Low Food Levels Research Articles

LIFE-HISTORY RESPONSES TO VARIABLE ENVIRONMENTS: STARVATION AND REPRODUCTION IN PLANKTONIC ROTIFERS

Zooplankton live in environments where resource abundance and quality can vary drastically over time. Success in nonequilibrium environments depends in part on the ability to store energy and control its allocation during periods of extreme food limitation or outright starvation. Starvation resistance is one measure of the ability of a species to persist when energy intake is less than energy expenditure. The life-history responses of nine species of planktonic rotifers to food deprivation were compared using cohort life- table experiments. Allometric patterns of energy storage and respiration rate lead to the prediction that larger species should have greater starvation resistance than smaller species. Contrary to this prediction, when rotifers were acclimated to high resource levels and starved as young adults, body mass did not predict starvation time. Rather, there was a trade-off between survival and reproduction during starvation. Some species did not reproduce during starvation and had high starvation times (up to 5.0 d). Other species maintained or increased fecundity relative to fed controls and had low starvation times (as low as 0.4 d). Species with more rapid senescence when fed tended to have shorter starvation times. However, the interspecific-trade-off between survival and reproduction remained after removing the effect of control survivorship. Differences in life-history responses to starvation may be critical in determining competitive outcome and community structure in variable environ- ments. When acclimated to low food levels prior to food deprivation, simulating conditions of declining resource abundance in nature, rotifer starvation time decreased. Juveniles had longer starvation times than adults, further supporting the idea that allocating energy to reproduction decreases starvation resistance.

Effects of pollution on the scope for growth of mussels ( Mytilus galloprovincialis) from the Venice Lagoon, Italy

Mussels ( Mytilus galloprovincialis) were sampled from six sites in the Venice Lagoon in May 1993 for measurement of their physiological responses (clearance rate, absorption efficiency and respiration rate) under standard laboratory conditions. Scope for growth (SFG) calculated for a ‘standardised ration level’ (0.4 mg particulate organic matter l −1) reflected the underlying pollution induced stress caused by the toxicants accumulated in the body tissues. Mussels from Chioggia had the highest SFG (16 J g −1 h −1) with slightly lower rates at Lio Grande, Crevan and Alberoni (9–10 J g −1 h −1), whereas mussels from the inner part of the lagoon (Salute and Canale Vittorio Emanuele) had significantly reduced SFG values (2 and −4 J g −1 h −1) which indicated severe pollution induced stress. There were significant negative correlations between SFG and the tissue concentrations of petroleum hydrocarbons, PCBs, DDT and HCH, but there were no significant correlations between SFG and the tissue concentrations of metals (Cd, Co, Cr, Cu, Fe, Hg, Mn, Fe, Ni, Pb and Zn). SFG was also calculated on the basis of measured POM levels prevailing at each site and this demonstrated that the inhibitory effects of toxicants on the growth of mussels living in the more polluted inner sites may be partially offset by the positive effects of eutrophication and the higher food/ration levels at these sites. There were also differences in the physiological responses and SFG of mussels from the two main cultivation sites within the Venice Lagoon (i.e. Alberoni < Chioggia) and these reflected the higher contaminant levels and lower food levels at Alberoni.

Predator–prey instability: individual‐level mechanisms for population‐level results

1. In previous work we established that increasing temperature led to a destabilization of the population dynamics of the invertebrate carnivore Mesostoma ehrenbergii and its prey Daphnia pulex, which ultimately resulted in the local extinction of Daphnia at higher temperatures. Two mechanisms are proposed to explain the population‐level phenomena: (1) quantitative changes in carnivore vital rates with increasing temperature led to stronger functional and numerical response and (2) qualitative changes in the dynamic allocation of energy to reproduction by the predator with increasing temperature introduces inverse density dependence in the predator’s response. 2. The growth of individual M. ehrenbergii was monitored under various food conditions to determine the effect of two temperatures (18 and 24 °C) and five food levels on rates of growth, prey consumption and reproduction and on reproductive allocation patterns. 3. The first mechanism was supported by both higher consumption rates (stronger functional response) and faster growth rates with earlier age at maturity and shorter generation time (stronger numerical response). 4. Evidence for mechanism two was also provided by an alteration of the reproductive allocation pattern with temperature. Viviparous (subitaneous) eggs were more likely to be produced by this carnivore at low food levels at 24 °C, while at 18 °C, high food levels were required before individuals made this switch. This shift actually introduces inverse density dependence in the predator’s numerical response which is highly destabilizing. 5. Based on the results of this study, the differential effect of M. ehrenbergii on the dynamics and structure of its D. pulex prey populations can be attributed to changes in both physiological rates and reproductive allocation patterns with temperature.

