Abstract
The flatifish Otharichthys sordidus (Pacific Sanddab) is a bottom
dweller dependent on camouflage and quick escape response to protect it
from danger. In an attempt to determine the nature of the substrates role
in the quick liftoff of the sanddab from the ocean bottom, an experimental
fish lifted off alternately from a porous and a solid substrate. The sanddab
displayed similar displacement, velocity, and acceleration vectors in both x¬
and y-directions when lifting off from either substrate. The similarity of
movement on both substrates indicates that a ground effect, or force caused
by hydraulic flows redirected by the nearby solid substrate, is of littie
conséquence to the sanddab as it lifts off from its substrate. I suggest three
possible mechanisms, each independent of substrate, that may contribute to
the liftoff of the sanddab.
Introduction
The sanddab Stharichthys sordidus is a bottom dwelling flatfish
common in the waters of Monterey Bay. Like other flatfish, the distance
between its pectoral fins is quite short, while it is quite wide between its
dorsal and anal fins. Since it has evolved to living on its right side, it thus
has à depressiform shape. Its right eye moves during development to the
left side of its body so both can be used in observing its surroundings. It is
known to eat squid and bottom-dwelling crustaceans (B. Hopkins, personal
communication)
The dorsal and anal fins of the Pacific sanddab reach nearly from its
nose to its caudal fin, their rays just long enough to touch the bottom around
the margin of the fish. Its left pectoral fin is large and used for steering and
propulsion, while its right is reduced. This fin seems to serve the fish chiefly
as as prop, supporting its anterior end off the substrate. It is well
camouflaged in its favored sandy bottom habitat, featuring a highly
developed system of chromatophores and iridophores on its upper surface
for improving its ability to blend with different substrates (A. Deck, personal
communication). In addition, sanddabs throw sand on their bodies to further
blend with their substrate.
The posture of the sanddab has a marked effect on its mode of
locomotion. In contrast to fishes that swim freely in the water column, a
sanddab commonsy rests for long periods of time on the bottom, moving only
as danger or food approaches. Its reduced right pectoral fin has lost its
powerful thrusting ability, and its dorsal and anal fins may retard lifting off
the substrate by creating a partial seal with the bottom. It also might pay
for its highly developed camouflage techniques: the namesake sand it
throws on its left side to blend with its substrate may retard its escape when
threatened.
In contrast, the sanddab may gain locomotory advantages from living
on the bottom. Ueland (1984) studied how the sanddab is able to maintain
its position in swiftly moving water, avoiding locomotion altogether, by
positioning itself in certain postures on the substrate. Interaction between
seawater moved by the swimming sanddab and the solid substrate, called
ground effect, may allow it to move more efficiently than without an
interaction between the water and the substrate. Blake (1979) showed that
the mandarin fish (Synchropus picturatus is able to hover more efficiently
near a solid substrate due to the ground effect than it could without the
substrate.
This experiment sought to describe and quantify the effect of the
substrate on the movement of the sanddab through comparing the liftoff of a
sanddab from a solid substrate, which would allow a ground effect to occur,
and a mesh substrate, whose porosity would prevent a ground effect from
aiding in liftoff.
Materials and Methods
All experiments took place using a single sanddab (19 em long, 101.5
g) in a 0.80 by 0.40 m tank of still sea water, changed daily. The fish was
caught in the waters of Monterey Bay in the vicinity of Hopkins Marine
Station, Pacific Grove, California. The fish ate squid several times a week
during the experimental period.
A 9 volt electric prod, discharged at the extreme posterior end of the
caudal fin of the sanddab, caused it to lift off from the substrate on which it
was lying. A tripod-mounted Sony Handycam video camcorder, filming at 30
frames per second and 1/250 sec per frame, recorded a liftoff sequence on
each of two different substrates. In each case, the depth of water above the
substrate was 0.17 m. No sand was available for the sanddab to bury itself
Recording continued until the camera captured the fish lifting off from both
substrates in a similar fashion.
