Increasing Food Concentration Research Articles

Effects of food quantity on the ingestion and egestion of MPs with different colors by Daphnia magna

Aquatic organism uptake and accumulate microplastics (MPs) through various pathways, with ingestion alongside food being one of the primary routes. However, the impact of food concentration on the accumulation of different types of MPs, particularly across various colors, remains largely unexplored. To address this gap, we selected Daphnia magna as a model organism to study the ingestion/egestion kinetics and the preference for different MP colors under varying concentrations of Chlorella vulgaris. Our findings revealed that as the concentration of Chlorella increased, the ingestion of MPs by D. magna initially increased and then showed a decline. During the egestion phase within clean medium without further food supply, an increase in food concentration during the ingestion phase led to a slower rate of MP discharge; while when food was present during the egestion phase, the discharge rate accelerated for all treatments, indicating the importance of food ingestion/digestion process on the MPs bioaccumulation. Furthermore, in the presence of phytoplankton, D. magna demonstrated a preference for ingesting green-colored MPs, especially at low and medium level Chlorella supply, possibly due to the enhanced food searching activities. Beyond gut passage, we also examined the attachment of MPs to the organism's body surface, finding that the number of adhered MPs increased with increasing food concentration, likely due to the intensified filtering current during food ingestion. In summary, this study demonstrated that under aquatic environment with increasing phytoplankton concentrations, the ingestion and egestion rates, color preferences, as well as surface adherence of MPs to filter feeding zooplanktons will be significantly influenced, which may further pose ecological risks. Our results offer novel insights into the unintentional accumulation of MPs by zooplankton, highlighting the complex interactions between food availability and MPs accumulation dynamics.

Cumulative reproduction model to quantify the production of the invasive species Arctodiaptomus dorsalis (Calanoida, Copepoda)

Arctodiaptomus dorsalis is an invasive calanoid copepod originally from America and has recently established a population in Lake Taal in the Philippines. It has been proposed that eutrophication due to aquaculture contributes to the establishment and success of this species. This study aimed to clarify this hypothesis by quantifying the effect of increasing food concentration on the reproduction of A. dorsalis using an Individual-Based Model. We parameterized food concentration-dependent equations to simulate the effect of different food concentration on the reproductive traits (latency time, embryonic development time, clutch size, and hatching success) of this species using data from the literature and simulated the reproduction of 1000 independent females for 20 days at different food concentration, taking into account the variability among individuals. The simulation results showed that the cumulative production of eggs and offspring increased continuously with time when the food concentration was above 3.2 × 104 cells mL−1, suggesting that this value was the threshold concentration required for stable reproduction of A. dorsalis. This value is not particularly low compared to other copepods. Hence, our result revealed that the food use efficiency of A. dorsalis is not particularly outstanding and urge the need of a comparison with native species to confirm a possible superiority. We suggest that A. dorsalis has filled the ecological niche left vacant by the disappearance of the original species due to the deterioration of water quality caused by the eutrophication of the lake. The reproductive cycle model developed in this study constitutes a necessary step in the development of a complete population dynamics model, which can help clarify the invasive abilities of A. dorsalis.

Ingestion and impact of microplastics on arctic Calanus copepods

Microplastics (MPs) are contaminants of emerging concern in the Arctic, but knowledge of their potential effects on Arctic plankton food webs remains scarce. We experimentally investigated ingestion and effects of MPs (20 μm polyethylene spheres) on the arctic copepods Calanus finmarchicus, C. glacialis and C. hyperboreus. These species dominate arctic zooplankton biomass and are relevant target species to investigate the potential impacts of MPs on the Arctic marine ecosystem. Females of each species were exposed to two concentrations of MPs (200 and 20,000 MPs L−1) in combination with different food (diatom) concentrations, reflecting high (3000–5000 cells mL−1, spring phytoplankton bloom) and low (50–500 cells mL−1, pre/post bloom) food conditions. MPs did not affect negatively fecal pellet production rates in any of the species at the studied exposure concentrations. However, egg production rates of copepods exposed to MPs were 8 times higher compared with the controls, which suggests that MP exposure can cause stress-induced spawning in arctic copepods. Microscopic examination of the fecal pellets confirmed ingested MPs in the three species (up to aprox. 1000 MPs cop−1 d−1). The number of MPs per pellet decreased exponentially with increasing food concentration. The daily ingestion of MPs per copepod was higher at low- food concentrations (250–500 cells mL−1). At our exposure conditions, the presence of MPs inside C. hyperboreus fecal pellets did not affect their sinking rates. Overall, our experimental research show that 1) acute exposure to virgin polyethylene MPs has a low impact on arctic Calanus species at environmentally relevant MP concentrations, independent of food availability, and 2) arctic copepods influence the environmental fate of plankton-sized MPs by exporting buoyant MPs from the surface layer to the sea floor via fecal pellets.

