Low Food Levels Research Articles

Factors controlling juvenile growth and population structure of the starfishAsterias rubensin intertidal habitats: field and experimental approaches

The dynamics of intertidal populations of the starfishAsterias rubens, living in contrasted habitats and over a broad geographical range, were studied from March 2000 to November 2002 using modal analysis. As only 1 juvenile (first year after recruitment) and 1 adult (subsequent years) modes could be distinguished; only juvenile growth was quantified. Concomitantly, experiments were carried out to test several factors assumed to influence juvenile growth: food quantity and quality, emersion, salinity variations and temperature. Three different juvenile growth patterns were evidenced: (1) a fast and protracted growth linked to high food availability and lack of disturbance; (2) a winter cessation of growth likely due to a seasonal increase of emersion-related stress and salinity variations; and (3) disrupted juvenile dynamics, which was encountered in 2 populations. In the first one, estuarine salinity conditions limited growth and, combined with food depletion, led to the extinction of the population. In the second one, wave action confined most of the population to a restricted area with low food levels. In the third scenario, intraspecific competition for food was probably at the source of an unusual growth pattern in which most juveniles did not grow while a small proportion achieved a medium growth rate.

Early food and social environment affect certain behaviours but not female choice or male dominance in bluefin killifish

Environmental conditions during development can have long-lasting effects on morphology, physiology and behaviour. In contrast to human personality, our understanding of how the early environment influences the development of animal personality is limited. In bluefin killifish, Lucania goodei, a male’s mating behaviour is highly repeatable through time and across interactions. To examine whether these stable behavioural types are influenced by early nutritional and social conditions, we reared individuals on either high- or low-food levels in combination with either one adult male, one adult female or no adult. Individuals reared on high food were larger at the earliest measurements and remained so as adults. The high-food treatment also had the highest mortality, particularly if individuals were reared with an adult male. Despite these growth and survival consequences, the food and social treatments did not affect whether males were preferred by females or became dominant as adults. While the outcome of social interactions was not influenced by the early food and social environment, particular mating behaviours were; males reared on low food were more aggressive to females and showed more courtship, and males reared without an adult were more aggressive to females than those reared with either an adult female or male. Our results suggest that some of the behaviours that contribute to a male’s behavioural type within the mating context and that influence mating success in this species are sensitive to aspects of the early environment.

Potential synergistic effects of microcystins and bacterial lipopolysaccharides on life history traits of Daphnia galeata raised on low and high food levels

Metastudies have found no consistent effects of the cyanobacterial toxin microcystin on Daphnia, and there are discrepancies between field observations and experiments. Confounding factors include absence or presence of alternative high quality food or the presence of bioactive compounds, other than microcystins in cyanobacteria. Of specific interest are lipopolysaccharides (LPS) on the outer cell wall. LPS may have a number of biological effects, including reduced detoxication of microcystins in plants and animals. When grazing seston in the field, filterfeeders take up heterotrophic bacteria attached to cyanobacteria, as well as free-living bacteria. The LPS produced by heterotrophic bacteria have been shown to be much more harmful than cyanobacterial LPS. We performed two experiments in which we tested for potential synergistic effects between bacterial LPS and microcystins. Full-factorial experiments separated the main effects and interactions between (i) food quantity as well as food quality (addition of the green alga Scenedesmus), (ii) presence or absence of strains that vary in amount and composition of microcystins (microcystin free strain NIVA-CYA43, moderate microcystin producing strain NIVA-CYA140 and high microcystin producing strain PCC7820), and (iii) presence or absence of bacterial LPS on different life history traits of Daphnia galeata. We measured juvenile growth rate, age and size at first reproduction, death before first reproduction and standard carbon content of Daphnia. From the experiments we conclude that microcystin-producing Microcystis had deleterious effects on the life history of D. galeata, but especially when the availability of high quality green algal food was limited in comparison to the supply of microcystin producing strain PCC7820. In the experiment in which PCC7820 was used as microcystin-producing strain, addition of LPS lowered SCC of Daphnia, but had no effects on other life history parameters. The interaction between Microcystis strain, Microcystis concentration and LPS was highly significant in case of PCC7820, but not in case of CYA-140, indicating that the effects of LPS and its interactions with microcystin on Daphnia life history were strongly context dependent.

