Feeding efficiency of Chaoborus flavicans (Insecta, Diptera) under turbulent conditions

Turbulence can affect predator–prey interactions. The effect of turbulence on the feeding efficiency of an ambush predator was tested with laboratory experiments. The experiments were conducted in 100-L aquaria in which ten individuals of fourth instar Chaoborus flavicans larvae were placed as predators. Two prey densities (3 and 10 ind. of Daphnia pulex L^?1) and two durations (30 and 120 min) were tested in a nonturbulent treatment and five different turbulence levels [average root-mean-square (RMS) velocities ranging from 0 to 7.3 cm s^?1, corresponding dissipation rates from 7.2 × 10^?7 to 1.3 × 10^?3 m² s^?3]. We hypothesized that the feeding rate of C. flavicans would be enhanced by turbulence due to increasing encounter rates up to a turbulence level above which a disturbance in post-encounter processes would lead to reduced feeding efficiency. However, the results showed no significant increase in the feeding rate of C. flavicans at intermediate turbulence. At high turbulence we found the expected significant negative response in the feeding rate of Chaoborus larvae. The feeding rate declined below the rates at nonturbulent and intermediate turbulence conditions as the average RMS velocity exceeded 3.1 cm s^?1 (dissipation rate 9.9 × 10^?5 m² s^?3, respectively).     

Grid-generated turbulence in a mesocosm experiment

In two mesocosm experiments designed to study the effect of nutrients and turbulence on trophic levels below fish larvae in the ecosystem, turbulence was created by oscillating grids. Utilizing two different frequencies, two levels of turbulence (`high' and `low') were generated in a total of eight mesocosms in both experiments. In each mesocosm a two-layer density structure was created. We succeeded in generating turbulence that mimics turbulence found in natural ecosystems, with turbulent eddies on larger scales than those relevant for organism interactions. Energy dissipation rates () were calculated from the frequency spectrums of the measured turbulent velocities. In the second experiment the average energy dissipation rates in the upper layer were 1.9×10^–7 W kg^–1 and 5.5×10^–8 W kg^–1 in the `high' and `low' enclosures, respectively. Finding `background' turbulence levels between 5.3×10^–9 and 3×10 ^–8 W kg^–1, we suggest that future experiments consider turbulent measurements as a standard variable.     

Feeding ecology and trophic impact of the hydroid Obelia dichotoma in the Kongsfjorden (Spitsbergen, Arctic)

Obelia dichotoma is a thecate hydroid with a worldwide distribution, occurring mainly on shallow water hard substrates. Since the trophic ecology of hydroids in polar waters is badly understood, the aim of the present work was to study qualitatively and quantitatively the diet of these organisms in an Arctic environment and to determine their trophic significance. For this purpose, the density of the hydroid population was documented, and simultaneously, zooplankton was sampled in two different years (1997 and 1998). Prey capture rates were estimated by analysing the gastrovascular content of the polyps in a diurnal cycle. Additionally, the digestion time of O. dichotoma was measured by laboratory feeding experiments using diatoms as food items. The analyses of the gastrovascular cavities of the polyps sampled during the diurnal cycles showed that O. dichotoma fed mainly on faecal pellets, organic matter and microalgae. Zooplankton prey was also observed, but gastrovascular contents and zooplankton abundance did not show any correlation in both years. The consumption rates of the hydroid populations differed between the 2 years. It was almost double (8.9 mg Carbon m^?2) in 1998 compared to 1997 (5.5 mg Carbon m^?2). The significance of the environmental variability in the feeding ecology and population dynamics of hydroids under Arctic conditions is discussed.     

