Single dietary amino acids control resting egg production and affect population growth of a key freshwater herbivore

The enormous success of the genus Daphnia in freshwater ecosystems is at least partially due to their cyclical parthenogenetic life cycle, in which asexual and sexual reproduction alternate periodically. This temporal change between reproductive strategies allows for (1) rapid population growth via subitaneously developing eggs when environmental conditions are appropriate and (2) the maintenance of genetic diversity via sexual reproduction and the production of resting eggs when environmental conditions deteriorate. We show here that dietary amino acids are involved in triggering the switch between reproductive modes in Daphnia pulex. Supplementation experiments demonstrate that specific dietary amino acids, in particular arginine and histidine, avert crowding-induced resting egg production, enhance subitaneous reproduction by increasing algal food quality and, as a combined effect of both processes, increase population growth rates. These findings suggest that the availability of single dietary amino acids potentially affects the seasonal dynamics and long-term persistence of Daphnia populations in the field, which may have consequences for the efficiency of carbon transfer and thus the trophic structure of freshwater food webs.     

Carbon and nitrogen relationships in the feeding and growth of the Pacific oyster, Crassostrea gigas (Thunberg)

Bivalve molluscs, in common with consumers in general, use behavioral and physiological mechanisms to balance metabolic requirements with available nutrients. This study considered how the Pacific oyster, Crassostrea gigas, meets the demands of growth and maintenance, measured in terms of carbon and nitrogen, in a variable food environment. Stoichiometry theory helped to evaluate: a) whether feeding behaviour modifies the intake of C and N given seasonal variability in food quality: b) how rates of metabolism and excretion, and C and N growth efficiencies, respond to mismatch between nutrient intake and the oysters' needs. Two field experiments in the Port Stephens estuary, near Sydney, Australia, measured feeding behaviour, metabolic and growth rates relative to seasonal changes in food supply. In a laboratory experiment, relationships between physiological rates and growth were measured to test a model of growth as a function of absorption of C and N. Potential metabolic targets for compensation were the C/N ratios of body tissues, maintenance and/or of soft tissue added as growth. C/N of whole soft issues varied little during the year (mean 5.4). In July (a time of low food availability of poor quality) growth was negligible and the C/N (maintenance) target was 6.7. In March (abundant food of high quality) growth was rapid with a high N-demand; the C/N of growth was 3.9. In November (medium food quality) there was an enhanced C-demand for glycogen storage; the C/N of growth was 7.9. Feeding behaviour changed the balance between C and N intake across months, primarily due to changes in the selection efficiency for nitrogen, which was highest at low filtration rates on particles of high C/N ratio. Nitrogen intake was favoured over C in July. In November, C-intake increased relative to N. In March, when abundant food nitrogen coincided with a high demand for growth, feeding behaviour was neutral with respect to C/N ratios. In all cases C/N of absorbed matter was greater than the C/N of growth. Growth efficiencies for carbon declined with increased C/N of ingested matter due to higher metabolic increments (SDA) when feeding on lower food quality; the metabolic costs of growth did not vary. In contrast, growth efficiencies for nitrogen did not alter with C/N for ingested matter, due in part to increased nitrogen losses, relative to tissue nitrogen content, when feeding on low C/N food. Nitrogen was therefore conserved metabolically relative to C. Both feeding and metabolic processes contributed to compensation for the mismatch between seasonally variable food quality and the demands of growth.     

Sex-specific response to nutrient limitation and its effects on female mating success in a gift-giving butterfly

Animals with complex life cycles respond to early food limitation by altering the way resources are allocated in the adult stage. Response to food limitation should differ between males and females, especially in organisms whose mating systems include nutritional nuptial gifts. In these organisms, males are predicted to keep their allocation to reproduction (sperm and nuptial gift production) constant, while females are predicted to sacrifice allocation to reproduction (egg production) since they can compensate by acquiring nuptial gifts when mating. In this study, we investigated how dietary nitrogen limitation during the larval stage affects sex-specific resource allocation in Pieris rapae butterflies. Also, we tested whether nutrient-limited females increased nuptial gift acquisition as a way to compensate for low allocation to reproduction. We found that as predicted females, but not males, sacrifice allocation to reproduction when larval dietary nitrogen is limited. However, females were unable to compensate for this low reproductive allocation by increasing their mating rate to acquire additional gifts. Females reared on low nitrogen diets also reduced wing coloration, a potential signal of female fecundity status. We suggest that female mating frequency is constrained by male mate choice based on females’ wing coloration. This study provides new insights into how larval dietary nitrogen, a key nutritional resource for all herbivores, alters male and female allocation to reproduction as well as to ornamentation.     

