A class of statistical models for the motion of Daphnia over small time scales.

A common question in the aquatic sciences is that of how zooplankter movement can be modeled. It is well-established in the literature that there exists a randomness to this movement, but the question is how to characterize this randomness. The most common methods for doing this involve the random walk and correlated random walk (CRW) models. Here, we present a time series model that allows a better description the randomness in Daphnia motion when the amount of time that elapses between observations of their position is small. Our approach is adaptable to description of tracks of a multitude of animal species through re-estimation of model parameters. The model we propose uses information about how the animal moved during the previous two time intervals to explain how it moves currently. We demonstrate that the proposed model provides better predictive accuracy and fit than do the CRW and random walk models.

Comparative analysis of Microcystis buoyancy in western Lake Erie and Saginaw Bay of Lake Huron.

Microcystis is the predominant genus of harmful cyanobacterium in both Lake Erie and Saginaw Bay of Lake Huron and has the capacity to regulate the buoyancy of its colonies, sinking under certain conditions while floating towards the surface in others. Understanding the factors that control buoyancy is critical for interpretation of remote sensing data, modeling and forecasting harmful algal blooms within these two systems. To determine if Microcystis colony buoyancy in the two lakes responds similarly to diurnal light cycles, colony buoyant velocity (floating/sinking terminal velocity in a quiescent water column) and size were measured after manipulating the intensity of sunlight. Overall, there were more positively buoyant (floating) colonies in Lake Erie while most of the colonies in Saginaw Bay were negatively buoyant (sinking). In Lake Erie the colonies became less buoyant at increased light intensities and were less buoyant in the afternoon than in the morning. In both lakes, apparent colony density was more variable among small colonies (< 200 µm), whereas larger colonies showed a diminished response of density to light intensity and duration. These findings suggest that colony density becomes less plastic as colonies increase in size, leading to a weak relationship between size and velocity. These relationships may ultimately affect how the bloom is transported throughout each system and will help explain observed differences in vertical distribution and movement of Microcystis in the two lakes.

Teamwork in the viscous oceanic microscale.

Nutrient acquisition is crucial for oceanic microbes, and competitive solutions to solve this challenge have evolved among a range of unicellular protists. However, solitary solutions are not the only approach found in natural populations. A diverse array of oceanic protists form temporary or even long-lasting attachments to other protists and marine aggregates. Do these planktonic consortia provide benefits to their members? Here, we use empirical and modeling approaches to evaluate whether the relationship between a large centric diatom, Coscinodiscus wailesii, and a ciliate epibiont, Pseudovorticella coscinodisci, provides nutrient flux benefits to the host diatom. We find that fluid flows generated by ciliary beating can increase nutrient flux to a diatom cell surface four to 10 times that of a still cell without ciliate epibionts. This cosmopolitan species of diatom does not form consortia in all environments but frequently joins such consortia in nutrient-depleted waters. Our results demonstrate that symbiotic consortia provide a cooperative alternative of comparable or greater magnitude to sinking for enhancement of nutrient acquisition in challenging environments.

Active hydrodynamic imaging of a rigid spherical particle.

A body with mechanical sensors may remotely detect particles suspended in the surrounding fluid via controlled agitation. Here we propose a sensory mode that relies on generating unsteady flow and sensing particle-induced distortions in the flow field. We demonstrate the basic physical principle in a simple analytical model, which consists of a small spherical particle at some distance from a plate undergoing impulsive or oscillatory motion. The model shows that changes in pressure or shear on the plate can be used to infer the location and size of the sphere. The key ingredient is to produce strong shear or strain around the sphere, which requires careful tuning of the viscous boundary layer on the moving plate. This elucidates how some organisms and devices may control their unsteady dynamics to enhance their range of perception.

Oscillations in the near-field feeding current of a calanoid copepod are useful for particle sensing.