The importance of small-scale turbulence in the feeding of herring larvae

Herring larvae form a large, persistent and well-defined aggregateoff southwest Nova Scotia coincident with a region of strongtidally generated turbulence. Our objective is to test the hypothesisthat turbulent, tidally well-mixed regions constitute a preferentialfeeding environment for herring larvae, and are thus the proximatecause for the apparent retention of larvae in these areas. Dimensionalanalysis, coupled with relevant biological parameters and fielddata, are used to derive two non-dimensional numbers from existingpredator-prey encounter models, and to propose a third numberwhich incorporates the minimum encounter rate required for larvalmaintenance. The results of this analysis show that when root-mean-squareturbulent velocities are of the same order of magnitude as theprey velocities, they start to influence encounter rates significantly;and that when turbulent velocities are an order of magnitudegreater than the predator velocities, they dominate the encounterprocess. At reported natural food concentrations, food itemsare on average one order of magnitude away from the larvae'sreach. The importance of turbulence as a mechanism to bringfish larvae closer to their prey is revealed by the findingthat in tidally well-mixed regions herring larvae would requireone order of magnitude less food than their counterparts inthe stratified regions. The model shows that the feeding environmenton the adjacent stratified waters off southwest Nova Scotiais detrimental for herring larvae, and that high mortality wouldbe expected if larvae had to overwinter in this region. Ouranalysis also shows that the growth pattern observed for autumn-spawnedlarvae is explained by the combination of the relevant physicaland biological scales. The overall results of this analysislend support to the hypothesis that turbulent, tidally well-mixedareas provide ideal feeding conditions for herring and may bethe proximate cause for an apparent retention.     

The effects of ambient flow velocity,colony size,and upstream colonies on the feeding success of bryozoa. I. Bugula stolonifera Ryland,an arborescent species

The effects of ambient flow velocity, colony size, and the presence of upstream colonies on the feeding success of the arborescent bryozoan, Bugula stolonifera (Ryland), were studied. Faster ambient flow velocities were found to reduce feeding of zooids of small colonies but not of large colonies. Zooids from different regions of colonies dominated in feeding at different ambient flow velocities: upstream zooids dominated in feeding from slow ambient flow: zooids from central regions dominated in feeding from fast ambient flow. These results are interpreted with respect to the branching morphology of colonies. Finally, evidence that upstream colonies interfere with the feeding success of zooids of colonies downstream was obtained.     

Effect of larval ontogeny, turbulence and light on prey attack rate and swimming activity in herring larvae

The effects of ontogeny (larval size), light and turbulence on the attack rate and swimming activity (proportion of time swimming and duration of swimming bout) of herring larvae (15-28 mm TL) have been investigated. Emphasis was put on the experimental design in order to create a set-up where the turbulence intensity distribution could be accurately measured as well as controlled in the entire experimental tank.Both larval size (ontogeny) and light had a significant positive effect on prey attack rate. Likewise, an intermediate increase in turbulence had a positive effect on prey attack rate, but this effect was dependent of light intensity and larval size.At low light (1.5 μE m² s⁻¹) intermediate turbulence increased the prey attack rate significantly for larger larvae (26 and 28 mm), while at high light (18 μE m² s⁻¹) intermediate turbulence had only a significant positive effect on the attack rate of smaller larvae 20 and 23 mm.In general, our data show a dome-shaped response of turbulence on attack rate and a U-shaped response of turbulence on swimming activity.For herring larvae >20 mm, the maximum (attack rate) and minimum (swimming activity) response of turbulence were found at intermediate turbulence intensities (energy dissipation rates between 7∗10⁻⁸ and 1∗10⁻⁶ W/kg). The highest turbulence level tested (8∗10⁻⁶ W/kg) showed only negative effects, as attack rates where at the lowest and swimming activity at the highest.Swimming activity increased with larval size or light, and decreased at intermediate turbulence. Compared to turbulent intensities under natural conditions this implies that larger herring larvae at 10 m depth have to be exposed to wind speeds of more than 17 m/s before negative effects on attack rate and swimming activity occurs.     

