Lecture 11

Lecture 11
Muscle blocks, Drag and lift-based propulsion,
Labriform swimming, flight by birds and insects
This course is about the adaptiveness of form, looking at a structure, thinking about it, not
just accepting it – and so perhaps guessing at some of its history.
I joke about enhancing your cocktail party conversation with what you learn in 325. But
knowledge is the mark of an educated person: you are a better conversationalist if you’re
knowledgeable. Imagine yourself in a restaurant eating fish. When somebody complains
about ‘y bones’ maybe you can explain. Maybe you have an answer for why fins have
leptotrichia (bony pointy things) or why fish myotomes look like tortured letter Ws.
• A knowledgeable person who has some idea of myotomes
and axial skeletons also
to eat aknows
fish how to eat a fish.
• When its cooked properly your first step can be to extract the
entire intact bony axial skeleton if you’re careful. Axial
skeleton vertebrae and ribs are no problem, they all connect.
• But there are still the Y-bones as a lurking throat-clogging
danger -- because they’re not attached to the rest of the
You might also not be
surprised that different fish
species have different
skeletal structure; this goes
with expecting the
diversity that is typical of
Butterfly fish skeleton (Wikki)
What is the function of y bones?
Ray-finned fishes:
Ribs connect to the backbone giving the axial skeleton
lepidotrichia or ‘fin rays’
support the fin and allow for variable surface area in deployment
White muscle and anerobic ‘predation function’
• Rate of oxygen supply to a muscle can be the
limiting factor in its activity. During critical
moments of predation (either capture or escape)
the normally supplied oxygen via lungs and
bloodstream can be inadequate.
• Bony fishes have a separate set of ANEROBIC
WHITE muscles (pink in salmon).
• These muscles convert glucose to lactic acid to
get their energy for contraction.
• The energy obtained in this way comes via a less
efficient metabolic process and the accumulation
of lactic acid is also a negative effect. But for
short periods a fish can make a highly adaptive
‘burst of speed’.
Text for this slide and next are taken from:
Environmental Science Investigation, an organization concerned
about declining salmon stocks in the Fraser River
Most of the [muscle blocks] in a fish
… [are] white muscles. In most
salmon species these myotomes are
pink due to a pigment salmon get
from their diet. So not really ‘white’
and not really ‘red’.
Bodybuilding estore
The red muscle is often a band along the side of the fish. The red
muscle contains a lot of myoglobin, capillaries and also a lot of
glycogen and lipids. The red muscle mass is somewhere between
0.5 to 30% of the total muscle mass in a fish, depending on the
species. Active fish, such as bluefin tuna, have a higher proportion
compared to sedentary species, like catfish. The red muscles are
aerobic while the white muscle is mostly anaerobic. As long as a
fish swims within the sustained swimming speed only the red
muscles are used, while in prolonged swimming at high swim
speeds, some of the white muscles are used, and this is what
eventually leads to fatigue. During burst swimming the white
muscles are used at maximum capacity, and this leads to a rapid
Why are the axial muscles of fish so strangely shaped? They look like zig-zag ‘W’s.
Univ. of Michigan Museum of Zoology, UMMZ
Adaptive fibre orientation in white muscle fibres in teleost fishes, taken from p. 210211, R. McNeill Alexander, 'Exploring Biomechanics', Mc Neill’[s figure redrawn (gkm).
• “…the commonest pattern has white fibers running at angles of up to 35 ̊to the
long axis of the body. The [muscles are] partitioned into segments called
myotomes and each fiber runs only the length of a myotome, from one
partition (septum) to the next. But if you follow a series of fibers, connected
end to end through the partitions [from one myotome to the next] you will find
a pattern: these chains of fibers run helically, like the strands of a rope." In
other words these muscle fibres describe helices and lie at changing distances
from the vertebral column.
Zig-zag blocks of muscle
myotomes separated by myocommas
"Imagine that the fibers were not so arranged, but instead all ran parallel to the long
axis of the body. Imagine the fish bending to such an extent [in producing body
waves] that the outermost fibers of the bend, just under the skin, had to shorten by
10 %. Fibers halfway between this peripheral position and the backbone would have
to shorten by only 5% and fibers right alongside the vertebrae would have to shorten
hardly at all. In each tail beat, the outermost fibers would have to shorten quite a lot
and relatively fast, whereas the innermost fibers would shorten much less in the
same time and therefore more slowly.“ This would be very inefficient. You’re not
getting good force production out of all your muscle.
• "Now consider how the actual arrangement of white fibers affects the
shortening of the muscles.“ Helical sequences of fibres run across muscle
blocks like the strands of a rope (represented as red ribbons in the
illustration). Each ‘fibre-chain’ lies close to the backbone for part of its
course and nearer the skin of the fish's side for others. The result is that
when the fish bends, say to the right, all the white fibers on the right
side have to shorten by about the same percentage of their length.“
• Easy to say: a little hard to visualize.
• The axial muscles on the left of the vertebral column are antagonized by
those on the right and vice versa. These left or right side 'chains' of fibres
(running across a series of 'zig-zag' myotomes) will all contract and shorten
in phase with each other, reaching the same % shortening all at the same
time and relaxing maximally at the same time. In other words they go
through their cycle of contracting and relaxing together. But they are
located at different points between the skin and the backbone as they
follow their helical pattern. Thus at the time these 'functional myotome
series' contract simultaneously they are at different phases of the body
wave; if they were not at different phases they could not shorten by a
uniform per cent.
Myotomes of longitudinally aligned
muscle fibres separated by septa and
with chevron shape, perhaps for the
same reasons as fish myotomes are Wshaped: obtaining simultaneous
shortening relative to phase of a body
wave and distance from the notochord.