Growth uncoupling and the relationship between shell growth and metabolism in the soft shell clam Mya arenaria

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 158:177-189 (1997) - doi:10.3354/meps158177 Growth uncoupling and the relationship between shell growth and metabolism in the soft shell clam Mya arenaria Daniel E. Lewis, Robert M. Cerrato* Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794-5000, USA *Addressee for correspondence.E-mail: rcerrato@notes.cc.sunysb.edu The goal of this study was to investigate whether shell growth reflects metabolic rate rather than overall or somatic tissue production in the soft shell clam Mya arenaria. This was accomplished by conducting laboratory experiments exposing clams to different levels of food, immersion period and temperature in order to uncouple shell and soft tissue growth, and then determining if shell growth and oxygen consumption remained positively correlated. Soft shell clams were raised in a simulated intertidal system for 2 to 3 wk to acclimate them to experimental conditions. After acclimation, oxygen consumption, a measure of metabolic activity, ingestion rate, a measure of energy intake, shell growth and soft tissue growth were estimated for all individuals. Tissue and shell growth were successfully uncoupled in all experiments. In the 2 experiments where significant changes in shell growth, soft tissue growth and oxygen consumption occurred, shell growth was positively correlated to oxygen consumption. Soft tissue growth was either uncorrelated or negatively correlated to oxygen consumption and shell growth. Shell transparency in thin sections increased with metabolic rate, while shell growth line clarity decreased in those treatments most stressful to the clams (i.e. low food level and high temperature). Our results are consistent with the hypothesis that shell growth is coupled to metabolic activity and is not a measure of somatic tissue production. We suggest that information on physiological rate processes is recorded in the shell of bivalves, and that growth line patterns may be used to reconstruct metabolic rates from field collected individuals. Microgrowth · Shell growth · Uncoupling · Metabolic rate · Bivalve Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 158. Publication date: November 17, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Tail spine length in the Daphnia galeata complex: costs and benefits of induction by fish

We studied the combined effect of fish kairomones and food conditions on the relative tail spine length (RTL) of five Daphnia taxa, and the interaction of these factors with the vertical distribution of the daphnids. The experiment was done in two large-scale indoor containers, the so-called plankton towers in Plön, Germany. We conducted a competition experiment in which food level and the presence of fish chemicals and later fish were varied. A strong response of RTL to fish kairomones (e.g., longer tail spines), significant differences in RTL between species, but no differences in RTL with water depth were found. Further, we observed that these Daphnia taxa produced a higher RTL only under high food conditions. This suggests that there is a cost related to the production of longer tail spines. In a preliminary study in lake Plußsee, we found that Daphnia had longer average RTL than in the towers. Further, we noted significant differences in RTL between the two sampling dates, which may be related to a lower food level. We also detected a strong inverse correlation between RTL and depth. We discuss the implications of these findings for the co-existence of co-occurring Daphnia species and their hybrids.

Effects of food level and straw bedding during pregnancy on sow performance and responses to an ACTH challenge

The interactive effects of straw provision and food level during pregnancy on performance, and the adrenal response to a challenge with ACTH (as a measure of chronic physiological stress) were studied over two parities. Ninety-six gilts were allocated to a 2 × 2 factorial experiment comparing food level (L = 1.6–1.8 kg (20–23 MJ DE) day−1; H = 3.0–3.2 kg (38–40 MJ DE) day−1) and the provision of straw (S = straw, N = no straw) during pregnancy. All gilts were loose housed in groups of 6 with individual feeding stalls. Significantly more N sows failed to start the second parity than S sows. H sows gained more weight and back fat over both pregnancies than L sows. There was a tendency for LS sows to gain more weight and back fat over both pregnancies than LN sows. Litter performance was not influenced by the straw treatment. HN sows had the highest concentration of basal cortisol, but no treatment effects were found in increase of salivary cortisol concentrations in response to the ACTH challenge in second parity sows. It is concluded that provision of straw may buffer the adverse effects of a low food level on weight and back fat gain in group-housed pregnant sows, but has no effect on reproductive performance. The cortisol data suggest differences between treatments to be of an acute, rather than a chronic nature.