Calanoid copepods are small crustaceans that constitute a major element of aquatic ecosystems. Key to their success is their feeding apparatus consisting of sensor-studded mouth appendages that are in constant motion. These appendages generate a feeding current to enhance the encounter probability with food items. Additionally, sensing enables the organism to determine the position and quality of food particles, and to alter the near-field flow to capture and manipulate the particles for ingestion or rejection. Here we observe a freely swimming copepod Leptodiaptomus sicilis in multiple perspectives together with suspended particles that allow us to analyse the flow field created by the animal. We observe a highly periodic motion of the mouth appendages that is mirrored in oscillations of nearby tracer particles. We propose that the phase shift between the fluid and the particle velocities is sufficient for mechanical detection of the particles entrained in the feeding current. Moreover, we propose that an immersed algal cell may benefit from the excitation by increased uptake of dissolved inorganic compounds.

Going with the flow: hydrodynamic cues trigger directed escapes from a stalking predator.

In the coevolution of predator and prey, different and less well-understood rules for threat assessment apply to freely suspended organisms than to substrate-dwelling ones. Particularly vulnerable are small prey carried with the bulk movement of a surrounding fluid and thus deprived of sensory information within the bow waves of approaching predators. Some planktonic prey have solved this apparent problem, however. We quantified cues generated by the slow approach of larval clownfish ( Amphiprion ocellaris) that triggered a calanoid copepod ( Bestiolina similis) to escape before the fish could strike. To estimate water deformation around the copepod immediately preceding its jump, we represented the body of the fish as a rigid sphere in a hydrodynamic model that we parametrized with measurements of fish size, approach speed and distance to the copepod. Copepods of various developmental stages (CII-CVI) were sensitive to the water flow caused by the live predator, at deformation rates as low as 0.04 s-1. This rate is far lower than that predicted from experiments that used artificial predator-mimics. Additionally, copepods localized the source, with 87% of escapes directed away (greater than or equal to 90°) from the predator. Thus, copepods' survival in life-threatening situations relied on their detection of small nonlinear signals within an environment of locally linear deformation.

Predatory posture and performance in a precocious larval fish targeting evasive copepods.

Predatory fishes avoid detection by prey through a stealthy approach, followed by a rapid and precise fast-start strike. Although many first-feeding fish larvae strike at non-evasive prey using an S-start, the clownfish Amphiprion ocellaris feeds on highly evasive calanoid copepods from a J-shaped position, beginning 1 day post-hatch (dph). We quantified this unique strike posture by observing successful predatory interactions between larval clownfish (1 to 14 dph) and three developmental stages of the calanoid copepod Bestiolina similis The J-shaped posture of clownfish became less tightly curled (more L-shaped) during larval development. Larvae were also less tightly curled when targeting adult copepods, which are more evasive than younger copepod stages. Strike performance measured as time to capture and as peak speed improved only slightly with larval age. Therefore, the J-posture may allow first-feeding larvae to minimize disturbance during their approach of sensitive prey, and may represent an alternative predatory strategy to the prototypical S-start.

Copepod manipulation of oil droplet size distribution.

Oil spills are one of the most dangerous sources of pollution in aquatic ecosystems. Owing to their pivotal position in the food web, pelagic copepods can provide crucial intermediary transferring oil between trophic levels. In this study we show that the calanoid Paracartia grani can actively modify the size-spectrum of oil droplets. Direct manipulation through the movement of the feeding appendages and egestion work in concert, splitting larger droplets (Ø = 16 µm) into smaller ones (Ø = 4-8 µm). The copepod-driven change in droplet size distribution can increase the availability of oil droplets to organisms feeding on smaller particles, sustaining the transfer of petrochemical compounds among different compartments. These results raise the curtain on complex small-scale interactions which can promote the understanding of oil spills fate in aquatic ecosystems.

Anti-EGFR-resistant clones decay exponentially after progression: implications for anti-EGFR re-challenge.

