on growth rate variations. Within the range of 25−35 °C, lizard RMR increased with increasing test ambient temperature, but food intake, apparent digestive coefficient (ADC, food energy minus faecal energy divided by food energy), and assimilation efficiency (AE, food energy minus faecal and urinary energy divided by food energy) were less thermally sensitive in both populations. Higher-elevation lizards tended to eat more food and have a lower RMR than lower-elevation ones, despite the lack of differences in ADC and AE. Our result showed that more energy intake and reduced maintenance cost may be associated with the higher growth rate of higher-elevation lizards. Accordingly, inter-population differences in energy acquisition and expenditure could act as potential sources for geographic variation in growth rate. Highlights • High-elevation toad-headed lizards had greater food intake (but similar assimilation efficiency) than low-elevation ones. • High-elevation lizards had lower metabolic rates than low-elevation ones, probably reflecting lower maintenance cost. • More energy intake but lower maintenance cost could potentially lead to a higher growth rate for high-elevation lizards. [ABSTRACT FROM AUTHOR]

Individual diet specialisation (IS) is frequent in many animal taxa and affects population and community dynamics. The niche variation hypothesis (NVH) predicts that broader population niches should exhibit greater IS than populations with narrower niches, and most studies that examine the ecological factors driving IS focus on intraspecific competition. We show that phenotypic plasticity of traits associated with functional trade‐offs is an important, but unrecognised mechanism that promotes and maintains IS. We measured nitrogen isotope (δ15N) and digestive enzyme plasticity in four populations of sparrows (Zonotrichia capensis) to explore the relationship between IS and digestive plasticity. Our results show that phenotypic plasticity associated with functional trade‐offs is related in a nonlinear fashion with the degree of IS and positively with population niche width. These findings are opposite to the NVH and suggest that among individual differences in diet can be maintained via acclimatisation and not necessarily require a genetic component. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the trophic transfer of 137 Cs in marine and freshwater food chains, focusing on phytoplankton, zooplankton, and planktivorous fish. Algal concentration factors were 278 in freshwater and 69 in seawater. The weight-normalized uptake rate constants of 137 Cs were similar for both freshwater daphnids and marine copepods. Most of the 137 Cs in marine copepods was in the exoskeleton followed by polar and non-polar components. In freshwater daphnids, 137 Cs was highest in the polar fractions followed by exoskeleton and low amounts in the non-polar components. Fish that fed on contaminated marine copepods assimilated 137 Cs with an efficiency of 88%, while 91% was assimilated from freshwater daphnids. A bioaccumulation model demonstrated that diet accounted for ≤3% of the total body burden of 137 Cs in marine zooplankton and ≤12% in freshwater zooplankton, but ≥99% of the total body burden in fish. Rate constants of 137 Cs loss from fish following aqueous exposure were 0.2 d −1 and 0.4 d −1 in marine and freshwater conditions, respectively, but only 0.06 d −1 and 0.3 d −1 , respectively, following dietary exposure. This model also indicates that trophic transfer factors from zooplankton to fish are up to 2.2 for marine conditions and up to 0.6 for freshwater. 137 Cs is unusual among metals in that it enters marine food chains primarily from the aqueous phase into zooplankton, from which it is highly assimilated by fish, resulting in detectable 137 Cs in fish tissues. [ABSTRACT FROM AUTHOR]

The trophic transfer of radiocesium ( 134 Cs) was investigated in two tropical fish, the silver moony Monodactylus argenteus and the spotted scat Scatophagus argus . Juveniles of both species were exposed to dietary 134 Cs using the pulse-chase feeding methodology. The food was brine shrimp ( Artemia salina ) previously exposed to the dissolved radiotracer. Depuration kinetics of 134 Cs were followed for 45 d. Results showed that Cs was similarly efficiently assimilated by both species (AE > 50%). The estimated trophic transfer factors in the two species ranked from 1 to 2, suggesting that 134 Cs could be biomagnified in both omnivorous species. In complement, dissections of 7 body compartments were carried out at three different times in order to highlight 134 Cs organotropism. 134 Cs organotropism was similar in both species: more than 50% of 134 Cs was quickly distributed in the muscles and skeleton (after 3 days of depuration), which is likely related to the analogous behavior between Cs and K, an essential element for muscle contractions and bone formation. [ABSTRACT FROM AUTHOR]