Effects of algal and larval densities on development and survival of asteroid larvae

Effects of larval and algal culture density and diet composition on development and survival of temperate asteroid larvae were studied in the laboratory at Santa Cruz, California, USA, during summer and fall of 1990. Larvae of Asterina miniata were reared at two densities, 0.5 or 1.0 ml-1, and fed one or two species of cultured phytoflagellates — Dunaliella tertiolecta alone or mixed with Rhodomonas sp. — at three concentrations of 5x102, 5x103, and 5x104 total cells ml-1. Algal concentration strongly influenced larval development; however, larval density also had a marked effect. Development progressed further with increasing algal concentration. Larval growth and differentiation were sometimes uncoupled; i.e., growth measures were directly related to food level, while differentiation indicators were less so. At the lowest food level, growth was negative and differentiation was arrested at early precompetent stages; these larvae never formed juvenile rudiments or brachiolar attachment structures. Development times of larvae given more food ranged from 26 to 50 d and depended directly on food availability. Development time to metamorphosis at the highest food concentration was similar for siblings fed D. tertiolecta alone or mixed with Rhodomonas sp. In contrast, when food level was an order of magnitude lower, larvae fed the algal mixture metamorphosed significantly earlier than larvae fed the unialgal diet. This suggests interactive effects of food quantity and food quality. Survival was little affected by larval or food density, except at the lowest ration. Feeding experiments in well-controlled laboratory conditions are useful to predict and compare the physiological or developmental scope of response of larvae to defined environmental factors; however, results from such studies should not be extrapolated to predict rates and processes of larval development in nature.

Effects of prey density on the growth and mortality of weakfish Cynoscion regalis (Bloch and Schneider) larvae: Experiments in field enclosures

Weakfish larvae ( Cynoscion regalis (Bloch and Schneider)) were reared in large enclosures (1.4 m 3) deployed near adult spawning grounds in Delaware Bay, USA. Larvae were fed wild zooplankton, and experimental prey levels encompassed naturally occurring densities (10, 100 and 1000 prey items 1 −1). Separate experiments were conducted for early-stage and late-stage larvae in 1990, and the late-stage experiment was repeated in 1994 using an improved statistical design. In 1990, there was a trend toward elevated growth rates with increasing prey levels. For early-stage larvae, larval notochord lengths ( NL) varied with prey density and ranged from 3.65 to 4.30 mm. Larval dry weight ( W) also varied with prey density and ranged from 39.5 to 61.3 μg. Calculated growth rates varied accordingly. Growth among late-stage larvae followed a similar trend, and NL ranged from 4.65 to 5.25 mm, while W ranged from 73.7 to 139.7 μg. In the repeated late-stage experiment (1994), a nested ANOVA showed significant effects of prey level on growth. Notochord length (4.33 mm) and dry weight (72.3 μg) were significantly depressed at the lowest prey density. Rates of instantaneous mortality were variable among individual enclosures and were not related to prey density in either year. Our results differ from previous field enclosure studies, which have shown little effect of prey density on the growth of larval fish. By contrast, our data indicate that low food levels may result in depressed growth rates among weakfish larvae in nursery areas like Delaware Bay. These lower growth rates may lead to an increased duration of larval development, potentially culminating in high mortality from increased exposure to predation or from advection from suitable juvenile habitats.

Genetic compatibility between sexual and clonal genomes in local populations of the hybridogeneticRana esculenta complex