Effect of diet on the coupling of ingestion and egg production in the ubiquitous copepod, Acartia tonsa

The ability of consumers to convert ingested carbon into growth is critical for secondary production and trophic transfer. We conducted laboratory experiments to investigate the effect of different prey and concentration on the ingestion rate (IR), egg production rate (EPR) and egg production efficiency (EPE) of the ubiquitous copepod, Acartia tonsa. Experiments were run at several prey concentrations, ranging from 11 to 1132 μgC L–1, of the diatom Thalassiosira weissflogii, the autotrophic dinoflagellate Prorocentrum minimum, the heterotrophic dinoflagellate Oxyrrhis sp., the flagellate Dunaliella tertiolecta, and the bacterivorous scuticociliate Uronema sp. IR increased curvilinearly with concentration for all diets. EPR also increased curvilinearly with increasing food concentration similar to IR, with the exception of the flagellate diet, for which EPR decreased linearly with food concentration. EPR ranked as T. weissflogii > P. minimum > Oxyrrhis sp. = Uronema sp. > D. tertiolecta. IR and EPR were linearly related, except for flagellate diet. The slope of the carbon-based relationship between IR and EPR, the egg production efficiency (EPE), was highest for the diatom (77.5%) and lowest for the scuticociliate (4.2%). Egg production was not correlated to ingestion of the flagellate offered to A. tonsa. We conclude that of the five prey species, the diatom T. weissflogii is the best prey to promote A. tonsa reproduction, to optimize trophic transfer efficiency, and to increase mass cultivation of this species.

Relative impacts of the invasive Pacific oyster, Crassostrea gigas, over the native blue mussel, Mytilus edulis, are mediated by flow velocity and food concentration

The ecological impacts of invasive species can be severe, but are generally viewed as highly unpredictable. Recent methods combiningper capitafeeding rates, population abundances and environmental contexts have shown great utility in predicting invader impacts. Here, clearance rates of the invasive Pacific oyster,Crassostreagigas, and native mussel,Mytilusedulis, were investigated in a laboratory experiment where oscillatory water flow and algal food concentrations were manipulated. Invasive oysters had lower clearance rates than native mussels in all experimental groups and did not differ among flow velocities or food concentrations. Native mussel clearance rates were higher at 5 cm s-1compared to 0 and 15 cm s-1flow velocities and increased with increasing food concentration. The Relative Impact Potential (RIP) metric was used to assess (i) the influence of flow velocity and food concentration on potential impacts ofC.gigason plankton resources and, (ii) the impacts of coexisting reefs, containing both species, on resources compared to monospecific native mussel beds. Greatest Relative Impact Potential of invasive oysters was seen at the lowest flow velocity, but became reduced with increasing flow velocity and food concentration. Relative Impact Potentials of coexisting reefs were generally greater than monospecific native mussel beds, with greatest impacts predicted at lowest flow velocity. We suggest that the greatest ecological impacts and competition potential ofC.gigaswill occur in areas with low flow velocity, but that increased flow will mediate co-existence between the two species.

Comparative feeding rates of native and invasive ascidians

Ascidians have a recent history of species introductions globally, often with strong ecological impacts. Comparisons of per capita effects of invaders and comparable natives are useful to assess such impacts. Here, we explore ingestion rates (IR) and clearance rates (CR) of Ciona intestinalis and Ciona robusta, co-occurring native and non-native ascidians, respectively, from Brittany, France. IR was positively related to food concentration, with the invader responding more strongly to increasing food concentration. CR also differed by species, with the invader demonstrating higher values. C. robusta exhibited a higher functional response (Type I) than did C. intestinalis (Type II). Relative impact measured using seasonal abundance and IR revealed that C. robusta has a much greater impact than C. intestinalis at all food concentrations tested, though the former has a constrained distribution which limits its regional impact. Nevertheless, when abundant, we expect C. robusta to exert a greater impact on algal foods.