Importance of dietary nitrogen and carbohydrates to survival, growth, and reproduction in adults of the grasshopper Ageneotettix deorum (Orthoptera: Acrididae)

Key demographic traits in insect herbivores (survival, growth, and egg production) are often responsive to variation in diet quality, especially to dietary nitrogen (N) levels. Soluble carbohydrates may also be limiting. Using defined diets under controlled laboratory conditions, we examined survival, growth, and egg production in response to a range of diet qualities in adult females of a grass-feeding grasshopper Ageneotettix deorum (Scudder). Diets varied factorially within naturally occurring ranges of total N (1–7%) and carbohydrate (4–27%) levels. N concentrations significantly impacted weight gain, egg production rate, the elapsed time until the first egg pod, and the time between the first and second egg pod. These responses were typically quadratic in nature with a maximum response near 4–5% total N. The rate of pod production rather than number of eggs per pod best explained changes in reproductive rate. Dietary carbohydrate levels seldom exerted a significant impact on demographic parameters except when interacting with N on survival, egg weight, and the period between egg pods. Clearly, factors that alter the availability of quality diet, especially total N levels, can contribute to demographic responses in A. deorum. Received: 5 August 1996 / Accepted: 26 April 1997     

Seasonal variability in feeding behaviour, metabolic rates and carbon and nitrogen balances in the Sydney oyster, Saccostrea glomerata (Gould)

To establish if nutrients limit the growth of bivalves requires information not only on the quality of food available, but also the animals' feeding behaviour and endogenous metabolic demands. We hypothesized that growth of the Sydney rock oyster (Saccostrea glomerata) would vary in response to seasonal changes in food quality rather than quantity. We also predicted that the oysters would show feeding preferences for nitrogen over carbon, and this behaviour would result in carbon/nitrogen (C/N) ratios for ingested and absorbed matter that would be lower than the C/N of both the seston and the oysters' estimated metabolic maintenance requirements. The experiments were done in two phases under natural conditions. In phase 1, feeding behaviour was assessed on a single occasion and the results used to pose hypotheses for testing in phase 2, which included measurements made on three occasions encompassing autumn, winter and spring conditions. Growth rates varied with changes in ambient food quality and not with the concentration of total suspended matter. Feeding behaviour responded to food quality and, in most cases, resulted in nitrogen enrichment. For example, when nitrogen was potentially limiting to growth and/or maintenance, due to high food C/N (July) or high nitrogen demand (March), pre-ingestive selection ensured nitrogen enrichment of ingested matter and C/N ratios of ingested matter which were below the maintenance requirement. However, in November, when endogenous demands indicated an increased requirement for carbon, feeding behaviour resulted in carbon enrichment, an increase in carbon conversion efficiency, and ingested C/N ratios greater than the maintenance requirement. The results support the assertion of variable feeding physiology in oysters, responsive to both exogenous (seasonal differences in carbon and nitrogen availability) and endogenous (cycles of reproduction and growth) factors.     

Diet, fecundity and egg size in some polyphagous predatory carabid beetles

Egg size was measured at different rates of egg laying in three polyphagous carabid species, known to be useful predators of cereal aphids; the small Bembidion lampros Herbst and the medium-sized Pterostichus cupreus L. and P. melanarius Illiger. Variations in fecundity, as well as the ability of the medium-sized species to also build up fat reserves, were obtained when beetles were subjected to different dietary regimes consisting of aphids, or foods with a lower or higher protein content. Egg size was found to be dependent on the rate of egg laying within a species. A diet of cereal aphids appeared to be adequate for egg production in these polyphagous carabids, but female P. cupreus were unable to build up fat reserves when they ingested aphids contaminated with the aphicide pirimicarb. Beetles were able to devote resources to more and larger eggs (B. lampros), or to larger eggs and/or fat reserves (P. melanarius/P. cupreus) when given access to a carbohydrate-rich food with low protein content. The highest rate of egg laying was obtained when female P. cupreus and P. melanarius were given a more varied diet at frequent intervals; including regular shifts between unsprayed aphids, carbohydrate-rich food and protein-rich maggots. Within the varied diet treatment a negative relationship was obtained between egg size and egg number among similar-sized individuals of P. cupreus and P. melanarius; females producing the largest number also laid the smallest eggs. Egg size affected larval survival, since first instars hatching from large eggs were found to survive longer than those hatching from small eggs. The influence of differences in food intake on reproduction, maintenance metabolism, and survival of fieldinhabiting carabids is discussed.     

