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First published online January 8, 2007
Journal of Experimental Biology 210, 299-310 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02633
Quantitative analysis of tethered and free-swimming copepodid flow fields
1 School of Civil and Environmental Engineering, Georgia Institute of
Technology, Atlanta, GA 30332-0355, USA
2 School of Biology, Georgia Institute of Technology, Atlanta, GA
30332-0230, USA
* Author for correspondence (e-mail: dwebster{at}ce.gatech.edu)
Accepted 7 November 2006
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Summary |
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Key words: copepod, Euchaeta antarctica, hydrodynamics, tethering, sensory systems
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), the
energetic costs of feeding and locomotion (e.g.
Stamhuis et al., 2002), and
the flow disturbance created by organism motion (e.g.
Yen and Fields, 1992). Flow
fields generated by copepods are of particular interest because of the
ecological significance of the interaction with other organisms. The flow
field around a copepod is a complex structure comprising an anterior feeding
current and a lateral and ventral propulsive current
(Fields and Yen, 1993). A
strong anterior feeding current can maximize intake volume to the feeding
appendages, as might be needed by a particle feeding plankter. It follows that
accurate estimates of volume processed in the feeding current are useful to
assess feeding rates and ultimately the impact of copepod grazing on trophic
energy transfer in planktonic communities. Alternatively, a predatory copepod
may construct a feeding current with a weaker velocity gradient so as not to
reveal its presence to rheotactic prey
(Yen and Strickler, 1996).
Thus, crypsis of the predator from its prey may be an important mechanism in
determining predation success. When generating the propulsive current,
trimming of the wake can be useful when hiding from mechanoreceptive predators
[e.g. fish (Coombs et al.,
1988)] or when creating a minimally disturbed trail of the
pheromone needed to attract a mate (Yen et
al., 1998).
Because of the interest in visualizing and quantifying the flow fields
created by copepods, several methods have been employed in recent decades:
Schlieren optics (Strickler,
1977), high-speed micro-cinematography
(Alcaraz et al., 1980;
Koehl and Strickler, 1981;
Strickler, 1982;
Gallager, 1993), manual
particle tracking (Yen et al.,
1991; Yen and Fields,
1992; Fields and Yen,
1993; Bundy and
Paffenhöfer, 1996), planar particle image velocimetry (PIV)
(van Duren et al., 1998;
van Duren et al., 2003;
Stamhuis et al., 2002;
van Duren and Videler, 2003),
and three-dimensional digital holography
(Malkiel et al., 2003).
Because copepods are (generally) small, visualization of the flow field
created by copepods requires high resolution. As a result, copepods are often
tethered in order to maintain a fixed position within the small field of view
during flow visualization. In the majority of the studies listed above, the
copepod was tethered during the flow visualization process. Researchers
generally acknowledge the distortion of the flow field induced by tethering
the organism but tethering is often considered a necessary step for acquiring
flow field data. For instance, during PIV measurements the copepod and
surrounding fluid must be imaged when the body position coincides with a thin
laser sheet. In absence of tethering, the researcher may have to wait
patiently for the organism to swim through the imaging region of the laser
sheet and hope that the body orientation is ideal during the pass. Obviously,
this is a potentially tedious experimental procedure. As an alternative,
researchers have suggested adding a fixed translational velocity to the
measurements around tethered copepods to account for the differences in the
flow field (e.g. Koehl and Strickler,
1981). It also has been suggested that placing a tethered copepod
in a moving current (at a speed that matches a typical swim velocity)
eliminates the potential influence of the tether on the flow field
(Bundy and Paffenhöfer,
1996).
Despite the common use of tethering of zooplankton during flow studies, a
comparison between untethered and tethered larvae showed that the flow pattern
is altered by the presence of the tether
(Emlet, 1990). Further, the
magnitude and location of high velocity regions are different in tethered
versus untethered copepods and larvae
(Emlet, 1990;
Bundy and Paffenhöfer,
1996). In addition to the physical differences in the flow field,
the organism behavior is also potentially modified by the addition of the
tether. Hwang et al. found similar mean time allocation habits of tethered
copepods compared to untethered copepods
(Hwang et al., 1993), but
there was a significant difference in the individual variability.
The objective of this study was to quantitatively compare the flow fields created by tethered and untethered copepodids using the particle image velocimetry (PIV) technique. As described in detail below, the fields of velocity, vorticity, dissipation rate and strain rate created by the copepodid Euchaeta antarctica were quantified for free-swimming and tethered specimens. Because events occur at time scales of milliseconds for organisms in the millimeter range, the measurements require high precision in timing and high spatial resolution.
