The fluid dynamics of barnacle feeding.

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.

Coral tentacle elasticity promotes an out-of-phase motion that improves mass transfer.

Corals rely almost exclusively on the ambient flow of water to support their respiration, photosynthesis, prey capture, heat exchange and reproduction. Coral tentacles extend to the flow, interact with it and oscillate under the influence of waves. Such oscillating motions of flexible appendages are considered adaptive for reducing the drag force on flexible animals in wave-swept environments, but their significance under slower flows is unclear. Using in situ and laboratory measurements of the motion of coral tentacles under wave-induced flow, we investigated the dynamics of the tentacle motion and its impact on mass transfer. We found that tentacle velocity preceded the water velocity by approximately one-quarter of a period. This out-of-phase behaviour enhanced mass transfer at the tentacle tip by up to 25% as compared with an in-phase motion. The enhancement was most pronounced under flows slower than 3.2 cm s-1, which are prevalent in many coral-reef environments. We found that the out-of-phase motion results from the tentacles' elasticity, which can presumably be modified by the animal. Our results suggest that the mechanical properties of coral tentacles may represent an adaptive advantage that improves mass transfer under the limiting conditions of slow ambient flows. Because the mechanism we describe operates by enhancing convective processes, it is expected to enhance other fitness-determining transport phenomena such as heat exchange and particle capture.

A novel mechanism of mixing by pulsing corals.

The dynamic pulsation of xeniid corals is one of the most fascinating phenomena observed in coral reefs. We quantify for the first time the flow near the tentacles of these soft corals, the active pulsations of which are thought to enhance their symbionts' photosynthetic rates by up to an order of magnitude. These polyps are approximately 1 cm in diameter and pulse at frequencies between approximately 0.5 and 1 Hz. As a result, the frequency-based Reynolds number calculated using the tentacle length and pulse frequency is on the order of 10 and rapidly decays as with distance from the polyp. This introduces the question of how these corals minimize the reversibility of the flow and bring in new volumes of fluid during each pulse. We estimate the Péclet number of the bulk flow generated by the coral as being on the order of 100-1000 whereas the flow between the bristles of the tentacles is on the order of 10. This illustrates the importance of advective transport in removing oxygen waste. Flow measurements using particle image velocimetry reveal that the individual polyps generate a jet of water with positive vertical velocities that do not go below 0.1 cm s-1 and with average volumetric flow rates of approximately 0.71 cm3 s-1 Our results show that there is nearly continual flow in the radial direction towards the polyp with only approximately 3.3% back flow. 3D numerical simulations uncover a region of slow mixing between the tentacles during expansion. We estimate that the average flow that moves through the bristles of the tentacles is approximately 0.03 cm s-1 The combination of nearly continual flow towards the polyp, slow mixing between the bristles, and the subsequent ejection of this fluid volume into an upward jet ensures the polyp continually samples new water with sufficient time for exchange to occur.

The Levantine jellyfish Rhopilema nomadica and Rhizostoma pulmo swim faster against the flow than with the flow.

Jellyfish locomotion and orientation have been studied in the past both in the laboratory, testing mostly small jellyfish, and in the field, where it was impossible to control the seawater currents. Utilizing an outdoor water flume, we tested the locomotion of jellyfish when swimming against and with currents of up to 4.5 cm s-1. We used adult jellyfish from two of the most abundant species in the eastern Mediterranean, Rhopilema nomadica and Rhizostoma pulmo, and measured their pulsation frequency and swimming speed relative to the water. While pulsation frequency was not affected by the water velocity, jellyfish swam faster against the current than with it. This finding suggests that jellyfish possess a sensory ability, whose mechanism is currently unknown, enabling them to gauge the flow and react to it, possibly in order to reduce the risk of stranding.

The nematocyst's sting is driven by the tubule moving front.

