Jan 24 Lecture
Bivalve mistakes
Chordate diagnostic characters
Notochord: axial hydrostatic organ, myotomes
Undulatory swimming in fishes
Today’s aside is
Bivalve Blunders
*there are probably many different
variations of hinges among different spp.
Previous lecture’s diagram hydrostatic forces affecting foot dilation is misleading
when applied to the razor clam. The hinge ligaments (I drew them as if the same as
a scallop which also might be wrong*) would not be visible in a section of a razor
clam showing the foot and valves at this angle.
The hinge does indicate the mollusc’s dorsum and, with the clam oriented foot-down
for burrowing, its hinge ligament is positioned to the side (90º different to the
diagram). So the diagram is useful only for showing the effects of the different
muscles on the haemolymph (blood) within the sinuses.
A second error was to involve the siphons in creating the quicksand. Careful reading
of Winter shows that he never mentions siphons (which in this clam are relatively
short): it is the movement of the valves that “draws pore water towards the animal,
which mixes with the failed soil to create a region of fluidization”.
The siphons of the razor clam are used for filter feeding when the tide is in: for this
purpose the animal is never deep down. It can burrow very fast compared to other
shellfish (i.e., its unusual form probably reflects some ‘streamlining’ for rapid
burrowing) and quickly uses its very well-developed foot to escape bird, crab, fish or
human predators that try to ‘dig it out’.
Chordata and the notochord as a hydrostatic* axial skeleton
*I thought Kier missed this example of a hydrostatic structure, but he does mention it
under ‘Additional examples’
Name of the phylum – Chordata** -- comes from notochord; chordates are
diagnosed by certain body features shared by all species in the phylum:
pharyngeal gill slits: a perforated pharynx
dorsal tubular (hollow) nerve cord (dorsal to the digestive tract)***
tail, body continues postanal (anus is not terminal)
circulation occurs from the heart forward in a ventral vessel and then up
around the pharynx and rearward in a dorsal vessel***
jointed endoskeleton
--- and at some time in their development all chordates have a notochord,
a long cylindrical tube bounded by helical connective tissue fibres.
surrounding a core of cells and fluid; this is also functioning as a
hydrostatic structure.
**Within chordata are subphyla: Urochordata (sea squirts), Cephalochordata
(amphioxus/lancelet), Vertebrata et al.
***contrast with annelids: ventral solid nerve cord; anterior blood flow in dorsal vessel
Subphylum Cephalochordata
“These animals [cephalochordates] are so often used to illustrate the fundamental
features of vertebrate organization that it is only too easy to forget that they are not
vertebrates at all (Barrington 1965)”. Point is, they are not ‘little fish’. Adult
amphioxus are benthic (bottom burrowing) filter feeders. Though interested here in
their notochord and axial musculature -- which enables them to swim in the water
column by undulatory body waves, ‘sort of like a fish’, their myotomes and notochord
are best not considered adapted for swimming: they use their metameric musculature
for burrowing more than swimming.
myotomes semitransparent
evident here dorsad, just below fin
Morgan Mccomb
Barrington E.J.W. 1965. The Biology of the Hemichordata and Protochordata.
Oliver & Boyd, Edinburgh & London. (see for feeding mechanism of amphioxus)
There is a median dorsal ‘fin’, a caudal ‘fin’, and a
median ventral ‘fin’ (stabilizers like canoe keel?) all
three continuous to tip of tail.
These are very different than the mobile shapeshifting fins of fishes.
A pair of metapleural folds run along the body, one
on each side from the anterior to the atriopore
(atriopore is a separate exit for filtered seawater).
These structures should lend stability by avoiding
rolling around the long axis, i.e., tend to keep the
dorsum up top while swimming.
The notochord is located just below the nerve
chord, an intimate structural relationship because
the the nervous system co-ordinates body bending
via the notochord.
Long, J.H. Jr. et al. 2002. The notochord of hagfish Myxine glutinosa:
visco-elastic properties and mechanical functions during steady
swimming. J. exp. Biol. 205: 3819-3831.
• “Chordates have evolved an unique hydrostatic axial skeleton, the
notochord, that is present in all taxa of that phylum early in
development; it is retained in the adults of some taxa and modified
by vertebral elements in others... Notochords are hypothesized to
have evolved to stiffen the body (Goodrich, 1930) and to prevent
body compression during muscle activation (Clark, 1964). In
addition, notochords may adjust function by means of dynamically
variable mechanical properties (Long et al., 1998).
Notochord is another example of a hydrostat, and given that it involves muscle
perhaps it is reasonable to think of it as a muscular hydrostat, at least in part.
(In fact isn’t the foot of a bivalve satisfying the definition of a muscular
hydrostat at least in part?)
Cell structure of
Muscles of amphioxus are arranged in a series of
V-shaped blocks known as myotomes, separated
by sheets of connective tissue, myocommas.
Because they are v-shaped several appear in any
single transverse section.
The muscle fibres within these v-shaped myotomes
run longitudinally (as is also the case with fish).
One way of describing notochord function: ‘it is a
laterally flexible fixed-length skeletal element that
makes the axial muscles on the right side of the
body antagonists to those of the other side’.
If no notochord and fibres on both sides contract
simultaneously, the longitudinal dimension of the
animal would shorten – head approaches tail.
All quotes from older Kardong 2nd edition 1998
(Kardong 6th edition 2012, is on reserve) and must deal with notochord
• “Such mechanical structures, in which the outer wall encloses a fluid
core, are called hydrostatic organs.”
• “The notochord is a hydrostatic organ with elastic properties that
resist axial compression.”
