1. Bio 325 Jan 17 Muscular hydrostats Mollusca Jetting

2. Argiope aurantia Spiders have flexor muscles in their jointed legs, but no extensor muscles: that is they have muscles that decrease the angle between one leg segment and another, but no muscles that increase that angle. They rely on fluid pressure for leg extension just as starfish do with their podia. Observation of ‘dead spider behaviour’: always die with their legs flexed, never with legs extended (as muscles go into rigor?) Why no extensor muscles? Think how the legs function in predation. Ted Mcrae Beetles in the Bush stabilimentum (later)

3. Muscular hydrostats Kier p.1252 Tongues, tentacles, trunks: “lack the fluid-filled cavities and fibre-reinforced containers that characterize... hydrostatic skeletal support systems” You don’t need a fluid-filled cavity to exploit incompressibility

4. Transverse sections showing the muscular arrangement of three examples of muscular hydrostats. Kier W M J Exp Biol 2012;215:1247-1257 ©2012 by The Company of Biologists Ltd A. Squid tentacle: T, transverse muscle fibres; L, longitudinal; transverse in the tentacle core, “and extend to interdigitate with bundles of longitudinal muscle fibres, notice the suckers.

5. Transverse sections showing the muscular arrangement of three examples of muscular hydrostats. Kier W M J Exp Biol 2012;215:1247-1257 ©2012 by The Company of Biologists Ltd B. Elephant Trunk: R, radials ‘extend from centre of the trunk between bundles of longitudinal muscle that are more superficial, notice nasal passages.

6. “The muscle fibers are typically arranged so that all three dimensions of the structure can be actively controlled, but in several cases such as the mantle of the squid [of which more later] and some frog tongues, one of the dimensions is constrained by connective tissue fibers.” “Because muscle tissue, like most animal tissues lacking gas spaces has a high bulk modulus [*bulk modulus again!], selective muscle contraction that decreases one dimension of the structure must result in an increase in another dimension. This simple principle serves as the basis upon which diverse deformations and movement of the structure can be achieved” (Kier 2012). Read carefully all the section on muscular hydrostats by Kier: complex bending achieved by interplay of contracting muscles --more subtle than a passive uniform fluid in a chamber – i.e., some muscles by contracting can affect the bulk modulus presented to other muscles that are acting upon its incompressibility *bulk modulus of a substance measures its resistance to uniform compression Wikki

7. Muscular hydrostats (Kier contin.) Selective contraction: “This simultaneous contractile activity is necessary to prevent the compressional forces generated by the longitudinal muscle from simply shortening the structure, rather than bending it, and can actually augment the bending by elongating the structure along the outside radius of the bend.” “The longitudinal muscle bundles are frequently located near the surface of the structure, as this placement away from the neutral plane increases the bending moment.” Helically arranged muscle fibres can be present and generate torsion. Best example you have available is your own tongue Frogs use their tongue to catch insects. Muscular hydrostat is involved.

8. Jet propulsion: sedentary polyp to pulsating medusa Animal Literature

9. Phylum Mollusca Characterized by lack of segmentation in contrast to the Annelids Strong cephalization (elaborate head) Thick muscular body wall Specialization of the ventral body wall as a muscular foot Mantle: dorsal body wall extended as folds, termed a mantle, this enclosing a mantle cavity Mantle secretes a shell and encloses gills nudibranch bivalve body plan drawn

10. Bivalve: Pecten, scallop: water-jet swimming propulsion Seawater exits from 2 openings near the hinge as the valves are adducted: this jets the scallop forward. anterior adductor lost, posterior relocated more centrally

11. An example of an antagonist to a muscle that is not another muscle; the Pecten adductor stores the energy of distortion that will later restore it to its precontracted state.