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Experimental setup
Flow fields created by free-swimming and tethered E. antarctica
were visualized in a clear, glass cubic tank (15 cmx15 cmx15 cm)
filled with artificial seawater of salinity 34.85 p.p.t. in a dark room. The
temperature of the tank was maintained at 0°C during the experiment by
immersing the glass tank in a recirculating bath of propylene glycol and
deionized water, which was surrounded by insulating foam with small windows to
provide optical access for the cameras. The recirculating bath fluid passed
through a Fisher Scientific chiller in order to maintain the desired
temperature. Images of free-swimming copepodids were recorded only when the
copepodids swam through the laser sheet. A second camera, connected to a
television monitor and possessing a perspective perpendicular to the PIV
camera, was used to monitor the position of the free-swimming copepodids.
To restrain the copepodid, we tethered it to a 38-gauge copper wire attached by cyanoacrylate glue. Attachments were made to the dorsal side of the copepodid near the junction between the cephalic and thoracic segments. A 0.5 mm bend at the end of the wire provided enough surface area for attachment. The end of the wire, dipped in glue and air-dried for 30 s until the glue was tacky, was attached to a copepodid, which was restrained in a drop of water in a cooled Petri dish. Just prior to attachment, the copepodid was blotted dry of seawater for less than 1 s. Once the tacky end of the wire made contact with the dorsal side of the copepodid, ambient seawater was added to harden the glue and ensure attachment. The copepodid was fully submerged in seawater, and the copepodid-wire bond was checked for proper attachment and positioning away from the cephalic appendages. Copepodids with a poorly positioned tether were not used for flow field imaging. The bend directed the wire away from the copepodid at a right angle to minimize interference with the deployment of the locomotory appendages, although as noted below the presence of the tether appeared to influence the symmetry of the resultant biologically generated flows. The wire was attached via a glass rod to a 3-axis precision position manipulator, which was used to position the copepodid in the laser sheet in the center of the field of view of the camera.
Fluid velocity measurements
Flow fields were measured for free-swimming and tethered Euchaeta
antarctica using the non-intrusive particle image velocimetry (PIV)
technique. The PIV technique measures the displacement of small tracer
particles suspended in the fluid over a short time period (e.g.
Westerweel, 1997;
Raffel et al., 1998). The
advantage of this technique is that the position of many particles can be
recorded with a digital camera; hence, a field of simultaneous velocity
vectors is measured. Because the particles were illuminated by a laser sheet,
the displacement vectors corresponded to the plane of illumination. One
important assumption for PIV is that the particles follow the fluid flow over
the measurement interval; therefore, tracer particles must be small and nearly
neutrally buoyant. In the current experiments, titanium dioxide particles with
a mean diameter of less than 5 µm were homogeneously seeded into the
fluid.
The particles were illuminated with an Oxford pulsed infrared laser (model
HSI-500, Shirley, MA, USA). The laser illuminated particles in a 1 mm thick
sheet with a row of laser diodes that produced monochromatic light at a
wavelength of 808 nm and maximum pulse energy of 15 mJ. Many copepods are
phototactic, respond to light by swimming towards it, and are typically most
sensitive to wavelengths centered around 500 nm
(Stearns and Forward, 1984;
Cohen and Forward, 2002). The
near IR wavelengths employed in the current measurements did not change the
organism behavior. Specifically, the copepodids swam freely into and out of
the laser sheet with no observable avoidance or preference and with no
observable change in swimming characteristics. The laser pulse repetition
period was variable within the range of 1-80 ms with the longer delay period
corresponding to greater output energy and illumination. In this set of
experiments, the period between the laser pulses was 8 ms and a timing control
circuit synchronized the camera shutter with the laser pulses. A VDS
Vosskühler CMC-1300 CMOS digital camera and a Datacube MaxRevolution
image acquisition board acquired the images. Image pairs (i.e. images of the
laser pulses separated by 8 ms) were collected at 50 Hz. The width of the
laser sheet (1 mm) is relatively large compared to the size of the copepodid
body (prosome length of 4.6 mm). As a result, the `planar' velocity
measurements correspond to a finite width of the three-dimensional flow. Based
on the camera lens focal length (105 mm), aperture (f/2.8) and location, the
depth of field was calculated to be 0.725 mm. Therefore, the reported velocity
fields correspond to the average over the depth of field rather than true
planar velocity fields.
