Particle size dependent feeding by the rotifer Brachionus plicatilis

Size selective feeding by Brachionus plicatilis was investigated with algae and bacteria (0.3–3.5 µm) and mono-disperse latex beads (0.3–3.0 µm) in short term feeding experiments. B. plicatilis demonstrated maximum clearance rate of particles with diameter 2µm, but particles with diameter down to 0.3 µm were also ingested. The clearance rate of bacteria was 15–55% of that obtained for optimal sized particles (2 µm), and was related to particle size. The relative reduction in retention of particles with diameter < 2 µm was more pronounced for latex beads than for natural food particles, suggesting other mechanisms than size to be important for the particle retention by the rotifer. This is emphasized by the fact that the clearance rates were much lower for latex beads than for natural food particles of comparable size. Efficient retention of bacteria was observed for rotifers in poor physiological condition, i.e. rotifers with low maximum clearance rate. This may reflect a strategy to optimize energy utilization by reducing locomotion costs and increasing energy intake.The results indicate that B. plicatilis has a low to medium ability to feed on bacteria. In natural ecosystems, its importance as a bacterial grazer is of limited importance. At high population densities, such as in live feed cultures, the rotifer may, however, efficiently remove bacteria from the culture.     

Feeding biology of two brachionid rotifers: Brachionus quadridentatus and Brachionus plicatilis

Brachionus quadridentatus and Brachionus plicatilis were examined in laboratory cultures to investigate their (1) food requirements and (2) ability to select particles on the basis of size. Growth experiments showed that the Chlorophyceans Nannochloris sp. and Chlorella vulgaris possess good nutritional value for these rotifers. However, B. quadridentatus declined in cultures with Microcystis firma, detritus of Enteromorpha sp. or Pseudomonas sp. (bacteria) as a sole source of food, thus indicating that these foods were inadequate. Utilization of latex microspheres by these two species indicate that they respond to particle size in different ways: B. quadridentatus had a higher selectivity index for 3 and 5 µm beads, while B. plicatilis had a higher selectivity index for 2 µm beads.     

Size-selective grazing of coastal bacterioplankton by natural assemblages of pigmented flagellates,colorless flagellates,and ciliates

Fluorescently-labelled bacteria (FLB) were used to study the feeding strategies of a natural assemblage of estuarine protozoans and to examine whether the protozoan grazing could account for the in situ size structure of the bacterioplankton. The FLB, DTAF-stained enterococci, ranging in volume from 0.01 to 0.30 × 10^–1 µm³, were added to a natural planktonic assemblage at a density of 5.5% of the natural bacterioplankton. Initial densities (individuals ml^–1) were as follows: total natural bacteria, 2.2 × 10⁶; FLB, 1.2 × 10⁵; pigmented flagellates, 300; colorless flagellates, 250; and ciliates, 30. FLB consumption rates were determined by examining the contents of protozoan food vacuoles, and the long-term effect of grazing (over a period of 100 hours) was determined by monitoring the decline in the FLB density in experimental vessels. The average consumption rates of FLB by pigmented flagellates were similar to those by flagellates that lacked chloroplasts (0.9 and 0.6 FLB protozoan^–1 hour^–1, respectively). The ciliates consumed bacteria at an average rate that was 17-fold higher (per cell) than flagellates, and they displayed a greater preference for larger bacteria than did the flagellates. FLB of the mid-size classes (0.025–0.100 µm³) were heavily grazed by the entire protozoan assemblage; the smallest (<0.025 µm³) and the largest (>0.100 µm³) FLB escaped protozoan grazing. This had a profound effect on the resulting size distribution of FLB. At the end of a 100-hour incubation, the percentage of mid-size FLB (0.025 to 0.100 µm³) decreased 2.0–2.2-fold, while the percentage of the smallest and the largest FLB increased 2.0–2.5-fold. Resultant densities of FLB were consistent with initial clearance rates determined for the protozoan groups. The grazing rates of protozoans on FLB were species-specific; whereas some species consumed FLB, others did not demonstrate bacterivory. The results suggest that protozoan grazing has a major effect on the size distribution of coastal bacterioplankton. By selectively feeding on a particular size-class of bacteria, planktonic ciliates may consume 15–90% day^–1 of the standing stock of largest size classes of bacterioplankton. Thus, ciliates, which were present in low abundance in the field, could not balance the production of the entire bacterial community, but they may strongly influence the portion of the bacterial community represented by the largest bacterial class. The direct effect of flagellates (e.g., grazing) was limited to smaller bacteria.Offprint requests to: M. P. Shiaris.     