Effects of food concentration on the life table demography and morphology of three Keratella quadrata morphotypes

Keratella quadrata with two (2PS), one (1PS) and none posteolateral (0PS) spines were cultured under four food levels (0.75 × 106, 1.5 × 106, 3.0 × 106 and 6.0 × 106 cells·mL−1 of Scenedesmus obliquus) to test the differences in the life table demography and the morphological characteristics among these three morphotypes. The results showed that each K. quadrata morphotype could produce 2PS, 1PS and 0PS offsprings. The frequencies of 0PS were extremely low (<5%) and could be considered as a small probability event, suggesting that the 0PS morphotype might be an abnormal status. The following life table demographic tests suggested that 0PS morphotype had a relatively lower intrinsic rate of population growth at high food concentrations and a relatively lower average lifespan, in comparison to 2PS rotifers. These results further supported that the 0PS K. quadrata might be an abnormal development. Along with the elevating food concentration, 1PS morphotype reproduced more 2PS offsprings, suggesting that high energy input might be helpful to grow more posterolateral spines. However, in response to the increasing food concentration, 0PS rotifers produced more 1PS offsprings. The underlying mechanisms required further investigations. The posterolateral spine length of offsprings of 0PS K. quadrata was significantly longer than those of 2PS and 1PS rotifer parents at the four food concentrations, which probably help the offsprings of 0PS rotifer parents to survive in natural environments, since long and more posterolateral spines offer rotifers high ability to compete with other rotifers and cladocerans for food or to resist predators.

Modulation of digestive physiology and biochemistry in Mytilus californianus in response to feeding level acclimation and microhabitat.

ABSTRACTThe intertidal mussel Mytilus californianus is a critical foundation species that is exposed to fluctuations in the environment along tidal- and wave-exposure gradients. We investigated feeding and digestion in mussels under laboratory conditions and across environmental gradients in the field. We assessed whether mussels adopt a rate-maximization (higher ingestion and lower assimilation) or a yield-maximization acquisition (lower ingestion and higher assimilation) strategy under laboratory conditions by measuring feeding physiology and digestive enzyme activities. We used digestive enzyme activity to define resource acquisition strategies in laboratory studies, then measured digestive enzyme activities in three microhabitats at the extreme ends of the tidal- and wave-exposure gradients within a stretch of shore (<20 m) projected sea-ward. Our laboratory results indicated that mussels benefit from a high assimilation efficiency when food concentration is low and have a low assimilation efficiency when food concentration is high. Additionally, enzyme activities of carbohydrases amylase, laminarinase and cellulase were elevated when food concentration was high. The protease trypsin, however, did not increase with increasing food concentration. In field conditions, low-shore mussels surprisingly did not have high enzyme activities. Rather, high-shore mussels exhibited higher cellulase activities than low-shore mussels. Similarly, trypsin activity in the high-shore-wave-sheltered microhabitat was higher than that in high-shore-wave-exposed. As expected, mussels experienced increasing thermal stress as a function of reduced submergence from low to high shore and shelter from wave-splash. Our findings suggest that mussels compensate for limited feeding opportunities and thermal stress by modulating digestive enzyme activities.

Type III functional response in the zebra mussel,Dreissena polymorpha

Distinguishing between functional response types requires observations at low food concentrations, but surprisingly these tend to be rare. A paucity of feeding observations at low food concentrations is especially acute for dreissenid mussels, despite their importance as invading species in fresh waters. We assessed the functional response of zebra mussels (Dreissena polymorpha) via feeding experiments and behavioral observations conducted at low food levels. Critically, food levels were chosen to be in the vicinity of minimum concentrations found in lakes with established D. polymorpha populations. Results of two feeding experiments show clear evidence of a Type III functional response for D. polymorpha feeding on Ankistrodesmus falcatus — clearance rate increased with increasing food concentration at low food levels. Mussels were always open and feeding during these experiments. An independent set of behavioral observations further showed that the fraction of mussels actively feeding (valves open, siphons extended) decreased as food concentrations decreased. We also measured somatic growth of juvenile mussels at varying levels of A. falcatus over 27 days and found that mussels were able to grow at food levels where Type III behavior was observed in the feeding experiments.