Pairs of PIV images were analyzed to determine the particle displacement in
the image plane via a cross-correlation calculation (e.g.
Raffel et al., 1998). The
images were divided into interrogation subwindows of 32x32 pixels.
Particle locations in a subwindow in the first image were compared to the
corresponding subwindow in the second image by calculating the
cross-correlation function in phase space. The average particle displacement
in the subwindow region was determined by locating the peak value of the
cross-correlation function relative to the center of the subwindow. The
location of the peak in the correlation was identified to subpixel accuracy
via a Gaussian function fit. This process was repeated for the entire
image with a 50% overlap of each subwindow. The velocity was calculated by
dividing the displacement vector by the time delay between consecutive laser
pulses. The velocity data were validated by calculating the median velocity of
a 3x3 grid of neighboring points and comparing the local velocities to
the median velocity (Westerweel,
1994; Nogueira et al.,
1997). Velocity vectors outside of an acceptable range were
identified as bad vectors and replaced by a spatially interpolated value.
Velocity vectors that coincided with the location of the organism body were
removed during post-processing by creating a blanking template from the
original image pair. Based on the accuracy of the peak correlation location
estimate and other considerations, the uncertainty of the velocity vector is
estimated to be ±3%.
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The spatial gradient of velocity is important from a mechanosensory
perspective and previous studies suggest that strain rate is the quantity that
most closely correlates with copepod behavior (e.g.
Fields and Yen, 1997;
Kiørboe et al., 1999;
Woodson et al., 2005). Strain
rate is a measure of the deformation of a fluid element as it flows. The
strain rate components that can be directly calculated based on the measured
planar velocity fields are:
 | (1) |
Vorticity 
is another quantity based on the velocity gradient and
hence also could be important to define the perturbation created by an
organism, although currently there is no direct evidence to support the idea
that predators or prey are sensing and responding to vorticity. Vorticity is a
point measure of the rotation of the fluid, and the components of the
vorticity vector for the measured planes are:
 | (2) |
Another quantity involving spatial gradients of the velocity field is the
viscous dissipation rate of kinetic energy. This quantity is of interest
because it relates to the costs of propulsion, the time that a flow
perturbation persists, and the ecological significance of predator avoidance.
The rate of energy dissipation due to viscosity, 
, is defined as (e.g.
Schlichting and Gersten,
2000):
 | (3) |
uz/z term was estimated
using the incompressible continuity equation:
 | (4) |
 | (5) |
 | (6) |
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To compare among free-swimming and tethered flow fields, it was important to check that the swimming behavior of the copepodids was similar. Several criteria were used to confirm that all tested specimens were behaving in a typical cruising mode. First, the raw image sequences of the free-swimming and tethered copepodids were viewed to verify that the second antennae were being used to propel the organism. The second antennae are used for propulsion during cruising, whereas the antennules (first antennae) and swimming legs are used for propulsion during escaping. Occasionally, we observed the appendage motion associated with escape behavior in both the free-swimming and tethered specimens, but the data presented herein are exclusively associated with cruise swimming behavior. Second, we confirmed that the swimming speeds of the free-swimming copepodids for the reported PIV data (0.83 cm s-1 for the dorso-ventral view, and 0.82 cm s-1 for the side view) were consistent with typical swimming speeds for cruising copepodids. Table 1 shows the average and standard deviation of the swimming speed and Reynolds number measured for a total of 67 individual specimens. Typical swimming speeds of cruising organisms are an order of magnitude smaller than the typical escaping speeds. Third, we measured the appendage paddling frequency and found that both the tethered and free-swimming copepodids were paddling at approximately 50 Hz.
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Flow field
The streamlines of the flow field for the free-swimming copepodid in both
the dorso-ventral and side views slightly converged in front of and slightly
diverged behind the body of the organism
(Fig. 2A,C). These images were
created by tracking tracer particle movement in the frame of reference of the
copepodid. While the streamlines around the free-swimming copepodid were
nearly parallel to the body of the specimen
(Fig. 2A,C), the streamlines
around the tethered copepodid curved sharply into the feeding appendages and
abdomen (Fig. 2B,D). More
pronounced convergence (upstream) and divergence (downstream) of the
streamlines was evident for specimens in the figures and the replicates that
are not shown. Qualitatively, the volume of fluid influenced by the tethered
copepodid appeared to be greater than the volume of fluid influenced by the
free-swimming copepodid in agreement with previous observations for larvae of
bivalves and gastropods (Emlet,
1990) and for larvae of mollusks
(Gallager, 1988). Specifically,
Emlet noted that the particle paths were much wider upstream for the tethered
specimen compared to the free-swimming organism
(Emlet, 1990), which agrees
with Fig. 2.