Hybridogenetic species possess a hybrid genome: half is clonally inherited (hemiclonal reproduction) while the other half is obtained each generation by sexual reproduction with a parental species. We addressed the question of whether different hemiclones of the hybridogenetic water frogRana esculenta are locally adapted for genetic compatibility with their sexual parental hostRana lessonae. We artificially crossedR. esculenta females of three hemiclones (GUT1, GUT2 and GUT3) from a pond near Gutighausen, Switzerland and one hemiclone (HEL1) from near Hellberg, Switzerland each toR. lessonae males from both populations. We also created primary hybrids by crossing the sameR. lessonae males from both populations toR. ridibunda females from Poznan, Poland (POZ). Tadpoles were then reared in the laboratory at two food levels to assess their performance related to early larval growth rate, body size at metamorphosis and length of the larval period. Tadpoles from hemiclones GUT1, GUT3 and POZ had higher growth rates than those from hemiclones GUT2 and HEL1 at the low food level, but at the high food level all growth rates were higher and diverged significantly between hemiclones GUT2 and HEL1. Tadpoles from the intrapopulational crosses GUT2 × GUT and HEL1 × HEL were larger at metamorphosis than those from the interpopulational crosses GUT2 × HEL and HEL1 × GUT. A high food level increased the size at metamorphosis in all tadpoles. A high food level also decreased the days to metamorphosis and tadpoles from GUT1, GUT3 and POZ had the shortest larval period whereas those from GUT2 and HEL1 had the longest. These results indicate that the differential compatibility of clonal genomes may play an important role in hybridogenetic species successfully using locally adapted sexual genomes of parental species and that interclonal selection is likely important in determining the distribution of hemiclones among local populations.

Vertebral number in adders,Vipera berus: direct and indirect effects on growth

Given the importance of body size, and thereby growth rate, for many reproductive parameters in snakes, morphological traits conferring an advantage in terms of growth may be important targets of selection. Studies have demonstrated effects of vertebral number of growth rate in garter snakes. In this study effects of total number of body vertebrae and of number of abnormal body vertebrae (obtained by counting number of ventral scutes and number of abnormal scutes) on growth rate in free-ranging male and female adders,Vipera berus(L.), are examined by calculating directional performance gradients (estimating linear effects) and stabilizing performance gradients (estimating curvilinear effects). After controlling for body size (SVL) female adders demonstrated a significant positive directional gradient for vertebral number, and a significant interaction between body size and vertebral number, showing that females with more vertebrae have higher size-specific growth rates, and that this effect is strongest among small, fast growing individuals. Females also showed a weak stabilizing effect of abnormal vertebrae. Males, on the other hand, showed a positive directional gradient for number of abnormal vertebrae, whereas no effect of vertebral number was observed. Indirect effects of the same variables were evaluated by use of path analysis. Generally, indirect effects were weak and did not substantially increase the amount of explained variance in growth rate. Field data showed that the correlation between vertebral number and growth rate in females was stronger in years with higher overall growth rate. To evaluate whether vertebral number and food availability show an interactive effect I used captive born juvenile adders in an experiment with two different food levels. The experiment confirmed the field data. No relationship between vertebral number and growth was observed in the low food level group, whereas in the high food level group a significant positive correlation was demonstrated. Finally, the heritability of vertebral number was examined using a mother-offspring regression and a full-sib analysis. The estimated heritabilities were 0·30 and 0·39, respectively. From these results it is concluded that both vertebral number and abnormal vertebral number may significantly affect growth in adders, but that this effect may differ between sexes and among years.

Vertebral number in adders, Vipera berus: direct and indirect effects on growth

Abstract Given the importance of body size, and thereby growth rate, for many reproductive parameters in snakes, morphological traits conferring an advantage in terms of growth may be important targets of selection. Studies have demonstrated effects of vertebral number of growth rate in garter snakes. In this study effects of total number of body vertebrae and of number of abnormal body vertebrae (obtained by counting number of ventral scutes and number of abnormal scutes) on growth rate in free-ranging male and female adders, Vipera berus (L.), are examined by calculating directional performance gradients (estimating linear effects) and stabilizing performance gradients (estimating curvilinear effects). After controlling for body size (SVL) female adders demonstrated a significant positive directional gradient for vertebral number, and a significant interaction between body size and vertebral number, showing that females with more vertebrae have higher size-specific growth rates, and that this effect is strongest among small, fast growing individuals. Females also showed a weak stabilizing effect of abnormal vertebrae. Males, on the other hand, showed a positive directional gradient for number of abnormal vertebrae, whereas no effect of vertebral number was observed. Indirect effects of the same variables were evaluated by use of path analysis. Generally, indirect effects were weak and did not substantially increase the amount of explained variance in growth rate. Field data showed that the correlation between vertebral number and growth rate in females was stronger in years with higher overall growth rate. To evaluate whether vertebral number and food availability show an interactive effect I used captive born juvenile adders in an experiment with two different food levels. The experiment confirmed the field data. No relationship between vertebral number and growth was observed in the low food level group, whereas in the high food level group a significant positive correlation was demonstrated. Finally, the heritability of vertebral number was examined using a mother-offspring regression and a full-sib analysis. The estimated heritabilities were 0·30 and 0·39, respectively. From these results it is concluded that both vertebral number and abnormal vertebral number may significantly affect growth in adders, but that this effect may differ between sexes and among years.

