Feeding behavior in freshwater bryozoans: function,form, and flow

Bryozoans are impressively active suspension feeders, with diverse feeding behaviors. These have been studied extensively in marine bryozoans, but less so in their freshwater counterparts. Here we identified 16 distinct behaviors in three phylactolaemate species and classified them into behaviors involving separate tentacles, groups of tentacles, lophophore arms, the introvert, or multiple zooids. We examined (1) the repertoire of behaviors in each species, and each behavior's (2) absolute frequency, (3) relative frequency and (4) duration in each of the three species, at two flow velocities (0 and 0.2 cm s^?1). Nine feeding behaviors were shared by all three species, but the occurrence of other behaviors in a given species was limited by its morphology. Behaviors involved in particle capture were the most frequent, and were often faster than the reactions involved in particle rejection. By contrast, the absolute frequency of behaviors varied widely among species without clear associations with species form, or function of the behavior. Flow velocity had only minor effects on the feeding behaviors exhibited by a species, or their frequencies or durations. Our results show that phylactolaemates have the same key feeding behaviors of the individual polypides (especially involving separate tentacles) as previously described in gymnolaemate and stenolaemate bryozoans, although their behaviors tend to be carried out more slowly than those of stenolaemates or gymnolaemates. Feeding behaviors involving multiple zooids were nearly absent in the studied phylactolaemates, but are common in gymnolaemates. Freshwater bryozoans appear to be intermediate between stenolaemate and gymnolaemate bryozoans in terms of richness of the repertoire of feeding behaviors.     

Feeding ecology of the earliest vertebrates

Based on protochordates and extant fish, the earliest Palaeozoic vertebrates were microphagous suspension-feeding animals that pumped food-carrying water very slowly and thus required highly concentrated suspensions. Such conditions exist in benthic (not open water) aquatic environments. Feeding modes which on the basis of extant fish are closely related to benthic microphagous suspension feeding include deposit feeding, epilithic algal scraping, and macrophagous suspension feeding; early jawless vertebrates are predicted to have included all these feeding types. The gnathostome condition is predicted to have followed an initial switch from feeding on suspensions to taking tiny individual food particles (microphagous suspension-feeding → microphagous particulate-feeding → macrophagous particulate-feeding).     

Cinefilms of particle capture by an induced local change of beat of lateral cilia of a bryozoan

A local disruption of the metachronal wave always accompanies capture of algal cells by tentacles of Flustrellidra hispida (Fabricius). Beat changes for ≈0.2 s over ≈100μm of the ciliated band during capture of a 10-μm particle. The halted parcel of water is therefore larger than the particle of food but much smaller than the flow that continues past the tentacles elsewhere. These events are consistent with the hypothesis that an induced local reversal of beat concentrates particles for those suspension feeders that retain particles upstream from a band of simple cilia (adults or larvae of bryozoans, brachiopods, phoronids, hemichordates, and echinoderms). These events are not explained by other hypotheses that have been advanced for concentration of particles by these suspension feeders. Aerosol filtration models of direct interception are not applicable to this type of ciliary suspension feeder because retention depends on the magnitude of a stimulus and response to it. The stimulus will not be the same function of diameter of the food particle, and response is unlikely below a threshold stimulus.     