Combined Effects of Turbulence and Different Predation Regimes on Zooplankton in Highly Colored Water—Implications for Environmental Change in Lakes

In aquatic ecosystems, predation is affected both by turbulence and visibility, but the combined effects are poorly known. Both factors are changing in lakes in the Northern Hemisphere; the average levels of turbulence are predicted to increase due to increasing wind activities, while water transparency is decreasing, e.g., due to variations in precipitation, and sediment resuspension. We explored experimentally how turbulence influenced the effects of planktivorous fish and invertebrate predators on zooplankton when it was combined with low visibility caused by high levels of water color. The study was conducted as a factorial design in 24 outdoor ponds, using the natural zooplankton community as a prey population. Perch and roach were used as vertebrate predators and Chaoborus flavicans larvae as invertebrate predators. In addition to calm conditions, the turbulent dissipation rate used in the experiments was 10⁻⁶ m² s⁻³, and the water color was 140 mg Pt L⁻¹. The results demonstrated that in a system dominated by invertebrates, predation pressure on cladocerans increased considerably under intermediate turbulence. Under calm conditions, chaoborids caused only a minor reduction in the crustacean biomass. The effect of fish predation on cladocerans was slightly reduced by turbulence, while predation on cyclopoids was strongly enhanced. Surprisingly, under turbulent conditions fish reduced cyclopoid biomass, whereas in calm water it increased in the presence of fish. We thus concluded that turbulence affects fish selectivity. The results suggested that in dystrophic invertebrate-dominated lakes, turbulence may severely affect the abundance of cladocerans. In fish-dominated dystrophic lakes, on the other hand, turbulence-induced changes in planktivory may considerably affect copepods instead of cladocerans. In lakes inhabited by both invertebrates and fish, the response of top-down regulation to turbulence resembles that in fish-dominated systems, due to intraguild predation. The changes in planktivorous predation induced by abiotic factors may possibly cascade to primary producers.     

Green Fluorescence of Cytaeis Hydroids Living in Association with Nassarius Gastropods in the Red Sea

Green Fluorescent Proteins (GFPs) have been reported from a wide diversity of medusae, but only a few observations of green fluorescence have been reported for hydroid colonies. In this study, we report on fluorescence displayed by hydroid polyps of the genus Cytaeis Eschscholtz, 1829 (Hydrozoa: Anthoathecata: Filifera) found at night time in the southern Red Sea (Saudi Arabia) living on shells of the gastropod Nassarius margaritifer (Dunker, 1847) (Neogastropoda: Buccinoidea: Nassariidae). We examined the fluorescence of these polyps and compare with previously reported data. Intensive green fluorescence with a spectral peak at 518 nm was detected in the hypostome of the Cytaeis polyps, unlike in previous reports that reported fluorescence either in the basal parts of polyps or in other locations on hydroid colonies. These results suggest that fluorescence may be widespread not only in medusae, but also in polyps, and also suggests that the patterns of fluorescence localization can vary in closely related species. The fluorescence of polyps may be potentially useful for field identification of cryptic species and study of geographical distributions of such hydroids and their hosts.     

基于物理模型实验的光倒刺鲃生态行为学研究

建立了基于天然河道的物理模型,通过控制实验研究水环境因子对鱼类行为的影响。文中选取了金沙江下游2.6 km河段建立物理模型,以南方水系的经济鱼类光倒刺鲃为研究对象,进行了鱼的行为对底质和流速的响应实验。分析得出,光倒刺鲃对砂卵石底质的选择明显大于其它底质且差异极其显著(P<0.01),光倒刺鲃2龄幼鱼期的喜好流速范围为0.3-0.6 m/s;研究同时发现水流紊动强度对光倒刺鲃行为具有重要影响。该研究结果可以为光倒刺鲃栖息地模型提供参考。     

Sedimentation loss of phytoplankton cells from the mixed layer: effects of turbulence levels

