Phytoplankton aggregate formation: observations of patterns and mechanisms of cell sticking and the significance of exopolymeric material

Flocculation of ‘sticky’ phytoplankton cells intorapidly sinking aggregates has been invoked as a mechanism explainingmass sedimentation of phytoplankton blooms in the ocean. Phytoplanktonstickiness, defined as the probability of adhesion upon collision,is one key factor determining the potential for aggregate formation.In the laboratory, we examined variation in stickiness in fivespecies of diatoms and two species of flagellates grown in batchcultures. We also investigated the production of paniculatemucus by phytoplankton cells and its role in aggregate formation,and we studied the effects of solute exudates on cell stickiness.Four of the five diatoms investigated were significantly sticky,while one diatom and both of the flagellates were not sticky.Stickiness varied considerably within species. In the diatomSkeletonema costatum, the typical but not entirely consistentpattern was that stickiness decreased with age of the batchcultures. We were otherwise unable to establish consistent relationshipsbetween cell stickiness and the growth stage of the algae, environmentalconcentrations of inorganic nutrients, and abundances of suspendedand epiphytic bacteria. We showed that the diatom S.costatumat times excretes a solute substance that depresses flocculation.This may reduce cell losses from the euphotic zone during thegrowth phase due to flocculation and sedimentation. We demonstratedtwo different mechanisms of phytoplankton aggregate formation.In the diatom S.costatum, the cells are sticky in themselves,and coagulation depends on cell-cell sticking and does not involvemucus. Aggregates are composed solely of cells. Cells of thediatom Chaetoceros affinis, on the other hand, are not in themselvessticky. Transparent exopolymeric particles (TEP), produced bythe diatom, cause the cells to aggregate and coagulation dependson TEP-cell rather than cell-cell sticking. Aggregates are formedof a mixture of mucus and cells. We found several species ofdiatoms and one flagellate species to produce copious amountsof TEP. TEP from some species (e.g. Coscinodiscus sp.) is stickyand may cause other, non-sticky particles to coagulate. Thisemphasizes the potential importance of diatom-derived paniculatemucus for particle flocculation in the ocean.     

Haramonas dimorpha gen. et sp. nov. (Raphidophyceae), a new marine raphidophyte from Australian mangrove*

A new raphidophyte flagellate is described from the river mouth of the Daintree River, north-east Australia where there are extensive mangrove forests. The organism has two distinct phases: a club-shaped motile form, and a more or less spherical benthic non-motile form. The motile cell is metabolic and possesses 10–20 chloroplasts. The chloroplasts are imbricated, somewhat reminiscent of roofing tiles. A unique structure has been found at the posterior end of the cell. It is a tubular invagination, which consists of a single membrane supported by many small flattened vesicles. In addition, the structure is surrounded by many fibril-containing vesicles. The tubular invagination is different from both the contractile vacuoles and the pusules of dinoflagellates in both behavior and structure. Based on the possession of these unique features, it is concluded that the this mangrove flagellate should be described as a new species in a new genus and the name Haramonas dimorpha Horiguchi gen. et sp. nov. is proposed.     

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     

Producing mechanism of an algicidal compound against red tide phytoplankton in a marine bacterium γ-proteobacterium

Strain MS-02-063, γ-proteobacterium, isolated from a coast area of Nagasaki, Japan, produced a red pigment which belongs to prodigiosin members. This pigment, PG-L-1, showed potent algicidal activity against various red tide phytoplanktons in a concentration-dependent manner. An understanding of a mechanism of PG-L-1 production by this marine bacterium may yield important new insights and strategies for preventing blooms of harmful flagellate algae in natural marine environments. Therefore, we analyzed the mechanisms of PG-L-1 production. In our previous study, the pigment production by this marine bacterium was completely inhibited at 1.56 μg/ml of erythromycin or 3.13 μg/ml of chloramphenicol, while minimal inhibitory concentrations for cell growth of erythromycin and chloramphenicol against this bacterium were >100 and 25 μg/ml, respectively. It is interesting to note that the ability of the pigment production in erythromycin-treated bacterium recovered by an addition of homoserine lactone. In fact, the pigment production was inhibited by β-cyclodextrin that inhibits autoinducer activities by a complex with N-acyl homoserine lactones. N-acyl homoserine lactones with autoinducer activities are ubiquitous bacterial signaling molecules that regulate gene expression in a cell density dependent process known as quorum sensing. Therefore, it was suggested that PG-L-1 produced by strain MS-02-063 is controlled by the homoserine lactone quorum sensing. It is speculated that this quorum sensing is involved in the production of algicidal agents of other marine bacteria. This bacterium and other algicidal bacteria might be concerned in regulating the blooms of harmful flagellate algae through the quorum sensing system.     