Effects of particle concentration and season on the filtration rates of the freshwater clam,Sphaerium striatinum Lamarck (Bivalvia: Pisidiidae)

The effects of particle concentration and season on the filtration rates of the freshwater clamSphaerium striatinum Lamarck were assessed by measuring clearance rates of small (2.02 μm) latex beads from dilute suspensions. Filtration rates decreased as particle concentration increased over a range of 2–64 mg 1⁻¹, with rates decreasing in similar proportion for clams of all sizes. For a 1-mg clam, rates decreased from approximately 8.4 to 0.57 ml clam⁻¹ h⁻¹. Seasonal filtration rates for adult clams peaked during periods of greatest reproduction. The patterns for smaller clams are similar, though proportional changes in filtration rates differ for various sizes of clams. It is estimated that clams occupying 1 m² of stream substrate removed about 3.67 gCa⁻¹. This is equivalent to 0.0004% of the carbon that flows past them annually. Filter-feeding provided only 24% of the calculated energy needs of the population, suggesting that another mode of feeding (e.g. deposit-feeding) may provide an important energy source for these forms. Funded in part by a grant-in-aid to D. J. Hornbach from Sigma-Xi, The Research Society. Funded in part by a grant-in-aid to D. J. Hornbach from Sigma-Xi, The Research Society.     

Feeding rate inhibition in crowded Daphnia pulex

Feeding rates of Daphnia pulex fed a range of levels of the alga Chlamydomonas reinhardi of 15 °C are strongly density-dependent. At lower densities, Daphnia (30 1^–1) fed at higher rates than crowded (270 1^–1) Daphnia which manifest a relatively depressed saturation feeding response. At 30 individuals/liter, Daphnia consumed 8.5 – 15.7 × 10⁴ cells d^–1h^–1 (on a volume basis, 12.1 – 22.2 × 10⁶ m³), at 270 L^–1 3.7 – 3.9 × 10⁴ (5.2 – 5.5 = 10⁶ m³ cells d^–1h^–1 when feeding on algae at 80 000 cells ml^–1 (11.3 × 10⁶ m³ ml^–1). The feeding rate data best fit an Ivlev feeding function. An autoallelopath might be causing the repression. Water preconditioned with crowded Daphnia completely repressed feeding in uncrowded Daphnia after six hours.     

Suspension feeding in Bithynia tentaculata (Prosobranchia,Bithyniidae), as affected by body size,food and temperature

The suspension feeding of Bithynia tentaculata was tested in laboratory experiments. The animals were fed in 1-1 aerated glass beakers, and filtration rates were calculated from changes in cell concentrations during the 6-h experiment. Temperature influenced the filtering rate, with minimum values of 5ml · ind^–1 · h^–1 at 5° C and maxima of 17.2 ml · ind^–1 · h^–1 at 18° C. Three food species of different size, motility and cell surface characteristics (Chlamydomonas reinhardii, Chlorella vulgaris and Chlorogonium elongatum) did not affect filtration rates. Suspension feeding increased with increasing food concentrations up to 12 nl · ml^–1, above which feeding rate was kept constant by lowering the filtering rates. Even the smallest animals tested (<4 mm body length) were found to be feeding on suspended food at a rate of 2.7 ml · ind^–1 · h^–1, and increasing rates up to 8.4 ml were found in the 6–7 mm size class. All size classes of Bithynia showed a circannual fluctuation of their filtration rates. The ecological consequences of Bithynia's ability to switch between two feeding modes, grazing and suspension feeding, are discussed.     

Can suspension feeding by bivalves regulate phytoplankton biomass in Lake Waccamaw, North Carolina?