Specific insulin-like peptides encode sensory information to regulate distinct developmental processes

An insulin-like signaling pathway mediates the environmental influence on the switch between the C. elegans developmental programs of reproductive growth versus dauer arrest. However, the specific role of endogenous insulin-like peptide (ILP) ligands in mediating the switch between these programs remains unknown. C. elegans has 40 putative insulin-like genes, many of which are expressed in sensory neurons and interneurons, raising the intriguing possibility that ILPs encode different environmental information to regulate the entry into, and exit from, dauer arrest. These two developmental switches can have different regulatory requirements: here we show that the relative importance of three different ILPs varies between dauer entry and exit. Not only do we find that one ILP, ins-1, ensures dauer arrest under harsh environments and that two other ILPs, daf-28 and ins-6, ensure reproductive growth under good conditions, we also show that daf-28 and ins-6 have non-redundant functions in regulating these developmental switches. Notably, daf-28 plays a more primary role in inhibiting dauer entry, whereas ins-6 has a more significant role in promoting dauer exit. Moreover, the switch into dauer arrest surprisingly shifts ins-6 transcriptional expression from a set of dauer-inhibiting sensory neurons to a different set of neurons, where it promotes dauer exit. Together, our data suggest that specific ILPs generate precise responses to dauer-inducing cues, such as pheromones and low food levels, to control development through stimulus-regulated expression in different neurons.

Calcein staining of calcified structures in pearl oyster Pinctada margaritifera and the effect of food resource level on shell growth

Marine mollusc shell growth has been widely measured using fluorochrome marking. In order to test the efficiency and reliability of calcein staining on Pinctada margaritifera shells and pearls, the present study examined two administration methods, different concentrations and several immersion times. Immersion in a 150 mg L −1 calcein solution for 12 h to 24 h appeared to be the best method for marking P. margaritifera shells. For pearl marking, injection of a 200 mg L −1 calcein solution into the pearl pouch was the optimal method. Calcein marking was then used to measure the influence of food resource levels on the shell growth. Groups of 23-month-old P. margaritifera were fed at three trophic levels for two months. The two highest food levels tested (6000 cell mL −1 and 15 000 cell mL −1) induced uniform growth between the dorsal and ventral sides of shell, whereas the lowest food level (800 cell mL −1) induced greater growth on the dorsal side. Shell deposits from the ventral side were observed using a scanning electron microscope, revealing that the difference of the trophic level over two months had modified the thickness of the aragonite tablets formed. These results showed that the trophic level is a major factor conditioning P. margaritifera development.

Experimental assessment of the probabilistic maturation reaction norm: condition matters

The probabilistic maturation reaction norm (PMRN) describes an individual's probability of maturing at a given age as a function of size and other relevant phenotypic traits. Population-level shifts in the PMRN are often interpreted to indicate genetic as opposed to phenotypic changes in maturation in fish. Inferences derived from trends in the PMRN have been challenged, warranting an experimental assessment of the method. This was accomplished in a laboratory experiment using zebrafish (Danio rerio). Fish were reared under different food levels to induce variation in growth and maturation. Plasticity in maturation was not entirely captured by the demographic age- and length-based PMRN. Adding condition to the PMRN captured a greater amount of environmental variation in maturation probability. Nevertheless, significant differences in the PMRNs among the food levels remained after accounting for the influences of age, size and condition on maturation probability indicating plasticity of the PMRN. This was particularly pronounced between fish held on low food levels as compared with fish experiencing abundant resources, with the latter experiencing higher size-specific maturation probabilities. Our analysis emphasizes the need for incorporating salient physiological traits influencing maturation, such as condition, to make accurate inferences about documented shifts observed in the position of PMRNs on maturation trends in wild fish stocks.

