AMER. ZOOL., 22:329-342 (1982)
Locomotor Patterns in the Evolution of
Actinopterygian Fishes'
PAUL W. WEBB
The University of Michigan, School of Natural Resources, Ann Arbor, Michigan 48109 and
National Marine Fisheries Service, Southwest Fisheries Center, Lajolla, California 92038
SYNOPSIS. Locomotor adaptations in actinopterygian fishes are described for (a) caudal
propulsion, used in cruising and sprint swimming, acceleration, and fast turns and (b)
median and paired fin propulsion used for slow swimming and in precise maneuver.
Caudal swimming is subdivided into steady (time independent) and unsteady (time dependent acceleration and turning) locomotion.
High power caudal propulsion is the major theme in actinopterygian swimming morphology because of its role in predator evasion and food capture. Non-caudal slow swimming appears to be secondary and is not exploited before the Acanthopterygii.
Optimal morphological requirements for unsteady swimming are (a) large caudal fin
and general body area, (b) deep caudal peduncle, often enhanced by posterior dorsal and
anal fins, (c) an anterior stabilizing body mass and/or added mass, (d) flexible body and
(e) large ratio of muscle mass to body mass. Optimal morphological requirements for
steady swimming are (a) high aspect ratio caudalfin,(b) narrow caudal peduncle, (c) small
total caudal area, (d) anterior stabilizing body mass and added mass, and (e) a stiff body.
Small changes in morphology can have large effects on performance.
Exclusive morphological requirements for steady versus unsteady swimming are partially
overcome using collapsible fins, but compromises remain necessary. Morphologies favoring unsteady performance are a recurring theme in actinopterygian evolution. Successive
radiations at chondrostean, halecostome and teleostean levels are associated with modifications in the axial and caudal skeleton.
Strength of ossified structures probably limited maximum propulsion forces early in
actinopterygian evolution, so that specializations for fast cruising (carangiform and thunmform modes) followed structural advances especially in the caudal skeleton. No such
limits apply to eel-like forms which consequently recur in successive actinopterygian radiations.
Slow swimming using mainly non-caudal propulsion probably first occurred among
neopterygians in association with reduced and neutral buoyancy. Slow swimming adaptations can add to and extend the scope of caudal swimming, but specialization is associated
with reduced caudal swimming performance. Marked exploitation of slow swimming opportunities does not occur prior to the anterodorsal location of pectoral and pelvic girdles
and the vertical rotation of the base of the pectoral fin, as found in the Acanthopterygii.
INTRODUCTION
At some stage in their lives, all fish swim,
and water is a dense, viscous medium that
places a premium on effective propulsion
mechanisms. Among fishes, the actinopterygians show greatest diversity of propulsive mechanisms that involve the body
and caudal fin, dorsal and anal fins, and
. c
i
inor-s
n
/r>
paired fins (Breder, 1926), as well as specializations for benthic life where swim,
,
.
, ...
, „
ming may be great y reduced (Marshall,
196
19711
'•
..
1
, .
From the Symposium on Evolutionary Morphology
Changes in skeletal elements provide the
major basis for assessing phylogeny. The
major actinopterygian trends relevant to
locomotor mechanics and swimming performance are as follows (Greenwood el ai,
1966; Patterson, 1968a, b; Gosline, 1980;
Rosen, 1982).
. .
.
•
r u
•i
c
(a) progressive ossification ot the axial
K. i
... S , ? e n ' •
,
i » -i .
r ,
v(b) abbreviation or the heterocercal tail to
'
, ,
, -,
attain an external homoceral tail.
(c) emphasis of caudal ray support on a
few hypural bones (modified hemal
arches).
(d) reduction in the number of vertebrae
•
,
,
•,
of the ActinopteigiL Fishes presented at the AnnJal
Supporting the hypural bones.
Meeung of the American Society of Zoologists, 27- (e) reduction in the number ot vertebrae.
30 December 1980, at Seattle, Washington.
(f) reduction in scale mass and armor, ex329
330
PAUL W. WEBB
cept for their secondary reappearance
in some modern teleosts.
(g) dorsal migration of the pectoral girdle
and pectoral fin insertion,
(h) forward migration of the pelvic girdle,
anus and origin of the anal fin.
The question to be addressed here is
how these phylogenetic trends are related
to the major trends in external body form
that determining how water is moved in
generating thrust, and how efficiently that
thrust is generated. The major trends in
body form pertinent to locomotor mechanics are:
(a) a recurring fusiform carnivore-type
with a large tail and caudal area. This
generalized form is dominant in primitive groups.
(b) recurring radiation from the generalized form towards elongate and benthic forms.
(c) progressive specialization of forms
with forked tails and a narrow caudal
peduncle.
(d) appearance of specialized deep-bodied
forms with dominant non-caudal propulsors.