The solid substrate was a Plexiglas sheet serving as the bottom of the
tank. The porous substrate was a sheet of 1 1/2 mesh galvanized
chickenwire resting 0.08 m off the bottom of the tank. The sanddab
appeared to be equally comfortable on each substrate, although the mesh did
tend to fray the dorsal, anal, and caudal fins of the fish slightly over time
Playback of the two video sequences on a large-screen television
provided large, clear images easily traceable onto acetate film. A digitizing
pad, recording at centimeter intervals the position of the curved back of the
fish, converted the shape of the fish into Cartesian coordinates. These
coordinates provided values for the radius of curvature at each point along
the back of the fish using the following formula from Thomas (1953).
(14(dy/dx)2)3/2)
rho - — - radius of curvature atx
d2y/dx?
By hanging each of six frozen dead sanddabs from each of three points
along the perimeter of their bodies, the intersections of the three vertical
lines showed where the approximate center of gravity of the fish was
located. The three points of intersection were always within several
millimeters of each other. The average ratio of distance from the nose to the
center of gravity divided by total length of the fish served to locate the
center of gravity of the experimental fish without needing to measure it
directly. This center of gravity provided a convenient reference point for
measuring the displacement, velocity, and acceleration of the sanddab as it
lifted off.
Results
The typical center of gravity for a sanddab is 388 of the way from its
nose to its tail (Figure 1). This point is the reference point for all further
calculations of displacement, velocity, and acceleration.
The sanddab moved its body in almost identical fashion when lifting
off from each substrate (Figure 2). The speed at which it undulated its body
was virtually identical between substrates (Figure 3). The shape of the
wave, characterized by the radius of curvature at any given point along the
wave, appears consistent between liftoffs from each substrate (Figure 4).
Thus, direct comparison of displacements, velocities, and accelerations
between liftoffs appears to be valid.
In each liftoff, the fich held its body off the substrate, balancing as a
tripod on the posterior part of the dorsal and anal fins and the right pectoral
fin. Once lateral movement had begun, the displacement in the X direction
(parallel to the substrate) was slightly greater at any given time for the
experiment on the solid substrate than for the experiment on the porous
substrate. In contrast, the displacement of the center of gravity in the Y
direction ( e perpendicular to the substrate) was indistinguishable
between substrates even after the fish began moving rapidly upward (Figure
5). In each case, the center of gravity of the fish moved about 7 em in each
direction, for a total of about 10 cm along the diagonal, before beginning to
glide about O. 167 s after beginning liftoff.
Taken as the first derivative of the displacement versus time graph,
the velocity versus time graph also shows little difference between the solid
and porous substrates (Figure 6). Similarly, the acceleration versus time
graph differs littie between substrates (Figure 7).
Discussion
It seems reasonable to hypothesize that the bottom habitat of the
sanddab might give it a kinematic advantage over more freely swimming
fishes. The ground effect provided by the solid ocean floor might allow the
undulatory motions of the sanddab during liftoff to move it more quickly
than similar motions could when at a distance from a solid substrate. To
gain an equal acceleration, fish living in the water column would have to
move more water at a higher velocity than flatfish due to the extra
dissipation of energy in open water that would not occur against a solid
substrate.
Since the sanddab appeared to be consistent in both the rate of wave
propagation along its body and the type of wave generated when lifting off
trom each substrate, it is appropriate to asum
attempting' to execute the same liftoff from each type of substrate. Thus
any difference in displacement, velocity, or acceleration noted is precumably
due to the difference in substrate. Surprisingly, all the data collected point
to the conclusion that the fish is equally effective in lifting off from both the
porous and solid substrates and thus gains littie through any ground effect it
may expérience. The porosity of the substrate thus appears to have little
effect on the movement of the sanddab.
The sanddab thus appears to be lifting itself off the substrate using a
force unaffected by the choice of substrate and great enough to overshadow
any ground effect that the fish may also be experiencing. There seem to
remain three possibilities, or combinations of possibilities, for the chief
method by which the sanddab lifts itself off the substrate. One possibility is
that a backwards, horizontal thrust by the sanddab's tail against the
surrounding water is mainly responsible for its movement. This force would
push the fish horizontally through the water. Depending on the angle of
attack-the angle that the bottom of the fish makes with the horizontal
substrate-the fish would gain differing amounts of vertical lift from its
horizontal thrust. The displacement, velocity, and acceleration of the
sanddab would thus not depend directly on the substrate. A ground effect
caused by the solid substrate, would still affect lift but the experimental data
indicate that such a contribution is negligible
Alternatively, the main thrust may be to the rear but not be hydraulic
at all. It may be that the dorsal and anal fin rays, whose tips lie along the
bottom like two bulsdozer treads, move peristaltically to the rear to force the
fish forward. The reduced right pectoral fin might also give some small
shove against the bottom. In this case, as in the above possibility, horizontal
movement would produce vertical lift because of the angle of attack the fish
presents to the water.