The effect of fine grinding or sodium hydroxide treatment of wheat, offered as part of a concentrate supplement, on the performance of lactating dairy cows

AbstractA study was conducted to examine the effect of fine grinding or sodium hydroxide treatment of wheat, and increasing concentrate food level, on milk production. Two concentrates based either on ground wheat (450 g/kg, GW) or sodium hydroxide treated wheat (500 g/kg, SW) were offered at four concentrate levels of 2·5, 5·0, 7·5 and 10·0 kg dry matter (DM) per day to 24 dairy cows in a three-period, change-over design experiment. On average, across all concentrate food levels, silage DM intake was significantly (P &lt; 0·01) higher with the SW concentrates, reflecting a significantly lower silage substitution rate with SW relative to GW concentrates (P &lt; 0·01). Milk yield was also significantly higher with the SW concentrates (P &lt; 0·05), although marginal responses to increased concentrate food level were similar (P &gt; 0·05). Milk protein concentration increased linearly with increasing concentrate food level (P &lt; 0·001), with a significantly greater response with the GW relative to the SW concentrate (0·59 v. 0·24 g/kg (P &lt; 0·05) increase per kg additional concentrate). However, milk protein concentration was also significantly lower with the GW concentrate at low food levels (P &lt; 0·05). Milk fat concentrations were similar with the two concentrate types with significant reductions in milk fat concentration with increasing concentrate food level (P &lt; 0·05). Blood urea (P &lt; 0·001) and β-hydroxybutyrate (P &lt; 0·05) concentrations were significantly lower in animals offered the SW concentrate. Apparent digestibility coefficients were unaffected by either concentrate type or concentrate food level (P &gt; 0·05), although modified acid-detergent fibre apparent digestibility was significantly reduced with increasing concentrate food level (P &lt; 0·001). Results indicate that, at similar levels of concentrate feeding, silage intake and milk yield were significantly greater with SW compared with GW concentrates (P &lt; 0·05 or greater). Increases in milk protein concentration with increasing concentrate food level were significantly greater with GW than with SW concentrates (P &lt; 0·05).

Uptake, distribution and excretion of magnesium inOreochromis mossambicus: dependence on magnesium in diet and water

The euryhaline Mozambique tilapia (Oreochromis mossambicus) shows a more marked ability to adapt to low magnesium levels in food and water than many other fish species. Nonetheless, the internal distribution of magnesium is altered under low-magnesium conditions. The amount of magnesium in scales, and to a lesser extent the vertebral bone, is reduced and hence serve as magnesium reservoirs. The magnesium concentration of muscle is only marginally reduced by low external magnesium, suggesting that magnesium is partitioned to ensure normal muscle functioning. The water magnesium is of vital importance, as exposure to low-magnesium water markedly deminished the ability of tilapia to adapt to low-magnesium feeding. However, magnesium intake from the water, eithervia the integument or drinking, does not increase in lowmagnesium fed fish, despite an increased opercular chloride cell density. The growth related magnesium accumulation of tilapia under low-magnesium conditions approximates the total intake of the element (from the food and from the water), indicating that magnesium losses are minimized and that the magnesium absorption from the gastrointestinal tract may be highly efficient and very important.

The developmental rate ofEuphausia crystallorophias larvae in Ellis Fjord, Vestfold Hills, Antarctica

Euphausia crystallorophias is the dominant Zooplankton species in the neritic seas of Antarctica, where it occurs in similar abundances to those ofEuphausia superba in more offshore areas. Despite its great abundance and probable ecological significance, few details are known of this species' development, life history and ecology. This study found thatE. crystallorophias spawned in Ellis Fjord from late November to early December and completed its larval development under the sea ice during the Antarctic winter. The mean time forE. crystallorophias eggs to develop to furcilia stage VI was 235.5 days, which is virtually identical to the developmental time already reported in the laboratory, but almost twice that ofE. superba. This slow development rate is likely to be due either to the low water temperatures (<0°C) in whichE. crystallorophias lives, or to low levels of food being available over winter.