Influence of low and decreasing food levels on Daphnia-algal interactions: Numerical experiments with a new dynamic energy budget model

Based on numerical experiments with a new physiologically structured population model we demonstrate that predator physiology under low food and under starving conditions can have substantial implications for population dynamics in predator–prey interactions. We focused on Daphnia-algae interactions as model system and developed a new dynamic energy budget (DEB) model for individual daphnids. This model integrates the κ-rule approach common to net assimilation models into a net-production model, but uses a fixed allocation of net-productive energy in juveniles. The new DEB-model agrees well with the results of life history experiments with Daphnia. Compared to a pure κ-rule model the new allocation scheme leads to significant earlier maturation at low food levels and thus is in better agreement with the data. Incorporation of the new DEB-model into a physiologically structured population model using a box-car elevator technique revealed that the dynamics of Daphnia-algae interactions are highly sensitive to the assumptions on the energy allocation of juveniles under low food conditions. Additionally we show that also other energy allocation rules of our DEB-model concerning decreasing food levels and starving conditions at the individual level have strong implications for Daphnia-algae interactions at the population level. With increasing carrying capacity of algae a stable equilibrium with coexistence of Daphnia occurs and algae shifts to limit cycles. The amplitudes of the limit cycles increase with increasing percentage of sustainable weight loss. If a κ-rule energy allocation is applied to juveniles, the stable equilibrium occurs for a much narrower range of algal carrying capacities, the algal concentration at equilibrium is about 2 times larger, and the range of algae carrying capacities at which daphnids become extinct extends to higher carrying capacities than in the new DEB-model. Because predator–prey dynamics are very sensitive to predator physiology under low food and starving conditions, empirical constraints of predator physiology under these conditions are essential when comparing model results with observations in laboratory experiments or in the field.

The shape of things to come: linking developmental plasticity to post‐metamorphic morphology in anurans

Development consists of growth and differentiation, which can be partially decoupled and can be affected by environmental factors to different extents. In amphibians, variation in the larval environment influences development and causes changes in post-metamorphic shape. We examined post-metamorphic consequences, both morphological and locomotory, of alterations in growth and development. We reared tadpoles of two phylogenetically and ecologically distant frog species (the red-eyed treefrog Agalychnis callidryas and the African clawed frog Xenopus laevis) under different temperatures with ad libitum food supply and under different food levels at a constant temperature. Low temperature and low food levels both resulted in similarly extended larval periods. However, low temperature yielded relatively long-legged frogs with a lower degree of ossification than warm temperature, whereas low food yielded relatively short-legged frogs with a higher degree of ossification than high food levels. Such allometric differences had no effect on locomotor performance of juveniles. Our results provide a basis for understanding the relationship between growth, differentiation and post-metamorphic shape in anurans and help explain many of the discrepancies reported in previous studies.

Annual reproductive cycle of the oyster, Saccostrea kegaki (Torigoe & Inaba 1981) on the southern coast of Jeju island, Korea

Gametogenesis of the oyster Saccostrea kegaki (Torigoe & Inaba, 1981) on the southern coast of Jeju (Cheju) Island, Korea was assessed monthly (March 2001 to February 2002). Gametogenesis in females and males commenced in April when the water temperature reached 16.5°C. Ripe gametes were observed from June to September in both sexes. Partially spawned oysters were evident from July to October when the water temperature ranged 24.6–26.7°C. Most oysters collected in July and August exhibited partially empty follicles with large eggs (>35 μm in diameter), suggesting that these two months are the major spawning periods. From November to March, most female oysters were in resting stage exhibiting no gametes in the follicles. In contrast, spawning or spent male oysters could be seen during November and January. Monthly mean oocyte diameters ranged from 10.3 ± 1.7 μm in April (early developing stage) to 36.9 ± 5.7 μm in August (ripe stage) and the mature oocyte was comparatively smaller than the size found in other oysters. The relatively slow rate of gametogenesis observed in S. kegaki off southern Jeju could be in part explained by low level of food in the water column rather than by water temperature.