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.     

High Motility Reduces Grazing Mortality of Planktonic Bacteria

We tested the impact of bacterial swimming speed on the survival of planktonic bacteria in the presence of protozoan grazers. Grazing experiments with three common bacterivorous nanoflagellates revealed low clearance rates for highly motile bacteria. High-resolution video microscopy demonstrated that the number of predator-prey contacts increased with bacterial swimming speed, but ingestion rates dropped at speeds of >25 μm s⁻¹ as a result of handling problems with highly motile cells. Comparative studies of a moderately motile strain (<25 μm s⁻¹) and a highly motile strain (>45 μm s⁻¹) further revealed changes in the bacterial swimming speed distribution due to speed-selective flagellate grazing. Better long-term survival of the highly motile strain was indicated by fourfold-higher bacterial numbers in the presence of grazing compared to the moderately motile strain. Putative constraints of maintaining high swimming speeds were tested at high growth rates and under starvation with the following results: (i) for two out of three strains increased growth rate resulted in larger and slower bacterial cells, and (ii) starved cells became smaller but maintained their swimming speeds. Combined data sets for bacterial swimming speed and cell size revealed highest grazing losses for moderately motile bacteria with a cell size between 0.2 and 0.4 μm³. Grazing mortality was lowest for cells of >0.5 μm³ and small, highly motile bacteria. Survival efficiencies of >95% for the ultramicrobacterial isolate CP-1 (≤0.1 μm³, >50 μm s⁻¹) illustrated the combined protective action of small cell size and high motility. Our findings suggest that motility has an important adaptive function in the survival of planktonic bacteria during protozoan grazing.     

Plunder of Human Blood Leukocytes Containing Ingested Material,by Other Leukocytes: Where Is the Fusagen That Allows Preservation of Membrane Integrity and Motile Function?

In studying phagocytosis of zymosan particles by human blood monocytes in phase-contrast videomicroscopy, we found that monocytes loaded with zymosan particles became chemotactic for polymorphonuclear leukocytes (PMN) which closed on them and purloined their particle content. This despoliation usually occurred in monocytes that had begun to swell—prefiguring their death. The violent seizure of their contents by the aggressing PMN often tore the monocytes apart. However, some apparently healthy monocyte survived the removal of zymosan content by PMN or, more commonly, its removal by another monocyte. PMN—a much hardier cell in slide preparations—that were similarly loaded with zymosan particles, also attracted PMN. The latter could remove zymosan from the target cell without killing it. Thus, leukocytes were sacrificing significant portions of themselves without losing residual membrane integrity and motile function. Their behavior with respect to other particles (e.g., bacteria) will be of interest. We suggest that the membrane fusagen resides in the inner membrane leaflets when they are brought together in an extreme hourglass configuration. This event may be similar to the fragmentation of erythrocytes into intact pieces, the formation of cytokineplasts, the rear extrusion of content by migrating cells on surfaces, and the phagocytic process itself.     

Sinking rates of heterogeneous, temperate phytoplankton populations

Throughout the summer of 1978, the sinking rates of phytoplanktonwithin the Controlled Experimental Ecosystems (CEE's) were monitoredusing a technique based upon measurement of the transit timeof radioactively (¹⁴C) labeled cells. The experimental frameworkof FOODWEB 1 offered an unprecedented opportunity to documentthe sinking rates of heterogeneous phytoplankton of diversetaxonomic composition, growing under a variety of nutrient regimes;the absence of advective exchange in the CEE's provided knowledgeof the preconditioning history of the phytoplankton sampledat any given time. Sinking rates of whole phytoplankton assemblages (not size-fractioned)ranged from 0.32 – 1.69 m·day^–1; the averagerate (± s.d.) observed was 0.64 ± 0.31 m·day^–1.The most notable deviations from the mean value occurred whenthe population size distribution and taxonomic composition shifteddue to blooms. The relationship between phytoplankton sinkingand ambient nutrient levels was studied by following the ratesof a given size fraction (8–53 µm) for ten daysfollowing nutrient enrichment of a CEE. Over this time sinkingrates ranged from 1.08– 1.53 m·day^–1; decreasedrates occurred after nutrification, yet over the course of theentire trial sinking rates were not significantly (p >0.05)correlated to the ambient levels of any single nutrient species. The sinking rate implications of spore formation were also studied,and showed that sinking rates of Chaetoceros constrictus andC. socialis spores (2.75 ± 0.61 m·day^–1)were ca 5-fold greater than rates measured when the vegetativestages of these species dominated the population, reflectingthe influence of physiological mechanisms in controlling phytoplanktonbuoyancy. An example of the potential influence of colony formation uponbuoyancy was noted in observations of C. socialis which occasionallywas found to exist in large spherical configurations made ofcoiled cell chains and having low (0.40 m·day^–1)sinking rates. A hydrodynamic rationale is presented to showhow such a colony together with enveloped water may behave asa unit particle having lower excess density, and therefore lowobserved sinking rate, despite its conspicuously large size. Overall, sinking rates were not significantly correlated withany of the measured nutrient or photic parameters. There were,however, trials and comparisons which showed evidence for: (1)higher sinking rates in populations dominated by large cells,(2) decreased sinking rates after nutrient enrichment, and (3)buoyancy response to light levels. It is suggested that correlationof sinking rates with synoptic environmental measurements atany given time is not explicit because the associations mayinvoke lag times of physiological response. The interpretationmade from these findings is that the preconditioning historyof the population, rather than the prevailing conditions atthe time of a given measurement, is most closely associatedwith population buoyancy modifications.     