The nematocyst is the explosive injection system of the phylum Cnidaria, and is one of the fastest delivery systems found in Nature. Exploring its injection mechanism is key for understanding predator-prey interactions and protection against jellyfish stinging. Here we analyse the injection of jellyfish nematocysts and ask how the build-up of the poly-γ-glutamate (pγGlu) osmotic potential inside the nematocyst drives its discharge. To control the osmotic potential, we used a two-channel microfluidic system to direct the elongating nematocyst tubule through oil, where no osmotic potential can develop, while keeping the nematocyst capsule in water at all times. In addition, the flow inside the tubule and the pγGlu concentration profiles were calculated by applying a one-dimensional mathematical model. We found that tubule elongation through oil is orders of magnitude slower than through water and that the injection rate of the nematocyst content is reduced. These results imply that the capsule's osmotic potential is not sufficient to drive the tubule beyond the initial stage. Our proposed model shows that the tubule is pulled by the high osmotic potential that develops at the tubule moving front. This new understanding is vital for future development of nematocyst-based systems such as osmotic nanotubes and transdermal drug delivery.

Myxozoan polar tubules display structural and functional variation.

BACKGROUND: Myxozoa is a speciose group of endoparasitic cnidarians that can cause severe ecological and economic effects. Although highly reduced compared to free-living cnidarians, myxozoans have retained the phylum-defining stinging organelles, known as cnidae or polar capsules, which are essential to initiating host infection. To explore the adaptations of myxozoan polar capsules, we compared the structure, firing process and content release mechanism of polar tubules in myxospores of three Myxobolus species including M. cerebralis, the causative agent of whirling disease. RESULTS: We found novel functions and morphologies in myxozoan polar tubules. High-speed video analysis of the firing process of capsules from the three Myxobolus species showed that all polar tubules rapidly extended and then contracted, an elasticity phenomenon that is unknown in free-living cnidarians. Interestingly, the duration of the tubule release differed among the three species by more than two orders of magnitude, ranging from 0.35 to 10 s. By dye-labeling the polar capsules prior to firing, we discovered that two of the species could release their entire capsule content, a delivery process not previously known from myxozoans. Having the role of content delivery and not simply anchoring suggests that cytotoxic or proteolytic compounds may be present in the capsule. Moreover, while free-living cnidarians inject most of the toxic content through the distal tip of the tubule, our video and ultrastructure analyses of the myxozoan tubules revealed patterns of double spirals of nodules and pores along parts of the tubules, and showed that the distal tip of the tubules was sealed. This helical pattern and distribution of openings may minimize the tubule mechanical weakness and improve resistance to the stress impose by firing. The finding that myxozoan tubule characteristics are very different from those of free-living cnidarians is suggestive of their adaptation to parasitic life. CONCLUSIONS: These findings show that myxozoan polar tubules have more functions than previously assumed, and provide insight into their evolution from free-living ancestors.

Benefit of pulsation in soft corals.

Soft corals of the family Xeniidae exhibit a unique, rhythmic pulsation of their tentacles (Movie S1), first noted by Lamarck nearly 200 y ago. However, the adaptive benefit of this perpetual, energetically costly motion is poorly understood. Using in situ underwater particle image velocimetry, we found that the pulsation motions thrust water upward and enhance mixing across the coral-water boundary layer. The induced upward motion effectively prevents refiltration of water by neighboring polyps, while the intensification of mixing, together with the upward flow, greatly enhances the coral's photosynthesis. A series of controlled laboratory experiments with the common xeniid coral Heteroxenia fuscescens showed that the net photosynthesis rate during pulsation was up to an order of magnitude higher than during the coral's resting, nonpulsating state. This enhancement diminished when the concentration of oxygen in the ambient water was artificially raised, indicating that the enhancement of photosynthesis was due to a greater efflux of oxygen from the coral tissues. By lowering the internal oxygen concentration, pulsation alleviates the problem of reduced affinity of ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) to CO2 under conditions of high oxygen concentrations. The photosynthesis-respiration ratio of the pulsating H. fuscescens was markedly higher than the ratios reported for nonpulsating soft and stony corals. Although pulsation is commonly used for locomotion and filtration in marine mobile animals, its occurrence in sessile (bottom-attached) species is limited to members of the ancient phylum Cnidaria, where it is used to accelerate water and enhance physiological processes.