• Kardong does an ‘IMAGINE IT AS IT ISN’T’
• “To understand the notochord’s mechanics, imagine what would
occur if one block of muscle contracted on one side of an animal
without a notochord. As the muscle shortens it shortens the body
wall of which it is a part and telescopes the body [animal collapses
longitudinally like an accordion] . In a body with a notochord, the
longitudinally incompressible cord resists the tendency of a
contracting muscle to shorten the body. Instead of shortening the
body, the contraction of the muscle sweeps the tail to the side.”
That is, the notochord functions to enable undulatory (rear-directed
body waves). [plastic ruler illustration]
Summarizing the working and functions of the notochord: it is not just a
simple fluid-filled chamber surrounded by connective tissue acting to
translocate muscle forces
• It is a hydrostat and at least in part a muscular hydrostat -- because
a tunic of helical connective tissue fibres works against muscle and
fluid within.
• It stiffens the body longitudinally, preventing it from shortening when
axial muscles contract.
• It makes the longitudinally alligned muscle fibres of the right and left
sides antagonists.
• It promotes lateral (transverse) body bending, which is the basis of
undulatory body waves.
• Since it is composed in part of muscle cells (capable of contracting)
there is a dynamic quality to its function as a skeleton: it can change
its stiffness topographically to facilitate adaptive bending
From amphioxus to fish and body undulating propulsion: there are
differences in the anatomy and effectiveness of the body wave
• Streamlined body, bone-supported flexible fins, notochord early in
embryology but then replaced by a bony series of interlocking
vertebrae, the vertebral column; axial musculature of zig-zag
blocks of muscle, myotomes separated by myocommas: the phasing
of their contraction creates locomotion by backward-directed
Undulation styles
How much of body oscillates,
How much stays stable (more
or less)
Some classifications
1. anguilliform: eels, sea snakes
undulate entire body, largest
head displacement (same as
snake ‘serpentine movement’
2. subcarangiform: cod,
salmonids: undulation involves
more body than just peduncle
and tail
3. carangiform: oscillating only
tail fin and peduncle
4. Tunniform: tunas etc.
Thrust: forces that tend to advance the fish; drag: forces that tend to retard it.
Retrograde body wave pushes back against
the water; creates a reaction force resolvable
into two vectorial components: one lateral and the other
thrust propelling the animal forward; lateral forces of
the right and left sides tend to cancel out.
‘Propulsive element’: small segment of body
Inclination of reaction force is toward the head
The farther back a propulsive element is located the
steeper this incline toward the head; thus more of the
reaction force is thrust as one moves posteriorly out
along the tail.
Rearward elements are moving through a greater
displacement – in the same time – so they must go
Webb, Paul W. 1984. Form and function
in fish swimming. Scientific American 251: 58-68.
Sfakiotakis M., Lane D.M., Davies J.B.C. 1999.
Review of fish swimming modes for aquatic
locomotion. IEEE Journal of Oceanic Engineering
24: 237-252
Change medium to ‘pegs’: snake locomotion
serpentine on land
From fish swimming in water to snakes in water, to snakes on land
Boundary Waters BWCA
Matthieu Billaud
Serpentine undulatory movement requires contact points where thrust
can be obtained; animal uses prominences in irregular substrate to push
against: only a little of its body makes contact whereas underwater all its
body makes contact.
Only certain
portions of the
wave are placed
in contact with
the substratum
Crotaline movement
in snakes
Could a snake employ
sidewinding underwater?
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 from p. 210-211, R. McNeill
Alexander, 'Exploring Biomechanics', figure
redrawn (gkm).
Why are fish muscles of two colours?
The rate at which oxygen can be supplied to a muscle may
become the limiting factor in the muscle’s activity during escape.
So many animals have a separate set of anerobic muscles that
work without oxygen limitation. These muscles convert glucose to
lactic acid to get the energy for contraction. Energy thus obtained
is via a less efficient metabolic process and the lactic acid that
accumulates is yet another limitation, but where a burst of speed
is important, anerobic muscles make this possible. Later the
animal is able to oxidize the incompletely used products of
anerobic metabolism.
In vertebrates the muscles that function in these two ways are of
different colours. Aerobic muscles are generally reddish, while
anerobic muscles are whiter.
The swimming muscles of fish: the bulk of a fish fillet, is white
anerobic muscle used only for short bursts of speed. In teleost
fish there is generally a band of red aerobic muscle running along
the side of the body close under the skin. This is the muscle that
powers sustained swimming.
“Aerobic axial muscles of fish, red muscles, run lengthwise along the sides of the
body." The white fibres (anerobic) " have different patterns in different fish species,
"but the commonest pattern in teleost fishes... has white fibers running at angles of
up to 35 degrees to the long axis of the body. The muscle is partitioned into segments
called myotomes and each fiber runs only the length of a segment, from one partition
(septum) to the next. 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 fibre
'chains' 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, 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.
"Now consider how the actual arrangement of white fibers affects the shortening of
the muscles." Sequences ('chains') "of fibres run between muscle blocks helically,
like the strands of a rope. Each 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."
Remember that the axial muscles on the left of the vertebral column are
antagonized by those on the right and vice versa. These '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.
Density is mass per unit volume; so regulating your density is a matter of losing
weight or increasing your volume. Some swimming animals regulate body density.
You soon realize as a scuba diver that swimming is much easier when you are not
working to stay down or to keep from sinking, when you are neutrally buoyant.
A diver achieves neutral buoyancy by regulating body density with a buoyancy
compensator (BC). Bony fishes do the same thing.
The BC is a rubber vest into which air can be introduced from the air tank – it swells,
occupies more volume and reduces the diver’s density. It can be adjusted (air in/air
out) until the diver stays steadily at depth, not rising or sinking.
[Buoyancy compensators give buoyancy because they add only relatively little weight
but when inflated they displace much more water.]

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