Production of ketocarotenoids by microalgae

Among the highly valued ketocarotenoids employed for food coloration, astaxanthin is probably the most important. This carotenoid may be produced biotechnologically by a number of microorganisms, and the most promising seems to be the freshwater flagellate Haematococcus pluvialis (Chlorophyceae), which accumulate astaxanthin in their aplanospores. Many physiological aspects of the transition of the flagellate into aplanospores have been described. Mixotrophic cultivation and suitable irradiance may result in fairly good yields (up to 40 mg/l; 43 mg/g cell dry weight) within a reasonable time, under laboratory conditions. In order to compete with synthetic astaxanthin, suitable scaling-up is required. However, large-scale production in open ponds has proved unsatisfactory because of severe contamination problems. A selective medium might overcome this difficulty. Further research for the development of suitable strains is thus warranted. Received: 8 July 1998 / Received revision: 12 November 1998 / Accepted: 14 November 1998     

Photokinesis and Photophobic Responses in the Gliding Flagellate, Euglena mutabilis

A pronounced photokinesis (as indicated by increases in bothpercentage of motile cells and average speed of movement) aswell as step-up and step-down photophobic responses at light/darkboundaries in the gliding flagellate Euglena mutabilis was studiedusing time-lapse video-microscopy. The spectral sensitivitiesof all the observed lightdependent motor responses were similarto each other and showed an activity throughout the whole visiblespectrum with maximum peaks at about 410, 450, 470, 530, 580and 650 nm and a pronounced minimum at about 600 nm. Thus, thephotoreceptor pigments markedly differ from the closely relatedswimming flagellate Euglena gracilis in which a flavin typeblue light receptor is supposed to be responsible for the stimulusperception. Light microscopic studies of mucilage distributionand regeneration suggested that the gliding movements are effectedby parallel gliding of adjacent pellicular strips while themucilage produced on the cell surface and transported to therear end seems to be responsible for adhesion of the cell onthe substrate. (Received November 21, 1985; Accepted January 30, 1986)     

The passage of two species of micro-algae through the gut of Mytilus edulis L.

The contents of fecal ribbons from Mytilus edulis L. fed either the unicellular flagellate Tetraselmis suecica (Kylin) Butch., or the diatom Thalassiosira pseudonana (Hasle and Heimdal) were examined for their particle size spectra. Contents of fecal ribbons were dispersed by shaking in filtered sea water and analyzed by a Coulter Counter. The flagellate can pass through the mussel's gut without any significant volume changes, implying that its digestion is largely intracellular (after phagocytosis). In contrast, the diatom does undergo volume changes, probably a result of disintegration of cells and aggregation of cells and cell fragments. Undigested flagellate cells can become resuspended in significant quantities after their defecation by the mussel.     