Suspension feeding by bivalves has been hypothesized to control phytoplankton biomass in shallow aquatic ecosystems. Lake Waccamaw, North Carolina, USA is a shallow lake with a diverse bivalve assemblage and low to moderate phytoplankton biomass levels. Filtration and ingestion rates of two relatively abundant species in the lake, the endemic unionid, Elliptio waccamawensis, and an introduced species, Corbicula fluminea, were measured in experiments using natural phytoplankton for durations of 1 to 6 days. Measured filtration and ingestion rates averaged 1.78 and 1.121 ind.^–1 d^–1, much too low to control phytoplankton at the observed phytoplankton biomass levels and growth rates. Measured ingestion rates averaged 4.80 and 1.50 µg chlorophyll a ind.^–1 d^–1, too low to support individuals of either species. The abundance of benthic microalgae in Lake Waccamaw reaches 200 mg chlorophyll a m^–2 in the littoral zone and averages almost an order of magnitude higher than depth-integrated phytoplankton chlorophyll a. Total microalgal biomass in the lake is therefore not controlled by suspension feeding by bivalves.     

Strain-Specific Differences in the Grazing Sensitivities of Closely Related Ultramicrobacteria Affiliated with the Polynucleobacter Cluster

Ultramicrobacteria (cell volume < 0.1 μm³) are the numerically dominant organisms in the plankton of marine and freshwater habitats. Flagellates and other protists are assumed to be the most important predators of these ultramicrobacteria as well as of larger planktonic bacteria. However, due to controversial observations conducted previously, it is not clear as to whether fractions of the ultramicrobacteria are resistant to flagellate predation. Furthermore, it is not known if closely related bacteria vary significantly in their sensitivity to flagellate predation. We investigated the sensitivity of ultramicrobacteria affiliated with the cosmopolitan Polynucleobacter cluster to grazing by Spumella-like nanoflagellates. Laboratory grazing experiments with four closely related (≥99.6% 16S rRNA gene sequence similarity) bacteria and three closely related (100% 18S rRNA gene sequence similarity) flagellates were performed. In comparison to larger bacteria, predation on the ultramicrobacterial Polynucleobacter strains was weak, and the growth of the predating flagellates was slow. Specific clearance rates ranged between 0.14 × 10⁵ and 2.8 × 10⁵ units of predator size h⁻¹. Feeding rates strongly depended on the flagellate and bacterial strain (P < 0.001). Grazing mortality rates of the three flagellate strains investigated varied for the same prey strain by up to almost fourfold. We conclude that (i) ultramicrobacteria affiliated with the Polynucleobacter cluster are not protected from grazing, (ii) strain-specific variations in grazing sensitivity even between closely related bacteria are high, and (iii) strain-specific differences in predator-prey interaction could be an important factor in the evolution and maintenance of microbial microdiversity.     

Grazing and assimilation rates of natural populations of planktonic rotifersKeratella cochlearis,Keratella quadrata andKellicottia longispina in a eutrophic lake (Aydat,France)

The filtering rates of¹⁴C labelledChlamydomonas sp. by 3 dominant species of rotifers were studied in eutrophic Lake Aydat. They varied from 4 to 53 µl ind^–1 h^–1 forKeratella cochlearis, from 2 to 56 µl.^–1h^–1 forKeratella quadrata and from 3 to 52 µl ind^–1 h^–1 forKellicottia longispina. Their maximum assimilation efficiency was 32%. At the measured grazing rates, these populations could clear the water in less than two days during July. In Lake Aydat, the rotifers community could play an important role in the regulation of seasonal succession of phytoplankton and bacteria.     

Optimisation of the cell release from immobilised cells of Bacillus simplex cultivated in culture media enriched with Cd2+: influence of Cd2+, inoculum size, culture medium and alginate beads characteristics

Calcium alginate beads were used to entrap a Bacillus sp. that has the ability to biosorb cadmium. During the batch incubation of alginate beads in a `rich' or a `poor' liquid medium, cell release out of the beads was noticed with a lag phase which was inversely proportional to the inoculum size (2×10⁷ or 2×10⁸ cells ml^–1 alginate), to the medium content, and proportional to the alginate concentration (10 or 15 g l^–1) and to the cadmium concentration (1, 5 or 10 mg l^–1). In addition, the cell release occurred more quickly when the medium was renewed. When the concentration was below 5 mg l^–1, the alginate matrix seemed to protect the bacteria against Cd²⁺ toxicity.     

