Jim Rohr

Strouhal Numbers and Optimization of Swimming by Odontocete Cetaceans

SUMMARY Swimming efficiencies of fish and cetaceans have been related to a certain synchrony between stroke cycle frequency, peak-to-peak tail/fluke amplitude and mean swimming speed. These kinematic parameters form a non-dimensional wake parameter, referred to as a Strouhal number, which for the range between 0.20 and 0.40 has been associated with enhanced swimming efficiency for fish and cetaceans. Yet to date there has been no direct experimental substantiation of what Strouhal numbers are preferred by swimming cetaceans. To address this lack of data, a total of 248 Strouhal numbers were calculated for the captive odontocete cetaceans Tursiops truncatus, Pseudorca crassidens, Orcinus orca, Globicephala melaena, Lagenorhynchus obliquidens and Stenella frontalis. Although the average Strouhal number calculated for each species is within the accepted range, considerable scatter is found in the data both within species and among individuals. A greater proportion of Strouhal values occur between 0.20 and 0.30 (74%) than the 0.25–0.35 (55%) range predicted for maximum swimming efficiency. Within 0.05 Strouhal increments, the greatest number of Strouhal values was found between 0.225 and 0.275 (44%). Where propulsive efficiency data were available (Tursiops truncatus, Pseudorca crassidens, Orcinus orca), peak swimming efficiency corresponded to this same Strouhal range. The odontocete cetacean data show that, besides being generally limited to a range of Strouhal numbers between 0.20 and 0.40, the kinematic parameters comprising the Strouhal number provide additional constraints. Fluke-beat frequency normalized by the ratio of swimming speed to body length was generally restricted from 1 to 2, whereas peak-to-peak fluke amplitude normalized by body length occurred predominantly between 0.15 and 0.25. The results indicate that the kinematics of the propulsive flukes of odontocete cetaceans are not solely dependent on Strouhal number, and the Strouhal number range for odontocete cetaceans occurs at slightly (∼20%) lower values than previously predicted for maximum swimming efficiency.

Bioluminescent response of individual dinoflagellate cells to hydrodynamic stress measured with millisecond resolution in a microfluidic device

SUMMARY Dinoflagellate bioluminescence serves as a model system for examining mechanosensing by suspended motile unicellular organisms. The response latency, i.e. the delay time between the mechanical stimulus and luminescent response, provides information about the mechanotransduction and signaling process, and must be accurately known for dinoflagellate bioluminescence to be used as a flow visualization tool. This study used a novel microfluidic device to measure the response latency of a large number of individual dinoflagellates with a resolution of a few milliseconds. Suspended cells of several dinoflagellate species approximately 35 μm in diameter were directed through a 200 μm deep channel to a barrier with a 15 μm clearance impassable to the cells. Bioluminescence was stimulated when cells encountered the barrier and experienced an abrupt increase in hydrodynamic drag, and was imaged using high numerical aperture optics and a high-speed low-light video system. The average response latency for Lingulodinium polyedrum strain HJ was 15 ms (N>300 cells) at the three highest flow rates tested, with a minimum latency of 12 ms. Cells produced multiple flashes with an interval as short as 5 ms between individual flashes, suggesting that repeat stimulation involved a subset of the entire intracellular signaling pathway. The mean response latency for the dinoflagellates Pyrodinium bahamense, Alexandrium monilatum and older and newer isolates of L. polyedrum ranged from 15 to 22 ms, similar to the latencies previously determined for larger dinoflagellates with different morphologies, possibly reflecting optimization of dinoflagellate bioluminescence as a rapid anti-predation behavior.

Hydrodynamic stimulation of dinoflagellate bioluminescence: a computational and experimental study

SUMMARY Dinoflagellate bioluminescence provides a near-instantaneous reporter of cell response to flow. Although both fluid shear stress and acceleration are thought to be stimulatory, previous studies have used flow fields dominated by shear. In the present study, computational and experimental approaches were used to assess the relative contributions to bioluminescence stimulation of shear stress and acceleration in a laminar converging nozzle. This flow field is characterized by separate regions of pronounced acceleration away from the walls, and shear along the wall. Bioluminescence of the dinoflagellates Lingulodinium polyedrum and Ceratocorys horrida, chosen because of their previously characterized different flow sensitivities, was imaged with a low-light video system. Numerical simulations were used to calculate the position of stimulated cells and the levels of acceleration and shear stress at these positions. Cells were stimulated at the nozzle throat within the wall boundary layer where, for that downstream position, shear stress was relatively high and acceleration relatively low. Cells of C. horrida were always stimulated significantly higher in the flow field than cells of L. polyedrum and at lower flow rates, consistent with their greater flow sensitivity. For both species, shear stress levels at the position of stimulated cells were similar to but slightly greater than previously determined response thresholds using independent flow fields. L. polyedrum did not respond in conditions where acceleration was as high as 20 g. These results indicate that shear stress, rather than acceleration, was the stimulatory component of flow. Thus, even in conditions of high acceleration, dinoflagellate bioluminescence is an effective marker of shear stress.

Stimulation of bioluminescence by turbulent pipe flow

Abstract The response of in situ luminous marine organisms is studied over a wide range of flow agitation associated with laminar and turbulent pipe flow. Because a fully developed pipe flow is hydrodynamically well characterized, it is particularly suited for studying bioluminescence stimulation. Results show a dramatic increase in bioluminescent activity when transition occurs from laminar to turbulent flow. Above transition, as the flow rate (and turbulence) increases, an accompanying rise in bioluminescence is observed that results primarily from more organisms being stimulated. The hydrodynamic parameters for the 6.35 mm diameter pipe, where significant bioluminescent stimulation began, are: wall shear stress = 22–27 dyn cm −2 , wall strain rate = 1915–2350s −1 , and an average shear stress throughout the pipe of 11–13.5 dyn cm −2 . The smallest turbulent length scale at bioluminescent onset is on the order of 33 μm.

Glowing with the Flow

Bioluminescent marine organisms such as dinoflagellates enable engineers and oceanographers to study complex fluid dynamics and demonstrate the important role of light emission in ecology.

A novel flow visualization technique using bioluminescent marine plankton II. Field studies

The broad scope of this study is to investigate whether the flow induced stimulation of naturally occurring luminescent plankton can be effectively utilized as a method of flow visualization in the field. The primary goal is to determine the nature of flow around a moving dolphin based on the luminescent response of in situ plankton to hydrodynamic stimulation. >