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Vorticity field
For the free-swimming copepodid, the vorticity field
(Fig. 5) also demonstrated
(anti)symmetry about the y=0 axis. The vorticity magnitude was
largest in the boundary layer region near the body. Because of the opposite
orientation of the velocity gradient, the vorticity on the left and right
sides of the body had opposite signs; in both cases the maximum magnitude of
the vorticity was roughly 20 s-1. In the side view, the region of
high vorticity magnitude also was confined to the near body boundary layer and
in particular along the ventral surface of the copepodid. In this plane, the
maximum vorticity magnitude also was roughly 20 s-1.
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) and two
orders of magnitude larger than the estimate for Temora longicornis
(van Duren et al., 2003). The
difference may result from the fact that E. antarctica is larger than
the other species. Further, increased resolution of the measurements may
improve (and increase) the dissipation rate estimate due to better resolution
of the spatial variation in velocity.
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Strain rate field
The exx component of the strain rate tensor is shown in
Fig. 7 for the free-swimming
copepodid. As with the other quantities described above, the location of the
largest values of the strain rate was in the boundary layer region near the
copepodid body. The peak value was approximately 10 s-1 along the
appendages and the ventral surface of the copepodid. A peak negative value of
-10 s-1 occurred along the antennae in front of the copepodid (i.e.
x=-0.05 cm). Other components of the strain rate tensor showed a
similar spatial distribution and will be discussed below for a specific
profile location.
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As described for the field plots in Fig. 3, the velocity direction and magnitude were altered by the presence of the tether. In Fig. 8A,B, the peak value of the ux-component of velocity was greater in the tethered case, and the uy-component differed in magnitude and direction over much of the profile. The vorticity (Fig. 8C) and shear strain rate (Fig. 8F) profiles in this case agreed fairly well between the specimens, except very close to the copepodid body (y'<0.05 cm), where the magnitude of both quantities was greater in the tethered case. The normal strain rate components (shown in Fig. 8D,E) were different between the tethered and free-swimming profiles. This reflected both a change in the velocity components (Fig. 8A,B) and a change in the spatial variation (i.e. gradient) of the velocity field.
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;
Bundy and Paffenhöfer,
1996), the flow field around a tethered organism differs from that
around a free-swimming animal. On this note, van Duren et al. write
"there is no doubt that the morphology of flow fields [around
tethered copepods] will be to some extent different from those around moving
animals" (van Duren et al.,
2003). At first thought, it seems intuitive to suggest that the
difference is merely due to a translational velocity difference due to the
fixed position of the tethered specimen compared to the moving specimen.
However, the particle trajectories shown in
Fig. 2 demonstrate that the
difference is not solely due to the addition of a uniform velocity field
corresponding to the translation of the organism. In both sets of images, the
particles are moving past a copepodid fixed in the photograph, and it is clear
that the particle paths are very different. Further, the data in Figs
3,
4, and
8 provide quantitative evidence
that the presence of the tether greatly influences the flow field
characteristics. To explain the fundamental difference between the tethered and free-swimming flow fields, it is illuminating to consider the forces. The free body diagrams for the organism in the free-swimming and tethered cases are sketched in Fig. 9. For an organism swimming in the horizontal direction (as shown in the sketch), the relevant forces on the copepod are the drag and thrust. For an organism that is cruising, i.e. not accelerating, the forces acting on the body are in equilibrium, which means they are equal and opposite in direction. Each force acts on the organism in one orientation and on the fluid in the opposite orientation. On the organism, Fthrust acts in the direction of copepod motion, and Fdrag acts opposite to the direction of copepod motion. On the fluid, Fthrust acts opposite to the direction of copepod motion and induces a fluid jet away from the organism, and Fdrag acts in the direction of copepod motion and causes fluid to be dragged along with the organism.
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The force balance changes meaningfully with the addition of a tether. The
(non-accelerating) organism can now impart an unbalanced force on the fluid
because it can push against the tether. The resistance to acceleration is not
provided by a drag force, but is given rather by the force and moment on the
tether. The force and moment on the tether balance Fthrust
and act out of, and not on, the fluid (Fig.