BACKGROUND: Colorectal cancer (CRC) has been shown to acquire RAS and EGFR ectodomain mutations as mechanisms of resistance to epidermal growth factor receptor (EGFR) inhibition (anti-EGFR). After anti-EGFR withdrawal, RAS and EGFR mutant clones lack a growth advantage relative to other clones and decay; however, the kinetics of decay remain unclear. We sought to determine the kinetics of acquired RAS/EGFR mutations after discontinuation of anti-EGFR therapy. PATIENTS AND METHODS: We present the post-progression circulating tumor DNA (ctDNA) profiles of 135 patients with RAS/BRAF wild-type metastatic CRC treated with anti-EGFR who acquired RAS and/or EGFR mutations during therapy. Our validation cohort consisted of an external dataset of 73 patients with a ctDNA profile suggestive of prior anti-EGFR exposure and serial sampling. A separate retrospective cohort of 80 patients was used to evaluate overall response rate and progression free survival during re-challenge therapies. RESULTS: Our analysis showed that RAS and EGFR relative mutant allele frequency decays exponentially (r2=0.93 for RAS; r2=0.94 for EGFR) with a cumulative half-life of 4.4 months. We validated our findings using an external dataset of 73 patients with a ctDNA profile suggestive of prior anti-EGFR exposure and serial sampling, confirming exponential decay with an estimated half-life of 4.3 months. A separate retrospective cohort of 80 patients showed that patients had a higher overall response rate during re-challenge therapies after increasing time intervals, as predicted by our model. CONCLUSION: These results provide scientific support for anti-EGFR re-challenge and guide the optimal timing of re-challenge initiation.

Olfaction in a viscous environment: the "color" of sexual smells in Temora longicornis.

We investigate chemical aspects of mating in the marine copepod Temora longicornis (Copepoda, Calanoidea). Our emphasis is the female pheromone signaling in form of well-defined trails for males to follow, observed in Doall et al. (Phil Trans R Soc Lond B 353:681-689, 1998). The viscous environment and the properties of the odorants play important roles as the spread of the pheromone trail limits the time during which it is useful for tracing. A key observation from our earlier work is the ability of a searching male to detect the direction of the female and to correct its swimming direction if necessary. We propose a simple mathematical model for the spread of a pheromone from a moving source and carry out numerical simulations of two possible detection mechanisms. We find that a searching agent that is capable to detect a ratio outperforms a searcher that depends on the gradient of a single compound. This suggests that copepod sex pheromones consist of blends of chemical compounds, and that a ratio detection mechanism similar to that in airborne insects is at work.

Escapes in copepods: comparison between myelinate and amyelinate species.

Rapid conduction in myelinated nerves keeps distant parts of large organisms in timely communication. It is thus surprising to find myelination in some very small organisms. Calanoid copepods, while sharing similar body plans, are evenly divided between myelinate and amyelinate taxa. In seeking the selective advantage of myelin in these small animals, representatives from both taxa were subjected to a brief hydrodynamic stimulus that elicited an escape response. The copepods differed significantly in their ability to localize the stimulus: amyelinate copepods escaped in the general direction of their original swim orientation, often ending up closer to the stimulus. However, myelinate species turned away from the stimulus and distanced themselves from it, irrespective of their original orientation. We suggest that faster impulse conduction of myelinated axons leads to better precision in the timing and processing of sensory information, thus allowing myelinate copepods to better localize stimuli and respond appropriately.

Convex Grooves in Staggered Herringbone Mixer Improve Mixing Efficiency of Laminar Flow in Microchannel.