Cave colonization without fasting capacities: An example with the fish Astyanax fasciatus mexicanus

Subterranean animals have commonly evolved hypoactivity, hypometabolism and/or the sequential use of energetic reserves to tolerate long fasting periods imposed by the low food levels found in subterranean environments. However, some tropical caves are characterized by a potential high level of nutriments. By using the tropical fish Astyanax fasciatus that presents both populations subterranean (Astyanax fasciatus mexicanus) and epigean (Astyanax fasciatus fasciatus) populations, we described behavioral, metabolic and biochemical responses during a long-term fasting period followed by a refeeding period. The results demonstrated that fed hypogean fishes exhibited different energy stores together with a hypometabolism. But, despite drastic decreases in locomotory activity and oxygen consumption during fasting, hypogean fishes consumed significantly more glycogen, triglycerides and proteins during the starvation period than epigean fishes. This lower fasting capacity showed by hypogean fishes is confirmed by the higher activation of the compensatory metabolic pathways (ketogenesis and gluconeogenesis). After the refeeding period, cave fishes did not recover from the “food deprivation” stress, and resume fed levels in glycogen, triglyceride reserves and proteins, in contrast to epigean ones. This study thus demonstrates that starvation adaptations are not necessary for cave life, but are rather correlated to the “energetic state” of each ecosystem, and that troglomorphism is not linked to starvation capacities and thus not to the impoverished food availability.

Potential for increasing the zinc content in common bean using genetic improvement

A large proportion of the world’s population is at risk of developing symptoms of zinc deficiency due to its low levels in food. The objective of this research was to estimate the heritability in early hybrid generations for the zinc content character in seeds of common bean (Phaseolus vulgaris L.), to investigate possible maternal effects on zinc contents and to evaluate the potential for genetic improvement. Reciprocal F1, and F2 generations as well as backcross (RCP1 and RCP2) populations were produced by crosses of three cultivars. The seed zinc content was measured by the nitric-perchloric digestion method and using the atomic absorption spectrophotometer. The range of variation in zinc content among tested progenies was from 21.76 mg kg−1 dry matter to 53.48 mg kg−1 dry matter, and no significant maternal effect was discovered. Narrow sense heritability was of an intermediate 57.5% to high 77.84% value and transgressive segregation was observed. These results suggest that breeding for increased zinc content in the seeds of common bean can be quite successful; an increase of 37.3% was obtained using just the three parents tested in this study.

Demographic characteristics of the copepod Acanthocyclops americanus (Sars, 1863) (Copepoda: Cyclopoida) fed mixed algal (Scenedesmus acutus)-rotifer (Brachionus havanaensis) diet

The predatory copepod Acanthocyclops americanus is commonly found in shallow ponds and lakes in Mexico. We tested the survivorship and reproduction-related variables of this copepod, isolated from Lake Huetzalin (Xochimilco, Mexico City), on four mixed diets comprising two algal concentrations (0.8 × 106 and 1.6 × 106 cells ml−1 of Scenedesmus acutus) and two rotifer densities (1 and 8 ind. ml−1 of Brachionus havanaensis). Survivorship patterns of naupliar and adult stages of A. americanus were evaluated at 24 ± 1°C. The demography experiments were initiated with five females and five males (1:1 ratio, 5th copepodites) into each of the 16 test [= 4 diet combinations × 4 replicates (cohorts)] jars containing 50 ml medium and with one of the chosen algal-rotifer diets. Data on the survival of naupliar and copepodites (up to V) showed a high death rate (80%) in A. americanus by the time nauplii reached the adult stage. The duration of developmental time for nauplii and the copepodite V (C-V) was about 28 days by which time A. americanus became adults. Food combination had a significant effect on both survival and duration of the naupliar and C-V but not on the development time in C-I, C-II, C-III, and C-IV. Naupliar stages showed highest life expectancy at low food levels (rotifers or alga). Adult male and female copepods showed better survival under higher availability of rotifers in the medium, while higher algal density resulted in steep mortality during the first days. Average adult lifespan and life expectancy of A. americanus varied from 19 to 23 days, depending on the diet combination. Age-specific reproductive output (m x ) of female A. americanus showed low reproduction (<10 offspring per female on any given day) when raised on low rotifer density, regardless of the algal concentration. On the other hand, enhanced offspring production (average of about 50 nauplii per female) was recorded when the rotifer density was higher (i.e., 8 ind. ml−1). Gross and net reproductive rates varied from 48 to 438 and 27 to 318 offspring per female, respectively. In general, higher availability of rotifers in the medium resulted in higher (8–10 times as compared to those in low rotifer density) gross and net reproductive rates. There was no significant difference in the generation time (13–14 days) in relation to the diet combination. The rate of population increase (r) (range: 0.29–0.69 per day) increased with greater abundance of B. havanaensis in the medium. Thus, our study showed that inclusion of animal protein in the diet is necessary for high population growth rates of A. americanus.