9B). Hence, adding a translational velocity to the tethered flow
field (a kinematic operation) does not take into account the force
Fdrag that results from fluid viscosity and causes fluid
to be dragged along with the organism (a dynamic effect). [Note that an
equally valid description of this phenomenon has been made with regard to a
`momentumless wake' for a self-propelled object (e.g.
Naudascher, 1965;
Sirviente and Patel, 2000).
The term `momentumless wake' refers to the momentum distribution in the wake
of the self-propelled object having the same momentum flux as the approaching
flow upstream of the object. The addition of a tether alters the momentum
distribution in the wake due to the addition of the unbalanced force on the
fluid.]
The difference between the flow fields is largely explained by the
unbalanced force in the tethered case. An analytical solution of the
Navier-Stokes equations for the laminar flow induced by a point force was
first reported by Landau (Landau,
1944) and Squire (Squire,
1951). The analysis begins by locating a force at the coordinate
origin within an infinitely large fluid domain. The point force and polar
coordinate system are shown in Fig.
10A. The solution for the flow velocity components is:
 | (7) |
 | (8) |
 | (9) |
):
 | (10) |
are the polar coordinates, 
is
the fluid kinematic viscosity, 
is the fluid density, and F is
the magnitude of the applied force. Sherman
(Sherman, 1990) provided an
alternative interpretation of C in which the radial velocity
component, ur, is evaluated along the downstream axis
(i.e. =0) at an arbitrary distance r=R:
 | (11) |
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Note that the left hand side appears like a local Reynolds number evaluated
at r=R and =0. We used the measured velocity field
for the tethered copepodid to estimate the velocity at a distance of 0.75 cm
downstream of the tether location. The resulting value for C was
0.4.
Fig. 10B shows the streamline pattern for the analytical flow solution. The streamlines converge upstream of the force location and diverge more gradually downstream. Comparison of the analytical streamline pattern with the particle paths for the tethered copepodid in Fig. 2B,D reveals a remarkable similarity. Despite the presence of the organism body and the fact that the force on the fluid is more broadly distributed in the organism case compared to the theoretical case, the general agreement suggests that the addition of a force on the fluid at the tip of the tether provides an explanation of the modified flow field for the tethered case compared to the free-swimming case.
Based on this Discussion, we can draw some important practical conclusions.
The flow field in the tethered case cannot be `corrected' by adding a uniform
translational velocity (Koehl and
Strickler, 1981). Rather, the addition of the unbalanced force in
the tethered case modifies the flow field due to the viscous flow effects. The
modified flow field has different spatial gradients and hence different fields
of vorticity, strain rate and dissipation rate. Further, calculations
performed during previous investigations of the filtering rate, volume of
fluid entrained by zooplankton, and spatial extent of the fluid disturbance
are influenced by the modification of the flow field. One solution to this
dilemma is to perform the velocity field measurements on free-swimming
organisms, as done in the current study. An alternate solution is to place a
tethered organism in a moving current
(Bundy and Paffenhöfer,
1996), but implementing this strategy raises difficult practical
issues. To eliminate the unbalanced force effect described above, the drag
force due to the fluid moving past the organism body must exactly balance the
self-generated thrust of the organism. The flow velocity in the test channel
must be adjusted such that force on the tether equals zero, which in practical
application requires that the force on the tether be measured (continuously).
Because the tethered copepodid specimens demonstrate unsteady thrust
generation (i.e. their swimming behavior and thrust force varies in time),
data collection should be limited to periods when the measured force on the
tether equals zero.
Flow field of the free-swimming copepodid
Researchers have quantified the flow fields around tethered copepods
(Bundy and Paffenhöfer,
1996; van Duren et al.,
1998; van Duren et al.,
2003) and free-swimming copepods
(Tiselius and Jonsson, 1990;
Yen et al., 1991;
Fields and Yen, 1993;
Bundy and Paffenhöfer,
1996; Malkiel et al.,
2003). The copepodids in these studies exhibited three locomotive
modes: feeding, cruising and escaping. Both simulations
(Jiang et al., 2002) and
experiments (Tiselius and Jonsson,
1990; Bundy and
Paffenhöfer, 1996) have shown that the geometry of the flow
field is dependent on the locomotive mode. The current discussion is limited
to the cruise mode of swimming.
Flow fields for free-swimming copepods have not been measured previously
with planar PIV, but data from lower resolution particle tracking and
holography methods provide useful information for comparison to the current
results. In light of the discussion above regarding the modification of the
flow field due to the presence of the tether, we did not make detailed
comparisons to previous planar PIV data for tethered copepods despite the fact
that the data in those studies have superior resolution than the particle
tracking and holography methods. Malkiel et al.