The liquid streams in a microchannel are hardly mixed to form laminar flow, and the mixing issue is well described by a low Reynolds number scheme. The staggered herringbone mixer (SHM) using repeated patterns of grooves in the microchannel have been proved to be an efficient passive micro-mixer. However, only a negative pattern of the staggered herringbone mixer has been used so far after it was first suggested, to the best of our knowledge. In this study, the mixing efficiencies from negative and positive staggered herringbone mixer patterns as well as from opposite flow directions were tested to investigate the effect of the micro-structure geometry on the surrounding laminar flow. The positive herringbone pattern showed better mixing efficiency than the conventionally used negative pattern. Also, generally used forward flow gives better mixing efficiency than reverse flow. The mixing was completed after two cycles of staggered herringbone mixer with both forward and reverse flow in a positive pattern. The traditional negative pattern showed complete mixing after four and five cycles in forward and reverse flow direction, respectively. The mixing effect in all geometries was numerically simulated, and the results confirmed more efficient mixing in the positive pattern than the negative. The results can further enable the design of a more efficient microfluidic mixer, as well as in depth understanding of the phenomena of positive and negative patterns existing in nature with regards to the surrounding fluids.

Statistical Mechanics of Zooplankton.

Statistical mechanics provides the link between microscopic properties of many-particle systems and macroscopic properties such as pressure and temperature. Observations of similar "microscopic" quantities exist for the motion of zooplankton, as well as many species of other social animals. Herein, we propose to take average squared velocities as the definition of the "ecological temperature" of a population under different conditions on nutrients, light, oxygen and others. We test the usefulness of this definition on observations of the crustacean zooplankton Daphnia pulicaria. In one set of experiments, D. pulicaria is infested with the pathogen Vibrio cholerae, the causative agent of cholera. We find that infested D. pulicaria under light exposure have a significantly greater ecological temperature, which puts them at a greater risk of detection by visual predators. In a second set of experiments, we observe D. pulicaria in cold and warm water, and in darkness and under light exposure. Overall, our ecological temperature is a good discriminator of the crustacean's swimming behavior.

Nickel nanoparticle-chitosan-reduced graphene oxide-modified screen-printed electrodes for enzyme-free glucose sensing in portable microfluidic devices.

A facile one-step strategy is reported to synthesize nanocomposites of chitosan-reduced graphene oxide-nickel nanoparticles (CS-RGO-NiNPs) onto a screen-printed electrode (SPE). The synthesis is initiated by electrostatic and hydrophobic interactions and formation of self-assembled nanocomposite precursors of negatively charged graphene oxide (GO) and positively charged CS and nickel cations (Ni(2+)). The intrinsic mechanism of co-depositions from the nanocomposite precursor solution under cathodic potentials is based on simultaneous depositions of CS at high localized pH and in situ reduced hydrophobic RGO from GO as well as cathodically reduced metal precursors into nanoparticles. There is no need for any pre- or post-reduction of GO due to the in situ electrochemical reduction and the removal of oxygenated functionalities, which lead to an increase in hydrophobicity of RGO and successive deposition on the electrode surface. The as-prepared CS-RGO-NiNPs-modified SPE sensor exhibited outstanding performance for enzymeless glucose (Glc) sensing in alkaline media with high sensitivity (318.4µAmM(-1)cm(-2)), wide linear range (up to 9mM), low detection limit (4.1µM), acceptable selectivity against common interferents in physiological fluids, and excellent stability. A microfluidic device was fabricated incorporating the SPE sensor for real-time Glc detection in human urine samples; the results obtained were comparable to those obtained using a high-performance liquid chromatography (HPLC) coupled with an electrochemical detector. The excellent sensing performance, operational characteristics, ease of fabrication, and low cost bode well for this electrochemical microfluidic device to be developed as a point-of-care healthcare monitoring unit.

Indium tin oxide-coated glass modified with reduced graphene oxide sheets and gold nanoparticles as disposable working electrodes for dopamine sensing in meat samples.