Most of these interrelationships cannot
be experimentally tested because they frequently involve extinct animals. Therefore, the usual methods of considering
functional-morphology in an evolutionary
context are employed. It is assumed that
similar structures or suites of characters
are functionally similar. These functions
can be determined by experiment on living
fish, fleshed out in most cases by the
known laws of physics. Where a change in
structure or suites of structures is observed
through time, then functional hypotheses
for the changes can be formulated. These
functional hypotheses will evolve as data
accumulate, and also as the phylogenetic
reference framework evolves. This is because phylogenies are also hypotheses of
relationship among animals (Atz et al.,
1980) that not only cannot be experimentally verified or rejected, but which also
evolve as data accrue and opinions change.
Also, the limited meager knowledge on
fish locomotion mechanisms must be recognized. To a large extent research has
concentrated on body/caudal fin locomotion, particularly steady (constant speed)
swimming. Many fish rarely swim. Fish
rarely swim steadily (Nursall, 1958; McCutcheon, 1976). They frequently use
non-caudal fins. Indeed, research may
have concentrated on biologically and behaviorally less important swimming modes!
In spite of the inevitable constraints, a
general, and necessarily preliminary, analysis of locomotor patterns in actinopterygian phylogeny seems possible.. These
patterns can be discussed under two categories; (a) caudal propulsion adaptations
mainly involved in fast-swimming, high acceleration and fast turns and (b) non-caudal adaptations used mainly in slow swimming and precise maneuver. A major
theme throughout actinopterygian evolution is a trend towards high power caudal
swimming (Gosline, 1971); slow swimming
is not exploited in any great degree prior
to the percoid level of organization among
the higher teleosts. The two patterns will
be discussed separatedly in relation to phylogenic trends in actinopterygians.
CAUDAL PROPULSION
Occurrence
Body and caudal fin propulsion appears
to be a common denominator throughout
the evolution of fish. This propulsion system has been retained in all groups, with
the exception of some specialized benthic
fish. The conservatism in caudal swimming
probably occurs because it is mechanically
most efficient (compare Wu, 1971; Wu
and Newman, 1972; Webb, 1971, 1975a,
c; Blake, 1980a, b, c) and it is the mechanism for achieving maximum acceleration
rates, maximum turning couples and high
sprint speeds. Most research has been performed in this area (see Lighthill, 1975;
Wu et al., 1975; Pedley, 1977; Hoar and
Randall, 1978; Sharp and Dizon, 1979).
A recurring morphology in actinopterygian fishes is the fusiform, generalized
carnivore, with a large caudal fin and caudal area and a deep caudal peduncle, e.g.,
the chondrostean Cheirolepis, early haleco-
ACTINOPTERYGIAN LOCOMOTION
PALEOZOIC
ACTINOPTERYGIANS
"LOWER" TELEOSTS
(c)
331
"HIGHER
(E)
TELEOSTS
(?)
(D)
Gasteropetecus
Pyntocepholus
Xiphistor
Conaroqadus
Fic. 1. A diagrammatic representation of some of the typical body and fin forms seen at various times in
selected radiations in actinopterygian fishes. Three general "grades" are illustrated. These show functional
varieties about a recurring generalized carnivore form (Cheiroiepis, Salmo, Serranus) and are not intended to
show phylogenetic lines. A, C, and E show variations among elongate and fusiform forms. Examples of body
form along the anguilliform to thunniform continuum are illustrated; Anguilla is the type for the anguilliform
mode and Thunnus for the thunniform mode. B, D and F show radiations towards deep-bodied non-caudal
forms usually specializing in slow swimming. More specialized forms are shown above less specialized forms.
thrust have considered potential (inviscid)
flow around a swimming fish, but outside
Salmo, etc. and higher teleosteans Mugil, the boundary which is a major source of
Perca, Serranus, Micropterus, etc. Some ex- drag. Because of this distinction, it is deamples are illustrated in Figure 1. Regular sirable to consider separately the mechanradiations have occurred towards more ics of thrust and drag, and the functionalelongate forms at each level of actinopte- morphological correlates for each.
rygian organization. Radiation towards
In addition, distinction must be made
specialized short-bodied forms, with forked between steady swimming (time indepentails and a narrow caudal peduncle are rel- dent motion at cruising and sprint speeds)
atively rare before the acanthopterygians, and unsteady swimming (time dependent
although there is a greater tendency in this motion involving linear and angular acceldirection in more recent groups. The re- eration). This is because the theory treatcurrence of the generalized body forms ing thrust mechanics (see Weihs, 1972,
raises several questions: What is its func- 1973; Lighthill 1975, 1977; Wu, 1977; Wu
tional significance? What underlies re- and Yates, 1979) and experimental studies
placement of similar forms in successive (Webb, 1977a, 1978; Webb and Smith,
actinopterygian radiations? Why do elon- 1980) show that morphological requiregate forms recur but not short streamlined ments for maximum performance in
forms?
steady and in unsteady swimming not only
These questions can be approached us- differ, but optimal designs are mutually
ing arguments based on recent studies on exclusive.
the locomotor mechanics of modern teTherefore, the following discussion
leosts. A major emphasis of this research treats first thrust and then drag, and in
has been problems of thrust generation each case, mechanics and functional-morand resistance to motion (drag) that must phology related to steady and to unsteady
equal that thrust. Theories concerning swimming are considered.
stome Parasemionotus, halecomorph Lepidotus, lower teleosteans Hiodon, Engraulis,
332
PAUL W. WEBB
X A
tangent at Q
(I) Steady Swimming (Lighthill 1971 )
-.--L...,.
