Jan 24 Lecture

1. Jan 24 Lecture Bivalve mistakes Chordate diagnostic characters Notochord: axial hydrostatic organ, myotomes Undulatory swimming in fishes Buoyancy

2. 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’.

3. 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

4. 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)

5. Romer • 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 shape-shifting 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.

6. 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?)

7. Cell structure of notochord Kardong

8. 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.

9. 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]

10. 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

11. 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 (RETROGRADE) waves krisweb

12. Undulation stylesHow 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. Mackerell

13. 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 faster. 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

14. From fish swimming in water to snakes in water, to snakes on land Boundary Waters BWCA Equisearch Matthieu Billaud

15. 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. Albertawow.com

16. Sidewinders Only certain portions of the body wave are placed in contact with the substratum

17. Crotaline movement in snakes Could a snake employ sidewindingunderwater?

18. 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).

19. 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.

20. “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

21. "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.

22. "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."

23. 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.

24. Buoyancy • 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.]