Food concentration and temperature effects on the demography of Latonopsis cf. australis Sars (Cladocera: Sididae)

Latonopsis cf. australis Sars, a rare cladoceran, originally collected from a shallow temporary water body in the State of Chihuahua (Mexico), was cultured using Scenedesmus acutus. Population growth data and selected demographic variables (average lifespan, gross and net reproductive rates, generation time, and the rate of population increase per day) of L. australis cultured under different food levels (0.015 × 106–1.0 × 106 cells ml−1 of S. acutus) and at 15 and 30°C were measured. Population growth of the cladoceran was evaluated at six food concentrations (0.015, 0.03, 0.06, 0.12, 0.24, and 0.5 × 106 cells ml−1), while life table demography was studied at three food levels (0.25 × 106, 0.5 × 106, and 1.0 × 106 cells ml−1). Population growth data indicated that L. australis is well adapted to living under low food levels, corroborating the fact that this species was originally collected in a shallow pond with high transparency. Demographic data showed that Latonopsis had higher survivorship at 15°C but greater reproductive output at 30°C. Average lifespan ranged from 12 to 25 days, depending on the temperature and food availability. The population growth rate was close to 0 at 15°C and between 0.2 and 0.4 per day at 30°C. The net reproductive rate was less than two individuals per female per day, significantly lower than values observed in other cladoceran taxa.

Pathogen Dose Infectivity Curves as a Method to Analyze the Distribution of Host Susceptibility: A Quantitative Assessment of Maternal Effects after Food Stress and Pathogen Exposure

Stress conditions have been found to change the susceptibility of hosts or their offspring to infection. The usual method of testing at just one parasite dose level does not allow conclusions on the distribution of susceptibility. To better understand the epidemiology and evolution of host-parasite systems, however, knowledge about the distribution of host susceptibility, the parameters that characterize it, and how it changes in response to environmental conditions is required. We investigated transgenerational effects of different stress factors by exposing Daphnia magna to standard conditions, to low food levels, or to a high dose of the bacterial pathogen Pasteuria ramosa and then measuring the susceptibility of the offspring to different spore doses of the parasite. For the analysis we used a mathematical model that predicts the fraction of infected hosts at different parasite doses, allowing us to estimate the mean and variance of host susceptibility. We find that low food levels reduce both the mean and the variance of offspring susceptibility. Parasite exposure, on the other hand, widens the offspring's susceptibility distribution without affecting its mean. Our analysis uncovered previously unknown transgenerational effects on the distribution of susceptibilities. The finding of an alteration in the variance of susceptibility to infection has implications for host and parasite dynamics and can contribute to our understanding of the stability of host-parasite interactions.

Effect of Temperature and Food on Development and Growth of Daphnia similoides (Cladocera: Daphniidae) from Lake Donghu

ABSTRACT Growth, reproduction, and life history parameters of Daphnia similoides from a subtropical eutrophic Chinese lake (Lake Donghu) were measured at four temperatures (10, 15, 25 and 35°C) and two food concentrations (0.2 and 2 mg carbon L−1 of a green alga) in the laboratory. D. similoides had five juvenile instars and up to 28 adult instars. The number of adult instars was greater at the low food level than at the high food level. The number of adult instars with maximal number of offspring, the cumulative number of offspring per female, development time, and body lengths of females and offspring were higher or longer at the low temperatures than at the high temperatures. Body length of females was significantly correlated with offspring number per adult instar at all combinations of temperature and food. Innate capacities of increase (0.13–1.01) linearly increased with temperature.