(2) Unsteady Swimming ( Weihs 1973)
F--&
mwnda-E
L
FIG. 2. Notation and summary of thrust forces acting on a fish during steady and unsteady swimming
as predicted from hydromechanical theory.
The figure shows the hypothetical outline of a
swimming fish seen from above (dotted lines) with
the body axis shown as a solid line. For analytical
purposes, the body is divided into "propulsive segments" along the whole length, 1. One such segment
is shown at a, following the usual convention of measurement from the trailing edge. This segment moves
laterally and forward with resultant velocity V(a,t>
resolved into a normal component, W(a,t) and tangential component, U(a.t). Each segment influences
a mass of water in its vicinity, the added mass, m.
In steady swimming (equation 1), the instantaneous
force acting on the body depends on the sum of contributions from all propulsive segments both anterior
to the trailing edge (the integral term) and at the
trailing edge (terms in parentheses).
In unsteady swimming (equation 2), the instantaneous force depends on the sum of contributions of
propulsive segments anterior to the trailing edge (the
integral term) plus a lift force, L), due to discrete fins
i to k. Further discussion is given in the text.
In both cases, n is the coordinate normal to the fish
centerline, and s the tangential coordinate. The figure is based on equations 1 and 3 and figure 1 in
Weihs (1973).
Thrust. Hydromechanical theories concerning the generation of thrust by fish
swimming motions have been widely discussed (see Weihs, 1973; Lighthill, 1975;
Wu, 1977; Webb, 1978). Space does not
permit a detailed development of these
concepts. Instead, main principles are
summarized, and to facilitate reading, formal mathematical descriptions are limited
to key equations summarized in Figure 2.
Steady swimming. During optimally efficient steady swimming, the body is bent
into a propulsive wave which passes backwards over the body at a velocity near to,
but of necessity greater than the swimming
speed. Any point along the body (a propulsive "segment" sensu Gray, 1968) oscillates laterally relative to the head. The amplitude of these lateral oscillations increases
along the body length to reach maximum
values of approximately 0.2L (L = body
length) at the tip of the caudal fin (the
trailing edge). The depth of the caudal fin
should be large to maximize the mass of
water affected (the added mass) which is
proportional to the square of the depth of
the body and/or fins.
The force acting on a steadily swimming
fish at any instant due to the effect of these
body movements on the water in the vicinity of the fish is formally described by
equation 1 in Figure 2. The mechanism
whereby thrust is generated can be visualized by considering that all the propulsive segments along the body interact in
series to gradually accelerate water in the
vicinity of the fish. The water reaches a
maximum velocity just as it is discharged
into the wake at the trailing edge. The total
work done depends on the momentum
gain of this water, but the thrust component is less than this because of the loss of
energy involved in the acceleration of the
water.
Thus the instantaneous force acting on
the body is the sum of the forces of the
segments anterior to the trailing edge (the
integral term in equation 1) plus that due
to the momentum discharged across the
trailing edge. However, because of interaction between segments, those anterior to
the trailing edge have little effect on the
ACTINOPTERYGIAN LOCOMOTION
mean force (i.e., the integral term has a
zero mean value). The mean force is given
by the rate of change of water momentum
discharged into the wake across a plane at
the trailing edge (the terms in parentheses). Mean thrust is the difference between
the total energy discharged to the wake
and the kinetic energy associated with the
increased water velocity (i.e., the difference between the two terms in parentheses
in equation 1).
There is a wide range of morphologies
associated with steady swimming in bodycaudal fin modes, originally classified by
Breder (1926). In mechanical terms (Lighthill, 1975), various locomotor types fall
along a continuum ranging from eel-like
fish to mackerel-like fish (anguilliform
modes to carangiform modes of Breder).
The range is associated with larger specific
wavelength (propulsive wavelength/L), a
more rapid rise in amplitude posteriorly,
larger caudal fin depth but smaller tail
area (high aspect ratio tail), reduced caudal peduncle depth (narrow necking) and
more streamlined body (see Fig. IE). Larger specific wavelengths and caudal concentration of increases in amplitude both reduce the time available for secondary
viscous-related growth in the added water
mass which contributes to energy losses
(Lighthill, 1975). The trends towards narrow necking of the caudal peduncle and a
large body and fin depth over the center
of mass reduce the magnitude of rapidly
fluctuating side forces that can increase lateral recoil (the tail "wagging the head")
which also increases energy loss (Lighthill,
1977). The increased depth of the trailing
edge increases the added mass of water
accelerated in each tail beat, increasing
thrust (Lighthill, 1975) or efficiency (Alexander, 1968). These kinematic features
presumably result in higher swimming
speeds and/or higher efficiency at a given
speed as the carangiform mode is approached. The anguilliform to carangiform continuum includes all non-lunate
tail swimmers. A further mode is the mechanically distinct thunniform mode
(Lindsey, 1978), represented by the tunas
(family Thunnidae) among the actinopte-
333
rygians. Thunniform swimmers have the
most advanced adaptations for improved
thrust and locomotor efficiency (see Lighthill, 1975, 1977; Hoar and Randall, 1978;
Wu and Yates, 1979) such that the slender
body theory applied along the anguilliform to carangiform continuum is no
longer applicable. Instead, separate hydromechanical theories are being developed
that take into account the unique morphology and kinematics associated with the
half-moon (lunate) tail.
