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Research on the swimming behavior of copepods
This is where we meet Dr. François-Gaël Michalec. He is a postdoctoral researcher in the group of Environmental Fluid Dynamics at the Institute of Environmental Engineering, working under Prof. Dr. Markus Holzner. His research focuses on calanoid copepods and their swimming behavior in response to the physical and chemical characteristics of their environment.
Copepods are small crustaceans inhabiting virtually all aquatic environments from hot hydrothermal vents to cold polar seas. Copepods of the order Calanoida usually outnumber all other marine zooplankton and represent a major food source for organisms from higher trophic levels such as fish larvae and small crustaceans.
This fundamental role at the basis of the trophic network has prompted great effort to better understand their ecology. Owing to their small size, copepods have limited swimming abilities and generally drift along currents in the ocean. At small scale however, they can move independently of the surrounding flow and will display a variety of complex behaviors which constitute a central component of their ecology. Motility allows them to capture preys, escape predation or unfavorable conditions and find mates; the outcome of these processes influences indirectly the dynamics of the local aquatic community.
Diverse environmental factors from natural and anthropic origins affect the behavior of calanoid copepods. Examples encompass salinity, chemical signals released by a conspecific animal or a predator, phytoplankton exudate, light intensity or waterborne pollutants. Hence, there is a strong interest in quantifying the behavior of copepods under different environmental conditions for accurate ecological prediction and modeling of population dynamics.
The high-speed 3D-PTV system at the Institute of Environmental Engineering
Dr. Michalec’s research group reconstructs the trajectories of swimming copepods by means of three-dimensional particle tracking velocimetry (3D-PTV), a flow imaging and measurement technique developed in the fluid dynamics community. The PTV system implemented at the Institute of Environmental Engineering represents a rare opportunity to couple high-speed recording frequency with long recording durations. The system consists of four calibrated monochrome EoSens® CL Mikrotron cameras positioned at different viewing angles. They record synchronously and capture the movements of copepods swimming freely within a measurement volume of approximately two liters.
At their full resolution of 1280 x 1024 pixels the cameras record at up to 506 frames per second. “The user-friendly control software makes things easy and the cameras are performing very well,” compliments Dr. Michalec. Images are transmitted to a large storage system composed of several arrays of hard disks. To obtain a large number of trajectories and to ensure reliable statistical analysis, Dr. Michalec conducts his experiments at 100 frames per second, stretching the recording time to one hour. Images are analyzed by a software solution developed at ETH Zurich. It reconstructs the trajectories of copepods from the synchronous image sequences provided by the four independent cameras. From the trajectories, Dr. Michalec’s group obtains Lagrangian quantities of interest such as velocity, acceleration and motion complexity.
Comparison with control values quantifies the extent of the behavioral alteration. For instance, measurements have evidenced hyperactivity caused by sub-lethal concentrations of toxicants in a widespread estuarine species, raising concern about the effects of background levels of pollution. In these measurements and unlike the more classic mortality tests, behavior analysis could capture changes occurring in response to trace amount of pollutants, conveying important information about the impairment of subtle ecological processes.
The useful combination of high-speed imaging and long recording duration
Calanoid copepods can respond with rapid acceleration to local disturbances in the flow and alternate sudden jumps and periods of slow and fast swimming. Previous studies were conducted with a 3D-PTV system composed of two cameras recording at a lower frame rate, preventing the capture of swift movements and intermittent events which are a characteristic feature of the behavior of copepods.
With the complete 3D-PTV at the Institute of Environmental Engineering, Dr. Michalec’s group can couple high temporal resolution (up to 506 frames per second) with long recording durations. This is a requirement for a reliable description of copepod motion. Finally, the use of four cameras greatly improves the particle detection and tracking efficiency by reducing ambiguities occurring during the stereo-matching procedure. The group can now track the behavior of hundreds of copepods swimming simultaneously.
The movements of copepods are recorded at a speed rate of 100 frames per second and a resolution of 1,280 × 1,024 pixels. Copepods appear as white dots on a black background.
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The copepods are recorded by four Mikrotron EoSens® CL cameras, installed at different viewing angles to capture their movements in three dimensions. For each camera, the whole sequence consists of 30,000 frames taken within five minutes.
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The argument of photosensitivity
In the 3D-PTV technique, the flow is seeded with small, neutrally buoyant tracer particles and strong illumination is typically provided by a laser operating in the visible range. However, copepods are sensitive to visible light and experiments must be conducted at moderate light intensity to avoid temperature increase. For this reason, Dr. Michalec’s group uses near-infrared illumination. For these wavelengths, the EoSens® sensor provides high quantum efficiency, allowing high relative aperture, large depth of field and sharpness over the whole investigation volume despite low light intensity.
The main advantage for Dr. Michalec’s group is the extremely high light sensitivity of the Mikrotron EoSens® CL camera. It enables the imaging of copepods even at low light conditions. Recording swimming copepods at 100 frames per second does not fully resolve their acceleration. The recording duration, however, is also to consider and this is limited by bandwidth and storage requirements. “So at 100 Hz we can record for about one hour, at 500 Hz we can record for about 10 minutes. That allows us to increase the number of trajectories while resolving a good part of their intermittent behavior”, says Dr. Michalec.
Quantum efficiency of the Mikrotron EoSens® CL camera, mono and color without UV/IR cut filter. The camera performs very well in the near infrared light spectrum, which begins at a wavelength of 700nm.
Three-dimensional particle tracking velocimetry is a flow measurement technique developed for the study of turbulent flows. The method is based on the visualization of a flow seeded with small and neutrally buoyant particles, which faithfully follow the flow motion. Three or four digital cameras, installed in an angular configuration, synchronously record the illuminated particles. The cameras are calibrated; this allows the reconstruction of the object space geometry and the determination of the particles’ three-dimensional coordinates. The subsequent tracking procedure reconstructs the trajectories of the particles, enabling the measurement of the flow being studied.