A Novel Crustacean Swimming Stroke: Coordinated
Four-Paddled Locomotion in the Cypridoidean
Ostracode Cypridopsis vidua (Mu?ller)
GENE HUNT1,*, LISA E. PARK2, AND MICHAEL LABARBERA3
1Committee on Evolutionary Biology, University of Chicago, Chicago, Illinois 60637; 2Department of
Geology, The University of Akron, Akron, Ohio 44325-4104; and 3Department of Organismal Biology
and Anatomy, University of Chicago, Chicago, Illinois 60637
Abstract. Despite the diversity and ecological importance
of cypridoidean ostracodes, there have been no kinematic
studies of how they swim. We used regular and high-speed
video of tethered ostracodes to document locomotion in the
cypridoidean species Cypridopsis vidua. Swimming in this
species is drag-based, with thrust provided by both anten-
nulae and antennae. About 15 complete power and recovery
strokes occur per second; maximal speeds for the limb tips
were about 30 mm/s for the antennulae and 50 mm/s for the
antennae. These speeds correspond to Reynolds numbers on
the order of 10
1 to 100 for the limb tips and 10
2 to 10
1
for the setae that extend outward from the swimming limbs
and provide much of the surface area of the limb. The
strokes of the four thrust-producing limbs are coordinated in
a manner that seems to be unique among aquatic arthropods.
When viewed from the anterior, power strokes are synchro-
nized diagonally: left antennula and right antenna power
strokes start at the same time and terminate just as the power
strokes for the right antennula and left antenna begin. Be-
cause power strokes occur throughout the stroke cycle,
swimming in this species is smoothly continuous, without
the rapid accelerations and decelerations characteristic of
most small aquatic arthropods.
Introduction
The mechanics of swimming in ostracodes has received
relatively little attention compared with that of other aquatic
microcrustaceans such as copepods (Buskey, 1998; Lenz
and Hartline, 1999; Titelman, 2001; Buskey et al., 2002),
isopods (Alexander, 1988; Hessler, 1993), branchiopods
(Williams, 1994), and cladocerans (Zaret and Kerfoot,
1980; Kirk, 1985; Lagergren et al., 1997). Prior kinematic
research on ostracodes has focused on marine pelagic myo-
docopids whose relatively large size mitigates some of the
technical difficulties of studying locomotion in small swim-
ming animals (Lochhead, 1968; Minkina and Pavlova, 1987;
Davenport, 1990). These studies have demonstrated that swim-
ming in myodocopid ostracodes is powered by sweeping mo-
tions of the antennal exopod, a finding that supports more
casual observations (Sko?gsberg, 1920; Cohen, 1983).
Aside from the myodocopids, the other major group of
swimming ostracodes is the Cypridoidea, a mostly freshwa-
ter clade within the podocopids. A number of studies have
described the swimming behavior of species within this
group, but only in general terms (Sko?gsberg, 1920; Schrei-
ber, 1922; Canon, 1924; Kesling, 1951). For cypridoideans
that swim, the morphology of the limbs, and in some cases,
direct observations of swimming animals, have suggested
that both antennulae and antennae are involved in providing
thrust for locomotion (Sko?gsberg, 1920; Hoff, 1942). Figure 1
illustrates the morphology of these limbs and their position
within the ostracode carapace. Despite the ease with which
many cypridoideans can be maintained in the laboratory, there
have been no detailed studies of the swimming stroke in this
diverse and ecologically important group of ostracodes.
Here we document, using regular and high-speed video,
the swimming stroke of the cypridoidean ostracode Cypri-
dopsis vidua (Mu?ller 1776). This cosmopolitan Holarctic
species is highly mobile, nekto-benthic, phytophilous, and
well known to be parthenogenetic (Delorme, 1970, 1989;
Uiblein et al., 1996). Although the locomotory behavior of
Received 17 February 2006: accepted 31 October 2006.