Unsteady swimming. Unsteady swimming
(accelerations such as fast-starts and turns)
involves large amplitude lateral movements of the tail, with trailing edge amplitudes commonly greater than 0.5L (Webb,
1978). Movements are non-periodic, and
involve little more than two beats of the
tail.
The same general principles of steady
swimming apply to unsteady swimming
(Weihs, 1972, 1973) and forces acting on
the body at any instant are formalized by
equation 2 (Fig. 2). Unsteady swimming
differs from steady swimming in that there
is no significant temporal or spatial interaction between propulsive segments. As a
result, the trailing edge is of no particular
importance, so that this term of the steady
swimming model disappears. Instead, both
instantaneous and mean forces in turning
and acceleration depend on the sum of
contributions from all propulsive segments
(the integral term of equation 2) plus any
lift forces due to fins and body sections
with sharp edges (second term in equation
2).
Fast-start kinematics show little variability over a wide range of body forms (Gray,
1933; Hertel, 1966; Weihs, 1972, 1973;
Webb, 1975*, 1976, 1978; Eaton et al,
1977; Eaton and Bombardieri, 1978; Wardie, 1975), and those differences observed
do not affect acceleration rates (Weihs,
1972, 1973; Webb, 1976). As a result, the
major variable affecting thrust is the distribution of depth (and hence added mass)
along the body length (see the integral
term in equation 2). Thrust due to each
propulsive segment, and hence total
thrust, is maximized when the body depth
334
PAUL W. WEBB
is large along the whole length of the fish,
resulting in a large body and fin area
(Weihs, 1972, 1973; Webb, 1977a). However, because amplitudes of propulsive
movements are greatest caudally, the
greatest contributions to thrust are made
by the area distant from the center of
mass, the locus about which the forces act
(Weihs, 1972).
The large amplitude motions by large
fins not only generate thrust forces in the
direction of motion, but also substantial
side forces. Those forces may be particularly large early in a turn or a fast-start
because then thrust forces tend to be poorly aligned with the body axis (Weihs, 1972,
1973). As a result, a large couple acts on
the center of mass causing it to yaw (lateral
recoil) in proportion to the acceleration
rate (Weihs, 1973). The recoil, plus body
bending, is useful in a turn, but represents
wasted energy in a fast-start. Recoil energy
losses can be reduced by large body and
fin depth over the center of mass, as in
steady swimming. Such morphology would
also reduce side-slip in unpowered turns.
and unsteady swimming. In steady swimming, a is small and periodic or zero and
therefore frictional drag dominates. In unsteady swimming, a is often very large relative to U and mass resistance dominates.
These differences in the major components have distinct consequences for functional design.
Steady swimming. The question of the
drag on a steadily swimming fish is a topic
of continuing debate. It now seems generally agreed that frictional drag is dominant, with little increment from pressure
drag due to body shape. Frictional drag is
determined by the nature of the boundary
layer and is greater for turbulent boundary layer flow than for laminar boundary
layer flow. Irrespective of the actual
boundary layer flow conditions, swimming
movements change the velocity gradients
across the boundary layer compared to
those on a geometrically similar non-flexing body. As a result, frictional drag will
also be higher. There are several reasons
for this higher drag. First, the lateral motion of the body and fins increases the incident velocity on propulsive segments
Drag
above the mean swimming speed. Second,
The equation of drag for rectilinear mo- the mean velocity gradient and hence drag
tion of a body at constant depth in water coefficient will also be increased because of
boundary layer thinning (Lighthill, 1971)
is:
as
there is insufficient time for the boundD = M,a + V2pSU2(kCD)
(3) ary layer to reach the thickness that would
be expected on a steadily moving non-flexwhere
ing body. The increased incident velocity
D
drag = thrust,
of propulsive segments might increase
total mass of the body and water drag by a factor of about 1.5. Boundary
M,
resisting acceleration,
layer thinning probably increases the local
a = acceleration rate,
drag coefficient by 5 to 10 times (Lighthill,
1971; Webb, 1973a). The mean drag of a
p = density of water,
flexing fish appears to be about 2 to 5 times
S = wetted surface area,
greater than that of an equivalent nonU = swimming speed,
(kCD) = drag coefficient, where k is a flexing body (Alexander, 1967; Lighthill,
factor taking into account addi- 1975; Webb, 1973a, 1975a).
tions to the drag coefficient CD
The drag increment due to locomotor
of a rigid body due to body
movements
will be largest where the amshape and/or lateral body moplitude
of
propulsion
movements is greattions (Lighthill, 1975; Webb,
est.