In this paper, we describe a study of the role of turbulencein the loss by sedimentation of phytoplankton cells from themixed layer. The approach presented allows the quantificationof the sedimentation rate of phytoplankton in the whole rangeof turbulence levels of this layer. Two types of phytoplanktercan be distinguished according to the effect that turbulencecan exert on their sedimentation rate. The rate of those cellswhose settling velocity is lower than –1 m day^–1will not be modified by turbulence. The sedimentation rate ofcells with higher settling velocities can, however, be modifiedby the level of turbulence. A set of dimensionless numbers isgiven to delimit several processes that are important in thedynamics of phytoplankton sedimentation in a turbulent regime.The use of these dimensionless numbers suggests that an increasein the turbulence level in the mixed layer does not always implya decrease in the sedimentation rate of phytoplankton cells.     

Effects of small-scale turbulence on Phaeocystis globosa (Prymnesiophyceae) growth and life cycle

The response of Phaeocystis globosa to small-scale turbulence was studied in 5 l microcosms. Turbulence was generated by oscillating grids. The effect of small-scale turbulence was examined under 3 turbulence levels representative of the P. globosa natural environment, and in non-turbulent control cultures. Single cell numbers, nitrogen concentrations and colony formation (number and diameter) were followed over 13 days in each experimental culture. Small-scale turbulence decreased single cell growth and also influenced colony formation. More colonies were formed when turbulence increased to a given threshold, but above this turbulence level, fewer and smaller colonies were observed in P. globosa cultures. The ecological significance of these results, particularly, the potential influence of small-scale turbulence on competition mechanisms between P. globosa and diatoms are finally discussed and suggested as a key factor to understand phytoplankton successions in the Eastern English Channel.     

Commensal vs. parasitic relationship between Carapini fish and their hosts: some further insight through δC and δN measurements

In the Moorea Lagoon (French Polynesia), the pearlfish Carapus boraborensis, Carapus homei, Carapus mourlani and Encheliophis gracilis are generally found inside echinoderm hosts such as the holothurian Bohadschia argus and the starfish Culcita novaeguineae. At the end of their larval stage, these fish settle on the reef and directly enter their echinoderm host where they undergo an important metamorphosis. The aim of this study was to get further insight on the type of symbiosis (commensal vs. parasite) between these fish and their hosts. δ¹⁵N and δ¹³C measurements were determined in the tissues of invertebrate hosts (holothurians and starfish) and carapids (larvae, juveniles and adults). The obtained isotopic signatures reveal different kinds of associations: metamorphosing larvae, juveniles and adults of C. boraborensis and C. homei do not feed at all on host holothurian tissues, C. mourlani and its asterian host display a commensal relationship without any feeding association, while E. gracilis is likely to feed on the tissue of the holothurian.     

Giant scallop feeding and growth responses to flow

The relationship between ambient seawater flow velocity and growth of the giant scallop Placopecten magellanicus Gmelin is shown to be a reverse ramp function with growth inhibition at flow velocities of > 10–20 cm · s⁻¹. The mechanism of inhibition involves a reduction in ration as velocities around the scallop increase. In ambient flows which are sufficient to cause overloading of the scallop gill, the feeding/filtration rate is reduced by an unknown mechanism, possibly involving the mantle edge closing or a gill bypass mechanism operating. In ambient flows where the pressure at the exhalant opening exceeds the inhalant plus the pressure head created by the gill, as when the scallop is placed dorsal edge to the flow, the tendency for flow reversal is resisted by a similar mechanism involving a reduction in feeding/filtration rate.     

The effects of ambient flow velocity,colony size,and upstream colonies on the feeding success of Bryozoa. II. Conopeum reticulum (Linnaeus), an encrusting species

The effects of ambient flow velocity, colony size, and the presence of an actively-feeding colony upstream on the feeding success of the encrusting bryozoan Conopeum reticulum (Linnaeus) were studied. Zooids from both large and small colonies showed a reduction in feeding as flow velocity increased, however, the reduction in feeding was less for zooids from large colonies except at very fast ambient flow velocities. The greater pumping capacity of large colonies may result in a relatively greater per zooid feeding success from moving water. The presence of an actively-feeding colony upstream was found to enhance the feeding of zooids on downstream colonies. Diversion of flowing water by actively-feeding colonies upstream may account for the observed enhancement of feeding by zooids on colonies downstream.The results from this study on an encrusting species are compared with results from a previous study on feeding from flow by an arborescent bryozoan, and the feeding performances of these two colony types are related to their respective flow microhabitats.     