Remote detection of algae by copepods: responses to algal size, odors and motility

Selective feeding on large algae by copepods involves remotedetection of individual particles and subsequent active captureresponses In this study we use radiotracer experiments to quantifythe clearance rates of five coexisting freshwater copepods andto investigate the relative merits of the chemoreception andmechanoreception hypotheses of remote detection Tropocyclopsand three diaptomid copepods exhibited relatively high clearancerates when feeding on low concentrations of large algae, suggestingthat most previous studies with freshwater copepods have underestimatedmaximal clearance rates and the degree of size selectivity.All five species of copepods exihibited strong selection foran intermediate-sized flagellate (25 µm Cartena) or alarge-sized nonmotile alga (80 µ.m Pediastrum) over asmall-sized flagellate (6 µ.m Chlamydomonas). The weight-specificclearance rate for Tropocylops prasmus feeding on motile Cartena(271 ml mg^–1 h^–1) was about twice that of threediaptomid copepods and more than an order-of-magnitude higherthan the estimate for Epischura lacustris feeding on its preferredalga, Pediastrum Assuming that distance chemoreception (‘smell’)is important in remote detection, we predicted that the additionof high concentrations of ‘algal odors’ would obscureany chemical gradients emanating from individual algal cellsand would thereby hinder the remote detection and active captureof large algae Contrary to this hypothesis, the addition ofamino acids, sucrose, and algal extracts had no effect on theclearance rates and selectivity of Diaptomus birgei. These results,together with recent cinematographic studies (Vanderploeg etal.,1990), suggest that mechanoreception is the primary mechanismfor the remote detection of large particles by diaptomid copepods.A raptorial cyclopoid, Tropocyclops prasinus, exhibited strongpreferences for motile algae, whereas a suspension-feeding calanoid,D birgei, did not select between motile and nonmotile cells.Motility appears to be an important factor in algal detectionfor small cyclopoid copepods but not for suspension-feedingdiaptomids     

Clearance rates of bacteria by the rotifer Filinia longiseta (Ehrb.) measured using three tracers

The clearance rates (CRs) of bacteria by Filinia longiseta were measured at 19°C, both in situ in Lake Loosdrecht and in the laboratory during summer. The tracer particles used in the field were: (1) 0.51 µm fluorescent microspheres, and (2) fluorescently labelled bacteria (FLB). A third type of tracer particle, natural [methyl-³H]-thymidine-labelled bacteria (< 1.2 µm), were used as a radiotracer in a laboratory experiment. The uptake of the first two tracer-particle types was measured by microscopic examination of the rotifer guts. In the third case, the uptake of radioactivity was determined by liquid scintillation counting. The rate of uptake of the microspheres decreased 10 min after the start of the experiment, probably because the gut passage time was exceeded. Using a 5 min feeding time, the rate of uptake of microspheres was higher than that of the FLB, though the variation in the uptake in both cases was high. The ingestion rates and CRs of bacteria by F. longiseta based on the fluorescent tracers were: microspheres, 5115 bact.ind^–1 h^–1 and 0.368 µl ind^–1 h^–1; FLB, 2252 bact.ind^–1 h^–1 and 0.162 µl ind^–1 h^–1. The mean CR using the thymidine-labelled natural bacteria and a 10 min feeding time was 0.179 µl ind^–1 h^–1. Thus, the CR based on the microsphere method was twice as high as for the other two methods.     

Characteristics of plankton communities in Chinese integrated fish ponds: effects of excessive grazing by planktivorous carps on plankton communities

Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 10⁷ and 2.20 × 10⁷ cells ml^–1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m^–3 Chla on average) and rotifers (5372 and 20733 ind. 1^–1 on average) exceeded the maximum values reported for natural waters.The average [³H]thymidine uptake rates were 694 and 904 pmoles 1^–1 h^–1 (13.4 and 20.6 µgC 1^–1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [³H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m^–3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m^–2 d^–1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1^–1 and 445 µg 1^–1 and those of rotifers were 763 µg 1^–1 and 1186 µg 1^–1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.     