We studied the effect of larval density on food utilization by sediment-dwelling northern brook lamprey (Ichthyomyzon fossor) larvae in the Otter River, Baraga County, MI, USA using field surveys and in situ cage experiments. Field surveys found that food utilization was inversely proportional to density across the range of 1 to 10 larvae/m2. Compared to 1 larvae/m2, values at 10 larvae/m2 were lower by an average of 36% for gut fullness, by 32% for selective ingestion of organic matter, and by 71% and 58% for assimilation of diet organic matter and diet amino acids, respectively. In situ cage experiments and additional field studies in other nearby rivers revealed the same relationships. We hypothesize that physical disturbance of the sediment by adjacent larvae interferes with food utilization with the frequency of disturbance proportional to density. Condition factors were not correlated with density and indicate that individual larvae minimize the effect of crowding by moving away from higher density patches. We conclude that these findings are generalizable to other lamprey species in similar environments in which lower rates of growth, maturation, and survivorship of lamprey larvae at higher densities have been reported by other researchers. [ABSTRACT FROM AUTHOR]

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Uptake of polychlorinated biphenyls (PCBs) by fish is controlled by the bioavailability of ingested PCBs in the gut and the freely dissolved concentration in the water moving across the gills. The prediction of bioaccumulation in fish relies on models that account for these exposure routes; however, these models typically do not account for incidental ingestion of sediment by fish, which is not well studied. The literature values for the PCB assimilation efficiency in the gut have been reported for compounds in food matrices and not associated with sediment particles. It is also unclear how mitigation strategies that alter PCB bioavailability in sediments affect predictions made by the bioaccumulation models when sediment ingestion is involved. To test the bioavailability of PCBs from treated and untreated sediments, dietary assimilation efficiencies were measured for 16 PCB congeners in mummichogs (Fundulus heteroclitus) that were fed 4 experimental diets. Diets consisted of PCB-spiked earthworms, spiked untreated sediment mixed with earthworms, spiked activated carbon-treated sediment mixed with earthworms, and spiked activated carbon mixed with earthworms. Assimilation efficiencies were determined by calculating the ratio of PCB mass in the fish tissue to the PCB mass in the food after a pulse feeding experiment. Assimilation efficiencies of PCBs associated with earthworm diet were similar to the values reported in the literature. Fish that were fed the PCB-spiked untreated sediment and activated carbon particles exhibited the highest and lowest assimilation efficiencies, respectively, over a wide KOW range. Assimilation efficiencies of sediment-bound PCBs were significantly reduced (31-93% reduction for different congeners) after amendment with activated carbon. The present study indicates that assimilation of PCBs can be reduced by sorption to black carbon. Environ Toxicol Chem 2017;36:3480-3488. © 2017 SETAC [ABSTRACT FROM AUTHOR]

Bioaccumulation of radiocaesium in many marine organisms occurs through complex trophic transfer mechanisms. The present study addresses the trophic transfer of 134 Cs in the widely distributed marine bivalve, the Manila clam Ruditapes philippinarum , by experimentally determining the assimilation efficiency (AE) and the specific role of food quality or diet on the AE in this marine invertebrate. Pulse-chase feeding experiments were carried out on this clam using the phytoplankton species Tetraselmis chuii , Phaeodactylum tricornutum and Isochrysis galbana . Depuration kinetics of 134 Cs over 21 days were analysed using a two-component exponential model. Observed assimilation efficiencies were consistently less than 10% but slightly varied among individuals fed on the three different phytoplankton species diets ( T. chuii : AE = 8.4 ± 0.6%; P. tricornutum : AE = 9.8 ± 0.5%; I. galbana : AE = 5.3 ± 0.6%), although no statistical differences were observed. Comparing results from these experiments with existing data from the literature on the same species exposed to caesium through seawater, it appears that trophic transfer processes are the main accumulation pathway, contributing up to 96% of the global 134 Cs bioaccumulation in this bivalve species. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) is known to biomagnify in marine food chains, resulting in higher concentrations in upper trophic level animals than their prey. To better understand how marine copepods, an important intermediate between phytoplankton and forage fish at the bottom of the food chain, assimilate and release MeHg, the authors performed a series of laboratory experiments using the gamma-emitting radiotracer 203Hg2+ and Me203Hg with the calanoid copepod Acartia tonsa. Assimilation efficiencies of Hg2+ and MeHg ranged from 25% to 31% and 58% to 79%, respectively, depending on algal diets. Assimilation efficiencies were positively related to the fraction of Hg in the cytoplasm of the algal cells that comprised their diet. Efflux rates of Hg2+ (0.29/d) and MeHg (0.21/d) following aqueous uptake were similar, but efflux rates following dietary uptake were significantly lower for MeHg (0.11-0.22/d) than Hg2+ (0.47-0.66/d). The calculated trophic transfer factors in copepods were >1 for MeHg and consistently low (≤0.2) for Hg2+. The authors used the parameters measured to quantitatively model the relative importance of MeHg sources (water or diet) for copepods and to predict the overall MeHg concentrations in copepods in different marine environments. In general, MeHg uptake from the diet accounted for most of the body burden in copepods (>50%). For an algal diet with a MeHg dry weight bioconcentration factor ≥106, >90% of a copepod's MeHg body burden can be shown to derive from the diet. The model-predicted MeHg concentrations in the copepods were comparable to independent measurements for copepods in coastal and open-ocean regions, implying that the measured parameters and model are applicable to natural waters. Environ Toxicol Chem 2017;36:1287-1293. © 2016 SETAC [ABSTRACT FROM AUTHOR]