The adaptive pattern of growth and reproduction of the colonial hydroid Clavopsella michaeli

The effects of variations in temperature, salinity and water movement on the laboratory growth and reproduction of a single clone of the bougainvilliid hydroid Clavopsella michaeli were examined. Unfavourable conditions resulted in reproducible oscillations of growth rate. They ranged from overall stimulatory effects (hormesis) to irreversible inhibition and shrinkage of colonies. Exposure to low concentrations (0.1–5 µg l^–1) of copper and mercury ions produced similar responses, which were therefore regarded as non-specific. A control mechanism, dampening these growth rate oscillations, supported higher colonial tolerance, that is adaptation, to environmental stresses of low intensity. When the counteractive capacity of the control mechanism was exceeded, inhibition occurred. Higher frequencies of gonozooids were observed in stressed colonies and were interpreted as an adaptive response.     

Conversion and Distribution of Cobalamin in Euglena gracilis z, with Special Reference to Its Location and Probable Function within Chloroplasts

Cobalamin is essentially required for growth by Euglena gracilis and shown to be converted to coenzyme forms promptly after feeding cyanocobalamin. Concentrations of coenzymes, methylcobalamin, and 5′-deoxyadenosylcobalamin, reached about 1 femtomole/10⁶ cells 2 hours after feeding cyanocobalamin to cobalamin-limited cells. Cobalamins all were bound to proteins in Euglena cells and located in subcellular fractions of chloroplasts, mitochondria, microsomes, and cytosol. Incorporated cobalamin into chloroplasts was localized in thylakoids. Methylcobalamin existed in chloroplasts, mitochondria, and cytosol, while 5′-deoxyadenosylcobalamin was in mitochondria and the cytosol, 2 h after feeding cyanocobalamin to Euglena cells. Quantitative alterations of methylcobalamin and 5′-deoxyadenosylcobalamin in chloroplasts suggest their important functions as coenzymes in this organelle. The occurrence of functional cobalamins in chloroplasts has not been reported in other photosynthetic eukaryotes.     

The effects of algae species and densities on the population growth of the marine rotifer, Colurella dicentra

Colurella dicentra clones isolated from bay water in the Mississippi Gulf Coast were cultured with artificial seawater. Experiments were conducted to determine the effects of six algae species (Nannochloropsis oculata, Tetraselmis chuii, Chaetoceros gracilis, Rhodomonas salina, Isochrysis galbana, and Prorocentrum micans), six C. gracilis densities, and six N. oculata densities (25,000, 50,000, 100,000, 250,000, 500,000, and 1,000,000 cells ml^− 1) on C. dicentra population growth. Algae type influenced rotifer production (p < 0.0001). C. gracilis treatment (9120 ± 3351SD) produced the highest number of rotifers followed by N. oculata (5760 ±2232SD). P. micans had the lowest number of rotifers, although not significantly different from numbers in T. chuii, R. salina, and I. galbana treatments (p > 0.05).The population growth rate (r) varied with algae species treatment. The highest values were recorded for C. gracilis treatment (0.22 to 0.26 d^− 1), followed by N. oculata (0.21 to 0.24 d^− 1), and the lowest for P. micans (− 0.19 to 0.14 d^− 1). C. gracilis and N. oculata densities had significant effects (p < 0.0001) on C. dicentra population growth. The highest rotifer production was recorded at a C. gracilis density of 100,000 cells ml^− 1, followed by 250,000 cells ml^− 1 and 50,000 cells ml^− 1. Algae densities of 500,000 cells ml^− 1 and above produced the lowest rotifer numbers. Population growth rate (r) varied with C. gracilis densities. The highest values were observed for C. gracilis concentrations of 100,000 cells ml^− 1 (0.17 to 0.19 d^− 1), and the lowest for concentrations of 500,000 cells ml^− 1 and above (− 0.19 to 0.09 d^− 1). The 100,000 cells ml^− 1N. oculata density gave the highest rotifer production followed by 50,000, 250,000, 25,000, and 500,000 cells ml^− 1. Algae densities of 1,000,000 cells ml^− 1 produced the lowest rotifer numbers. Population growth rate (r) varied with N. oculata densities, with the highest values obtained for algae densities of 100,000 cells ml^− 1 (0.35 to 0.40 d^− 1), and the lowest for concentrations of 1,000,000 cells ml^− 1 (0.05 to 0.012 d^− 1). This is the first report of C. dicentra in Mississippi Coastal waters, and perhaps the smallest marine rotifer species (93 by 49 μm) ever cultured successfully.     