Species-specific and combined feeding rates and selectivity of dominant mysids from a subtropical estuary, Brazil

We studied the influence of mysid species number on their feeding rates to test how the structure of the mysid assemblage may affect carbon fluxes in the Cananeia estuary. Three types of experiments were conducted at the laboratory in order to: (i) evaluate mysid feeding rates and selectivity on the natural zooplankton assemblage; (ii) determinate mysid functional responses to varying food density using Artemia sp. nauplii as food; and (iii) check inter- and intraspecific predation between mysids. For the former two experiments, three mysid species were incubated isolated or combined (2 species: Metamysidopsis elongata atlantica+Mysidopsis coelhoi; 3 species: M. e. atlantica+M. coelhoi+Chlamydopleon dissimile), comprising 5 treatments. The three mysid species had opportunistic feeding behavior on the natural zooplankton assemblage, but there was evidence of avoidance of medium-size prey (300–400μm) mostly comprised of Oithona spp., probably due to a combination of small size and prey swimming patterns. Ingestion rates showed a non-linear increase with increasing food concentrations, suggesting a type II functional response for all mysid species and combination of species. There were both positive (complementarity effect) and negative effects of species combinations on the investigated response. Negative interactions were probably linked to intraguild predation as confirmed by experiment (iii), which resulted in a reduction of mysid predation over the zooplankton.

Ingestion and absorption of particles derived from different macrophyta in the cockle Cerastoderma edule: effects of food ration

We analyzed the capacity of the common cockle Cerastoderma edule to utilize detrital food particles obtained from three different macrophytes: the vascular plant Juncus maritimus and two green macroalgae (Ulva lactuca and Enteromorpha sp.). We measured feeding and digestive parameters at three concentrations of detritus (0.5, 1.0 and 3.0 mm(3) l(-1)), so that functional relationships between ingestive and digestive processes could be assessed. Increasing concentrations of detritus (food) resulted in a reduction in filtering activity (clearance rate l h(-1)), but an increase in ingestion rate. Consequently, gut content also increased with increasing food concentration, irrespective of food type. In contrast, the trend followed by absorption efficiency with increasing ingestion rate was determined by food type, being significantly reduced (from 0.63 to 0.11) with Juncus but remaining almost constant with the green macroalgae (0.58 ± 0.07 with Ulva) or only minimally reduced (from 0.66 to 0.48 with Enteromorpha). This differential response had clear consequences for energy uptake: absorption rate increased with increasing particulate organic matter with Enteromorpha but decreased with Juncus. We discuss the possible role of digestive parameters such as digestibility, gut content and gut-residence time in the differential utilization of detrital matter from different vegetal origins by cockles.

Growth of mussels Mytilus edulis at algal (Rhodomonas salina) concentrations below and above saturation levels for reduced filtration rate

Average filtration and growth rates of groups of juvenile Mytilus edulis (n=25–45 of 22–35 mm shell length) were measured at different concentrations of an algal cell monoculture in 9 laboratory experiments of duration 14–30 days, 4 experiments below and 5 above the limit of incipient saturation concentration (C sat≈6000–7000 Rhodomonas salina cells ml−1). From a nearly constant filtration rate (F≈30 ml min−1 for a 30 mm shell length) at measured algal concentrations below C sat the steady-state filtration rate decreased approximately as 1/C for increasing algal concentrations (C) above C sat to levels as low as 12–9% of the former value. Corresponding calculated gross ingestion rates (I=F×C) increased linearly below C sat and remained nearly constant above C sat. However, the measured weight-specific growth rates (µ) decreased sharply above C sat from a maximal value of about 9.5% day−1 to about 1.5% day−1. Below C sat on the other hand, measured µ values increased linearly with increasing algal concentration, which was in good agreement with an earlier advanced bioenergetic growth model. The overall functional response of M. edulis resembles a Type I in terms of gross ingestion, but with a rapid decrease instead of a constant above C sat in terms of actual ingestion and growth. The physiological implications of the functional response remain uncertain. The response to increasing food concentration with possible regulation of net ingestion appears only to come into play when C sat is exceeded and then as partial valve closure and reduced filtration and growth rates along with production of pseudofaeces. A survey of naturally occurring phytoplankton biomass in the sea shows that this is generally below C sat except for the short spring bloom periods; hence, mussels generally feed at optimal rates depending on the composition and concentration of biomass exceeding the minimal concentration below which the mussels close their valves and reduce or cease filtering.

