Invertebrate Locomotion

Invertebrate Locomotion

1. principles of locomotion

   • - diverse range of body structures  - perform mechanical work  - mobile or sessile - energy invested must result in a return for the animal  - must obey the same base principles as all moving objects - NEWTONS LAWS OF MOTION 
   • A moving animal must overcome resistive forces in the medium in which its movign - friction, drag  these can change the rate of movement 
   • Energy is conserved and mechanical energy can be converted to kinetic energy and ultimately lost as heat 
   • 1 - if a body is at rest to its environemnt it can be set in motion only by th Application Of An External force
   • 2 - The Application of an unbalanced force to a mass in motion results in acceleration or deceleration in the direction of the force
   • 3 - for every action there must be an equal and opposite reaction
   • 4 - Energy within a closed syatem remains constant but may be changed from one form to another
   • the internal workings
     • derived from energy  generated by chemical reactions of the cell but these are not proportional 
     • Efficiency of locomotion
       • expressed as the output of useful work divided by input of energy 
       • Efficient systems may have limited power  tradeoff between effieciency and absolute power 
   • importance of body size
     • have inertia and experience drag  affected by relative size, absolute velocity and the viscosity of the medium 
     • expressed as the Reynolds Number 
     • • measure of the relative importance of inertial and viscous forces 

2. GENERATION OF FORCE - BY ANIMAL CELLS

   • change in cell shape occuring against environmental resistance by the utilisation of metabolic energy 
   • 1. flagellated cells 2. ciliated cells 3. muscle cells
     • ATP-dependent formation of cross linkages between protien fibrils 
     • 1-2 changes in shape as outer bundle of tubular microfibrils slide along each other as a result of cross-linkages bewteen adjacent strands  differ in their absolute length and mode of deployment 
     • Flagellum wavelenght < total length, single flagellum can generate a constant force parallel to main axis  cilium is relatively short and describes a fixed cycle of movements with stiff power stroke and flexed recovery stroke 
     • cilia produce constant propulsive forces by sequenced and coordinated movements of large numbers of cilia
   • Muscle fibres
     • generate forces because they have the ability to shorten against resistance 
     • myofibrils - tubular contractive element - 1 nanometer in diameter 
     • functional unit - sarcomere thick and thin filaments - 2 protiens myosin and actin 
     • cross bridges are formed between myosin and actin fillaments - thin over thick 
     • repeated cycle of formation and detachment of cross-bridges and at each cycel high-energy phospahtes must be supplied by ATP 
     • Rate of shortening - rate of satcomere shortening and number of sacromeres 
     • force generated - number of filaments in parallel; as overlap of fibrils increases linkage sites become saturated and power drops 

3. Muscular activity and skeletal systems

   • skeletal systems are required to transm9it the generated forces 
   • muscles can only genrate power when they contract and force must be applied to restore fibres to original length 
   • Antagonism between sets of fibres, action of cilia or energy stored in elastic tissues 
   • Fluid Skeletons
     • wide range of soft-bodied invertebrates 
     • can be employed in variety of ways but basics are the same - a simple hydraulic system 
     • contraction of the muscles increase the pressure  internal pressure reisted by boundary surfaces 
     • more versatile if muscles arranged as circular and longitudinal sets  opposing muscles - cicular on the outside 
     • Undivided cylindrical animal
       • can peroform a wide variety of changes in shape = perfrom mechanical work 
       • hydrostatic lever 
   • Rigid skeletons
     • crustacea and some echinoderms 
   • no absolute divide

4. soft bodied organisms

   • walking
     • transmit a force through a fixed point 
     • many systems involve the propagation of waves of contaction ans relaxation of muscles logitudenal muscle parrael to the direction of travel 
     • Waves
       • direct, retrograde, monotaxic or ditaxic  
       • DIrect and mono = low geared systems with ability to move large mass or overcome large resistive forces 
       • ditaxic - higher speed, gretater manoeuvrability - patella 
       • Pedal retractor muscles - attach to shell and dorsal mantle and act to shorten or raise and lower the sole of the foot
         • Direct waves - depend on contraction of longitudinal and dorsoventral muscles beginning at posterior end 
         • Retrograde waves involve contraction of transverse muscles interacting with haemocoelic pressure to extend anterior part of foot forward, followed by longitudinal contraction 
     • Errant polychaete worms
       • exhibit a mode of locomotion that involves the movement of multiple limbs 
       • engage in 3 basic epibenthic locomotor patterns 
       • variations in length and amplitude of metachronal waves combine with parapodial movements to produce the different patterns 
       • parapodia and chaeta retract on recovery stroke so parapodia on opposite sides of any given segment are exactly out of phase 
   • Burrowing
     • lost most of their intersegmental septa  segemnts no loger constant volume - fluid shifts  reduced parapodia 
     • chaetae or surface or expanded body segments act as anchor points  burrow wall provides the antagonistic force resisting the hydraulic pressure 
     • Arenicola marina
       • embed and anchor anterior region in substratum 
       • contract posterior circular muscles = coelomic fluid = segments to swell 
       • anterior circular muscles contract and longitudinal relax  posterior edge protruded as anchors  proboscis thrust forward then retracts 
     • bivavle molluscs
       • blade like laterally compressed foot  pedal retractor muscles  foot directed anteriorly and used in burrowing and anchoring 
       • combination of muscles action and hydraulic pressure  extension of the foot by engorgement with haemolyph coupled with the action of paired pedal protractor muscles 
       • when foot id extended the valves are pulled together by the shell adductor muscles  haemolymph forced into foot haemocoel causing foot to expand and anchor in the substratum
   • swimming
     • jet propulsion and the cephalopods
     • Cephalopod
       • fins can be used for stabilisation but also undulate to assist steering and propulsion 
       • many large vertebrates exhibit, thrust & glide swimming - kinetic energy stored following intermittent power strokes 
       • cephalopods do the same - rapidly expel water from mantle cavity 
       • siphon is highly mobile and can be manipulated to piont in almost any direction - manoeuvre and steer  squids attain the highest speeds of marine invertebrates 
       • steps
         • contraction of radial muscles and relaxation of circular muscles - water into matle 
         • reversal of muscular action forces it out 
         • mantel edge clamp tightly to channel water through a ventral funnel or siphon 

5. jointed limbs

   • reducing in size toward the tip  allow movement in on plane  allow for extension and flexion of limb 
   • rotation of limb plane at basal joint often responsible for forward movement  body typically carried slung between laterally projecting limbs 
   • walking movements do not involve raisinf or lowering of centre of gravity  limb acts as a mechanical lever 
   • crustacea
     • characteristics by hardened exoskeletons 
   • swimming
     • rowing action of the limbs  higher - metachronal limb movements are restircted to certain appendages 
     • shrimp - pleopods
     • Amphipods - stomatopods
     • Euphausids - pereopods
     • surphace area of paddle is at maximum during power stroke
       • reduced due to folding and movements of setae during recovery stroke 
     • exceptions
       • planktonic copepods  portunid crabs - use posterior thoracopods - scull 
   • temporary burst swimming - escape response
     • contract ventral abdominal flexor muscles  shoot backwards with spread telson providing large propulsive surface  caridoid escape reaction 

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