Advances in the Study of Feeding Behaviors, Mechanisms, and Mechanics of Sharks

Sharks as a group have a long history as highly successful predatory fishes. Although, the number of recent studies on their diet, feeding behavior, feeding mechanism, and mechanics have increased, many areas still require additional investigation. Dietary studies of sharks are generally more abundant than those on feeding activity patterns, and most of the studies are confined to relatively few species, many being carcharhiniform sharks. These studies reveal that sharks are generally asynchronous opportunistic feeders on the most abundant prey item, which are primarily other fishes. Studies of natural feeding behavior are few and many observations of feeding behavior are based on anecdotal reports. To capture their prey sharks either ram, suction, bite, filter, or use a combination of these behaviors. Foraging may be solitary or aggregate, and while cooperative foraging has been hypothesized it has not been conclusively demonstrated. Studies on the anatomy of the feeding mechanism are abundant and thorough, and far exceed the number of functional studies. Many of these studies have investigated the functional role of morphological features such as the protrusible upper jaw, but only recently have we begun to interpret the mechanics of the feeding apparatus and how it affects feeding behavior. Teeth are represented in the fossil record and are readily available in extant sharks. Therefore much is known about their morphology but again functional studies are primarily theoretical and await experimental analysis. Recent mechanistic approaches to the study of prey capture have revealed that kinematic and motor patterns are conserved in many species and that the ability to modulate feeding behavior varies greatly among taxa. In addition, the relationship of jaw suspension to feeding behavior is not as clear as was once believed, and contrary to previous interpretations upper jaw protrusibility appears to be related to the morphology of the upper jaw-chondrocranial articulation rather than the type of jaw suspension. Finally, we propose a set of specific hypotheses including: (1) The functional specialization for suction feeding hypothesis that morphological and functional specialization for suction feeding has repeatedly arisen in numerous elasmobranch lineages, (2) The aquatic suction feeding functional convergence hypothesis that similar hydrodynamic constraints in bony fishes and sharks result in convergent morphological and functional specializations for suction feeding in both groups, (3) The feeding modulation hypothesis that suction capture events in sharks are more stereotyped and therefore less modulated compared to ram and bite capture events, and (4) The independence of jaw suspension and feeding behavior hypothesis whereby the traditional categorization of jaw suspension types in sharks is not a good predictor of jaw mobility and prey capture behavior. Together with a set of questions these hypotheses help to guide future research on the feeding biology of sharks.     

Spore and micro-particle capture on an immunosensor surface in an ultrasound standing wave system

The capture of Bacillus subtilis var. niger spores on an antibody-coated surface can be enhanced when that coated surface acts as an acoustic reflector in a quarter wavelength ultrasonic (3 MHz) standing wave resonator. Immunocapture in such a resonator has been characterised here for both spores and 1 microm diameter biotinylated fluorescent microparticles. A mean spatial acoustic pressure amplitude of 460 kPa and a frequency of 2.82 MHz gave high capture efficiencies. It was shown that capture was critically dependent on reflector thickness. The time dependence of particle deposition on a reflector in a batch system was broadly consistent with a calculated time of 35 s to bring 95% of particles to the coated surface. A suspension flow rate of 0.1 ml/min and a reflector thickness of 1.01 mm gave optimal capture in a 2 min assay. The enhancement of particle detection compared with the control (no ultrasound) situation was x 70. The system detects a total of five particles in 15 fields of view in a 2 min assay when the suspending phase concentration was 10(4) particles/ml. A general expression for the dependence of minimum concentration detectable on; number of fields examined, sample volume flowing through the chamber and assay time shows that, for a practical combination of these variables, the threshold detection concentration can be two orders of magnitude lower.     

Feeding activity, absorption efficiency and suspension feeding processes in the ascidian, Halocynthia pyriformis (Stolidobranchia: Ascidiacea): responses to variations in diet quantity and quality

The benthic suspension feeding ascidian, Halocynthia pyriformis (Rathke, 1806), is often exposed to high concentrations of resuspended sediment in the Bay of Fundy. Resuspended sediment can change diet quantity and quality that may alter the ascidian's ability to feed and gain energy. The feeding activity of H. pyriformis exposed to bottom sediment was examined using standard physiological techniques and video endoscopy. Ascidians were exposed to natural seston plus additions of bottom sediment ranging in concentration from 0 to 46 mg l(-1). For each sediment concentration, clearance rate, ingestion rate, and retention efficiency of the ascidians was estimated using flow-through feeding chambers. Samples of suspended particles and feces were collected to estimate absorption efficiency and absorption rate. Results indicate that with increasing sediment concentration, ingestion rate increased to a constant level, absorption rate increased linearly despite a logarithmic decrease in absorption efficiency, and the retention of small particles (2-5 &mgr;m) increased while retention of larger particles (5-15 &mgr;m) decreased. As sediment concentration increased, squirting frequency increased and diameter of the siphon was reduced. Endoscopic observation of feeding structures and processes and the measurement of particle velocity was performed on ascidians exposed to 0 and 10 mg l(-1) of bottom sediment. An increase in squirting frequency at the high concentration facilitated the rejection of unwanted material and altered the structure and transport velocity of mucus. Mucus velocity was five times slower at 10 mg l(-1) than at 0 mg l(-1), however, the overall distance of mucus travel and the probability of clogging was reduced at 10 mg l(-1). H. pyriformis appears to compensate for episodic changes in the quantity and quality of available food particles by altering siphon-opening diameter, squirting frequency, structure and transport of mucus, and retention efficiency to maintain constant clearance rates.     