The relationship between dietary specialism and availability of food and water on cannibalistic interactions among predatory mites in protected crops

The interplay between dietary specialism, the tolerance of food and water stress and level of cannibalism is likely to be important in determining the outcome of biological control using inundative releases of multiple natural enemies, such as phytoseiid mites in protected crops. The dietary specialist, Phytoseiulus persimilis, with a short immature development time (4-5 days) when plentiful food was available had a low ability to survive without food (5 days), even with access to water. The dietary generalists, Neoseiulus californicus, N. cucumeris and lphiseius degenerans, had longer immature development times (by up to 2 days) than P. persimilis. Survival ability differed amongst the generalist species when they were starved but provided with constant access to water. Both N. californicus and N. cucumeris survived the longest (8-10 days) and I. degenerans survived the shortest period (4 days). No negative intra-specific interaction between immatures was observed with P. persimilis when food was available and in the absence of food this species tended to starve rather than act cannibalistically. Both N. californicus and N. cucumeris showed a low degree of cannibalism between immatures, either when food was available, or when starved but given access to water. Even when food was available survival of I. degenerans fell by 30% in 4 days and remained at 60-70% for 3 further days; survival continued to decline rapidly when they were starved but provided with water. This indicates that immatures of I. degenerans could either feed on dead conspecifics or that they were capable of a degree of cannibalism. Adult females of P. persimilis did not feed on conspecific eggs even when deprived of food but provided with water. Adult female N. californicus and N. cucumeris did feed on conspecific eggs but at a low level (<1 egg per day), which occurred only after 48 h starvation. Although egg cannibalism occurred more consistently with adult female I. degenerans than with other mite species it was at a low level (<1 egg per day). If the tendency to cannibalism, not just of eggs but with more susceptible life stages such as larvae, is reduced when water is available freely this could be important in determining the interactions that occur under natural conditions.     

Omnivory in the calanoid copepod Temora longicornis: feeding, egg production and egg hatching rates

We measured in laboratory experiments the ingestion, egg production and egg hatching rates of female Temora longicornis as a function of diet. The diets consisted of a diatom (Thalassiosira weissflogii), an autotrophic dinoflagellate (Heterocapsa triquetra), and a bacterivorous ciliate (Uronema sp.) given as sole foods, or combinations of these single-food items: diatom+dinoflagellate, diatom+ciliate, dinoflagellate+ciliate, and diatom+ciliate+dinoflagellate. For the three single-item diets, the functional response was similar; i.e., ingestion rate increased linearly with food concentration (food range: ∼25 to ∼600 μg C l⁻¹). When all diets were considered, maximum daily carbon ration (∼70% of body weight) was independent of food type. However, the maximum daily egg production rate (12% of body carbon) was obtained with the diatom diet. For all diets, both ingestion and egg production rates increased with food concentration. Ingestion and egg production rates were affected differently by the interaction of food concentration and food type: at low food concentrations, ingestion rates were highest on diets containing the diatom. At high food concentrations, egg production rates were highest on the two phytoplankter diets and their combination. The presence of the ciliate in the diet did not enhance ingestion rate or egg production. Mixed-food diets did not enhance egg production relative to single-food diets. Hence, dietary diversity did not appear to be particularly advantageous for reproduction. Carbon-specific egg production efficiency (EPE; egg production/ingestion) was independent of food concentration and type, and equaled 9%. Egg hatching success was low (mean<30%) and independent of food concentration and type, and egg production rates. Our results are consistent with previous observations that egg production in T. longicornis is enhanced during diatom blooms. However, the relatively low EPE and egg hatching success suggest that reproduction and recruitment in this study were severely constrained by the biochemical composition of the diet, or the physiological condition of the females towards the end of their season of growth in Long Island Sound.     