* To whom correspondence should be addressed, at Department of
Paleobiology, National Museum of Natural History, Smithsonian Institu-
tion, Washington DC 20013-7012. E-mail: hunte@si.edu
Reference: Biol. Bull. 212: 67?73. (February 2007)
? 2007 Marine Biological Laboratory
67
this species has been studied (Roca et al., 1993; Uiblein et
al., 1996), little is known about the mechanics of how these
animals swim. Our analyses confirm previous suggestions
that both antennulae and antennae power swimming in this
species. Moreover, we show that the power strokes of these
limbs are coordinated into a four-paddled gait that seems to
be unique among aquatic arthropods.
Materials and Methods
A culture of Cypridopsis vidua was obtained from a
commercial supplier (Ward?s Natural Science, Rochester,
NY) and maintained in an acrylic plastic container at room
temperature. Replacement water from Lake Michigan was
added weekly, and boiled wheat grains were added every
few weeks as a food source. Specimens of C. vidua were
observed in their culture container and in a watch glass
under a dissecting microscope. In addition, the swimming
movements of two tethered and several freely swimming
adult specimens were filmed using both regular and high-
speed video through a dissecting microscope (Wild M5A).
Tethering is common in studies of microinvertebrate swim-
ming because freely mobile animals do not stay in a micro-
Figure 1. Morphology of Cypridopsis vidua. (A) Environmental scanning electron image of critical-point-
dried specimen with left valve removed. The anterior of the animal is to the left, and dorsal is toward the top
of the page. Drawings of right antennula (B) and right antenna (C), showing natatory setae. Drawings modified
from Kesling (1951), with permission from the author.
68 G. HUNT ET AL.
scope?s field of view long enough to permit detailed obser-
vations on limb movements (Hwang et al., 1993). We
tethered ostracodes by first using a pipette to remove one
from its container and transfer it to a dry microscope slide.
Once removed from water, the ostracode tightly closed its
valves and remained motionless. A single human eyebrow
hair was glued to the end of a 2-mm-diameter wooden
dowel for easy manipulation, and the tip of the hair was
dipped into a drop of cyanoacrylate (KrazyGlue). The hair
was then touched to the lateral side of a valve, where the
glue would bond the hair and valve together in a few
seconds. The tethered ostracode was then placed under a
dissecting microscope in a small water-filled container
(100 ml) several centimeters from the container?s edge.
After one to several hours, the ostracode would open its
carapace and attempt to swim vigorously. Limb movements
were continuous at first, but became increasingly intermit-
tent over the course of a few hours. Resting ostracodes
could often be induced to resume swimming by turning off
the light source for a few seconds, and then turning it back
on. Eventually, the ostracode no longer attempted to swim,
presumably due to fatigue or acclimation.
The ostracodes were filmed from several angles, using
regular video (30 frames/s [fps]), and high-speed video (250
fps; Redlake MotionScope 1000). About 20 sequences at 30
fps and about 50 at 250 fps were recorded to VHS video-
tape. This footage was later copied to digital videotape,
from which sequences were downloaded for analysis using
the software Adobe Premiere (ver. 5.1).
One representative high-speed sequence of 68 frames was
selected for detailed analysis. This sequence showed a series
of limb motions in lateral view, in which the movements of
both right and left antennulae and antennae could be seen.
For these frames, the tip and base of all four limbs were
digitized in NIH Image (ver. 1.62). From these digitized
points, the angle of each limb from the animal?s anterior-
posterior axis was calculated for each frame in the sequence.
We refer to this measure as the stroke angle, which in-
creases as limb tips move posteriorly. The digitized points
from the limb tips were smoothed, using the program
Quicksand (ver. 008), to reduce digitizing error, and limb
speeds were estimated by using the optimally regularized
smooth spline algorithm (see Walker, 1998). For a subset of
these 68 frames, points were digitized along the length of
the right antennula and left antenna to show the position and
orientation of the limb segments during the power and
recovery strokes.
Results
When tethered and attempting to swim, the ostracode
spreads its two valves to allow the movement of limbs. The
angle of the gape is about 30? in the anterior region near the
antennulae and antennae and about half as wide posteriorly.