(Malkiel et al., 2003)
observed large-scale recirculation of the fluid surrounding sinking, feeding
copepods. The current data do not reveal the recirculation pattern because the
data are locally focused around the organism body and the fact that the
copepodids were moving in cruise mode.
Analyses of the small-scale fluid motion of the pelagic copepodid offers
some insight into factors influencing the complexity of the
biologically-generated flow. For this free-swimming polar species, E.
antarctica, the magnitude of the velocity field shows a maximum (1.2 cm
s-1) surrounding the locomotory appendages, the second antennae.
Velocity fields of free-swimming copepods have maximum velocity magnitudes
ranging from 0.3 cm s-1 to 3.8 cm s-1
(Tiselius and Jonsson, 1990;
Yen et al., 1991;
Fields and Yen, 1993;
Bundy and Paffenhöfer,
1996) and 1.98 cm s-1 in the simulations of Jiang et
al. (Jiang et al., 1999). The
velocity fields for these copepods were symmetrical in the dorso-ventral view
with converging streamlines into the appendages and diverging streamlines to
the rear of the organism (Tiselius and
Jonsson, 1990; Yen et al.,
1991). The geometry of the flow field was similar in the current
study, and the maximum velocity (1.2 cm s-1) was in the same range.
The velocity distribution in the current study differed from that observed by
Bundy and Paffenhöfer (Bundy and
Paffenhöfer, 1996), who reported considerable variability
among trials. The resolution of the velocity field around the locomotory
appendages is superior in the current study, which could lead to better
estimates of the velocity and may explain the discrepancy. Overall, the
current flow field measurements are qualitatively consistent with the previous
data collected with other methods and provide improved quantitative details
while avoiding the issues of tethering.
As noted for the tropical congener of this copepod
(Lenz and Yen, 1993), the
intensity of the anterior flow field declines toward the distal tips of E.
antarctica. Hence, the mechanoreceptive sensors extend beyond the induced
flow field, which enables sensing of an approaching predator. In front of the
antennules, the anterior feeding current velocity has a double maximum where
the longest mechanosensory hairs are located
(Fig. 3A) (see also
Yen and Nicoll, 1990). The
structure of the feeding current appears to be optimized for detecting prey
escapes (closest to mechanosensors) and for aggregating prey where they can be
captured (i.e. within capture range of this carnivorous copepod).
With regard to the mechanoreceptive signal generated by E.
antarctica, Fig. 8D-F
suggests that exy is the largest component of the strain
rate in the induced flow field. Shear strain rates greater than 2
s-1 (maximum of around 10 s-1) surround the locomotory
appendage region, which demonstrates the intensity of the copepodid-generated
flow disturbance. The 0.5 s-1 contour of exy
for the free-swimming case (Fig.
11A,C), which is a likely threshold to induce an escape response
(Fields and Yen, 1997;
Kiørboe et al., 1999),
provides a measure of the spatial extent of the signal for other
mechanoreceptive predators. The area in the dorso-ventral view surrounded by
the 0.5 s-1 contour is 11 times the area of the exoskeletal form.
Hence, mechanoreceptive predators will perceive a much more spatially extended
signal than the body size. The 0.5 s-1 contour of
exy for the tethered specimen in the side view
(Fig. 11D) shows that the
extent of the strain rate field is much greater in the tethered case (the
contour extends beyond the boundaries of the measured field). Thus, the
spatial extent of the mechanoreceptive signal would be overestimated with
these data for the tethered specimen. Alternatively, the spatial extent of the
0.5 s-1 contour for the tethered copepodid in the dorso-ventral
view (Fig. 11B) is similar or
slightly reduced compared to the free swimming case
(Fig. 11A). The differences in
the strain rate fields for the free-swimming and tethered copepodid field
demonstrate the subtle influence of altering the spatial distribution of the
flow field due to the physical presence of the tether (described above) and
perhaps due to the behavioral changes of the organism. This comparison
highlights the importance of using a free-swimming flow field when making
sensory ecology conclusions.
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Acknowledgments |
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References |
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Bundy, M. H. and Paffenhöfer, G. A. (1996). Analysis of flow fields associated with freely swimming calanoid copepods. Mar. Ecol. Prog. Ser. 133,99 -113.
Cohen, J. H. and Forward, R. B., Jr (2002).
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