Lecture 6 Fluid as skeleton: hydrostatic, hydraulic skeletons and muscular hydrostats

1. Lecture 6Fluid as skeleton: hydrostatic, hydraulic skeletonsand muscular hydrostats In which we leave solid skeleton and leverage for skeletons made upon fluid and muscle itself. Animals with fluid-incompressible skeletons are many: cnidarian polyps, annelid worms, echinoderms, molluscs (4 differnent major phyla: Cnidaria, Annelida, Echinodermata, Mollusca). Assigned reading: Kier W.M. 2012. The diversity of hydrostatic skeletons. Journal of experimental Biology 215: 1247-1257. , Notice Glossary p. 1255 for terms you may not know: e.g., bulk modulus, mesoglea,siphonoglyph, etc.

2. The Introduction of a paper is often the best place to find useful general information as the writer explains the problem and what has been done in the past. • “Animal skeletons serve a variety of functions in support and movement. For example, the skeleton transmits the force generated by muscle contraction, providing support for maintenance of posture and for movement and locomotion. Also, because muscle as a tissue cannot actively elongate [muscles can’t push], skeletons provide for muscular antagonism, transmitting the force of contraction of a muscle or group of muscles to re-elongate their antagonists. In addition, the skeleton often serves to amplify the displacement, the velocity or the force of muscle contraction [mechanical amplification]. A wide range of animals and animal structures lack the rigid skeletal elements that characterize the skeletons of familiar animals such as the vertebrates and the arthropods. Instead these animals rely on a [fluid skeleton]... in which the force of muscle contraction is transmitted by internal pressure” (Kier 2012)

3. 3 main functions of skeleton • transmits/translocates force and is shaped and made of materials that lend themselves to this translocation (chitin, bone, collagen, resilin...) • muscular antagonism: muscles can’t push, they can only pull. So they typically function in pairs that are antagonists of each other; one pair member contracts and as it does stretches the antagonist back to its precontracted dimension. (mandible adductor and abductor are a good example: often there is a difference in the power/size of the two antagonists: opening mandibles doesn’t require the same amount of force as crushing food. • mechanical amplification: skeletal leverage via optimal moments of force can increase the force effect

4. The exoskeleton of a locust includes cranium, mandible, and the inflections of mandible cuticle, the two apodemes*; the mandible is an appendage jointed to the head. • Pinnately arranged muscle fibres originate on the inner cranium and angle downward, converging and inserting on the apodemes. Muscle contraction pulls on the apodeme which translocates forces and so moves the mandible. • The contraction of the adductor muscles is antagonized by the abductor muscles. • The moment of force (red perpendicular distance from apodeme insertion to axis ) is greater for the adductor than the abductor (blue), because the adductor inserts farther from the axis of mandible rotation, this ‘lever arm’ in effect amplifying its muscle power. *do not forget that the muscles have been omitted; apodemes don’t contract

5. An adaptation aside This insect, Romalea, conspicuous to colour-vision capable predators , is warningly coloured : it sequesters noxious chemicals.

6. Principles of support and movement (Kier 2012) hydrostatic skeletons • The fluid of hydrostatic skeletons is essentially water ; it has a high bulk modulus, i.e., resists significant volume change. Fluids are effectively incompressible. If you stress fluid (apply a force per unit area to it) its pressure increases without appreciably changing its volume. • When stresses (force per unit area) are applied to solid skeletons, non-fluids like the exoskeleton of an insect, the direction of the force matters: pull, push, or slide stresses give rise to different forces acting in different dirctions inside the skeleton: tensile, compressive, shear). But stress applied to a fluid is omnidirectional in effect :“press air into a tire and the tire inflates in any direction it can get away with” (Vogel). • With fluid skeletons ,“contraction of circular, radial or transverse muscle fibres will decrease [chamber] diameter, thus increasing the pressure, and because no significant change in [chamber] volume can occur, this decrease in diameter must also result in an increase in length.” The reverse occurs to re-expand the diameter and re-elongate the muscle fibres.

7. Muscle fibre orientations: Circular, radial and transverse affect cross-sectional area of a fluid-filled chamber, a hydrocoel: their contraction causes the fluid skeleton to lengthen and supports bending. Muscle fibre orientations: longitudinal muscle fibres shorten the hydrocoel Some muscle fibres run helically in muscular hydrostats (cavity less fluid skeleton) and create torsion, twisting about the long axis of the structure. torsion: twisting

8. Amplificationby hydrostatic skeletons.Consider relationship between diameter and lengthening. Muscle contraction forces can still be amplified by hydrostatic skeletons even in the absence of “fulcrums and lever arms”. For a constant volume (see Fig. 2 above) the percentage increase in length, brought about by shortening of circulars [or radial or transverse fibres], becomes y greater as diameters decrease.