The position of the swimming limbs in the carapace, along
with drawings of these limbs (from Kesling, 1951), is
shown in Figure 1. As reported previously, power strokes of
both the antennulae and antennae provide thrust for drag-
based swimming (Sko?gsberg, 1920; Schreiber, 1922;
Kesling, 1951; contra Canon, 1924, who reported that the
antennulae were not involved in swimming). The resulting
four-paddled locomotion is qualitatively distinct from that
of the myodocopid ostracodes that have been studied, all of
which rely exclusively on their antennae to power the swim-
ming stroke.
The position and orientation of the right antennula in
lateral view during one representative sequence are shown
in Figure 2. At the beginning of its power stroke, the
antennula is predominantly oriented anteriorward and some-
what dorsally. Through the power stroke, the limb sweeps
dorsally and posteriorly, spanning an arc of nearly 90?. The
stroke lies within the sagittal plane, with only slight outward
movement near the end of the power stroke. The antennula
is shown for nine consecutive frames, each 0.004 s apart.
During the power stroke, the long setae at the distal end of
the antennulae are fully extended in a fanlike configuration
(Fig. 1B).
As the power stroke ends and the recovery stroke begins,
these long setae collapse from a fan into a tight bundle. The
antennula is then pulled close to the carapace by bending at
the joints, and the limb moves forward in this ?folded?
position for about 4?5 frames (0.016?0.020 s, Fig. 2). The
entire limb then rotates around the base to return the anten-
nula to its position at the start of the power stroke (Frames
15?17, Fig. 2). Through the 68-frame sequence, each full
power and recovery stroke lasts on average about 17 frames
( 0.068 s), which translates into a beat frequency of about
15 cycles/s.
At the beginning of its power stroke, the antenna is
located in the anteroventral region of the carapace (Fig. 2).
Its power stroke sweeps posteriorly and somewhat ventrally
(Fig. 2). As in the antennulae, a group of long setae splay
out from the antennae during the power stroke. These an-
tennal setae originate relatively proximally on the limb, at
the distal end of the third articulating podomere (Fig. 1C).
During the power stroke they form a fanlike splay that
follows behind the axis of the limb at an angle of about
30?40?. Because of their relatively proximal placement,
these setae increase the effective breadth but not the distal
extent of the antennae. As the power stroke slows, the setae
collapse against the axis of the antenna, becoming invisible
in lateral view.
The orientation of the antenna during the recovery stroke
is rather similar to that of the power stroke, but because
more of the antenna is pulled inside of the carapace, less is
exposed to flow (Fig. 2). The antenna is pulled forward and
rotated about the base of the limb located between the
valves for about 6 frames ( 0.024 s). Then, the antenna
69OSTRACODE SWIMMING STROKE
moves outward, resuming its initial position at the begin-
ning of the power stroke (Fig. 2, frames 1?3).
During locomotion, the four limbs beat continuously to
provide the thrust for swimming. When viewed from the
anterior, these four limbs lie in four quadrants with anten-
nulae in the dorsal two quadrants and antennae in the ventral
two. Clearly, there are many possible ways to coordinate
these four paddles into a swimming stroke, and the manner
in which this coordination is accomplished has potential
implications for swimming performance.
In this species, the relative timing of limb movements is
consistent: power strokes of the left antennula and right
antenna are nearly synchronized, as are the strokes of the
right antennula and the left antenna. The coordination of
limb movements is clear when stroke angles for all four
swimming appendages are plotted with respect to time (Fig.
3). As defined here, stroke angle increases during the power
stroke and decreases during the recovery stroke. In Figure 3,
synchronized (diagonal) limbs are plotted together and are
clearly in phase. It can also be seen that the stroke of
opposing limb pairs is offset by one-half cycle. For exam-
ple, the left antennula and right antenna start their coordi-
nated power strokes just as the power strokes of the right
antennula and left antenna terminate. Close examination of
this plot and of the high-speed sequences revealed that
initiation of the antennula power stroke slightly precedes
that of its opposite antenna by one or two frames (0.004?
0.008 s). This overall pattern of antennula-antenna coordi-
nation is very consistent, occurring in all high-speed se-
quences examined.
