J. Rudi Strickler

Calanoid copepods, feeding currents, and the role of gravity.

Feeding currents of free-swimming calanoid copepods, observed through an expanded krypton laser beam and a back-focus dark-field optical system, show that these planktonic animals generate a double shear field to help in detecting food. The interrelation between flow field, perception of food items, and body orientation explains why these animals are generally negatively buoyant.

Advertisement and concealment in the plankton: what makes a copepod hydrodynamically conspicuous?

Euchaeta rimana, a pelagic marine copepod, roams a 3-dimensional environment and its antennular setal sensors are oriented to detect water-borne signals in 3 dimensions. When the copepod moves through water or moves water around itself, it creates a fluid dis- turbance distinct from the ambient fluid motion. As it swims and hovers, the copepods laminar feeding current takes the unstable nature of small-scale turbulence, organizes it, and makes the local domain a familiar territory within which signals can be specified in time and space. The streamlines betray both the 3-dimensional spatial location (x, y, z) as well as the time (t) separating a signal caught in the feeding current and the copepod receptor-giving the copepod early warning of the approach of a prey, predator, or mate. The copepod reduces the complexity of its environment by fixing information from a turbulent field into a simpler, more defined laminar field. We quantitatively analysed small-scale fluid deformations created by copepods to document the strength of the signal. Physiological and behavioral tests confirm (a) that these disturbances are relevant signals transmitting information between zooplankters and (b) that hydrodynami- cally conspicuous structures, such as feeding currents, wakes, and vibrations, elicit specific responses from copepods. Since fluid mechanical signals do elicit responses, copepods shape their fluid motion to advertise or to conceal their hydrodynamic presence. When swimming, a copepod barely leaves a trace in the water; the animal generates its flow and advances into the area from which the water is taken, covering up its tracks with the velocity gradient it creates around itself. When escaping, it sheds conspicuous vortices. Prey caught in a flow field expose their location by hopping. These escape hops shed jet-like wakes detected by copepod mech- anoreceptors. Copepods recognize the wakes and respond adaptively.

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.

The three-dimensional flow field generated by a feeding calanoid copepod measured using digital holography.

SUMMARY Digital in-line holography is used for measuring the three-dimensional (3-D) trajectory of a free-swimming freshwater copepod Diaptomus minutus, and simultaneously the instantaneous 3-D velocity field around this copepod. The optical setup consists of a collimated He-Ne laser illuminating a sample volume seeded with particles and containing several feeding copepods. A time series of holograms is recorded at 15 Hz using a lensless 2Kx2K digital camera. Inclined mirrors on the walls of the sample volume enable simultaneous recording of two perpendicular views on the same frame. Numerical reconstruction and matching of views determine the 3-D trajectories of a copepod and the tracer particles to within pixel accuracy (7.4 μm). The velocity field and trajectories of particles entrained by the copepod have a recirculating pattern in the copepods frame of reference. This pattern is caused by the copepod sinking at a rate that is lower than its terminal sinking speed, due to the propulsive force generated by its feeding current. Consequently, the copepod sees the same fluid, requiring it to hop periodically to scan different fluid for food. Using Stokeslets to model the velocity field induced by a point force, the measured velocity distributions enable us to estimate the excess weight of the copepod (7.2×10-9 N), its excess density (6.7 kg m-3) and the propulsive force generated by its feeding appendages (1.8×10-8 N).

Swimming of Planktonic Cyclops Species (Copepoda, Crustacea): Pattern, Movements and Their Control

The zooplankton of lakes and seas consists for the most part of copepods, some species grazing phytoplankton and some living as carnivores, hunting other copepods. Animals of the order Cyclopoida are between 0. 05 and 0. 3 cm long and show a very distinct jerky (hop and sink) swimming pattern, using an average of one powerstroke per second. This gives them a mean speed of 0. 1 to 0. 5 cm/sec so that their Reynolds number ranges from 1 to 50. Storch (1929) filmed the sequence of movements of the four ventrally situated pairs of legs during a powerstroke and found a 4-3-2-1 metachronical pattern. One stroke of all the legs together brings them back to the original position (ventral, front), supposedly leaving the animal with a forward velocity component. The decelerating phase of one hop has been used by Vlymen (1970) to calculate the drag for one copepod species, for which he found the energy expended in swimming to be only 0. 3% of the metabolic energy consumption. In order to avoid an encounter with a predator, Cyclops display an escape reaction, swimming up to 35 cm/sec for about one second. This translates to about 120 powerstrokes per second. In this speed range flapping of the abdomen is more effective (Re ~ 500), but requires more energy.

Zooplankton capture by a coral reef fish : an adaptive response to evasive prey

SynopsisHigh-speed cinematography and video using modified Schlieren optics and laser illumination helped elicit details of prey capture mechanisms used by Chromis viridis while feeding on calanoid copepods and Artemia. Chromis viridis is capable of a ram-jaw, low-suction feeding, as well as a typical suction feeding behavior described for other species of planktivores. By adjusting the degree of jaw protrusion and amount of suction used during a feeding strike, this fish can modulate its feeding strikes according to the prey type being encountered. The ram-jaw feeding mode enables C. viridis to capture highly evasive calanoid copepods within 6 to 10 msec. The use of specialized feeding behavior for evasive prey and the ability to vary feeding behavior are adaptations for feeding on evasive prey.