How do changes in parental investment influence development in echinoid echinoderms?

Understanding the relationship between egg size, development time, and juvenile size is critical to explaining patterns of life-history evolution in marine invertebrates. Currently there is conflicting information about the effects of changes in egg size on the life histories of echinoid echinoderms. We sought to resolve this conflict by manipulating egg size and food level during the development of two planktotrophic echinoid echinoderms: the green sea urchin, Strongylocentrotus droebachiensis and the sand dollar, Echinarachnius parma. Based on comparative datasets, we predicted that decreasing food availability and egg size would increase development time and reduce juvenile size. To test our prediction, blastomere separations were performed in both species at the two-cell stage to reduce egg volume by 50%, producing whole- and half-size larvae that were reared to metamorphosis under high or low food levels. Upon settlement, age at metamorphosis, juvenile size, spine number, and spine length were measured. As predicted, reducing egg size and food availability significantly increased age at metamorphosis and reduced juvenile quality. Along with previous egg size manipulations in other echinoids, this study suggests that the relationship between egg size, development time, and juvenile size is strongly dependent upon the initial size of the egg.

Measurement of Vitamin D in Foods and Nutritional Supplements by Liquid Chromatography/Tandem Mass Spectrometry

Vitamin D is a fat-soluble vitamin with great nutritional interest. An HPLC/MS/MS method was developed to measure vitamin D with atmospheric pressure chemical ionization. Under the experimental parameters used, the LOQ was 0.018 IU/g or 0.45 ng/g, which greatly enhances the capability of measurement of vitamin D at low levels in foods and supplements. This method was validated with spike recovery of 100 +/- 15% and the RSD of less than 10% for most sample matrixes, including infant formula, cheese, cereal and cereal-based foods, multivitamin supplements, and pet foods. The results for vitamin D were compared with those obtained by other methods.

Food level and sex shape predator-induced physiological stress: immune defence and antioxidant defence

Despite the potential impact on prey fitness and predator-prey interactions, most studies of predation risk ignore physiological responses and their dependence upon food level and sex. Therefore, we reared male and female larvae of the damselfly Lestes viridis under predator stress (dragonfly larvae) at high and low food levels, and subsequently scored for important variables of insect immune defence (i.e. phenoloxidase) and antioxidant defence [i.e. superoxide dismutase, and catalase (CAT)]. Under predation risk, larvae did not decrease growth rate or immune defence, and only slightly reduced food intake in the high food treatment, probably because of time stress, i.e. little time available to complete the larval development. However, larvae facing predator stress did show an upregulation of antioxidant enzymes. This upregulation was dependent upon food level for CAT and both food level and sex for SOD, consistent with energetic constraints and sex differences in the link between longevity and adult fitness. Our results illustrate that predator stress can influence life history, behavioural and physiological responses differentially and in a context-dependent way. This implies that non-consumptive physiological effects of predators on their prey show independent yet similar complexities in behavioural and life history response variables. In general, our results advocate that mechanistic studies on predator-prey interactions may benefit from including physiological variables.

Strong body mass decrease of the invasive clam Corbicula fluminea during summer

The filter-feeding clam Corbicula fluminea has widely spread from its Asiatic origin into freshwater habitats on several continents, where it often has a considerable impact on ecosystem processes. The present study was stimulated by the observation that C. fluminea can experience mass mortality during warm summers, even when temperatures are still far below the lethal level. We hypothesized that starvation due to low food quantities during summer is a main factor in this context. In order to test nutritional conditions in an environment where summer mortality occurred, the clams’ body mass was tracked in river bypass systems installed at the Lower River Rhine (Germany and The Netherlands). Two food levels were adjusted in the bypass channels: one corresponding to the original chlorophyll level in the river (ambient food) and one with a chlorophyll a level reduced by about 50% (low food). The clams kept at the ambient food level increased their shell length during summer, although growth rates decreased at low food levels in the River Rhine in late summer. In contrast to shell length, body mass decrease in late summer cumulated in 94% reduction from August until October. This trend was enhanced by the experimental food reduction, i.e., clams kept in the low food level treatments weighed 60% less than the clams in the ambient food treatment at the end of summer. However, mortality was low in both treatments. The data demonstrate a high plasticity in the body mass of C. fluminea. The corresponding possibility to respire body mass can be seen as one strategy to survive longer starvation periods.