Feasting in fresh water: impacts of food concentration on freshwater tolerance and the evolution of food × salinity response during the expansion from saline into fresh water habitats

Saline to freshwater invasions have become increasingly common in recent years. A key hypothesis is that rates of freshwater invasions have been amplified in recent years by increased food concentration, yet this hypothesis has remained unexplored. We examined whether elevated food concentration could enhance freshwater tolerance, and whether this effect evolves following saline to freshwater invasions. We examined physiological response to salinity and food concentration in a 2 × 2 factorial design, using ancestral brackish and freshwater invading populations of the copepod Eurytemora affinis. We found that high food concentration significantly increases low-salinity tolerance. This effect was reduced in the freshwater population, indicating evolution following the freshwater invasion. Thus, ample food could enable freshwater invasions, allowing subsequent evolution of low-salinity tolerance even under food-poor conditions. We also compared effects of food concentration on freshwater survival between two brackish populations from the native range. Impacts of food concentration on freshwater survival differed between the brackish populations, suggesting variation in functional properties affecting their propensity to invade freshwater habitats. The key implication is that high food concentration could profoundly extend range expansions of brackishwater species into freshwater habitats, potentially allowing for condition-specific competition between saline invaders and resident freshwater species.

Response of the pelagic tunicate appendicularian, Oikopleura dioica to controlled simulations of a strong bloom condition: A bottom‐up perspective

To test the hypothesis of bottom‐up regulation of appendicularians under high food conditions that occur during blooms, an experiment including measurements of fecundity, hatching success, and mortality at different quantity and quality of food was performed on Oikopleura dioica. Food level and fecundity were positively correlated. There was little effect of food on adult mortality, although higher food levels significantly decreased successful juvenile development. The positive effect of increased food concentration on fecundity was offset by a negative effect on hatching success; hence, the variance in quality and quantity of food explored in this study had little effect on the intrinsic rate of natural increase.

기수산 요각류 Paracyclopina nana 의 섭식, 성장 및 생산력에 관한 먹이농도의 영향

기수산 요각류 P. nana의 대량배양을 위한 최적 먹이공급량을 규명하기위해 먹이공급량에 따른 P. nana의 섭식량, 성장 및 성체암컷의 nauplius 생산수를 조사하였다. 실험을 위한 먹이생물로 해양 식물플랑크톤인 Tetraselmis suecica를 사용하였으며 실험은 발달 단계에 따라 6 그룹으로 나누어 실시하였다. 먹이공급량은 각 단계 그룹마다 0.5, 1, 2, 5, 10, 20, 40, 60 및 <TEX>$80{\times}10^4$</TEX> cells/mL로 설정하였고 P. nana 암컷의 nauplius 생산수 측정실험은 0.2, 0.5, 1, 2, 3 및 <TEX>$4{\times}10^4$</TEX> cells/mL의 먹이공급실험구를 두어 실시하였다. 실험결과, 먹이 공급량에 따른 P. nana 각 발달 단계의 시간 당 초식율은 모든 발달단계가 T. suecica의 농도가 높아짐에 따라 계속 증가하는 경향을 보였다. P. nana는 nauplius 기간에서, 먹이량이 증가함에 따라 개체 발달에 영향을 미치지 않는 것으로 나타난 반면, copepodite 단계에서는 먹이량이 <TEX>$1{\times}10^4{\sim}5{\times}10^4$</TEX> cells/mL을 기준으로 감소하거나 증가할 때, 개체 발달에 악영향을 미치는 것으로 나타났다. 먹이 공급 농도에 따른 암컷의 일일 nauplius 생산은 먹이 공급량이 증가함에 따라 증가하였고 3 및 <TEX>$4{\times}10^4$</TEX> cells/mL 실험구는 <TEX>$2{\times}10^4$</TEX> cells/mL 실험구와 유의적인 차이를 보이지 않았다. 결과적으로, P. nana의 대량배양을 위한 T. suecica 공급밀도는 nauplius 단계는 5,000 cells/mL, copepodite 단계와 성체수컷은 10,000 cells/mL, 성체암컷은 20,000 cells/mL이 가장 효율적인 먹이 공급 밀도라고 판단된다. We studied the effect of food concentration on grazing, growth and fecundity of cyclopoid copepod Paracyclopina nana. Marine phytoplankton Tetraselmis suecica was used as a livefood for the copepod. Six stage compositions were used and food concentrations for the experiment were 0.5, 1, 2, 5, 10, 20, 40, 60 and <TEX>$80{\times}10^4$</TEX> cells/mL. Range of food concentrations with 0.2, 0.5, 1, 2, 3 and <TEX>$4{\times}10^4$</TEX> cells/mL were used for nauplii production experiment. Grazing rates of P. nana in all developmental stages on the different concentrations were increased with increasing diet concentration. While the growth of nauplius was not affected by increase of food concentration, food concentration outside of <TEX>$1{\times}10^4$</TEX> to <TEX>$5{\times}10^4$</TEX> cells/mL range negatively affected that of copepodite. Daily nauplii production was increased with increasing food concentration but <TEX>$3{\times}10^4$</TEX> and <TEX>$4{\times}10^4$</TEX> cells/mL treatments were not significantly different with <TEX>$2{\times}10^4$</TEX> cells/mL treatment. As a result, optimum concentration of T. suecica for mass culture of P. nana was considered to be 5,000 cells/mL for nauplius stage, 10,000 cells/mL for copepodite stage and adult male, 20,000 cells/mL for adult female, respectively.