Therefore
drag
can
be minimized by
1975a; Bone, 1977; Alexander,
(1) holding the body rigid, permitting
1979).
good anterior stream-lining, and concenIn equation 3, the first resistance term trating large amplitude movements posteis that due to inertia and the second is that riorly, (2) minimizing body and fin area
due to friction. The magnitude of these anterior to the trailing edge where lateral
two components differs between steady amplitudes are large; i.e., narrow necking
ACTINOPTERYGIAN LOCOMOTION
335
(Lighthill, 1977), (3) scooping out the cen- steady swimming is a large aspect ratio tail,
ter of the caudal fin reducing the area narrow caudal peduncle and rigid stream(Lighthill, 1975). The trend from anguil- lined body to maximize thrust and miniliform to thunniform modes is therefore mize drag. For unsteady swimming, the
not only a trend towards greater power body should be flexible, with a large depth
and swimming efficiency, but also one to- along its whole length, with emphasis on
wards reduced drag by reduction of un- a large caudal area. Ray-finned fishes are
wanted area over the flexing portion of the unique in being able to modify body and
body and by streamlining the more rigid fin area by means of their erectile/collapsanterior portion (Walters, 1962; Fierstine ible fins, and therefore actinopterygians
and Walters, 1968).
have some capability to adjust body and fin
Unsteady swimming. In a fast-start, the ac- shapes according to the competing deceleration rate is large, with maximum val- mands of steady and unsteady propulsion.
ues of the order of 4-5 G and mean values Nevertheless, this ability to compromise is
of 1-2 G (Webb, 1976, 1978). Speed in- not perfect as increased caudal fin depth
creases through a fast-start and can reach usually results in little improvement of
values of 1—1.5 nvsec" 1 in 50 to 150 mil- steady swimming, but does improve accelliseconds, but during that period the eration and turning performance (Webb,
boundary layer will have to grow, and CD 1977a; Webb and Smith, 1980; Webb and
may be small. Conservative theoretical cal- Keyes, 1981).
culations (Webb, 1975a, 1978) and experThe early evolutionary appearance of
imental studies (Webb, 1982) show that large caudal area, often enhanced by a
the frictional drag term is a small per- posterior location of dorsal and anal fins,
centage of total drag during such pe- and the regular recurrence of this morriods of rapid acceleration. Thus, drag in phology at each actinopterygian grade
a fast-start is dominated by the mass of the must therefore be interpreted as design
system, and the large area required for ef- favoring unsteady swimming (Lauder,
fective thrust generation adds little cost to 1980; Webb and Smith, 1980). This was
drag.
probably due to the importance of fastDrag reducing mechanisms for acceler- starts and turns in avoiding predators and
ation involve a reduction of non-essential in catching elusive prey (Hoogland et al.,
mass. Reductions in non-muscle tissue 1956; Nursall, 1973; Neill and Cullen,
density reduce the dead weight to be ac- 1974; Eaton and Bombardieri, 1978;
celerated relative to the mass of the muscle Webb and Skadsen, 1980). All fish face
motor. The resultant improved ratio of predation at some time in their lives so that
power-to-load represents the resistance re- unsteady swimming performance is likely
duction. All density saving adaptations, to be important for survival to reproducusually attributed to neutral buoyancy, will tion.
also contribute to improved acceleration;
The recurring generalist form (fusiform
Webb and Skadsen (1979) describe mass body with large caudal area) during actireductions in the skin that appear to be nopterygian evolution thus appears to be
related specifically to reducing accelera- the most effective compromise for caudal
tion resistance.
propulsion providing adequate steady
Drag will be increased, probably sub- swimming performance (Beamish, 1978)
stantially, by large body and fin area dur- and good unsteady performance (Webb,
ing accelerations and turns by fish that are 1978).
already in motion.
Compromises in functional morphology of
caudal propulsion
Magnitude of effects of morphological
variation on swimming
performance and power
The hydrodynamic principles outlined
above lead to exclusive optimum morphologies for steady and unsteady swimming. The optimum morphology for
It is important to consider the locomotor
impact of variations in body and fin morphology. Hydromechanical theory (Lighthill, 1975; Weihs, 1972, 1973) and exper-
336
PAUL W. WEBB
imental observations (Webb, 1977a) have
clearly shown that relatively large increments in fast-start performance accrue
with relatively small increases in body and
fin depth, especially when that area is located caudally.
Equivalent experimental observations
have not been made for steady swimming.
However, Hunter and Zweifel (1971) have
measured tail beat frequencies and amplitudes for six teleosts from which some idea
of the importance of body form can be
deduced. The morphological variation
among the species studied was not large
(Fig. 3) and was towards the center of the
larger anguilliform to thunniform range.
Hunter and Zweifel showed that for fish
of a given size, tail beat frequencies at a
given speed varied among the six species
up to 30 to 40%. Since tail beat amplitudes
were constant (20% of length) the differences in tail beat frequency imply substantial variation in thrust, and hence drag,
over the narrow range of morphologies
studied. While it is apparent that external
morphology alone will not dictate swimming performance, it is clear from Figure
3 that small variations in morphology can
have large effects on locomotor mechanics
and performance. Observations on a wide
range of morphologies from gars to cetaceans suggest that more anguilliform
swimmers have relatively higher tail beat
frequencies than thunniform swimmers
(Kayan et ai, 1978). These observations
tend to support the arguments of increasing thrust efficiency and reduced drag associated with more thunniform morphologies.