The relative profitability of particulate- and filter-feeding in the herring, Clupea harengus L.

Analysis of laboratory video-recordings of herring feeding by biting and filtering on Calanus finmarchicus and three sizes of Artemia enabled the capture rates of the two methods to be estimated at different prey concentrations. At low concentrations the fish feed by selective capture of individual particles, but the capture rate achievable by this method is constrained by the maximum rate at which they can bite. Filter-feeding is not subject to this constraint because capture rate is directly proportional to prey concentration and above a critical prey concentration its capture rate exceeds that of biting. The possession of two feeding methods allows the fish to maximize prey intake over a wide range of concentrations and the phenomenon of switching between feeding methods can be explained by their relative profitabilities at different concentrations. The observation that less than 50% of fish are filtering when capture rates by the two methods are equal suggests that filtering is energetically more costly than biting. Estimates of the energy cost of filtering indicated that it may be from 1.4 to 4.6 times higher than that of biting.     

Time and extent of ciliary response to particles in a non-filtering feeding mechanism

Mechanisms of suspension feeding are usually described by the physics of inanimate filters. High-speed videorecordings in this study demonstrated that sea urchin larvae concentrate particles without filtration. They actively captured individual particles. At most times and places, the effective strokes of the swimming/feeding ciliary band were away from the circumoral field. Cilia of this band responded to particles by a reversal of beat that redirected the particle toward the circumoral field. A change of beat occurred along approximately 80 micro m of ciliary band during particle capture. Cilia responded 0.02 to 0.06 s after the particle was within reach of effective strokes and reversed beat, usually for about 0.1 to 0.2 s. The whole event (disruption of forward beat) generally lasted between 0.13 and 0.5 s. These observations imply reversed movement of a parcel of water much larger than the included captured particle, but particles are nevertheless greatly concentrated because water is directed toward the circumoral field only when and where a particle is sensed. Thus most of the concentration of particles occurs by a temporarily and locally redirected current, without filtration, and size and quality of particles captured depends on sensory capabilities, not the mechanics of filtration.     

Filter mesh size and food particle uptake by Daphnia

Food size selection of four Daphnia, species (D. magna, D. hyalina, D. galeata, D. pulicaria) was investigated using spherical plastic beads as artificial food and with small bacteria. The size of the particles ranged from 0.1 to 35 m with special emphasis to the particle diameters between 0.1 and 1 m. In one set of experiments a mixture of differently sized particles was offered as food suspension and the selectivity of filtering was determined by comparing the size spectrum of the particles found in the gut contents with the spectrum in the food suspension. In a second series of experiments suspensions of uniformly sized particles were offered to single animals and their feeding activity was observed directly. In both types of experiments the mesh sizes of the filtering apparatus of the respective animals studied were measured after the experiments by, scanning electron microscopy. The mean sizes of the filter meshes were about 0.4–0.7 m. In all experiments the size of the particles found in the gut or those which caused high feeding activities were larger than the smallest mesh sizes of the filters. As a consequence simple mechanical sieving provides a sufficient explanation for the mechanism of particle retention of the filtering process in Daphnia. D. magna was found to feed with high efficiency on suspended freshwater bacteria, the residual species investigated showed low filtering efficiencies when bacteria were offered as food.The present study was supported by Deutscher Akademischer Austauschdients     

Size and shape determination of fixed chylomicrons and emulsions with fluid or solid surfaces by three-dimensional analysis of shadows

Chylomicrons and chylomicron-sized emulsions are spherical particles in suspension but their shape and apparent size may be distorted by electron microscopy processing. To assess adsorption to grids, flattening, and shrinkage, chylomicrons and emulsions were fixed with osmium tetroxide and together with polystyrene beads were shadowed with platinum. Vertical profiles projected from particle shadows indicated that the chylomicrons and emulsions were slightly shrunken, slightly truncated, oblate spheroids while the polystyrene beads were spheres. Particle diameters were corrected by assuming that volumes of oblate spheroids on the grid surface were equal to volumes of spheres in the original lipid suspension. Because of the compensating effects of shrinkage (decreases diameter) and flattening (increases diameter) the differences between the means of measured diameters and corrected diameters were less than or equal to 5%.     