Diet is an important pathway for metal uptake in marine organisms, and assimilation efficiency is one of the most relevant parameters to quantify trophic transfer of metals along aquatic food webs. The most commonly used method to estimate this parameter is pulse-chase feeding using radiolabeled food. This approach is, however, based on several assumptions that are not always tested in an experimental context. The present study aimed to validate the approach by assessing single-feeding and multiple-feeding approaches, using a model species (the turbot Scophthalmus maximus). Using the kinetic data obtained from the single-feeding experiment, the reconstruction of a multi-feeding experiment was tested for consistency with data provided by an actual multi-feeding performed under the same experimental conditions. The results validated the single-feeding approach. Environ Toxicol Chem 2017;36:1227-1234. © 2016 SETAC [ABSTRACT FROM AUTHOR]

This laboratory study analyzes feeding preferences of a crayfish species with high invasive potentials, the red swamp crayfish Procambarus clarkii. We first conducted a series of multiple-choice tests among fresh plants (3 tests, one per each of three seasons, i.e. spring, summer, and autumn) and animals (1 test in summer). In the first tests, crayfish were simultaneously offered fresh pieces of seven plants of different growth forms and taxonomic groups, selected on the basis of their availability in the habitat. In the second test, the offered animals were dead earthworms, rainbow trout fry, and toad tadpoles. Then, a binary choice test was run in autumn between the most preferred plant (Urtica sp.) and earthworms. The percentages of organic C and N contents were measured for each food category using a gas-chromatography technique and the assimilation efficiency of both dry mass and organic content was assessed in a dedicated experiment. The results showed that P. clarkii is selective when offered fresh plants, consuming a relatively larger biomass of Urtica (in the three seasons examined), green algae (in spring), and Polygonum (in summer and autumn). Surprisingly, crayfish did not exhibit preference for any animal prey. Also, P. clarkii’s feeding decisions seemed not to be associated with either the availability of plants in the habitat or their occurrence in the gut contents of wild crayfish. Neither did their ingested biomass depend on the organic content or assimilation efficiency. Procambarus clarkii preferred Urtica sp. over earthworms, notwithstanding the higher nutritional value and assimilation efficiency of the latter. The above results might suggest that feeding preferences of the species often depend on factors other than nutritional value and digestibility. Future studies will analyze all the possible factors together in order to unravel the rules that govern feeding preferences in crayfish. In the case of invasive species, such knowledge will highly contribute to planning science-based actions for their containment and mitigation.

The energy budget of xylem-feeding Corymbia rubra (Coleoptera: Cerambycidae) larvae was estimated by determining bioenergetic parameters. The values of several of the life history and bioenergetic parameters of this wood-feeding insect are exceptional and associated with specific adaptations to living in and feeding on wood. The energy budget of C. rubra sheds light on the life strategy of a xylem-feeder. Its slow growth (0.012 mg × mg-1 × d-1) is associated with a low consumption rate (0.097 mg × mg-1 × d-1) and the low nutritional value of its food (the assimilation efficiency is 34%). The gross growth and net growth efficiency are 15% and 43%, respectively.