The influence of fungal food quality on the growth and fecundity of Folsomia candida (Collembola: Isotomidae)

Summary The Basidiomycete fungi Coriolus versicolor and Hypholoma fasciculare were grown in liquid media containing 2, 20, 200 and 2,000 ppm nitrogen (as asparagine) and fed to cultures of Folsomia candida. Collembola feeding on both species of fungi exhibited trends of increased moulting and egg laying rates up to 200 ppm N and an inhibition of growth and fecundity at 2,000 ppm N. The differences in moulting rates between individual treatments were small for both species of fungi and not all the pair wise comparisons of treatments were significantly different. Egg laying rates of collembola fed C. versicolor showed a highly significant response to all levels of N in the growth medium and egg production at 200 ppm N was over three times higher than at 2 ppm N. Collembola fed H. fasciculare showed a less marked fecundity response to the different nitrogen levels and egg production at 200 ppm was approximately 1.5 times higher than at 2 ppm N. Both moulting and egg laying rates were significantly affected by the species of fungus presented as food to the collembola. The patterns of growth and reproduction of starved control groups of F. candida as well as those fed the test fungi demonstrate the adaptability of this species to changes in the quality and quantity of available food.     

Evidence for biochemical limitation of population growth and reproduction of the rotifer Keratella quadrata fed with freshwater protists

The biochemical factors that determine the food quality of protistsfor rotifers are poorly understood. We evaluated populationgrowth rates and egg production of the rotifer Keratella quadratafed with four protist species growing on either an algal ora bacterial diet. The cryptomonad Cryptomonas phaseolus, consideredas a good quality prey, and assays without prey served as controls.Population growth rates and egg numbers of K. quadrata werecorrelated with single biochemical compounds (fatty acids, aminoacids and sterols) of the protists. Feeding on the alga C. phaseolusor the algivorous ciliates resulted in enhanced population growthrates and high egg production by K. quadrata, whereas feedingon bacterivores supported only moderate egg production but nopopulation growth. The rotifers’ egg production was correlatedwith the protist biochemical composition, including polyunsaturatedfatty acids (PUFAs), the sterol desmosterol, ergosterol, stigmastanoland the amino acid leucine. No significant relationships wereobserved between population growth rates of the rotifers andthe protists’ biochemistry, suggesting that populationgrowth and reproduction of K. quadrata may have different nutritionalrequirements. To our knowledge, this is one of the first studiesto analyze a large variety of biochemical compounds to determinethe food quality of protists for a zooplankton predator andthe first study to analyze the biochemical quality of protiststo a rotifer species.     

The effect of dietary amino acid composition on egg production in blue tits

Most studies on the interaction between food supply and reproduction in animals have assumed that energy is likely to be the factor limiting egg number and/or size. In this paper, we investigate whether dietary protein proximately constrains egg production in birds. We provisioned breeding blue tits with two food supplements that differed only in the concentration of five essential amino acids. Birds receiving a supplementary diet containing an amino acid balance close to that required for egg protein formation laid significantly larger clutches (18% greater) than control birds, whereas birds receiving an otherwise identical supplementary diet but without a favourable amino acid balance did not increase egg production. To our knowledge, this is the first demonstration that dietary amino acid composition may limit egg production in free-living birds.     

Development, growth, and egg production of Ageneotettix deorum (Orthoptera: Acrididae) in response to spider predation risk and elevated resource quality

Abstract.  1. Predation risk to insects is often size- or stage-selective and usually decreases as prey grow. Any factor, such as food quality, that accelerates developmental and growth rates is likely to reduce the period over which prey are susceptible to size-dependent predation.
2. Using field experiments, several hypotheses that assess growth, development, and egg production rates of the rangeland grasshopper Ageneotettix deorum (Scudder) were tested in response to combinations of food quality and predation risk from wolf spiders to investigate performance variation manifested through a behaviourally mediated path affecting food ingestion rates.
3. Grasshoppers with nutritionally superior food completed development ≈ 8–18% faster and grew 15–45% larger in the absence of spiders, in comparison with those subjected to low quality food exposed to spider predators. Growth and development did not differ for grasshoppers feeding on high quality food when predators were present in comparison with lower quality food unimpeded by predators. Responses indicated a compensatory relationship between resource quality and predation risk.
4. Surviving grasshoppers produced fewer eggs compared with individuals not exposed to spiders. Because no differences were found in daily egg production rate regardless of predation treatment, lower egg production was attributed to delayed age of first reproduction. Results compare favourably with responses observed in natural populations.
5. Risk of predation from spiders greatly reduced growth, development, and ultimately egg production. Increased food quality counteracts the impact of predation risk on grasshoppers through compensatory responses, suggesting that bottom-up factors mediate effects of spiders.     