Therefore, small morphological changes
are expected to be significant for both
steady and unsteady swimming.
Structural and functional patterns in
actinopterygian evolution
The importance of retaining good unsteady swimming performance and adequate steady swimming performance,
characteristic of the recurring actinopterygian "generalist" form does not explain
the succession of superficially similar external morphologies replacing each other
throughout actinopterygian evolution. The
succession from chondrosteans, through
various halecostome and neopterygian
levels to the teleosts shows remarkably little improvement in caudal propulsion
morphology (at least before acanthopterygians). The major structural patterns involve internal anatomical changes, particularly in ossification of the axial skeleton
and organization of the caudal skeleton
which transmit thrust forces to the body
(see Greenwood et al., 1966; Romer, 1966;
Shaeffer, 1967; Patterson, 1968a, b; Lauder, 1980).
The earliest chondrosteans, such as the
paleonisciforms had a large heterocercal
caudal fin. The caudal peduncle was deep,
and dorsal and anal fins were inserted towards the tail {e.g., Cheirolepis in Fig. 1A).
This is the basic "generalist" form favoring
unsteady swimming. The presence of leading edge scales and scutes, a heavy fin ray
skeleton, and more diffuse and weak axial
support of median fin skeletal elements
(Romer, 1966; Gosline, 1971; Moy-Thomas and Miles, 1971) suggest these early actinopterygians may have been less able to
collapse their median fins for steady swimming.
In addition, the notochord was unrestricted, lacked ossified centra, and had
relatively weak neural and hemal arches
(Schaeffer, 1967). These features would
probably reduce the magnitude of
compression and bending forces that
could be tolerated. Furthermore, extension of the notochord to the caudal fin
trailing edge and heavy scales probably reduced body flexibility (Aleyev, 1977) and
hence the magnitude of propulsive forces
that could be generated. Therefore, thrust
was probably relatively low. The heavy
ganoid scales would also increase mass resistance in acceleration (Webb and Skadsen, 1979) as well as side-slip forces during
turns. Finally, the paired fins probably extended ventro-laterally like stiff planes,
more analogous to modern sharks than
modern actinopterygians (Romer, 1966),
so that control in turns was probably relatively poor.
Overall, unsteady swimming was proably emphasized (Lauder, 1980) but performance at all activity levels was probably
poor in comparison to modern forms.
Maximum locomotor power output was
337
ACTINOPTERYGIAN LOCOMOTION
Sardinops sagax
Sal/no gairdneri
Carassius aura/us
Leuciscus leuciscus
Trachurus symmetricus
Scomber japonicus
0.5
1.0
SWIMMING
1.5
2.0
SPEED
2.5
(m.s"')
FIG. 3. Relationships between tail beat frequency and swimming speed for six species of teleost fish with
various body forms. Data were taken from Hunter and Zweifel (1971) and calculations were based on a fish
with a total length of 25 cm. With the exception of Sardinops, these species rank approximately from more
anguilliform at the top with higher tail beat frequencies at a given speed, to more carangiform at the bottom,
achieving higher speeds for a given tail beat frequency. All fish were tested individually except for Sarcknopi
which was tested in groups of five; this may have influenced swimming kinematics.
probably also limited by the anatomy of
these early fish. This may have limited the
evolution of specialized steady swimming
morphologies which may have required
compressive and bending forces over a
narrow caudal peduncle in excess of skeletal tolerances. However, more elongate,
anguilliform swimmers would not be excluded because thrust forces would undoubtedly be lower. Therefore, it is not
surprising to see eel-like forms among
chondrosteans (e.g., Tarrasius in Fig. 1A)
nor to find they recur at each level of actinopterygian organization.
If these arguments are correct, then the
significance of morphological trends in
successive levels of halecostome organization may be defined. Constriction of the
notochord by increasingly ossified centra
(Schaeffer, 1967) would increase compressive and bending strength. Greater body
and fin flexibility through abbreviation of
the notochord, reduction of skeletal ma-
terial in the fin rays, and reduction in cosmoid and ganoid layers in the scales
(Greenwood et al., 1966; Romer, 1966)
could allow greater body flexibility leading
to larger thrust forces for both steady and
unsteady swimming. The more anterior
insertion of the dorsal fin (Gosline, 1971)
implies greater control of lateral recoil.
The reduction in scale mass would reduce
resistance in unsteady swimming. Increased fin flexibility would allow greater
changes in caudal area to improve steady
swimming, which would be further improved by the more frequent occurrence
of "scooped out" tail fins {e.g., Caturus,
Hypsocormus, Pholidophorus).
The trends established among the halecostomes (greater constriction of the notochord by ossified centra, strong neural
and hemal arches, abbreviation of the notochord, reduction in the number of caudal rays, improved articulation of fin rays
with the axial skeleton and reduction in
338
PAUL W. WEBB
scales) continue into the teleosts (Greenwood et al, 1966; Gosline, 1971). However, these changes also tended to weaken
the support for the dorsal lobe of the caudal fin, by the formation of a "hinge" area,
necessitating major structural changes
which occurred at the pholidophorid/leptolepid level (Patterson, 1968a). At this
level, the uroneurals of the ural centra at
the base of the caudal fin extended over
the preural centra to strengthen the caudal
connection of the axial skeleton. At the
same time this permitted full external
homocercy as seen in the teleost tail fin.