Experimental study of the interspecific competition between two sibling marine herbivorous rotifers in relation to food availability and initial population density

Our laboratory study concerns the competitive interaction between two marine rotifer species Brachionus plicatilis and Brachionus rotundiformis at five algal (Nannochloropsis salina) concentrations (0.4×106 to 32.4×106 cells/ml) and at four initial inoculation densities (numerically, 100% B. plicatilis; 75% B. plicatilis and 25% B. rotundiformis, 50% each of the two species; 25% B. plicatilis and 75% B. rotundiformis and 100% B. rotundiformis. The initial biomass varied as 0.33 and 0.22 µg/ml for B. plicatilis and B. rotundiformis respectively. Experiments were performed at (25±1)°C. Population densities were enumerated and the medium was changed daily up to 8 d in the experiment. At the lowest food level tested, B. rotundiformis formed superior competitor than B. plicatilis, regardless of starting inoculation density. Generally when the food concentrations increased, B. plicatilis showed a greater increase in biomass than B. rotundiformis. B. rotundiformis formed the largest in population growth, regardless of increasing food concentrations. When grown alone, B. plicatilis reached peak abundances of (1.311 5±0.028) and (137.5±0.014) µg/ml at low and high food densities respectively. The corresponding values of B. rotundiformis were 0.724 5±0.016 and 18.15±0.021. The adverse effects of B. rotundiformis on the peak abundances of B. plicatilis were observed at the lowest food level and higher initial density. The rate of population growth in controls varied from (0.792±0.162) to (1.482±0.132) µm/d for B. plicatilis and (0.445±0.041) to (0.856±0.012) µm/d for B. rotundiformis, depending on food level. When both species were introduced together, low food levels favoured higher abundance of B. rotundiformis than B. plicatilis, suggesting that increased population density of the smaller B. rotundiformis was more successfull than larger B. plicatilis in brackish waters. Our work reveals that available food (type and quantity) along with starting inoculation density had significant effect on the interspecific competition between marine sibling rotifer species in zooplankton community structure.

Accumulation kinetics of arsenic in Daphnia magna under different phosphorus and food density regimes

In the present study, the dissolved uptake, dietary assimilation, and elimination of arsenic (initially added as arsenate) in the freshwater cladoceran Daphnia magna were examined. A biphasic correlation between the arsenic uptake rate and its ambient concentration, as well as a two-saturation-site arsenic uptake competition with phosphate was observed. The calculated uptake rate constant, as influenced by the ambient phosphorus concentration, ranged from 0.035 to 0.35 L/g/d. Food concentration substantially decreased (by 23.2-64.4%) the arsenic assimilation efficiency with the incipient limiting algal food concentration of 3.86 mg/L dry weight. Arsenic assimilation by the daphnids was independent of their own phosphorus status, but was lower when their algal diet was phosphorus-limited and thus contained a higher proportion of arsenite due to the enhanced biotransformation. Arsenic efflux rate constant ranged from 0.34 to 0.44 d(-1) with increased food concentration slightly facilitating its loss. Excretion, accounting for 51.3 to 60.6% of total loss, was the dominant pathway for arsenic elimination with a remarkable contribution from offspring production (24.7-29.8%), whereas molting (3.64-4.05%) and egestion (7.9-11.9%) had minor roles only. According to the well-established biokinetic model, dietary assimilation was predicted to be the main pathway for arsenic bioaccumulation in the daphnids, and arsenic has a great potential to be biodiminished along the food chain.