The effect of food limitation on gonad development and egg production of the planktonic copepod Calanus finmarchicus

Calanus finmarchicus, one of the dominant copepods species of the North Atlantic, often encounters low food concentration or quality during the reproductive period; however, our knowledge on the effect of these conditions on reproduction processes is scarce. The present study combines experiments with histological observations to describe the response of C. finmarchicus to limited food focussing on (1) oocyte maturation processes, (2) gonad morphology, (3) egg production rates as a function of spawning frequency and clutch size, and (4) the fuelling of egg production by measuring carbon and nitrogen content of the females. In the laboratory, C. finmarchicus females were exposed to 0, 10, 50, 150 or >300 μg C l⁻¹ for several days. To account for food quality and season, reproductive activity was compared in April and July 1999 between females feeding on diatoms or dinoflagellates. The effect of feeding history was studied in February with females fed and starving prior to the experiment. Feeding conditions had severe effects on oocyte maturation process. Hence, egg production varied significantly with food concentration and quality, season and feeding history due to variation in both clutch size and spawning frequency. Clutch size differed by a factor of 2-4 between food limited and well fed females, and is thus an important parameter for modelling egg production. Changes in clutch size were related to changes of the number of maturing oocytes in the females gonads indicating that the latter can be used to precise the prediction of egg production from preserved samples. The proportion of females carrying at least some mature oocytes was relatively high at low food availability. Apparently, these females used internal body reserves as the carbon and nitrogen content decreased significantly under these conditions. These results indicate that C. finmarchicus embarks on the strategy to enable reproduction in all or many females of a population at low rates when feeding conditions are unfavourable.     

Modelling interactions of food quality and quantity in homeostatic consumers

1. Boundaries between growth limitation by carbon (C) and phosphorus (P) in homeostatic heterotrophic consumers such as zooplankton will vary as demands on these two elements vary, as they should at different food quantities. At very low food quantity when production is close to zero, metabolic requirements (for carbon) become more important than growth requirements (for carbon and phosphorus in fixed proportion). Thus, the boundary separating C-and P-limited growth should be at a higher C : P ratio at low food quantity than at high food quantity.
2. A model including both metabolism and growth indicates that consumer growth should differ between foods of high vs. low phosphorus concentration only when food quantity is above a certain level. Thus, two foods might give identical consumer growth rates at low food quantity but give different consumer growth at higher quantity.
3. Solution of the model using parameters based on 2 mm Daphnia compared with a survey of C and P in seston of marine and freshwater sites supports earlier conclusions of the potential importance of food quality constraints on Daphnia growth.     

Endogenous contributions to egg protein formation in lesser scaup Aythya affinis

Lesser scaup Aythya affinis populations have declined throughout the North American continent for the last three decades. It has been hypothesized that the loss and degradation of staging habitats has resulted in reduced female body condition on the breeding grounds and a concomitant decline in productivity. We explored the importance of body (endogenous) reserves obtained prior to arrival on the breeding ground in egg protein formation in southwestern Montana during 2006–2008 using stable‐carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses of scaup egg components, female tissue, and local prey items. From arrival on the breeding grounds through the egg‐laying period, δ¹⁵N values of scaup red blood cells decreased while δ¹³C values became less variable; a pattern consistent with endogenous tissues equilibrating with local (freshwater) dietary sources. In 2006 and 2008, isotopic values for egg albumen and yolk protein indicated that most (>90%) protein used to produce these components was obtained on the breeding grounds. However, in 2007, a year with an exceptionally warm and dry spring, endogenous reserves contributed on average 41% of yolk and 29% of albumen. Results from this study suggest that female scaup can meet the protein needs of egg production largely from local dietary food sources. This highlights the importance of providing high‐quality breeding habitats for scaup. Whether this pattern holds in areas with similar breeding season lengths but longer migration routes, such as those found in the western boreal forest, should be investigated.     

Fermentation of Peptides by Bacteroides ruminicola B(1)4

The maximum growth rate of Bacteroides ruminicola B(1)4 was significantly improved when either Trypticase or acetate and C(4)-C(5) fatty acids were added to defined medium containing macrominerals, microminerals, vitamins, hemin, cysteine hydrochloride, and glucose. The organism was unable to grow with peptides as the sole energy source, but growth yields from glucose were significantly improved when Trypticase was added to batch cultures containing basal medium, acetate, and C(4)-C(5) volatile fatty acids. During periods of rapid growth, very little peptide was deaminated to ammonia, but after growth ceased there was a linear increase in ammonia. Fifteen grams of Trypticase per liter resulted in maximum ammonia production. In glucose-limited chemostats, ammonia production from peptides was inversely proportional to the dilution rate, and 87% of the variation in ammonia production could be explained by retention time in the culture vessel. Chemostats receiving Trypticase had higher theoretical maximum growth yields and lower maintenance energy expenditures than similar cultures not receiving peptide. Cells from the Trypticase cultures contained more carbohydrate, and this difference was most evident at rapid dilution rates. When corrections were made for cell composition and the amount of peptides that were fermented, it appeared that peptide carbon skeletons could be used for maintenance energy. B. ruminicola B(1)4 was unable to grow on peptides alone because it was unable to utilize peptides at a fast enough rate to meet its maintenance requirement.     