Context-dependent kin discrimination in larvae of the marbled salamander, Ambystoma opacum

Abstract. The context=dependent nature of kin discrimination was examined in larvae of the marbled salamander, Ambystoma opacum. In a graphical analysis, relationships between aggression and social and ecological factors were explored. Increasing relatedness between individuals was predicted to reduce aggressive encounters, except when correlates of fitness (e.g. body size) differed between individuals and/or when environmental conditions increased the profitability of aggression. These predictions were tested in a multifactorial experiment in which genetic relatedness, food quantity, relative body size and the potential for physical encounters were manipulated. The effects of each factor and their interactions on larval aggression and correlates of fitness (body size and timing of metamorphosis) were examined. Frequency of aggression depended upon an interaction between kinship, food quantity and relative larval size. Thus, in accordance with our graphical model, kinship=based aggression varied in a context=dependent fashion. Low food level reduced body size and increased the time required to reach metamorphosis. When larvae were not separated by a barrier, growth was inhibited in the smaller, but not the larger, individual of a pair. Moreover, the larval period was significantly shortened in chambers where barriers were absent. Neither kinship nor initial relative size, either alone or in combination with the other factors, significantly affected larval performance. These results show that the frequency of larval aggression in A. opacum depends upon prevailing social and environmental conditions; however, the influence of aggression on larval growth and metamorphosis remains unclear. Previous inferences regarding context=dependent kin discrimination in amphibians have been based on independent, interspecific studies or concurrent, intraspecific comparisons between individuals of alternative morphologies. The present results provide direct evidence that simultaneous variation in multiple socioecological factors may alter aggression in a single amphibian species in which kin discrimination is known to occur.

Viability costs of condition-dependent sexual male display in a drumming wolf spider

According to the conditional handicap models females use male ornaments as honest signals of male viability. The assumptions for honest signalling are that the traits are costly and that they reflect male phenotypic condition, and hence optimal trait size is largest in the most viable males. However, experimental evidence for the costs of signalling are scarce. In this study we experimentally tested whether acoustic signalling, drumming, in a wolf spider Hygrolycosa rubrofasciata is a condition dependent, costly trait, and thus offers an honest signal of quality to females. Males of this species court females by drumming dry leaves with their abdomen. Females prefer to mate with males of high drumming rate, but body mass of males does not affect female choice. We manipulated phenotypic condition of males by keeping them in high, intermediate and low food levels. Males in a high food level treatment maintained their drumming rate at a high level, while males with intermediate and low food levels exhibited a reduction in drumming rates. Thus, phenotypic condition of the males affects their sexual signalling. We induced another set of males to increase their drumming activity by presenting females in proximity. These males suffered higher mortality and lost significantly more weight than other males, confirming that drumming is costly. However, within the increased treatment group males that drummed most actively survived better than less active males. Thus, males vary in their ability to bear the costs of drumming, which suggests that drumming is an honest signal of male quality (= conditional handicap) for females.

Competition between cohorts of juvenile Daphnia magna: a new experimental model

Competition between two cohorts of juvenile Daphnia magma STRAUS was studied in a newly developed culture system, which allowed the cohorts to share resources while separated. The cohorts came from consecutive broods of a common mother cohort. Two different feeding regimes were applied: continuous feeding and daily pulses. Through body growth the cohorts created a situation of stable grazing pressure, where mortality was balanced by further growth. The larger animals appeared somewhat better competitors at constant low food levels, whereas smaller animals performed best at fluctuating food. On the basis of model simulations by KOOIJMAN et al. (1989) body lengths were expected to converge, causing synchronisation of reproduction and thereby internally generated population oscillations. However, the experiments showed almost parallel growth curves, i.e. body sizes did not converge. Under the present conditions this pattern is best described by a linear growth model for juveniles. Reproduction was substantially delayed and no indication of synchronisation was found.