Application of enriched stable isotope technique to the study of copper bioavailability in Daphnia magna

Abstract The biokinetics of Cu in Daphnia magna, including dissolved uptake, assimilation and efflux, has been determined using a gamma 67Cu radiotracer methodology. However, this gamma emitting radioisotope is not readily available due to its very short half-life. In the present study, we employed a stable isotope tracer ( 65Cu) to determine the Cu biokinetics and compared our results to those determined using 67Cu. The dissolved uptake rate constant of 65Cu was 3.36 L/(g-day), which is higher than that of 67Cu (1.32 L/(g-day)). With increasing food concentrations from 2(10 4 to 1(10 5 cells/mL, the Cu assimilation efficiency (AE) decreased from 46% to 11%, compared to a decrease from 27% to 16% when determined using 67Cu. The efflux of Cu from Daphnia magna was quantified following both dissolved and dietary uptake. The efflux of waterborne Cu was comparable to that of dietborne Cu and the efflux rate constant (0.32–0.52 day −1) was higher than that determined by 67Cu (0.19–0.20 day −1). By considering different water properties and handling procedure between the two experiments, we believe that these differences are reasonable. Overall, this study demonstrated that the enriched stable isotope tracer technique is a powerful tool to investigate metal bioavailability and maybe a good alternative to radioactive measurements.

Effect of food concentration on the production and viability of resting eggs of the rotiferBrachionus: implications for the timing of sexual reproduction

Summary1. Life‐table experiments withBrachionus calyciflorustest several hypotheses related to the idea that sexual reproduction in monogonont rotifers should occur when food resources are favourable.2. The food concentration necessary for a fertilised mictic female to produce one phenotypically normal resting egg was higher than that for an amictic female to produce one daughter. At the lowest concentration ofCryptomonas erosa(1.25 × 103 cells mL−1), the lifetime fecundity of these two types of females was 0.9 and 1.4, respectively.3. The lifetime fecundity of both fertilised mictic females and amictic females increased with food concentration to 3.4 resting eggs and 15.2 daughters female−1, respectively. The approach to maximal fecundity with increasing food concentration was more rapid for fertilised mictic females, such that their lifetime fecundity relative to that of amictic females gradually decreased from 0.64 (at 1.25 × 103 C. erosa mL−1) to 0.22 (at 2.5 × 104 C. erosa mL−1).4. The probability of a fertilised mictic female producing one or more abnormal resting eggs during her lifetime was high (approximately 75%). The mean proportion of abnormal eggs produced per female varied among the different food‐concentration treatments (26–38%) but was not higher at the low food concentrations.5. The proportion of normal resting eggs that hatched was high (51–71%); those produced at low food concentrations were no less likely to hatch than those produced at high food concentrations. No abnormal resting eggs hatched.6. The probability of a fertilised mictic female producing an abnormal resting egg increased rapidly with her age at all food concentrations. The probability of a normal resting egg hatching declined with maternal age at the low food concentration in one of two experiments.7. The results support the idea that induction of mictic females should occur when food resources are good. Coincidence of sexual reproduction with low food availability risks low production of resting eggs for several reasons. Population size may be small, with a low probability of encounters between young mictic females and males, and fertilised mictic females may be unable to mature and produce resting eggs.

Economic models of fish shoal (school) size: a near comprehensive view of single species shoaling strategy

This paper extends theory of shoaling presented by Landa (1998), which uses the economic theory of clubs (Buchanan 1965). The findings include that non-patchy feeding shoals formed for defense increase in size with increased predation and decline in size with increased food concentration. There is strong evidence for the former and a piece of evidence for the latter. The size of shoals formed to find patches of food is unaffected by food availability. When defense is also involved, increasing predation increases shoal size and increasing food availability decreases shoal size. The optimum size of migrating schools—synchronized and polarized swimming shoals—is often the whole population migrating, explaining the mammoth size of some schools. Further, mergers of schools are forecast along the migration route with larger schools at the end of the route than at the beginning. When there is no defense motive for shoaling, non-patchy feeders may form large schools under low food density and be solitary under high food density. Small schools might not be found. When there is also a defense motivation for shoaling, small schools are possible. For schools formed among patchy feeders, increased food availability decreases school size. The same finding holds for schooling among patchy feeders where an added shoal benefit is defense. Also under these circumstances, increased predation increases shoal size.