Feeding mechanisms and feeding strategies of Atlantic reef corals

The feeding behaviour of 35 species of Atlantic reef corals was examined in the laboratory and in the field. Observations were made during the day and at night, using freshly hatched brine shrimp nauplii and finely ground, filtered fresh fish as food sources. Three feeding strategies were observed: Group I–feeding by tentacle capture only; Group II–feeding by entanglement with a mucus net or mucus filaments; Group III–feeding by a combination of tentacle capture and mucus filament entanglement. Group I included corals of the families Poritidae and Pocilloporidae which were normally expanded during both day and night. Group II included corals of the family Agaricidae which were normally expanded at night and contracted during the day. Group III included corals of the other families examined which, with the exception of Dendrogyra cylindrus , were normally expanded only at night.
Feeding responses were elicited by both chemical and tactile stimuli. A preparatory feeding posture was assumed in response to chemical stimuli and consisted of horizontal positioning of the tentacles, elevation of the oral disk to form a cone-like mouth, a wide mouth opening and secretion of mucus by the epidermis of the oral disk. Following the assumption of the preparatory feeding posture, food capture and ingestive movements were elicited by tactile stimuli. However, food capture and ingestive movements were also elicited by chemical stimuli alone in those species which were normally contracted during the day.
While expanded corals captured food with their tentacles or with mucus filaments, contracted corals were able to feed by capturing fine particulate matter with mucus filaments only and thus acted as suspension feeders. By a combination of feeding strategies, reef corals were able to feed both day and night and a wide range of potential food ranging from fine particulate matter to large zooplankton was available to them.     

Effect of endocytosis upon acid phosphatase activity ofTetrahymena pyriformis

Summary Increased endocytosis inTetrahymena pyriformis, produced by presenting starved cells with either peptone-yeast extract medium or killed yeast cell suspension, results in increased cellular acid phosphatase activity.Tetrahymena, grown in peptone-yeast extract medium, showed increased acid phosphatase activity after phagocytosis of yeast cells. This increase was not apparent until about one hour after presentation and was maximal at about 2.5 hours.Tetrahymena, grown on yeast suspension, showed little increase in acid phosphatase activity on presentation with peptone-yeast extract medium. These results may indicate that endocytosis, of either particles or solutes, produces an adaptive increase in acid phosphatase activity (presumably lysosomal in nature) which is related to feeding.Histochemical examination failed to localise the increase in acid phosphatase activity cellularly, but small particles, of about 1 diameter, which showed acid phosphatase activity and were presumably lysosomes were noted. Closely orientated yeast cells showed varying intensities of lead deposition, from absence to intense staining. This suggests that newly ingested yeast cells may be ingested initially in a single phagosome and that thereafter one or more lysosomes may fuse with them.     

Numerical simulation of flow fields and particle trajectories in ciliary suspension feeding

A model describing the ciliary driven flow and motion of suspended particles in downstream suspension feeders is developed. The quasi-steady Stokes equations for creeping flow are solved numerically in an unbounded fluid domain around cylindrical bodies using a boundary integral formulation. The time-dependent flow is approximated with a continuous sequence of steady state creeping flow fields, where metachronously beating ciliary bands are modelled by linear combinations of singularity solutions to the Stokes equations. Generally, the computed flow fields can be divided into an unsteady region close to the driving ciliary bands and a steady region covering the remaining fluid domain. The size of the unsteady region appears to be comparable to the metachronal wavelength of the ciliary band. A systematic investigation is performed of trajectories of infinitely small (fluid) particles in the simulated unsteady ciliary driven flow. A fraction of particles appear to follow trajectories, that resemble experimentally observed particle capture events in the downstream feeding system of the polycheate Sabella penicillus, indicating that particles can be captured by ciliary systems without mechanical contact between particle and cilia. A local capture efficiency is defined and its value computed for various values of beat frequencies and other parameters. The results indicate that the simulated particle capture process is most effective when the flow field oscillates within timescales comparable to transit timescales of suspended particles passing the unsteady region near the ciliary bands. However, the computed retention efficiencies are found to be much lower than those obtained experimentally.     