The structural changes probably improved
transmission of thrust forces to the axial
skeleton and also ensured symmetry of
thrust forces in the vertical plane. Patterson (1968a) suggests the changes in caudal
skeleton were associated with greater elasticity and flexibility of the trunk leading to
more powerful and efficient swimming
and greater exploitation of the advantages
of neutral buoyancy.
However, in spite of these apparent advantages of more powerful swimming
forms, early teleostean external morphology shows little advance over the halecostomes. Radiation towards more specialized
steady swimming modes after the early
leptolepids is associated with greater ossification and strength in the preural vertebrate and in the articulation of reduced
hypural bones on a single posterior half
centrum. Reduced vertebral number, particularly in the trunk, probably serves to
concentrate propulsive movements caudally. Such modifications occur throughout the actinopterygians (Greenwood etal,
1966) but it is not until the acanthopterygian level of organization that a sufficiently
advanced stage is reached for large numbers of specialized steady swimming forms
to occur. These include the appearance of
the mechanically refined thunniform mode
of propulsion (Fig. IE), previewed by an
extinct shark, Cladoselache\
In summary, the recurring fusiform,
large tailed generalist morphology in actinopterygians is probably a compromise
between design criteria for unstead) and
steadv swimming. The former area of performance is judged most important. The
succession of these similar body forms
throughout actinopterygian phylogeny is
attributed to anatomical changes in
strength and flexibility of the skeleton.
Similarly, it is suggested that specialized
steady swimmers would exceed the strength
limits of the skeleton of early antinopterygians. As a result they are not common
before the acanthopterygians. Elongate
forms would not challenge design limits
and hence regularly occur at all levels of
actinopterygian radiation. Therefore, the
most recent teleosts include the most diverse caudal propulsion morphologies,
with some distinct new forms compared to
previous radiations. The morphologically
sophisticated fast cruising species that occur for the first time (Fig. IE) have evolved
at the cost of unsteady swimming capabilities (Webb, 1977a; Webb and Keyes,
1981). In the other direction, recurring
elongate forms are found swimming in anguilliform modes. Fish with this type of
morphology eschew locomotor sophistication; for example the spiny eel, Mastacembelus loennbergi exposed to threatening
stimuli withdraws its head instead of accelerating away (Eaton etal., 1977). Steady
swimming performance is probably low
(Beamish, 1978). However, the anguilliform body has permitted expansion into
a wide variety of labyrinthine niches, in
dense weeds, coral reefs, etc. and into soft
substrates by burrowing. These habitats,
and the associated body form, have not
excluded anguilliform swimmers from
making long pelagic migrations (e.g., Anguilla, Marshall, 1965), although the specialized thunniform cruisers are certainly
excluded from the habitat of eels. In this
respect, eel-like fish are more versatile, although less "glamorous."
MEDIAN AND PAIRED FIN PROPULSION
The most important and common functions of median and paired fin propulsion
mechanisms are slow swimming and precise maneuver (Gosline, 1980). Variations
in mechanisms have been classified in
terms of morphology (Breder, 1926; Lindsey, 1978). There are currently insufficient
comparative mechanical studies to provide
a functional classification analogous to that
ACTINOPTERYGIAN LOCOMOTION
of caudal propulsion (Lighthill, 1975), in
spite of major advances in theoretical and
experimental studies by Blake (see review,
1981). The best integrative studies at this
time remain those of Alexander (1967)
and Gosline (1971) and the following discussion draws heavily on their insights.
Mechanics and compromises
339
high-speed caudal turning and maneuver
by acting to increase drag on one side, and
as vanes, keels, etc. (Gosline, 1971; Aleyev,
1977). Some fish may even replace caudal
cruising with pectoral propulsion, as in the
embiotocids without cost to cruising performance or costs of transport (Webb,
19736; Dorn et ai, 1979; Fish, 1980). This
may permit specialization of the body and
caudal fin for sprints and acceleration.
Complete substitution of non-caudal median fin propulsion for caudal fin swimming (e.g., Mola, Fig. IF) is rare and the
general adaptive significance is unclear.
Perhaps greater maneuverability is possible at higher speeds.
Specialization for slow swimming using
non-caudal propulsion does appear to affect caudal propulsion. Such fish frequently have a deep body and caudal peduncle,
and a large caudal fin area enhanced by
the posterior insertion of dorsal and anal
fins (e.g., Chaetodontidae, Pomacentridae). This morphology probably provides
for good acceleration, but undoubtedly
impairs steady swimming at high speeds.
Data are not available, but observations by
Hobson and Chess (1978) are instructive.
They found that the body form of Enewetak Atoll (Marshall Islands) planktivores
varied with the distance of their feeding
stations above the reef. Fish with distant
feeding stations were dependent on sprint
swimming to return to the reef to escape
predators, and these had fusiform bodies
and forked tails. Fish with feeding stations
close to the reef, requiring no more than
a quick dart to reach refuge, had deep
bodies with large caudal areas. The only
exceptions were among some deep-bodied
fish that foraged further from the reef, but
which had longer spines than usual as passive predator defenses.