Naupliar and copepodite growth and survival of two freshwater calanoids at various food levels: Demographic contrasts, similarities, and food needs

Various demographic responses of Tropodiaptomus spectabilis and Metadiaptomus meridianus (Copepoda: Calanoida) were measured at 17°C at seven food levels of Cryptomonas (0.05–2.5 mg C liter−1). In both species, naupliar, copepodid, and total postembryonic durations declined monotonically, while survivorship, fecundity, and adult size increased asymptotically with rising food supply. M. meridianus matured faster than T. spectabilis up to ∼0.5 mg C liter−1, and its development responses were saturated at lower food levels (∼0.35 mg C liter−1). High naupliar mortality led to lower overall survival in M. meridianus (maximally 40%) than in T. spectabilis (90%). Survival asymptotes varied between ∼0.35 and 0.5 mg C liter−1. Clutch sizes were 5‐fold larger in M. meridianus than in T. spectabilis. Although respective regression estimates of food threshold levels for maturation (11 and 5), egg production (18 and 60), and population increase (46 and 79 µg C liter−1) in T. spectabilis and M. meridianus did not differ, the derived estimates of interspecific equivalence for egg production and population increase (82 and 165 µg C liter−1) suggest that T. spectabilis may be the better competitor below these food levels.

Combined effects of 3,4-dichloroaniune and food concentration on life-table data of two related cladocerans, Daphnia Magna and Ceriodaphnia Quadrangula

Life-table relations of two seasonally competing cladocerans, Ceriodaphnia quadrangula and Daphnia magna, were investigated, using 3,4-dichloroaniline (DCA) as stressor. Screening tests revealed that Ceriodaphnia was about one order of magnitude more sensitive to DCA than Daphnia. Therefore, in Daphnia life-table experiments were performed at 5, 20 and 80 μg/L DCA, whereas in Ceriodaphnia 2, 5 and 10 μg/L DCA were applied. Each concentration was combined with three food levels (1 ∗10 3, 5 ∗10 3, and 2.5 ∗10 4 cells/mL Scenedesmus subspicatus). Reproduction was found to be the most sensitive parameter to DCA. In comparison to controls, about the same egg numbers were produced but failed to hatch. Overall, reproduction was more affected at lower food levels. At low food levels, DCA delayed the first reproduction only in Daphnia. In contrast to Ceriodaphnia, growth and filtration rates of Daphnia were inhibited. DCA decreased the survival probability only in Ceriodaphnia. Consequently, the decrease in intrinsic rate of natural increase in Daphnia, evoked by DCA, was due to the inhibition in reproduction rather than to higher mortality, whereas in Ceriodaphnia both lower survival and lower reproduction contributed to a strong decline in intrinsic rate at DCA concentrations higher than 5 μg/L.

Exploitation Competition in Mobile Grazers: Trade‐offs in Use of a Limited Resource

Mechanisms underlying competitive effects between two species of marine snails (Tegula aureotincta and Tegula eiseni) were examined; results indicated that the interaction involved exploitation of microalgae and not interference. A behavioral experiment demonstrated that an individual of either species was less likely to feed on a recently grazed patch when the prior forager was T. aureotincta. Both snails responded only to the amount of food available on a patch, and, except at very low food levels, the likelihood that a snail would remain to feed was identical for the two species across microalgal densities. Where food was scarce, T. eiseni was more likely than T. aureotincta to feed in that patch. Density—dependent effects on microalgal standing stock differed between the snails; at low grazer densities, T. aureotincta depressed food levels to a comparatively greater degree, whereas at high grazer densities, T. eiseni had a relatively greater effect. The relative growth performance of the snails differed as a function of available food: T. aureotincta had superior body growth at high resource densities, while T. eiseni grew comparatively better at low microalgal densities. Differences in effects on and growth responses to microalgae arose from dissimilar foraging characteristics of the snails. Compared with T. eiseni, T. aureotincta moved more rapidly while feeding, harvested twice as great an area per unit time, but was unable to crop microalgae as close to rock surfaces. While T. eiseni grazed a smaller area per unit time, it removed relatively more microalgae per area grazed. These differing suites of foraging attributes may represent a trade—off to harvest food at different densities: cropping closer to the substratum (area intensive grazing) yields more food when microalgae are scarce, but precludes grazing a larger area per unit time (area extensive grazing) through fast movement, which provides a greater food intake when microalgae are abundant. If an area extensive (moves faster)—intensive (digs deeper) trade—off is a common feature of benthic grazer systems, knowledge of this aspect could provide great predictive insight into the patterns and causes of density dependence arising from exploitation.