High motility reduces grazing mortality of planktonic bacteria

We tested the impact of bacterial swimming speed on the survival of planktonic bacteria in the presence of protozoan grazers. Grazing experiments with three common bacterivorous nanoflagellates revealed low clearance rates for highly motile bacteria. High-resolution video microscopy demonstrated that the number of predator-prey contacts increased with bacterial swimming speed, but ingestion rates dropped at speeds of >25 microm s(-1) as a result of handling problems with highly motile cells. Comparative studies of a moderately motile strain (<25 microm s(-1)) and a highly motile strain (>45 microm s(-1)) further revealed changes in the bacterial swimming speed distribution due to speed-selective flagellate grazing. Better long-term survival of the highly motile strain was indicated by fourfold-higher bacterial numbers in the presence of grazing compared to the moderately motile strain. Putative constraints of maintaining high swimming speeds were tested at high growth rates and under starvation with the following results: (i) for two out of three strains increased growth rate resulted in larger and slower bacterial cells, and (ii) starved cells became smaller but maintained their swimming speeds. Combined data sets for bacterial swimming speed and cell size revealed highest grazing losses for moderately motile bacteria with a cell size between 0.2 and 0.4 microm(3). Grazing mortality was lowest for cells of >0.5 microm(3) and small, highly motile bacteria. Survival efficiencies of >95% for the ultramicrobacterial isolate CP-1 (< or =0.1 microm(3), >50 microm s(-1)) illustrated the combined protective action of small cell size and high motility. Our findings suggest that motility has an important adaptive function in the survival of planktonic bacteria during protozoan grazing.     

A Model for Bacterial Colonization of Sinking Aggregates

Sinking aggregates provide important nutrient-rich environments for marine bacteria. Quantifying the rate at which motile bacteria colonize such aggregations is important in understanding the microbial loop in the pelagic food web. In this paper, a simple analytical model is presented to predict the rate at which bacteria undergoing a random walk encounter a sinking aggregate. The model incorporates the flow field generated by the sinking aggregate, the swimming behavior of the bacteria, and the interaction of the flow with the swimming behavior. An expression for the encounter rate is computed in the limit of large Péclet number when the random walk can be approximated by a diffusion process. Comparison with an individual-based numerical simulation is also given.     

Swimming with protists: perception, motility and flagellum assembly

In unicellular and multicellular eukaryotes, fast cell motility and rapid movement of material over cell surfaces are often mediated by ciliary or flagellar beating. The conserved defining structure in most motile cilia and flagella is the '9+2' microtubule axoneme. Our general understanding of flagellum assembly and the regulation of flagellar motility has been led by results from seminal studies of flagellate protozoa and algae. Here we review recent work relating to various aspects of protist physiology and cell biology. In particular, we discuss energy metabolism in eukaryotic flagella, modifications to the canonical assembly pathway and flagellum function in parasite virulence.     

Determining the mass density of marine copepods and their eggs with a critical focus on some of the previously used methods

We collected Calanus finmarchicus copepodites CIV, CV and CVIfemales in a deep fjord on the west coast of Norway during April1996, May 1997 and November 1998. Eggs of C. finmarchicus andCalanus glacialis were collected during May 1989 in the BarentsSea. The sinking speeds of animals and eggs were measured ina homogeneous column with seawater of known density, and Stokeslaw was applied to estimate their mass density. Also the densitycontrast between the organisms and seawater was calculated.The mean mass density of C. finmarchicus ranged from 1.0274to 1.0452 g cm^–3. During spring copepodite stage CV hada significantly lower mean mass density (1.0345 g cm^–3)compared to CIV (1.0381 g cm^–3) and CVI females (1.0408g cm^–3). Copepods collected during winter had a distinctlylower mass density. The sinking speed of C. glacialis eggs followeda unimodal distribution, with a mean of 25.9 m day^–1,while sinking speeds of C. finmarchicus eggs were bimodal, thetwo groups of eggs having a mean sinking speed of 23.3 m day^–1and 35.4 m day^–1 respectively. Correspondingly the meanmass density was 1.0556 g cm^–3 for C. glacialis eggs andfor the two groups of C. finmarchicus eggs 1.0639 g cm^–3and 1.0812 g cm^–3. Results of earlier work, particularlyusing density gradient methods to determine mass density ofzooplankton, are critically reviewed, and it is suggested thatthis method should not be used to determine the mass densityof small organisms the size of C. finmarchicus.     