Clearance rates of sessile rotifers: in vitro determinations

We measured laboratory clearance rates of 10 rotifer and one unidentified bryozoan species from 3 different lakes using ³²P labeled algae (Chlamydomonas) or yeast (Rhodotorula). Clearance rates for all rotifers fed yeast ranged from < 2.0 to > 260 µl · animal^–1 · h^–1 depending on species. The in vitro clearance rates of two sessile rotifers (Ptygura crystallina and P. pilula) were not significantly different from previously measured in situ rates (Wallace and Starkweather 1983). Clearance rates for 5 rotifers fed algae ranged from < 5.0 to > 90.0 µl · animal^–1 · h^–1. Ptygura beauchampi, P. crystallina, P. pilula, Floscularia conifera, and F. melicerta ingested both cell types but their clearance rates varied substantially among species and between cell types. There was a substantial time-dependent loss of 32P from formalin-fixed animals (Sinantherina socialis) awaiting processing. This loss stabilized at approximately 20 hours and was estimated to be about 40% of the initial ingested label. Clearance rates for the bryozoan fed yeast or algae were highly variable, ranging from < 1.0 to > 3 000 µl · animal^–1 · h^–1.     

Zooplankton filtration rates in Lake Huron,Georgian Bay and North channel

On three research cruises in 1981, zooplankton community filtration rates were measured at 4 stations: Saginaw Bay, mid-Lake Huron, Georgian Bay and North Channel. For all four stations, the highest rates were observed during the late-September cruise. The maximum observed rate was 137 000 ml d^–1 m^–3, while the lowest rate was 7200 ml d^–1 m^–3. The grazing experiments were performed on three size classes of radioactively labelled algal food (0.45–5 µm, 5–20 µm and 20–64 µm). In 11 of 12 experiments, the smallest size class of food yielded the highest filtration rate. For the late-May cruise we used published data on phytoplankton biomass for the Georgian Bay and North Channel stations to calculate community feeding rates of 0.09 and 0.015 mg C mg C m^–3 d^–1, respectively, and percent cropping rates of 0.74 and 0.35 per day, respectively. A comparison of our feeding rates to literature values for zooplankton biomass suggests that algal food alone may not be sufficient to sustain zooplankton growth at those stations.     

Planktonic bacterial biomass and seasonal pattern of the heterotrophic uptake and respiration of glucose and amino acids in the shallow sandpit lake of Créteil (Paris Suburb,France)

Biomass and activity of planktonic bacteria were investigated during a one year study in a shallow sandpit lake. The shallowness of the lake helped keep the water column homogeneous regarding bacterioplankton. Small free-living bacteria (0.03 µm³ cell^–1) dominated the populations throughout the period studied. Bacterial abundances varied from 1 to 11 × 10⁶ cells ml^–1. Kinetic parameters (V _max, K + S and T) were determined with ¹⁴C labelled compounds (glucose and amino acids mixture). V _max values were high and averaged 0.056 and 0.050 µgCl^–1 h^–1 for glucose and amino acids respectively. Maximal V _max values were observed in summer at the highest temperatures, but also in early spring. T values were much greater in winter. K + S values were significantly higher for amino acids (3 µg Cl^–1) than for glucose (1 µg Cl^–1). A low percentage of mineralization (about 25% for both tracers) could be the expression of the high growth efficiency expected when bacteria are growing at the expense of low molecular weight compounds as phytoplankton exudates.     

Potential toxicity of chrysophytes affiliated with Poterioochromonas and related 'Spumella-like' flagellates

The chrysophyte genera Poterioochromonas and Ochromonas andtheir heterotrophic analogons, i.e. the ‘Spumella-like’flagellates, account for a significant and often dominatingfraction of the pelagic nanoplankton. Even though several osmotrophicallyand autotrophically grown strains of Ochromonas and Poterioochromonasare assumed to produce toxins, the potential toxicity has beeninvestigated neither for its association with bacterivorousnutrition nor within the related exclusively heterotrophic ‘Spumella-like’flagellates. We investigated the toxic potential of severalflagellate strains using cultures of flagellates, cell extractsand filtrate of flagellate cultures. The effect on potentialpredators was exemplarily tested for the cladoceran Daphniamagna and the rotifer Platyias sp. All tested heterotrophicand mixotrophic flagellate strains were toxic to zooplanktonat abundances exceeding 10⁴ flagellates mL^–1. For therotifers, survival on any of the flagellate strains was significantlylower than that in the control treatment (P < 0.001) alreadyafter 24 h. We conclude that (i) ‘Spumella-like’flagellates can be toxic to zooplankton, (ii) all tested flagellates,i.e. heterotrophic and mixotrophic flagellates, feeding phagotrophicallycan be toxic to zooplankton and (iii) sublethal effects maybe observed at typical field abundances, even though acute toxicityseems to be restricted to flagellate abundances observed onlyat peak events.     