Optimal co-allocation of carbon and nitrogen in a forest stand at steady state

Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.     

Interactions of temperature and dietary protein concentration in growth and feeding of Manduca sexta caterpillars

Abstract .Temperature and the protein content of food affect rates of consumption and growth in herbivorous insects in different ways: reduced temperature typically reduces both consumption and growth rates, whereas reduced dietary protein typically increases consumption rate but either reduces or has no effect on growth rate. The interactions between temperature and dietary protein concentration in affecting consumption, growth and efficiency in fifth-instar caterpillars of Manduca sexta were studied, using both short-term (4 h) and long-term (duration of fifth stadium) experiments. The short-term experiments examined constant temperatures between 14 and 42°C, whereas the long-term experiments examined constant temperatures between 18 and 34°C; both experiments considered two levels of dietary protein. In both experiments, caterpillars had significantly higher consumption and frass production rates on low-protein compared with high-protein diets at each test temperature between 18 and 34°C, thereby compensating for the lower diet quality. In contrast, at more extreme temperatures (14 and 42°C) in the short-term studies, consumption and frass production rates were lower on low-protein compared with high-protein diets. As a result, there were substantial interactions between temperature and dietary protein for consumption and frass production rates in the short-term experiments, but not in the long-term experiments. These results suggest that interactions between temperature and dietary protein may emerge because of the failure of compensatory feeding responses at low and high temperatures. It is hypothesized that the failure of compensatory responses is more likely to occur under diurnally fluctuating temperatures than under a constant temperature with the same mean, and it is proposed that interactions between temperature and dietary protein for consumption are relevant to M. sexta and other caterpillars that experience wide diurnal fluctuations in temperature in the field.     

Effects of nitrogen stressed algae on different Acartia species

We studied the reproductive response of two copepod species,Acartia tonsa and Acartia clausii, fed algae with differentC:N ratios (4.5 and 9.1 molar ratios respectively) in orderto investigate the influence of nutritional imbalances on calanoidcopepods egg production. Adult females were incubated with thecryptophyte Rhodomonas sp. at saturating concentrations. Thealgae were cultured under nitrogen depleted and sufficient conditions.Ingestion rates of the animals fed with different algae andtheir response in terms of egg production and hatching successof the eggs were quantified. Both species produced more eggwhen fed with nitrogen-limited algae. Ingestion rates and egghatching differed between species, but were not significantlyaffected by the quality of the food. The only difference betweenthe two species in their reaction to food quality was that A.tonsa increased the number of resting eggs, whereas no restingegg production was observed in A. clausii when fed with nitrogenlimited algae. These results support the recent suggestion thata moderately high prey C:N ratio (10–15) supports a higheregg production than a C:N ratio substantially <10.     

Threshold elemental ratios for carbon versus phosphorus limitation in Daphnia

1. The transition from carbon (C) to phosphorus (P) limited growth in Daphnia depends not only on the C : P ratio in seston, i.e. food quality, but also on food quantity. Carbon is commonly believed to be limiting at low food because of the energetic demands of basal metabolism. The critical C : P ratio in seston (otherwise known as the threshold elemental ratio, TER) above which P is limiting would then be high when food is scarce. 2. A new model that differentiates between the C : P requirements for growth and maintenance is presented that includes terms for both C and P in basal metabolism. At low food the calculated TERs for Daphnia of around 230 are only slightly higher than values of 200 or so at high intake. Seston C : P often exceeds 230, particularly in oligotrophic lakes where phytoplankton concentration is low and detritus dominates the diet, indicating the potential for limitation by P. 3. The analysis highlights the importance of P, as well as C, in maintenance metabolism and the overall metabolic budget, such that food quality is of importance even when intake is low. Further measurements of C and P metabolism at low food, in particular basal respiration and excretion rates, are needed in order to improve our understanding of the interacting roles of food quantity and quality in zooplankton nutrition.