The third possibility is that the fish gains a vertical normal force from
the substrate, but the nature of the substrate does not matter. The bottom
may act as a solid springboard to allow the fish to gain a vertical
displacement that would not be possible without a substrate to push against.
Freely swimming fish that accelerate using an undulatory motion like that of
the sanddab during liftoff initially slip laterally from their original axis of
orientation, then cross their original axis to a point displaced slightly
laterally and forward from their original position. (Figure 8) Since for a
sanddab the normal force of the bottom prevents the initial lateral slippage
it can end up with a greater net lateral displacement than freely swimming
fish. This might give it an advantage in striking prey or escaping from a
predator. Images from a camera filming at more than 30 frames per second
would help resolve the above three possibilities into a model for the sanddab
during liftoff.
A final, less obvious advantage the sanddab gains from its bottom
habitat is its ability to optimize itself as a 'transient swimmer. This type of
fish tends to remain stationary, conserving energy and swimming only
occasionally in short bursts of acceleration. Of advantage to this type of fish
is a large muscle to body mass ratio, a deep silhouette to maximize the
amount of water moved with a single stroke of the body, and a body flezible
enough to create large amplitude waves (Webb, 1984 and Daniel, 1988).
Because of its flattened posture and bottom habitat, the sanddab is able to
maximize all three variables. Fish enjoying a transiently swimming lifestyle
high in the turbulent water column, by contrast, cannot afford such a deep
silhouette lest it act as a sail to drive them through the water away from
their desired position.
Conclusion
Evolving to live flat on the bottom of the ocean has resulted in the
sanddab making a number of adaptations. Its camouflage disguises it from
predator and prey, while the shape it assumes allows it to stay in place
despite water moving swiftly over its body. While it seems to receive little,
if any advantage from a ground effect during liftoff from the substrate,
further work is required to determine the chief method used by the sanddab
for lifting off from the ocean bottom.
Acknowledgements
Thanks to Mark Denny of Hopkins Marine Station for providing laboratory
space and useful advice during the course of this study. Additional thanks to
Charles Goldman of UC-Davis for his encouragement and support.
Bibliography
Blake, R. W., 1979. The energetics of hovering in the mandarin fish
(Synchiropus pieturatus J. Erp. Biol 82: 25-33.
Blake, R. W., Median and Paired Fin Propulsion, in Webb, P. W., and Weihs, D.,
ed. Fiah Remechanis pp. 221-224. Praeger, New York, NY, 1983.
Daniel, T. L., 1986. Forward Flapping Flight from Flexible Fins. Can / 222
66: 630-638.
Thomas, George B., Jr. Clculus and Analytie Geometry p. 406. Addison-
Wesley, Cambridge, MA, 1953.
Ueland, F., 1984. Rheotaxis in dtharichthys sordidus Ciass papers, Stanford
University Biology department“ 175H.
Webb, P. W., Form and Function in Fish Swimming, S. Am, July 1984, p. 72.
Figures
1: Location of Center of Gravity of the Sanddab.
2: Liftoff Sequence of the Sanddab. Each frame 1/30 5.
3: Speed of Wave Propagation along the Sanddab during liftoff.
4: Radius of Curvature of Wave along the Sanddab during liftoff.
5. Displacement of the Center of Gravity of the Sanddab during liftoff.
6: Velocity of the Center of Gravity of the Sanddab during liftoff.
7: Acceleration of the Center of Gravity of the Sanddab during liftoff.
8: Undulatory motion of a sanddab during liftoff from a solid substrate (top)
and typical undulatory motion of a transiently swimming fish (bottom) (from
Webb, 1974)
Distance from Nose to Center of Gravitu
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X-Displacement
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Y-Displacement
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Time (3)
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Time (3)
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X-Accelerations
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time (s)
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time (s)
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