Feeding in a calcareous sponge: particle uptake by pseudopodia

Sponges are considered to be filter feeders like their nearest protistan relatives, the choanoflagellates. Specialized "sieve" cells (choanocytes) have an apical collar of tightly spaced, rodlike microvilli that surround a long flagellum. The beat of the flagellum is believed to draw water through this collar, but how particles caught on the collar are brought to the cell surface is unknown. We have studied the interactions that occur between choanocytes and introduced particles in the large feeding chambers of a syconoid calcareous sponge. Of all particles, only 0.1-microm latex microspheres adhered to the collar microvilli in large numbers, but these were even more numerous on the choanocyte surface. Few large particles (0.5- and 1.0-microm beads and bacteria) contacted the collar microvilli; most were phagocytosed by lamellipodia at the lateral or apical cell surface, and clumps of particles were engulfed by pseudopodial extensions several micrometers from the cell surface. Although extensions of the choanocyte apical surface up to 16 microm long were found, most were 4 microm long, twice the height of the collar microvilli. These observations offer a different view of particle uptake in sponges, and suggest that, at least in syconoid sponges, uptake of particles is less dependent on the strictly sieving function of the collar microvilli.     

A model of particle capture by bryozoans in turbulent flow: significance of colony form

A hydrodynamic model was developed to examine particle capture by lophophores of encrusting bryozoans. Particle capture rate is predicted to increase with increasing speed of the feeding current. There should be a large feeding advantage when lophophores are tightly packed and excurrents are vented through chimneys. This prediction contradicts conclusions of an earlier model study and suggests that selection for colony integration has a basis in the acquisition of food. If lophophores are not tightly packed, particle-capture patterns depend on two key ratios: the advection ratio (feeding current velocity to shear velocity) and the separation ratio (lophophore spacing to lophophore diameter). At high separation ratios, particle capture rates should be fairly uniform among zooids. At high advection ratios, lophophores located near the upstream colony edge should experience higher rates of particle capture. Rates of particle capture in turbulent flows should greatly exceed those in laminar flows (of identical speed) when excurrent waters are locally remixed into the flow above lophophores. However, when lophophores are tightly packed and excurrents are vented through chimneys, feeding rates should be identical in turbulent and laminar flows. Thus, colonies that vent excurrents through chimneys may be uniquely able to exploit weak laminar flows.     

Interfacial feeding behavior and particle flow patterns ofAnopheles quadrimaculatus larvae (Diptera: Culicidae)

The interfacial feeding behavior, mouthpart movements, and particle flow patterns of Anopheles quadrimaculatuslarvae were investigated, using videotape recordings, high-speed microcinematography, SEM, and laboratory experiments. While positioned at the water surface, larvae demonstrated 12 behaviors associated with movements of the head. In one of these, a larva rotated its head 180° and directed its mouthparts against the air-water interface. The larva rapidly extended and retracted its lateral palatal brushes (LPBs) at a rate of 5 cycles/s (5 Hz), creating currents and allowing for the collection of particles. Particles moved toward the head at a velocity of 4.31 mm/s, in discrete stops and starts, as the LPBs beat. Our analyses determined that particle movement toward the mouth was governed by very low Reynolds numbers (0.002–0.009). This finding indicated that viscous forces predominated in Anophelesfeeding and no inertial movement of particles occurred. According to this model, the LPBs cannot intercept particles directly, but function as paddles for particle entrainment. We did not observe the pharynx to function in particle filtration but, rather, in food bolus formation. We propose that the maxillary pilose area and midpalatal brush function as interception structures. It appeared that the LPBs do not break the surface film to feed, but collect particles from the surface microlayers. A plume of uningested particles emerged from the sides of the cibarium and descended into the water column. The plume consisted of alternately clear and dark, lenticular laminae formed beneath the larval head during the collecting filtering feeding mode. A comparison of particle sizes from surface microlayers and gut contents of fourth instars showed that larvae ingested mainly small particles in the range of 1.5 to 4.5 pm in diameter. The potential significance of interfacial feeding by anopheline larvae in their aquatic environment is discussed.     