Particle tracking velocimetry and particle image velocimetry study of the slow motion of rough and smooth solid spheres in a yield-stress fluid.

We report experimental evidence of an effect opposite to the "solidification" of small bubbles in liquid where the surface can become immobile. Namely, it is demonstrated that smooth solid spheres falling in a yield-stress fluid under the action of gravity can behave similar to drops. Particle tracking velocimetry was used to determine the shape of the yielded region around solid spherical particles undergoing slow stationary motion in 0.07% w/w Carbopol gel due to gravity under creeping flow conditions. The flow field inside the yielded region was determined by particle image velocimetry. It was found that the shape of the yielded region and the flow field around slow-moving rough particles is similar to the published results of numerical simulations, whereas those around smooth spheres resemble the experimental results obtained for viscous drops. The effect was explained by a slip of the gel on the smooth surface. Most likely, the slip originated from seepage of clean water from the gel, forming a thin lubricating layer near the solid surface.

The sponge pump: the role of current induced flow in the design of the sponge body plan.

Sponges are suspension feeders that use flagellated collar-cells (choanocytes) to actively filter a volume of water equivalent to many times their body volume each hour. Flow through sponges is thought to be enhanced by ambient current, which induces a pressure gradient across the sponge wall, but the underlying mechanism is still unknown. Studies of sponge filtration have estimated the energetic cost of pumping to be <1% of its total metabolism implying there is little adaptive value to reducing the cost of pumping by using "passive" flow induced by the ambient current. We quantified the pumping activity and respiration of the glass sponge Aphrocallistes vastus at a 150 m deep reef in situ and in a flow flume; we also modeled the glass sponge filtration system from measurements of the aquiferous system. Excurrent flow from the sponge osculum measured in situ and in the flume were positively correlated (r>0.75) with the ambient current velocity. During short bursts of high ambient current the sponges filtered two-thirds of the total volume of water they processed daily. Our model indicates that the head loss across the sponge collar filter is 10 times higher than previously estimated. The difference is due to the resistance created by a fine protein mesh that lines the collar, which demosponges also have, but was so far overlooked. Applying our model to the in situ measurements indicates that even modest pumping rates require an energetic expenditure of at least 28% of the total in situ respiration. We suggest that due to the high cost of pumping, current-induced flow is highly beneficial but may occur only in thin walled sponges living in high flow environments. Our results call for a new look at the mechanisms underlying current-induced flow and for reevaluation of the cost of biological pumping and its evolutionary role, especially in sponges.

Flow enhances photosynthesis in marine benthic autotrophs by increasing the efflux of oxygen from the organism to the water.

Worldwide, many marine coastal habitats are facing rapid deterioration due in part to human-driven changes in habitat characteristics, including changes in flow patterns, a factor known to greatly affect primary production in corals, algae, and seagrasses. The effect of flow traditionally is attributed to enhanced influx of nutrients and dissolved inorganic carbon (DIC) across the benthic boundary layer from the water to the organism however, here we report that the organism's photosynthetic response to changes in the flow is nearly instantaneous, and that neither nutrients nor DIC limits this rapid response. Using microelectrodes, dual-pulse amplitude-modulated fluorometry, particle image velocimetry, and real time mass-spectrometry with the common scleractinian coral Favia veroni, the alga Gracilaria cornea, and the seagrass Halophila stipulacea, we show that this augmented photosynthesis is due to flow-driven enhancement of oxygen efflux from the organism to the water, which increases the affinity of the RuBisCO to CO(2). No augmentation of photosynthesis was found in the absence of flow or when flow occurred, but the ambient concentration of oxygen was artificially elevated. We suggest that water motion should be considered a fundamental factor, equivalent to light and nutrients, in determining photosynthesis rates in marine benthic autotrophs.

