Anders Andersen

Barnacles capture prey with a cirral fan filter that is swept through the water. As the barnacles grow, the cirral fan experiences changing flow regimes, both at the level of the filter elements (Reynolds number ~ 0.01 to ~ 1) and of the cirral fan (~ 1 to ~ 100). Here, we examined the implications of barnacle size on feeding current and clearance rate as a function of filter characteristics and prey size in the barnacle Amphibalanus improvisus . The feeding current is near fore‐aft symmetric in newly settled individuals but becomes increasingly asymmetric with size. Thus, in large specimens, most flow arrives from the sides, uninterrupted by the beating fan, while in small individuals most incoming flow passes the beating fan, warning evasive prey. This change is consistent with reports of an ontogenetic change in the diet from passive phytoplankton to inclusion of evasive prey. The leakiness of the filter increases during development, from 52% to 83%, and it thus functions both as a paddle that moves water, and as a filter that strains prey from the water. Clearance rates increase with the size of the barnacle, from 20 to > 500 mL ind ⁻¹ h ⁻¹ , orders of magnitude higher than reported earlier for similarly sized barnacles. Clearance rates also increase with prey size, saturating at particle sizes of about 80 μ m or more. Barnacles thus occupy a different niche than most other co‐occurring benthic suspension feeders that retain prey down to a few microns in size.

Some planktonic organisms can remotely sense and evade predators by powerful escape jumps. Remote perception typically happens through the fluid disturbance generated by the approaching predator or its feeding current. In copepods and ciliates with mechanosensors, the perception and jump mechanisms are well understood. But how some flagellates perceive the fluid disturbance and achieve similar relative speeds with only two flagella is less explored. Here, we examined the ability of three haptophytes, Chrysochromulina simplex, Prymnesium polylepis, and Prymnesium parvum, to sense and evade the fluid disturbance generated by the feeding current of a copepod nauplius. Chrysochromulina simplex has a long haptonema (14 cell diameters), while the haptonema of the two other species are shorter (1 and 0.5 cell diameters). Only C. simplex responded to the fluid disturbance by fleeing at high speeds. The jump mechanism consists of two phases: the rapid coiling of the haptonema that pulls the cell about two cell diameters in the direction of the haptonema, followed by flagellar reversal and high‐speed swimming (70 cell lengths per second) in the opposite direction. We rationalize cell displacements and escape speeds from haptonema and flagellar kinematics and fluid dynamics. Using a microfluidic channel, we demonstrate that the component of the fluid signal that triggers the jumps is the maximum deformation rate rather than the magnitude of deformation. High‐speed escape jumps may be an avoidance mechanism evolved by haptophytes with long and coiling haptonema, while species with shorter haptonema may use other defense mechanisms, such as stealth and toxicity.

Sessile barnacles feed by sweeping their basket-like cirral fan through the water, intercepting suspended prey. A primary component of the diet of adult barnacles is copepods that are sensitive to fluid disturbances and capable of escaping. How do barnacles manage to capture copepods despite the fluid disturbances they generate? We examined this question by describing the feeding current architecture of 1 cm sized Balanus crenatus using particle image velocimetry, and by studying the trajectories of captured copepods and the escapes of evading copepods. We found that barnacles produce a feeding current that arrives both from behind and the sides of the barnacle. The flow from the sides represents quiescent corridors of low fluid deformation and uninterrupted by the beating cirral fan. Potential prey arriving from behind are likely to encounter the cirral fan and, hence, capture here is highly unlikely. Accordingly, most captured copepods arrived through the quiet corridors, while most copepods arriving from behind managed to escape. Thus, it is the unique feeding flow architecture that allows feeding on evasive prey. We used the Landau–Squire jet as a simple model of the feeding current. For the Reynolds number of our experiments, the model reproduces the main features of the feeding current, including the lateral feeding corridors. Furthermore, the model suggests that smaller barnacle specimens, operating at lower Reynolds numbers, will produce a fore–aft symmetric feeding current without the lateral corridors. This suggests an ontogenetic diet shift from non-evasive prey to inclusion of evasive prey as the barnacle grows.

We show how to construct and apply a setup to acoustically tether and enable behavioral observations of individual microorganisms using simple laboratory equipment and a standard light microscope. We explore the capability of the setup with the freely swimming dinoflagellate Alexandrium minutum as the study organism. The setup allows us to tether cells in focus in the mid-plane of the sample chamber and make observations of individual organisms at high magnification without affecting their flagellar beat frequencies. We discuss the prospect of the method to explore appendage motion and swimming kinematics of other flagellates and ciliates, and we argue that the method will be applicable to a broad range of cell sizes and shapes.

The important role of unicellular flagellated micro-organisms in aquatic food webs is mediated by their flagella, which enable them to swim and generate feeding currents. The flagellum in many predatory flagellates is equipped with hairs (mastigonemes) that reverse the direction of thrust compared to the thrust due to a smooth flagellum. Conventionally, the mechanism of such reversal has been attributed to drag-based thrust of individual hairs, neglecting their hydrodynamic interactions. However, at natural densities of hairs, hydrodynamic interactions are important. In fact, using fully resolved three-dimensional computational fluid dynamics, we show here that hydrodynamic interactions are key to thrust-generation and reversal in hairy flagellates, making their hydrodynamics fundamentally different from the slender-body theory governing smooth flagella. We reveal the significant role of the curvature of the flagellum, and using model analysis we demonstrate that strongly curved flagellar waveforms are optimal for thrust-generation. Our results form a basis for understanding the diverse flagellar architectures and feeding modes of predatory flagellates.

Heterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean. In their microscale world, viscosity impedes predator–prey contact, and the mechanisms that allow flagellates to daily clear a volume of water for prey corresponding to 10⁶ times their own volume is unclear. It is also unclear what limits observed maximum ingestion rates of about 10⁴ bacterial preys per day. We used high‐speed video microscopy to describe feeding flows, flagellum kinematics, and prey searching, capture, and handling in four species with different foraging strategies. In three species, prey handling times limit ingestion rates and account well for their reported maximum values. Similarly, observed feeding flows match reported clearance rates. Simple point force models allowed us to estimate the forces required to generate the feeding flows, between 4 and 13 pN, and consistent with the force produced by the hairy (hispid) flagellum, as estimated using resistive force theory. Hispid flagella can produce a force that is much higher than the force produced by a naked (smooth) flagellum with similar kinematics, and the hairy flagellum is therefore key to foraging in most nanoflagellates. Our findings provide a mechanistic underpinning of observed functional responses of prey ingestion rates in nanoflagellates.

Ciliates are ubiquitous in the marine environment and important consumers of phytoplankton and flagellates. The feeding on suspended food particles in ciliates is complex and relies typically on coordinated motion in bands of transversal rows of cilia known as membranelles. Many ciliates are upstream collectors that use a single membranelle band to generate feeding flow, retain food particles, and transport them to the mouth region. To explore a representative upstream collector, we consider the ciliate Euplotes vannus. We determine the clearance rate using particle tracking and estimate the flow-generating force by fitting a point force model to the observed flow. To obtain a mechanistic understanding of feeding flow and particle retention, we use high-speed video microscopy. The cilia move parallel to the membranelle band towards the mouth region in the power strokes, and a metachronal wave propagates away from the mouth region parallel to the band and outwards along the membranelles. A gap, therefore, opens between neighboring membranelles from the inner side of the band where the mouth region is located, and while food particles are retained, water is drawn in and pushed outwards across the membranelle band as the gap closes from the inner side. We suggest a model that rationalizes our observations and quantifies the mechanism by which the membranelle band can both generate feeding flow and retain food particles. The model compares favorably with our observations and suggests a trade-off between the clearance rate and the membranelle gap that determines the lower cutoff in the prey size spectrum. We quantify the trade-off and discuss the ecological significance of our findings.

Microscopic sessile suspension feeders live attached to surfaces and, by consuming bacteria-sized prey and by being consumed, they form an important part of aquatic ecosystems. Their environmental impact is mediated by their feeding rate, which depends on a self-generated feeding current. The feeding rate has been hypothesized to be limited by recirculating eddies that cause the organisms to feed from water that is depleted of food particles. However, those results considered organisms in still water, while ambient flow is often present in their natural habitats. We show, using a point-force model, that even very slow ambient flow, with speed several orders of magnitude less than that of the self-generated feeding current, is sufficient to disrupt the eddies around perpendicular suspension feeders, providing a constant supply of food-rich water. However, the feeding rate decreases in external flow at a range of non-perpendicular orientations due to the formation of recirculation structures not seen in still water. We quantify the feeding flow and observe such recirculation experimentally for the suspension feeder Vorticella convallaria in external flows typical of streams and rivers.

Many aquatic microorganisms are attached to solid surfaces while creating feeding flows that bring prey particles to them. To explore the effects of surface proximity and orientation of the flow-generating force, we analyze the low-Reynolds-number flow due to a point force above a plane no-slip surface. The presence of the surface reduces the feeding flow relative to the unbounded situation. We show that the reduction of the flow rate through a circular clearance disk perpendicular to the force and centered at its position is twice as large when the force is perpendicular to the surface as when it is parallel. When the force is perpendicular to the surface, the flow forms a toroidal eddy with closed streamlines, and the resulting flow recirculation may lead to refiltration of water that has already been cleared for prey. We prove that due to the nature of the far-field flow, the shortest recirculation time along a streamline through a circular clearance disk is inversely proportional to the flow rate to the power four. Finally, we discuss the biological advantages and disadvantages of perpendicular and parallel force orientation and the effects of prey diffusion and ambient flow, and we argue that recirculation is irrelevant in the typical perpendicular feeding flow since the recirculation time is long compared to the biologically relevant timescales.

Many planktonic suspension feeders are attached to particles or tethered by gravity when feeding. It is commonly accepted that the feeding flows of tethered suspension feeders are stronger than those of their freely swimming counterparts. However, recent flow simulations indicate the opposite, and the cause of the opposing conclusions is not clear. To explore the effect of tethering on suspension feeding, we use a low-Reynolds-number flow model. We find that it is favorable to be freely swimming instead of tethered since the resulting feeding flow past the cell body is stronger, leading to a higher clearance rate. Our result underscores the significance of the near-field flow in shaping planktonic feeding modes, and it suggests that organisms tether for reasons that are not directly fluid dynamical (e.g., to stay near surfaces where the concentration of bacterial prey is high).