To summarize, slow swimming capabilities can add to caudal propulsion performance increasing overall versatility, but
specialization incurs a cost to caudal swimming.
The essential morphological features of
slow swimming and precise maneuver are
low or neutral buoyancy and flexible fins.
Neutral buoyancy eliminates the need for
fish to either swim fast enough to generate
lift equal to their weight in water (Magnuson, 1978) or to rest on the bottom. Fin
flexibility is required to pass controlled
waves along the fins and to feather them
as necessary. Slow swimming with median
and paired fins is frequently associated
with a deep truncate body. This may improve the effectiveness of the dorsal and
anal fins when they are located along the
trailing edge of the body (Webb, 1975a).
The short body is postulated to reduce
turning resistance by reducing the mass of
the body and entrained water distant from
the turning axis (Alexander, 1967), but
would also allow fish to turn in a smaller
space.
Non-caudal slow swimming capabilities
seem to be a secondary development in
actinopterygians and marked radiation of
slow swimming forms is only seen among
higher teleosts. Some consideration must
therefore be given to the questions of why
median and paired fin propulsion occurs
late in actinopterygian phylogeny, and of
compromises and interactions between
non-caudal and caudal swimming.
Median and paired fin propulsion can
be additive to caudal locomotion. Modern
actinopterygians swim slowly to various
degrees, with no obvious cost to caudal
swimming performance (Webb, 1977a,
1978; Beamish, 1978; Dorn et ai, 1979).
The use of pectoral and pelvic fins in an
efficient "4 wheeled" braking system (Harris, 1937, 1953; Gosline, 1971) may enhance caudal propulsion because higher Phylogenetic trends
speeds are possible in cluttered habitats,
The paired fins of chondrosteans were
such as among weeds, corals, etc. The probably stiff and relatively inflexible. In
paired fins can also be extended to aid spite of the presence of various air spaces,
340
PAUL W. WEBB
these fish were probably negatively buoyant (Romer, 1966). They were therefore
unlikely to be capable of very slow swimming. Occasional deep-bodied forms occurred (Fig. IB) but they were probably
found in relatively open slow waters (Romer, 1966) and not in cluttered habitats.
With the evolution of an effective suite
of characters for neutral buoyancy (swimbladder, reduced armor) and greater fin
mobility (Romer, 1966), some neopterygians were probably true slow swimmers.
Initially, slow swimming and maneuver capabilities probably allowed the penetration
of more weedy habitats and new options
in stalking prey analogous to the behavior
of modern Lepisosteus. More options would
have become available in tortuous and labyrinthine habitats for more specialized
deep-bodied forms, which were more common among the halecostomes (e.g., Dapedium, Proscinetes) than chondrosteans.
Lower teleosts show little advance in
slow swimming morphology and behavior
over the halecomorphs, and deep-bodied
forms only evolved to any marked degree
in association with special habits, such as
the "flying fish" Gastropelecus (Fig. ID).
The major radiation of slow swimming
modes occurs among the Acanthopterygii
and to a small degree the Paracanthopterygii, associated with (1) the relocation of
the pectoral and pelvic girdles so that the
pectoral fins insert high on the body above,
or even slightly behind the pelvics, which
have migrated forward; and (2) the frequent rotation of the pectoral fin base towards the vertical plane (Greenwood et al,
1966; Romer, 1966; Gosline, 1971, 1980;
Nelson, 1976). The pectoral and pelvic fins
come to occupy strategic positions about
the center of mass to generate a wide variety of propulsion and braking forces
(Harris, 1937, 1953; Alexander, 1967;
Gosline, 1971, 1980; Lindsey, 1978). The
anus also migrates forward. The anal fin
advances and more closely mirrors the
dorsal fin, providing greater symmetry in
median fin propulsors and extending the
array of possible locomotor and maneuver
forces. The new fin locations and extensions are also used as brakes and keels and
increase versatility of caudal propulsion
maneuvers (Gosline, 1971; Alevev, 1977).
The rotation of the pectoral fin base to
the vertical plane also introduces possibilities of interacting with the bottom in ways
analogous to a helicopter. This ground effect can lead to substantial savings in energy costs by fish swimming close to the
bottom (Blake, 1979).
ACKNOWLEDGMENTS
Much of the research reported here was
made possible by grants from the National
Science Foundation; BMS75-18423 and
PCM77-14664. The manuscript was prepared during the tenure of an NRC/
NOAA fellowship at the National Marine
Fisheries Service, Southwest Fisheries Center. I thank Dr. J. R. Hunter and Mr. R.
S. Keyes for their hospitality and generous
use of their facilities. I am indebted to Dr.
W. A. Gosline for continuing to stimulate
my thinking and for reviewing the manuscript, to Drs. G. V. Lauder and K. Liem
for their comments on the manuscript,
and to G. R. Smith, J. Humphreys and D.
E. Rosen for interesting and conflicting
discussions on historical hypotheses and
confidence in cladograms.
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