Figure 2 suggests that the antennae move considerably
faster through the water during the power stroke than during
the recovery stroke; the limb travels much farther between
frames 4 and 5 than between any successive frames during
the recovery stroke (Fig. 2). This impression is supported by
direct estimates of the velocity of the distal tip of the limb
during swimming. During the power stroke, the antennal tip
reaches maximum speeds of about 50 mm/s, about twice the
maximum speed measured for the recovery stroke (25
mm/s). In contrast, there is almost no difference between
maximum speeds during power and recovery strokes of the
antennulae; the limb moves with a maximum speed of about
30 mm/s in both directions. Because the terminal
podomeres in both limbs have diameters of about 10 
m,
these velocities correspond to Reynolds numbers of about
0.3 and 0.5 for the tips of the antennulae and antennae,
respectively, during the fastest part of their power strokes.
The natatory setae, with diameters of about 2 
m, experi-
ence Reynolds numbers of about 0.06 (antennulae) and 0.1
(antennae).
In addition to filming extended sequences of tethered
animals, in a few cases we were also able to record freely
swimming ostracodes. Although these sequences are gener-
ally short because the animals swim out of view within a
few strokes, they are sufficient to show that the major
features of the swimming stroke that we have described on
the basis of tethered animals also appear in freely swimming
ones. Thus the movement and coordination of the swim-
ming limbs described here do not appear to be artifacts of
tethering (which forces all the energy from the animal?s
Figure 2. Schematic showing position of right antennula and left antenna in left lateral view during power
(left) and recovery (right) strokes. Numbers indicate high-speed video frames, each separated by 0.004 s.
Anterior is to the left, and dorsal is toward the top of the page.
70 G. HUNT ET AL.
power stroke to go into moving the external fluid rather than
producing relative motion between the animal and the fluid;
see Emlet and Strathmann, 1985; Emlet, 1990).
Discussion
Cypridopsis vidua, like nearly all small aquatic arthro-
pods, uses setose limbs as oars to provide thrust for swim-
ming. For net forward motion, the total forces exerted
against the fluid during each power stroke must exceed the
total forces exerted against the fluid during the recovery
stroke. This constraint is particularly acute at the relatively
low Reynolds numbers that govern swimming in this ani-
mal, where drag forces are proportional to wetted area (i.e.,
total surface exposed to flow, rather than projected area; for
a sphere, the wetted area [4r2] is 4 times greater than its
projected area) and speed (rather than speed squared) (Vo-
gel, 1994). The swimming stroke in this species exhibits
many features commonly employed to ensure that drag
during the power stroke exceeds that during the recovery
stroke. Important in this regard is the extension of the
natatory setae into a broad fan during the power stroke and
the collapse of this fan during the recovery stroke. Although
difficult to estimate precisely, probably over three-fourths
of the surface area of the antennulae and and at least half of
the surface area of the antennae is accounted for by the
fanlike arrangement of setae. Even though this fan is not a
solid surface, closely spaced cylinders behave like a slightly
leaky paddle at low Reynolds numbers (Nachtigall, 1980;
Koehl, 1995). Moreover, gaps between setae are partially
filled in by very fine setules extending from the setae. These
setules likely decrease fluid flow between setae, further
decreasing the leakiness of the limb as a paddle. During the
recovery stroke in both appendages, the setae collapse into
a cylindrical bundle and greatly reduce the surface area of
the ?paddle? relative to the power stroke. This common
technique used by many aquatic arthropods to decrease drag
during recovery strokes (Vogel, 1994) exploits the fact that
drag at low Reynolds number is proportional to the wetted
area in contact with the fluid.