Sensitive, rapid, and accurate detection of dopamine (DA) at low cost is needed for clinical diagnostic and therapeutic purposes as well as to prevent illegal use of DA in animal feed. We employed a simple approach to synthesize reduced graphene oxide sheets (rGOS) and gold nanoparticles (AuNPs) at room temperature on indium tin oxide-coated glass (ITO) slides as disposable working electrodes for sensing DA. Graphene oxide (GO) was directly reduced on ITO to remove oxygenated species via a rapid and green process without using chemical reducing reagents. AuNPs were electrochemically deposited in situ on rGOS-ITO with fairly uniform density and size. The sensitivity of the AuNPs-rGOS-ITO sensor for DA detection is 62.7 µA mM(-1) cm(-2) with good selectivity against common electrochemically interfering species such as ascorbic acid (AA) and uric acid (UA), and the detection limit measured by differential pulse voltammetry (DPV), at a signal-noise ratio of 3, was 6.0 × 10(-8) M. The electrochemical catalysis of DA was proven to be a surface process with an electron transfer coefficient (α) of 0.478 and a rate constant (k(s)) of 1.456 s(-1). It correlates well with the conventional UV-vis spectrophotometric approach (R = 0.9973) but with more than thrice the dynamic range (up to 4.5 mM). The sensor also exhibited good stability and capability to detect DA in beef samples, and thus is a promising candidate for simple and inexpensive sub-nanomolar detection of DA, especially in the presence of UV-absorbing compounds.

Plankton reach new heights in effort to avoid predators.

The marine environment associated with the air-water interface (neuston) provides an important food source to pelagic organisms where subsurface prey is limited. However, studies on predator-prey interactions within this environment are lacking. Copepods are known to produce strong escape jumps in response to predators, but must contend with a low-Reynolds-number environment where viscous forces limit escape distance. All previous work on copepod interaction with predators has focused on a liquid environment. Here, we describe a novel anti-predator behaviour in two neustonic copepod species, where individuals frequently exit the water surface and travel many times their own body length through air to avoid predators. Using both field recordings with natural predators and high-speed laboratory recordings, we obtain detailed kinematics of this behaviour, and estimate energetic cost associated with this behaviour. We demonstrate that despite losing up to 88 per cent of their initial kinetic energy, copepods that break the water surface travel significantly further than those escaping underwater and successfully exit the perceptive field of the predator. This behaviour provides an effective defence mechanism against subsurface-feeding visual predators and the results provide insight into trophic interactions within the neustonic environment.

The ciliate Paramecium shows higher motility in non-uniform chemical landscapes.

We study the motility behavior of the unicellular protozoan Paramecium tetraurelia in a microfluidic device that can be prepared with a landscape of attracting or repelling chemicals. We investigate the spatial distribution of the positions of the individuals at different time points with methods from spatial statistics and Poisson random point fields. This makes quantitative the informal notion of "uniform distribution" (or lack thereof). Our device is characterized by the absence of large systematic biases due to gravitation and fluid flow. It has the potential to be applied to the study of other aquatic chemosensitive organisms as well. This may result in better diagnostic devices for environmental pollutants.

Real-time assessment of fluid flow generated by appendage movements of Daphnia using standing square-wave chronamperometry.

Standing square-wave chronoamperometry (SSWCA) was applied to the analysis of the microfluid flow generated by the movement of the appendages of the Crustacea Daphnia. This novel approach provided for the first time real-time assessment and analysis of the breathing rate/fluid flow of individual organisms. An electrochemical tracer was delivered into the fluid inflow of the organism and a carbon fiber microelectrode placed in the fluid outflow's path. The variation of the net concentration/flux of the electroactive tracer, dopamine, at the electrode surface was measured with SSWCA. The observed chronoamperometric peaks (with fine structure) of the outflow are seen as a direct representation of appendage movement and, too, the workings and responses of the organism to its environment, e.g., external stimuli such as food or chemicals. It was concluded that SSWCA follows primarily the variation of the convective component of the Nernst-Plank equation for flux and, to lesser extent, diffusion and migration. In this work, SSWCA can clearly be used to monitor changes in the Daphnia-generated fluid outflow on a different time scale than was previously possible. This new application of SSWCA is faster and likely more accurate than using high-speed video.