Food competition between the benthic polychaeteHediste diversicolorand the semipelagic mysidNeomysis integerin the northern Baltic Sea

INTRODUCTION As the structure of benthic communities in the Baltic Sea is relatively simple, the area offers a unique opportunity to study the mechanisms and detect the driving forces of changes in biodiversity. Nevertheless, scientists are still debating about the relative contribution of biotic interactions and physical disturbance in determining the patterns of coastal communities in the northern Baltic Sea. Earlier studies demonstrated significant intra- and interspecific competitive links among several benthic algal and invertebrate species in the region (Kotta et a1., 2001, 2008; Orav-Kotta et al., 2009). Other studies, however, supported an alternative view that physical disturbance is the main mechanism of community dynamics in the northern Baltic Sea region (e.g. Kautsky & van der Maarel, 1990; Herkul et al., 2006). Earlier studies have focussed on the competitive links either between benthic or pelagic species. To date, there are virtually no studies where the competitive interactions between benthic and pelagic species are experimentally evaluated although such benthic--pelagic coupling might be relevant in many instances. Neomysis integer (Leach) is the most prevalent mysid species in the coastal areas of the northern Baltic Sea (Kotta & Kotta, 1999). In May-June the bulk of the population is concentrated in the shallow areas below the 10 m depth. In the less exposed bays the density of mysids may occasionally exceed 10 000 ind. [m.sup.2]. It is likely that in such shallow areas the density of plankton is low and the food should be primarily obtained from the upper layer of the sediment. Neomysis integer is considered to be omnivorous. It is known to feed besides phytoplankton and zooplankton on bottom detritus and filamentous algae (Mauchline, 1971). The polychaete Hediste diversicolor (O. F. Muller) is the key benthic invertebrate species in these shallow and isolated bays. The species is a deposit feeder at juvenile stages and a predator at the later stages of its development (Bonsdorff & Pearson, 1999). Hence, competition between the species is likely in these areas. The aim of this study was to investigate if the two species are competing for space and food during May-June when the densities of both species are very high in the shallow bays. We expect that at low food levels the competitive interactions reduce the growth and survival of the test organisms whereas at high food levels such negative effects do not occur. MATERIAL AND METHODS The in situ food competition between the prevalent mysid N. integer and the polychaete N. diversicolor was studied in a shallow (2 m) bay (59[degrees]50'N, 23[degrees]15'E) adjacent to the Tvarminne Zoological Station in the north-eastern Baltic Sea. The bay is moderately exposed to the sea and the prevailing sediment is silty sand. The test organisms and sediment were collected in the vicinity of the experimental site. The sediment was sieved through a 1 mm mesh and dried in the sun for 3 days to assure that it was free from living benthic invertebrates. To investigate interspecific interactions between the test organisms, 3 L plastic buckets ([empty set] 18 cm) were used. The buckets were filled with an 8 cm layer of sediment and a 7 cm layer of seawater and allowed to settle for 6 h. The test organisms were added to four replicate buckets per treatment (totalling 24 buckets). Four individuals of H. diversicolor and/or 15 individuals of N. integer were added to each bucket so that their densities corresponded to natural densities in the study area. The animals were fed by adding sediment rich in detritus and microphytobenthos. A more detailed setup of the experiment is given in Table 1. The buckets were closed by a mesh-net (0.2 mm mesh size) to minimize the risk of migration but at the same time to assure sufficient water exchange in the buckets. The experimental buckets were randomly placed on the seafloor at a depth of 0. …