Asymmetry in the net forces exerted during the power
and recovery strokes is further increased by the change in
orientation of the appendages and setae relative to the flow
during the swimming cycle. The antennules, antenna, and
their setae are all basically cylindrical objects. At low Reyn-
olds numbers, the drag coefficient of a cylinder with its long
axis parallel to the flow (as in the recovery stroke) is
significantly lower than the drag coefficient of the same
cylinder with its long axis perpendicular to the flow (as in
Figure 3. Stroke angle for all four swimming limbs through four complete stroke cycles. Diagonal limbs are
plotted together, with left antennula (L-a1) and right antenna (R-a2) in the top panel, and the right antennula
(R-a1) and left antenna (L-a2) in the bottom panel. Stroke angle is defined as the angle between the limb axis
and the anterior-posterior body axis; stroke angle increases during power strokes and decreases during recovery
strokes.
71OSTRACODE SWIMMING STROKE
the power stroke); depending on the Reynolds number of
the flow and the aspect ratio of the cylinder, the difference
may be as great as a factor of 2 (Happel and Brenner, 1991;
Vogel, 1994). Thus, during their power strokes, both the
antennulae and antennae (and their setae) are in the high
drag orientation (perpendicular to relative fluid movement)
and switch to a low drag orientation during the recovery
stroke.
Another mechanism that increases the asymmetry in net
forces between the power and recovery strokes is the ten-
dency for the limbs to remain close to the body of the
ostracode during the recovery stroke, rather than fully ex-
tended as during the power stroke. In the antennulae this is
accomplished by bending at the joints such that the limbs
are folded close to the body during the recovery stroke. This
same bending occurs in the antennae but to a lesser extent,
probably because there are fewer articulating segments than
in the antennulae (Fig. 1). In this situation, much of the
shear of the fluid (which generates drag forces) occurs
between the animal itself (body and valves) and the append-
age rather than between the appendage and the surrounding
fluid; although energetically expensive, the forces that op-
pose forward motion of the animal are thus minimized. In
addition, the antennae are withdrawn somewhat into the
carapace during the recovery stroke, which has the same
effect of reducing the limbs? exposure to flow and further
minimizing the forces exerted against the bulk fluid (as
opposed to against the animal itself).
Because the swimming limbs are diagonally synchro-
nized, there is always a (paired) power stroke occurring
while the animal is swimming. As a result, swimming in this
species is smoothly continuous, without the large accelera-
tions and decelerations commonly seen in most other mi-
crocrustaceans (Zaret and Kerfoot, 1980; Williams, 1994).
At the low Reynolds numbers in which these animals func-
tion, the dominance of viscous forces disallows effective
coasting between power strokes (Williams, 1994).
The observed coordination between limbs probably has
advantages over an alternative strategy in which all four
limbs start and complete power strokes at the same time.
Under a completely synchronized stroke, all four limbs
would be in recovery stroke at the same time, causing the
animal to come to a complete stop. Thus the mass of the
ostracode, and of the water in front of it, would have to be
accelerated again with each power stroke. The energetic
costs of this acceleration can be substantial relative to
locomotion at a constant velocity, even for small crusta-
ceans (Vogel, 1994, p. 365), making this hypothetical stroke
less efficient than one in which the animal never stopped
completely. There might be specific advantages to coordi-
nating diagonal limbs rather than those on the same side (for
example, both antennae, or left antennula and left antenna).
The power strokes of each limb produces thrust in non-
forward directions?ventral and lateral for the antennulae,
and dorsal and lateral for the antennae. Coordinating diag-
onal limbs may help to cancel out the non-forward thrusts,
leading to less wasted lateral movement and more efficient
forward motion.
Although the observed limb coordination might have
advantages over other conceivable synchronization
schemes, it may not be universally optimal or attainable in
this group of ostracodes. In fact, we have observed a dif-
ferent, unidentified cypridoidean species in which no power
strokes are coordinated. In this species, the power stroke of
the left antenna starts halfway through the power stroke of
the right antennula, and other limbs? motions are similarly
offset by one-quarter of a cycle. Mathematical models,
along with comparative study of stroke synchronization and
swimming capabilities among cypridoidean ostracodes,
should help to elucidate the functional and evolutionary
implications of this unusual swimming gait.
Acknowledgments
We thank S. Whittaker of the NMNH SEM facility for
preparing specimens and for assistance in SEM imaging,
and R. Kesling for permission to reproduce his limb draw-
ings that appear in Figure 1.
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