Effect of turbulence on the feeding and moulting of the cirripede Balanus Balanoides (L.) given an algal diet

The shell growth, moulting, and faecal pellet production of Balanus balanoides (L.) have been investigated experimentally in turbulent and quiet conditions. In turbulent conditions an algal food level of 50% was sufficient to maintain the same shell growth as at the 100% level, whereas at this level in quiet conditions growth was less than at 50% with turbulence. At the 10% food level growth was markedly reduced in quiet conditions. On the assumption that production of faeces is a measure of feeding activity, the latter is stimulated by turbulence as is the moulting rate. The relation between production of faeces and moulting is, however, independent of water movement. Faecal pellet production continues at higher algal food levels after the growth rate has become constant indicating that when assimilation is maximal there is no feed-back mechanism regulating food intake.     

Carnivorous pitcher plant facilitates its ant prey

Positive species interactions are ubiquitous in natural communities. However, positive species interactions are often overlooked or undetected because they tend to be more context dependent—that is, outcomes of interactions may change from positive to neutral/negative, depending on the ecological contexts in which these take place. It was recently shown that the pitchers of the carnivorous plant, Nepenthes gracilis, trapped more ant prey when found in close proximity to a congener, N. rafflesiana. As pitchers usually succeed in trapping only a fraction of all pitcher visitors, it was proposed that N. rafflesiana feeds ant colonies in its immediate vicinity with its copious nectar secretions. We tested this hypothesis using ex situ experiments. Philidris sp., Crematogaster treubi, and C. ferrarii colonies were reared in the presence or absence of N. rafflesiana plants in enclosed terraria. As such an interaction has the potential of being context dependent, the interaction was tested under high and low feeding conditions. Low feeding conditions resulted in strong declines in ant colony health, but this was mitigated by the presence of N. rafflesiana, although N. rafflesiana had no significant effect on ant colony health under high feeding conditions. This was observed even though pitchers of N. rafflesiana trapped large numbers of ant workers. Our results show that prey facilitation by a predator is possible, at least in eusocial insects like ants, and that prey facilitation conforms to the central prediction of the stress gradient hypothesis in being more positive at higher stress (lower resource) levels.     

The effect of density and mutual interference by a Predator: A laboratory study of predation by the Nudibranch Coryphella rufibranchialis on the hydroid Tubularia larynx

The nudibranch Coryphella rufibranchialis (JOHNSTON) feeds on a variety of hydroids, including Tubularia larynx Ellis & Solander. Experiments in which density of prey and predators were altered showed that more prey were eaten as prey density increased. However, more prey were consumed at low predator densities, presumably because of mutual interference among nudibranchs at the higher predator densities. The number of prey consumed per nudibranch was maximal with low predator densities and a ratio of 25–50 polyps per predator. Coryphella seems to show an opportunistic feeding strategy involving solitary predators rapidly depleting hydroid colonies and moving on to new colonies.