The notochord makes antagonists of the
muscle blocks of the right and left sides.
Quick mention of amphioxus and its notochord, precursor to the
vertebrate backbone
• See Sfakiotakis: Review of Fish Swimming Modes…
• Fish jump, burrow, fly, glide, jet -- but most use either BCF
or MPF.
• BCF propulsion: retrograde waves using BODY CAUDAL
Creole Wrasse, my favourite Caribbean reef fish, yellow-marked males
with females in schools; odd swimming habit attracted my attention.
Corey Fscher Bonaire
“15-20% of living fishes use their pectoral fins as their primary mode of locomotion”
(Thorsen & Westneat 2004); relatively slow swimmers creating thrust with pectorals.
Ask yourself ‘why evolve toward pectoral fin locomotion and away from BCF? BCF
associated with higher speeds.
“cichlids, damselfishes, parrotfishes, wrasses [above pictures of the creole wrasse,
Clepticus parrae, Bonaire], surfperches, many of angelfishes, butterfly fishes,
goatfishes, surgeonfishes and other coral reef families” emphasize pectorals
Labriidae is the family name of wrasses: and their family name is the basis of the term
labriform as a swimming mode.
An MPF swimming fish using primarily its
pectoral fins: is the creole wrasse.
Labriidae hence ‘labriform’ swimming.
Labriform swimming is a mode of fish swimming in which propulsion is achieved by
cyclic movement of just the pectoral fins; the body is kept straight like a projectile,
while the pectorals are oscillated up and down, abducted (away from body) and
adducted (toward the body) complexly. Pectoral propulsion occurs in a combination
of rowing and flapping that varies with speed (Sfakiotakis et al. 1999).
Rowing is ‘drag-based labriform mode’; flapping is ‘lift-based labriform mode’.
Labriform-swimming fish rarely exhibit a clearly rowing or flapping
movement (lift-based vs drag-based): they use a complex combination
of them that varies with speed. I think these movement models are not
exactly adhered to, but they help to explain the range of movement.
The fins also change shape: “the pectoral fins of the sea perch pass a
wave back over their length as a result of phase lags in the movement
of the individual fin rays” (Sfakiotakis 1999)
See Sfakiotakis et al. 1999, p. 248
Two main oscillatory types when swimming with the pectoral fins: Drag-based, Lift-based.
Thie first is a ‘rowing’ action the latter ‘flapping’ “similar to that of bird wings” .
Drag based swimming is more efficient than lift based at slow speeds (Vogel 1994).
Vogel, S. 1994. Life in Moving Fluids. Princeton Univ. Press, Princeton, N.J.
Figs from Sfakiotakis 1999
Drag-based labriform swimming
There are two phases: power stroke and
recovery stroke. In the power stroke the
fins move “posteriorly perpendicular to
the body at a high attack angle and with
a velocity greater than the overall
swimming speed. On the recovery the
fins are “feathered to reduce resistance
and brought forward”. “ Thrust is
generated due to the drag [on the fin]
encountered as the fin is moved
Feathered: turned edge on
Lift-based labriform pectoral
fin swimming
Lift forces are generated in the plane
perpendicular to the direction of fin
motion; with lift-based labriform pectoral
fin swimming this can occur during both
the upstroke and the downstroke. No
recovery stroke is necessary.
Lift-based fins can generate larger, more
continuous and more efficient thrust than
fins performing rowing motions.
Sea horse: imagine a trout as it isn’t
Sea horses: strangest of teleost fish
form: their caudal fin has evolved into
a prehensile tail, their body lost all
trace of ancestral streamlining.
Locomotion is taken over by the
dorsal and pectoral fins which move
with a sculling action. Swimming
speed is not important to this fish:
but it swims with great
manoeverability (Blake 1976).
Blake, R.W. 1976. On seahorse locomotion.
J. mar. biol. Assoc. U.K. 56: 939-
Sfakiotakis et al. (1999) on fin oscillation in seahorses (p. 250); and also Webb (1988)
p. 709 on “avoiding moving in water”
Seahorse crypsis
Dorsal fin oscillates at what is a very high frequency for a fish fin: up to 40 beats
per second [40 Hertz]. The fin rays achieve this to-fro movement .
“...other species utilizing fin undulations for propulsion” rarely exceed 10 Hz. “To
account for this, it has been suggested that it actually helps the seahorse avoid
potential predators because the fin beat frequency lies beyond the fusion
frequency of the predator’s [retinae], rendering the seahorse indistinguishable
from surrounding vegetation.”
The evolution of fish form is not always aimed at speed in swimming vs
manoeverability in swimming: it can be selected for predator avoidance: crypsis.
Reminder to engineers involved in biomimicry projects: “evolved designs are highly
effective for the fish adapting to their habitat [but] it should be kept in mind that
the locomotor methods employed cannot necessarily be considered optimal per
se. This is because their development has always been in the context of
compromises for various activities (feeding, predator avoidance, energy
conservation etc.)”
• Single-oar sculling is the process of propelling a watercraft by moving a
single, stern-mounted oar from side to side while changing the angle of the
blade so as to generate forward thrust on both strokes (think Venice and
gondolas). Canoeists scull or feather in this way: a seahorse can work its
dorsal fin in this way like a canoe paddle, but much better than a paddle
because it is so flexible: because of the leptotrichia (fin rays) and
intervening membranes. It is really important to the seahorse to maintain
its place in currents – because the sea is almost always in motion. The
animal has evolved to be cryptic; so as not to disrupt the ‘cryptic picture’ it
presents to predators there must be no departures from ‘seaweed’: if the
seaweed rocks in the current, the fish must rock in the same way or its
crypsis be penetrated. Manoeverability (but not speed) is really important
for this animal that is not actually swimming.

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