A cinematographic comparison of behavior by the calanoid copepod Centropages hamatus Lilljeborg: tethered versus free-swimming animals

Many previous cinematographic studies of copepod behavior have used animals tethered to dog or cat hairs to keep them in focus. We compared behavior of tethered and free-swimming specimens of the calanoid copepod Centropages hamatus Lilljeborg using cinematographic methods. Precise quantification was made of the time allocated to four modes of behavior: slow-swim (movement of feeding appendages only), break (no appendages moving), fast-swim (posteriorally-directed movement of first antennae and pereiopods), and groom (brushing of first antennae through feeding appendages). Ten copepods each were used for tethered and free-swimming filming. Under both experimental regimes, copepods spent < 1 % of total amount of time in fast-swimming and grooming behavior. Most of the time (50.7–95.5%) animals were on break. The rest of the time (3.8–48.9%) animals were in the slow-swimming mode, moving only feeding appendages. There were no significant differences between tethered and free-swimming animals in mean time allocations to slow-swimming and break behavioral modes. However, individual variability of tethered animals was higher than that of free-swimming ones. We conclude that, while mean time allocation to slow-swimming and break behaviors were similar between free-swimming and tethered animals, the variability between tethered individuals is a factor to be considered when designing experiments.

Hang on or run? Copepod mating versus predation risk in contrasting environments

Mating durations of copepods were found to differ significantly between fishless high-altitude waters and lowland lakes containing fish. In lowland species the whole mating process was completed within a few minutes, but it averaged over an hour in high-altitude species. Alpine copepods showed a prolonged post-copulatory association between mates, during which the male clasped the female for an extended period after spermatophore transfer, while in lowland species males abandoned their partner immediately after copulation. Prolonged associations also occurred after transfer of spermatophores to heterospecific females with shorter conspecific mating duration, suggesting that male interests largely dictate the time spent in tandem. The differences observed may be adaptations to environments with different predation pressure, as pairs in tandem are more conspicuous and less reactive than single animals. We argue that differences in mating behavior and mating duration evolved under sexual versus natural selection, reflecting trade-offs between enhancement of fertilization success and reduction of vulnerability to visual predation. In fishless mountain lakes with high intrasexual competition, guarding males can reduce the risk of spermatophore displacement or the risk that the female will accept sperm from rival males without increased risk of being eaten, thereby maximizing paternity. Populations from fishless alpine lakes further differed from lowland species by exhibiting higher female/male size dimorphism and more intense pigmentation. While these traits vary between populations according to predation pressure, mating duration appears to be more species-specific.

Seasonal adaptations of Daphnia pulicaria swimming behaviour: the effect of water temperature

Daphnia swimming behaviour is controlled by a variety of external factors, including light, presence of food and predators. Temperature represents a key driver in the dynamics of Daphnia populations, as well as on their motion. In this study, we have investigated the behavioural adaptations of adult Daphnia pulicaria to two different temperatures, representative of the mean winter (3°C) and summer (22°C) temperatures to which these organisms are exposed to in the real environment. Video observations were conducted both in the presence and in the absence of light to investigate possible day/night modifications in the motion strategy. Analyses of mean speed, velocity power spectral density and trajectory fractal dimension point out specific adaptations that allow D. pulicaria to successfully adjust to the changing conditions of the environment. Independently of the light conditions, in cold waters D. pulicaria swim almost vertically with defined motional frequencies, likely to increase the encounter with food items diluted in the fluid. A similar behaviour is displayed by the animals at summertime temperatures in the presence of light; however, in this case the vertical swimming is coupled with the absence of peaks in the power spectra and might be exploited to avoid predators. In contrast, at 22°C in dark conditions D. pulicaria move horizontally with lateral motions to take advantage of possible patches of phytoplankton. This information sheds new light into the complex and dynamic adaptations of D. pulicaria in response to external stimuli.

Copepod flow modes and modulation: a modelling study of the water currents produced by an unsteadily swimming copepod.

Video observation has shown that feeding-current-producing calanoid copepods modulate their feeding currents by displaying a sequence of different swimming behaviours during a time period of up to tens of seconds. In order to understand the feeding-current modulation process, we numerically modelled the steady feeding currents for different modes of observed copepod motion behaviours (i.e. free sinking, partial sinking, hovering, vertical swimming upward and horizontal swimming backward or forward). Based on observational data, we also reproduced numerically a modulated feeding current associated with an unsteadily swimming copepod. We found that: (i) by changing its propulsive force, a copepod can switch between different swimming behaviours, leading to completely different flow-field patterns in self-generated surrounding flow; (ii) by exerting a time-varying propulsive force, a copepod can modulate temporally the basic flow modes to create an unsteady feeding current which manipulates precisely the trajectories of entrained food particles over a long time period; (iii) the modulation process may be energetically more efficient than exerting a constant propulsive force onto water to create a constant feeding current of a wider entrainment range. A probable reason is that the modulated unsteady flow entrains those water parcels containing food particles and leaves behind those without valuable food in them.