12. functional axes of squid body are shifted visceral mass elongated dorsoventrally, shell internal funnel developed from posterior of foot primitive ventral surface became functional anterior end Chromatophores: pigment cells in skin, circlet of smooth muscle cells, disperse concentrate Class Cephalopoda squid, octopus, cuttlefish, Nautilus primitive dorsum

13. Source: Gosline J.M., & Demont M.E. 1984. Jet-propelled swimming in squids. Scientific American 252: 96-103. Well written, worth reading in detail. A swimming squid takes up and expels water by contracting radial and circular muscles in its boneless mantle wall. Elastic collagen “springs” in the muscle increase the power of the jet. Beware potential confusion: squids jet propel themselves and there is a fluid-filled cavity involved – the mantle cavity. If the seawater in this cavity were not incompressible the jetting wouldn’t work. But this cavity is NOT functioning as a hydrostatic skeleton. Rather the mantle wall itself is a muscular hydrostat.

14. The seawater within the mantle cavity of the squid is not functioning as a hydrostatic skeleton. But it is the basis of the animal's jet propulsion, which in turn depends upon the incompressibility of seawater. When the radial muscles of the mantle contract, the volume of the mantle cavity is increased and seawater is drawn in. When the circular muscles of the mantle contract, the volume of the mantle cavity is decreased and seawater is squirted out. The action-force of the jetted (incompressible) seawater creates a reaction force that pushes the squid in the opposite direction: opposite to whatever direction the funnel is pointing. One-way valves control intake of water into mantle cavity at sides. Pressure build up in seawater inside mantle cavity (circulars contract) forces the inner flaps of the funnel against the mantle wall water jets out funnel (hypostome)

15. In the mantle structures interact: 1) collagen fibres (connective tissue) make a tunic that prevents longitudinal dimension change 2) radial muscles contract to thin the mantle wall and 3) circular muscles of the mantle contract to thicken the wall. Circulars and radials are antagonists. The mantle (the actual wall) is a muscular hydrostat and its volume must stay constant (just as if it were a fluid-filled cavity). But (per Kier) the fibres are very critical: because of the collagen ‘tunic’ the mantle cannot get longer in the A to B dimension: it can change in girth. A B

16. 1.Radial muscles contract to cause: hyperinflation: seawater intake into mantle cavity: outside diameter of mantle increases by approximately 10% over resting diameter (girth increase); cavity volume increases 22% re relaxed volume, wall thins. 2. Circular muscles contract to bring mantle to about 75% of its relaxed diameter, radials restored to precontracted length (girth decrease): volume drops & pressure rises sharply, forcing the inlet valvesl against the mantle wall and leaving only the funnel as exit. relaxed Mantle wall Escape Jet Cycle of squid Internal organs contracted

17. The mantle wall functions as a muscular hydrostat -- a fluid-based skeleton [muscles mostly water] without a distinct fluid chamber -- it makes antagonists of the radial and circular muscles: contraction of one kind of muscle restores the other to its relaxed state via this type of fluid skeleton. The radial and circular muscles become coupled as antagonists by virtue of their own tissue being significantly water and so incompressible – and because the mantle cannot lengthen. Because the mantle is incompressible it must retain an overall constant volume; and it cannot get longer as mantle muscles contract because of the collagen fibre tunic that prevents any movement in that direction. Thus, it can only increase or decrease in thickness – at the same time changing its overall diameter and the capacity of the mantle cavity. When the radials contract the mantle walls must get thinner and the walls move apart -- to maintain hydrostat volume. Conversely when the circulars contract the mantle wall must get thicker as the overall outside diameter of the mantle decreases. If there were no inextensible fibres, if the animal’s mantle was not in a jacket of fibres preventing it from lengthening, then the radials and the circulars could not have an antagonistic effect on each other.

18. Razor clam burrowing Natural History Museum of Rotterdam Winter A.G. et al. 2012. Localized fluidization burrowing mechanics of Ensis directus. Journal of experimental Biology 215: 2072-2080. (See also Inside JEB, Kathryn Knight. 2012. Razor clams turn soil into quicksand to burrow. greatly elongated dorsoventral axis; hinge is dorsum, foot is ventral anterior posterior

19. Ensis directus burrowing motions. Winter A G et al. J Exp Biol 2012;215:2072-2080 ©2012 by The Company of Biologists Ltd