Feeding by benthic invertebrates: Classification and terminology for paleoecological analysis

A trophic terminology is proposed, based on the feeding function of benthic invertebrates. This function has four aspects; (I) food resources can bc characterized on the basis of composition, condition, size, and dispersion; (2) the location of benthic food resources is not random, but most are concentrated at the sediment-water interface with a decreasing amount above and below; (3) an organism accomplishes food resource selection on the basis of particle size, specific gravity relationships, and nutrient capacity; finally (4) organisms have developed food acquisition mechanisms which arc adaptations for dealing with small particles, large particles and masses, or fluids. The classification, comprising nine main groups of animals, makes clear the distinction between site-of-life and location of food acquistion. It accounts logically for the four aspects of feeding, and yet has the advantage of simplicity.     

Particle transport modeling in pulmonary airways with high-order elements

Inhaled particles can be either harmful (e.g., smoke, exhaust, viruses) or beneficial (e.g., a therapeutic drug). The accurate and computationally efficient simulation of particle transport and deposition remains a challenge because it requires the simultaneous solution of the Navier-Stokes equations and multiple advection-diffusion mass transport equations when the particles are modeled as multiple mono-dispersed populations. The solution of these equations requires that multiple length scales be resolved since the ratio of advection to diffusion varies among the different equations. Here, the spectral element method is examined because the high-order approximation provides greater flexibility in resolving multiple length scales. The problem geometry is based on the Weibel model A of the human airway for convergence tests and the first three generations of a typical rat airway for experimental validation. Particles in the size range 1 to 100 nm are simulated for deposition results. The particle concentration and flux were determined using meshes of varying coarseness to represent the geometry along with basis polynomials of order 5 to 11. The higher-order elements accurately propagate the short wavelengths contained in the advection-diffusion solution without sacrificing efficiency for the more computationally expensive Navier-Stokes solution. As the diffusion coefficient in the advection-diffusion equation decreases (i.e., particle size increases) the advantages of the spectral elements become apparent for the coupled system.     

Modulation in Feeding Kinematics and Motor Pattern of the Nurse Shark <Emphasis Type="Italic">Ginglymostoma cirratum</Emphasis>

Synopsis Studies of feeding in bony fishes have almost universally demonstrated the ability of individuals to modulate their method of capture in response to differing stimuli. Preliminary evidence indicates that morphologically specialized inertial suction feeding sharks are the most likely fishes to lack inherent modulatory ability. We examined the ability of the nurse shark, Ginglymostoma cirratum, to modulate its feeding behavior based on different food types and sizes. G. cirratum is an inertial suction feeding fish that is apparently stereotyped in its food capture behavior. Electromyography showed no statistical difference between feeding motor patterns based on food type (squid or fish) or size (gape width or twice gape width), although there were slight inter-individual differences in the onset of muscle firing for some muscles. Kinematic analysis showed a statistical difference in variables associated with durations for different food types, with the durations for all variables being faster for squid bites than fish bites, but no difference based on the size of the food item. This apparent lack of modulation may be associated with specialization of the morphology and behavior of G. cirratum for obligate suction prey capture. This functional specialization constrains the method in which G. cirratum captures prey but does not appear to result in dietary specialization. An unusual post capture spit-suck manipulation allows this shark to handle and ingest large prey.     

Sub-micron particle behaviour and capture at an immuno-sensor surface in an ultrasonic standing wave

The capture of 200 nm biotinylated latex beads from suspensions of concentration 10(7) to 2.5 x 10(8) particle/ml on an immuno-coated surface of the acoustic reflector in an ultrasound standing wave (USW) resonator has been studied while the acoustic pathlength was less than one half wavelength (lambda/2). The particles were delivered to the reflector's surface by acoustically induced flow. The capture dependencies on suspension concentration, duration of experiments and acoustic pressure have been established at 1.09, 1.46 and 1.75 MHz. Five-fold capture increase has been obtained at 1.75 MHz in comparison to the control (no ultrasound) situation. The contrasting behaviours of 1, 0.5 and 0.2 mum fluorescent latex beads in a lambda/4 USW resonator at 1.46 MHz have been characterized. The particle movements were observed with an epi-fluorescent microscope and the velocities of the particles were measured by particle image velocimetry (PIV). The experiments showed that whereas the trajectories of 1 mum particles were mainly affected by the direct radiation force, 0.5 mum particles were influenced both by the radiation force and acoustic streaming. The 0.2 mum latex beads followed acoustic streaming in the chamber and were not detectably affected by the radiation force. The streaming-associated behaviour of the 200 nm particles has implications for enhanced immunocapture of viruses and macromolecules (both of which are also too small to experience significant acoustic radiation force).