Trophic structure and productivity of the lagoonal communities of Tikehau atoll (Tuamotu Archipelago,French Polynesia)

Carbon standing stocks and fluxes were studied in the lagoon of Tikehau atoll (Tuamotu archipelago, French Polynesia), from 1983 to 1988.The average POC concentration (0.7–2000 µm) was 203 mg C m^–3. The suspended living carbon (31.6 mg C m^–3) was made up of bacteria (53%), phytoplankton < 5 µm (14.2%), phytoplankton > 5 µm (14.2%), nanozooplankton 5–35 µm (5.7%), microzooplankton 35–200 µm (4.7%) and mesozooplankton 200–2000 µm (7.9%). The microphytobenthos biomass was 480 mg C m^–2.Suspended detritus (84.4% of the total POC) did not originate from the reef flat but from lagoonal primary productions. Their sedimentation exceeded phytobenthos production.It was estimated that 50% of bacterial biomass was adsorbed on particles. the bacterial biomass dominance was explained by the utilisation of 1) DOC excreted by phytoplankton (44–175 mg C m^–2 day ^–1) and zooplankton (50 mg Cm^–2 day^–1)2) organic compounds produced by solar-induced photochemical reactions 3) coral mucus.50% of the phytoplankton biomass belongs to the < 5 µm fraction. This production (440 mg C m^–2 day^–1) exceeded phytobenthos production (250 mg C m^–2 day^–1) when the whole lagoon was considered.The zooplankton > 35 µm ingested 315 mg C m^–2 day^–1, made up of phytoplankton, nanozooplankton and detritus. Its production was 132 mg C m^–2 day^–1.     

The role of phototrophic sulfur bacteria as food for meiobenthic harpacticoid copepods inhabiting eutrophic coastal lagoons

Laboratory experiments were undertaken using Amonardia normani and Schizopera cf. compacta, two meiobenthic harpacticoid copepods commonly found in coastal lagoons. The first experiments were designed to determine if the phototrophic sulfur bacteria Chromatium gracile can be ingested by these copepods and at what concentrations. Egestion rate was used as an index of feeding rate. The response of the egestion rate, expressed in numbers of faecal pellets produced by copepod per day, as a function of bacterial concentration followed the functional model. A. normani attained constant feeding rates from the bacterial concentration of 1 × 10⁷ cells ml^–1 (5 µg C ml^–1) onwards, S. cf. compacta attained constant feeding rates from 2.6 × 10⁷ cells ml^–1 (13 µg C ml^–1) onwards. The faecal pellet volume changed significantly (p<0.05) between food concentrations for A. normani but not for S. cf. compacta (p>0.05). In order to investigate the effect of the phototrophic bacterial diet on the population dynamics of A. normani three groups of nauplii were maintained at 2 × 10⁷ cells ml^–1 and observed every day. The mortality of these nauplii was very high compared to those maintained on a diatom diet (Nitzschia constricta); only in one of the groups did some copepodites develop but no adults were ever observed. Adults fed on bacteria did not have different (p>0.05) survival rates compared to those fed on diatoms, nevertheless, the number of nauplii produced was significantly less (p<0.05) on the bacterial diet. These results lead us to suggest that although the phototrophic sulfur bacteria (Chromatium gracile) can be ingested by both copepod species it cannot sustain the full development of the A. normani population. Thus, a bloom of phototrophic sulfur bacteria does not seem to be a favourable situation for opportunistic benthic copepods to colonize eutrophic coastal lagoons after a dystrophic crisis.     

Feeding habits of larval Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret (Israel) II. Experimental study

Interactions between the larvae of Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret, Israel, and their zooplankton prey were studied experimentally. Prey species preference and size selectivities were measured. Larvae were hatched in the lab from eggs collected in the field, and fed different food items in various concentrations. The food items included lake zooplankton, algae, and commercial pellets. It was shown that small, first feeding larvae (7–8.5 mm SL) prefer small bodied zooplankters (< 180 µ). The effect of these food sources on larval growth was measured. It was found that larval Mirogrex grew at a higher rate when fed zooplankton prey sized from 63 µ–250 µ. Food items smaller than 63 µ, larger than 250 µ and Scenedesmus sp., produced less than optimal growth rates. The importance of Mirogrex feeding habits and their potential influence on the Kinneret ecosystem is considered.