9. Collagen an important material in hydrostatic skeletal systems as ‘crossed fibre helical connective tissue array’ Collagen is a important body material, a protein, important consitutent of fibrous connective tissue; it takes the form of long fibrils becoming the basis of tendons, ligaments and skin; it is made by fibroblasts during embryogeny Susan Barker

10. Connective tissue – collagen fibresDon’t confuse connective tissue fibres with muscle fibres: only muscle fibres can contract. • The walls of the hydrostatic chambers are often reinforced with connective tissue fibres that “control and limit shape change”. • These fibres are typically arranged in a “crossed fibre helical connective tissue array”. • Note his language: the fibres are “stiff in tension” meaning that when you pull on opposite ends there is negligable extension. But because they are structured as a helix (a ‘spring’ if you like) these relatively inextensible fibres in the wall of a structure can allow the structure to extend (see echinoderm tube foot). • “Elongation and shortening is possible because the pitch of the helix changes during elongation (the fiber angle, which is the angle relative to the long axis, decreases) and shortening (the fibre angle increases)...”

12. Phylum Coelenterata/Cnidaria • Hydras, jellyfish, sea anemones, corals. • Radial symmetry. • Gastrovascular (GV) cavity (internal space, filled with seawater) opening via a mouth; no anus, no assembly-line digestion. • Whorl of tentaclesare extensions of body wall and GV cavity aid in food capture using stinging organelles (nematocysts). • Diploblastic: epidermis and gastrodermis. • Mesoglea layer may give some elasticity “The mesoglea ranges from a thin, noncellular membrane to a thick, fibrous, jelly-like, mucoid material with or without wandering cells” (Barnes). • Two ‘structural types’ or morphs occur: polyp is sessile; medusa is free swimming. • Very limited development of organs: e.g. siphonoglyph might be called an organ.

13. DiveGallery Tubastrea Hydrostatic Skeletons Jennifer Goble Ryan Photographic

14. Fig. 5 Kier The body of a sea anemone is “a hollow column ...closed at the base ...at the top with an oral disc that includes a ring of tentacles surrounding the mouth and pharynx”. “By closing the mouth, the water in the internal cavity –the coelenteron/GV – cannot escape, and thus the internal volume remains essentially constant. The walls of an anemone include a layer of circular muscle fibres. Longitudinal muscle fibres are found on the vertical partitions called septa that project radially inward into the coelenteron, including robust longitudinal retractor muscles along with sheets of parietal longitudinal muscle fibres adjacent to the body wall.”

15. Phylum Cnidariasea anemones, corals, jellyfish etc. “With the mouth closed, contraction of the circular muscle layer decreases the diameter and thereby increases the height of the anemone. Contraction of the longitudinal (or R =retractor) muscles shortens the anemone and re-extends the circular muscle fibres.” “...with this simple muscular arrangement a diverse array of bending movements and height change can be produced.”

16. Metamerism and hydrostatic skeletons Phylum Annelidasegmented worms.Most species are marine, polychaetes.Annelida have a coelom. Coelomate animals Leeches are also annelids, freshwater, not marine, specialized for blood feeding. Nereis Metameres are segments grouped sometimes into tagmata: a tagma is a series of metameres specialized for a shared function. Transverse grooving along leech body does not represent its ancestral segmentation and is not homologous with grooving of Nereis. ABC News

17. Animals with a coelom are termed coelomate, animals without one are acoelomate. What is a coelom? It is defined as a fluid-filled cavity forming within mesoderm (mesoderm being one of the primary germ layers of the embryo). chaetae To burrow effectively through soil, searching out softer regions and crevices, going around or under rocks etc. the worm needs to twist and turn and push its body. For push you need purchase. For this it has chaetae; producible and retractable ‘hobnails’.

18. Schizocoel: splitting coelom formation in annelids Development, primary germ layers: ectoderm, endoderm, mesoderm (embryonic); Budding occurs of embryonic tissue behind the trochophore larva, in a series of segments; bilateral spaces appear in mesoderm and enlarge until the mesoderm becomes a layer applied against the gut (endoderm) and the skin (ectoderm). Mesoderm forms mesenteries, dorsal visceral and ventral. The mesoderm against the forming body wall differentiates into the circular and longitudinal muscles. Each somite space expands also to form fore and aft the worm’s septum. free-swimming trochophore larva dispersal stage cronodon.com

19. Lumbricus castaneus What was the primitive function of the segmentation of annelids? Why partition the coelom? Earthworm is adapted for burrowing, being able to change body shape locally: a cylindrical anteriorly pointed probing snout, backed with serial septa-partitioned hydrostatic skeletal units bounded by muscle: its body is a flexible digging machine for making its way through soil. clitellum Earthworm Society of Britain Annelida: 8800 spp. Triploblastic coelomate bilateria, body cavity a schizocoel, metamerically segmented, longitudinal and circular muscles around a hydrostatic skeleton, extracellular digestion in a straight digestive tract running from anterior mouth to posterior anus; gut supported by longitudinal mesenteries and septa, ventral nerve cord with segmental ganglia and anterior brain, circulatory system high pressure blood in vessels, excretion by metameric nephidrida. o Each compartment has its own pair of nephric tubules. Why?

22. For cylindrical fluid skeletons “at a given pressure, the stresses in the circumferential direction are twice those in the longitudinal direction” Kier 2012 Septa function in allowing large lateral forces useful in burrowing