Evidence for microbial attenuation of particle flux in the Amundsen Gulf and Beaufort Sea: elevated hydrolytic enzyme activity on sinking aggregates

Export of autochthonously produced particulate organic carbon (POC) is a globally important mechanism for sequestering carbon in the deep sea. The role of microbial hydrolytic activity in attenuating POC flux is generally understudied, and particularly complex on Arctic continental shelves influenced by other sources of POC. To evaluate this role, we used fluorogenic substrate analogs to measure extracellular enzyme activity (EEA) associated with particle size fractions considered suspended (1–70 μm) and sinking (>70 μm). Samples were collected by in situ filtration at depths of 25–100 m at ten stations (156–1,142 m deep) in the Amundsen Gulf and Beaufort Sea in June–July, 2008, during the Circumpolar Flaw Lead project. Significant positive correlations observed between EEA and both chlorophyll a and δ¹³C_POC suggest that EEA is elevated in waters dominated by marine-derived POC. No difference in bulk EEA was observed between size fractions, but POC- and cell-specific EEA was significantly elevated on sinking aggregates. Calculations show that 2–44% of carbon retention in surface waters could be attributed to mobilization by enzymes associated with sinking aggregates, and up to 57% if enzymes associated with suspended particles are included. Model results suggest that microbial attenuation of POC below the euphotic zone is a quantitatively important mechanism for carbon loss, especially when particles are sinking slowly. The role of microbes in attenuating POC flux on Arctic shelves appears to have been underestimated previously and may become increasingly important if climate warming brings increased marine productivity.     

The effect of electrostatic charge of food particles on capture efficiency by Oxyrrhis marina Dujardin (dinoflagellate)

Laboratory experiments were carried out to investigate the effect of food quality, measured as surface charge of the particles, on capture efficiency and ingestion rate by the heterotrophic dinoflagellate Oxyrrhis marina. Fluorescent particles in two size classes of around 1 and 4 microm and of 7 different qualities were offered to the flagellate: carbohydrate and albumin particles, the algae Synechocystis spec. and Chlorella spec., carboxylated microspheres, silicate particles and bacteria. Rates of particle uptake showed significant differences depending on particle size and quality, and ranged from 0 to 4 particles cell(-1) h(-1). Ingestion rates were up to 4 times higher for 4 pm particles than for 1 microm particles, which indicates strong size-selective feeding. Our main result is that the surface charge or zeta potential, of artificial particles, i.e. carboxylated microspheres (> or = -107 mV) and silicate particles, strongly differ from more natural and natural food (< or = -17 mV). For both size classes Oxyrrhis had ingestion rates up to 4 times higher for particles with less negative charge, such as albumin particles or algae. Thus, the zeta potential of the model food should be considered in experimental design. Particles with a zeta potential similar to that of natural food, e.g. albumin, seem to be the preferred model food.     

Unique features of the motility and structures in the flagellate polar region of Campylobacter jejuni and other species: an electron microscopic study

Similarly to Helicobacter pylori but unlike Vibrio cholerae O1/O139, Campylobacter jejuni is non‐motile at 20°C but highly motile at ≥37°C. The bacterium C. jejuni has one of the highest swimming speeds reported (>100 μm/s), especially at 42°C. Straight and spiral bacterial shapes share the same motility. C. jejuni has a unique structure in the flagellate polar region, which is characterized by a cup‐like structure (beneath the inner membrane), a funnel shape (opening onto the polar surface) and less dense space (cytoplasm). Other Campylobacter species (coli, fetus, and lari) have similar motility and flagellate polar structures, albeit with slight differences. This is especially true for Campylobacter fetus, which has a flagellum only at one pole and a cup‐like structure composed of two membranes.     

Remote prey detection in Oithona similis: hydromechanical versus chemical cues

We quantified prey encounter rates and prey reaction distancesin the ambush-feeding cyclopoid copepod Oithona similis by videorecording freely swimming copepods at different concentrationsof prey, the dinoflagellate Gymnodinium dominans. Prey encounterrate increased with prey concentration, and a maximal clearancerate of 0.42 ± 0.10 ml h^–1 was estimated. The averagedistance (from the antennules) at which O.similis reacts toprey is 0.014 ± 0.007 cm. A simple prey encounter modelwas used to combine observed predator and prey velocities andprey reaction distance, and yielded a clearance rate similarto that estimated directly from prey encounter rates. The observedprey reaction distance was consistent with that estimated froma published model of hydromechanical prey perception. The possibilityof remote chemodetection was examined by modeling the distributionof solutes leaking out of a swimming cell. The cell leaves along slender chemical trail in its wake. However, since theambush-feeding O.similis is essentially stationary when perceivingprey, it is the width rather than the length of the trail thatmatters. Owing to advection, the chemical signal vanishes almostinstantaneously off the sides of the swimming flagellate, andsolute concentrations are below any likely detection thresholdwithin 40–50 µm from the flagellate. Our observationsare thus inconsistent with remote chemodetection in O.similis.The considerations are generalized, and it is concluded thatambush-feeding copepods, unlike cruisers and suspension feeders,cannot utilize chemical signals for the detection of individualprey, but rely on either hydromechanical detection or directinterception of prey.