Model demonstrating the potential for coupled nitrification denitrification in soil aggregates.

A model of reactive, multi-species diffusion has been developed to describe N transformations in spherical soil aggregates, emphasizing the effects of irrigation with reclaimed wastewater. Oxygen demand for respiratory activity has been shown to promote the establishment of anaerobic conditions. Aggregate size and soil respiration rate were identified as the most significant parameters governing the existence and extent of the anaerobic volume in aggregates. The inclusion of kinetic models describing mineralization, nitrification, and denitrification facilitated the investigation of coupled nitrification/denitrification (CND), subject to O2 availability. N-transformations are shown to be affected by effluent-borne NH4+-N content, in addition to elevated BOD and pH levels. Their incremental contribution to O2 availability has been found to be secondary to respiratory activity. At the aggregate level, significant differences between apparent and gross rates of N-transformations were predicted (e.g., NH4+ oxidation and N2 formation), resulting from diffusive constraints due to aggregate size. With increasing anaerobic volume, the effective nitrification rate determined at the aggregates level decreases until its contribution to nitrification is negligible. It was found that the nitrification process was predominantly limited to aggregates <0.25 cm. Assuming that nitrification is the main source for NO3- formation, denitrification efficiency is predicted to peak in medium-sized aggregates, where aerobic and anaerobic conditions coexist, supporting CND. In effluent-irrigated soils, the predicted NO2- formation rate in small aggregates is enhanced when compared to freshwater-irrigated soils. The difference vanishes with increasing aggregate size as anaerobic NO2- consumption exceeds aerobic NO2- formation due to the coupling of nitrification and denitrification.

Quantifying ground water inputs along the Lower Jordan River.

The flow rate of the Lower Jordan River has changed dramatically during the second half of the 20th century. The diversion of its major natural sources reduced its flow rate and led to drying events during the drought years of 2000 and 2001. Under these conditions of low flow rates, the potential influence of external sources on the river discharge and chemical composition became significant. Our measurements show that the concentrations of chloride, calcium, and sodium in the river water decrease along the first 20-km section, while sulfate and magnesium concentrations increase. These variations were addressed by a recent geochemical study, suggesting that ground water inflow plays a major role. To further examine the role of ground water, we applied mass-balance calculations, using detailed flow rate measurements, water samplings, and chemical analyses along the northern (upstream) part of the river. Our flow-rate measurements showed that the river base-flow during 2000 and 2001 was 500 to 1100 L s(-1), which is about 40 times lower than the historical flow rates. Our measurements and calculations indicate that ground water input was 20 to 80% of the river water flow, and 20 to 50% of its solute mass flow. This study independently identifies the composition of possible end-members. These end-members contain high sulfate concentration and have similar chemical characteristics as were found in agricultural drains and in the "saline" Yarmouk River. Future regional development plans that include the river flow rate and chemistry should consider the interactions between the river and its shallow ground water system.

Sources and transformations of nitrogen compounds along the Lower Jordan River.

The Lower Jordan River is located in the semiarid area of the Jordan Valley, along the border between Israel and Jordan. The implementation of the water sections of the peace treaty between Israel and Jordan and the countries' commitment to improve the ecological sustainability of the river system require a better understanding of the riverine environment. This paper investigates the sources and transformations of nitrogen compounds in the Lower Jordan River by applying a combination of physical, chemical, isotopic, and mathematical techniques. The source waters of the Lower Jordan River contain sewage, which contributes high ammonium loads to the river. Ammonium concentrations decrease from 20 to 0-5 mg N L(-1) along the first 20 km of the Lower Jordan River, while nitrate concentrations increase from nearly zero to 10-15 mg N L(-1), and delta(15)N (NO(3)) values increase from less than 5 per thousand to 15-20 per thousand. Our data analysis indicates that intensive nitrification occurs along the river, between 5 and 12 km from the Sea of Galilee, while further downstream nitrate concentration increases mostly due to an external subsurface water source that enters the river.