Fish Locomotion

Water 800x denser and 55x more viscous than air - constrains fish design

3 main directions of movement fish can do in water:

• Roll - movement along a horizontal axis - forwards, backwards
• Yaw - movement along vertical axis - turning motions
• Pitch - movement pointing up and down

• Anguiliform - eels, larvae, lamprey
  • Most of body propels
  • Move by undulation
  • Slow to moderate speed
  • Elongate/round body shape
  • Small caudal fin aspect ratio
• Thunniform - salmon, jacks, mako shark, tuna
  • Posterior of the body propels
  • Move by undulation
  • Moderate to fast speed
  • Fusiform body shape
  • Large caudal fin, aspect ratio low
  • Pelagic habitat
• Ostraciform - boxfish, torpedo ray
  • Caudal region propels
  • Move by oscillation
  • Slow speeds
  • Variable body shape
  • Large caudal fin with low aspect ratio

• Tetradontiform - triggerfish, sunfish, porccupine fish
  • Pectoral or median fins are propulsive drivers
  • Osciliation is propulsive mode
  • Slow speeds
  • Variable, deep bodied shape
  • Small to medium caudal fin, aspect ratio low
  • Pelagic habitat
• Rajiform - rays, bowfins, knifefish
  • Propulsive drivers are pectoral or median fins
  • Move by undulation
  • Slow speeds
  • Elongate, often flat bodies
  • Variable caudal fin size, aspect ratio low
• Labriform - wrasses
  • Pectoral fins drive
  • Move by oscillation
  • Slow but fast bursts
  • Large caudal fins, ratio low

Body/caudal fin (BCF) locomotion - generate thrust by bending body into backward-moving propulsive wave that extends to caudal fin

Median/paired fin (MP) locomotion - median and paired fins generate propulsive power, enabling fish to swim accurately and stay still in fast flowing water

Swimming modes involve trade-offs to be effecitve in life history:

• High maneuverability = flat body with large median and paired fins
• Efficient cruising - streamline (tuniform) body
• Rapid acceleration - streamline body with large caudal fin

Some fish can switch and blend modes 

Transition from eel (anguiliform) which moves entire body to tuna (thunniform), which only moves the tail. 

Salmonids - intermediate of the graduation (subcarangiform swimmers)

Thunniform swimming - evolved 4 times in non-related groups

• Little head movement
• Advantagous - increases head stability and increases effectiveness of receiving sensory inputs

Gymnotiforms (knife fish) - evolved BCF swimming mode using elongated anal fin

Maintains a rigid body and remain stationary 

Detect changes in weak electric fields to provide info about the environment

Change/reverse beating fin to change propulsive forces and compensate for water movement = horizontal and vertical control

Swim by contracting serious of myotomes on 1 side of body in sequential pattern and relaxing muscles on other side. Each myotome spans multiple (1 or 2) vertebrae

Efficient swimming - sinusoidal motion along axis of flexible but incompressible spine

Tail/body motion generates lateral forces = forward thrust as they are cancelled by fish body shape and median fins

Vertebral column either described as:

• Monospondylous - 1 vertebra per body segment
• Dispondylous - 2 vertebrae per body segment

Muscle blocks are often described as body segments - more joints per muscle block and more fluid w/in vertebral discs = incompressible flexibility

• Numerous elongate spines for muscle attachment (primary structures)
• Neural arches provide protection for spinal cord
• Epipleural/epineural spines not directly connected to vertebrae (secondary structures) - stiffening in muscle blocks so they keep shape

Main body muscles - discrete myotomes either side of spinal column and forms a W shape

Adjacent myomeres seperated by collagenous myosepta - attached to backbone and skin in collagen sacs

Myosepta form larger units bound by median and horizontal septa = compartmentalisation = muscles can move together

Two muscle types, in different ratios in the body due to conflicting demans of low-speed economic cruising vs short high speed bursts (more red muscle in active fish)

• White muscle - anaerobically for short burst swimming
  • Consists of majority of body in most fish
  • Lacks myoglobin and little vascularisation = limited O2 supply 
• Red muscle - hard to fatigue, used in sustained swimming
  • Thin, lateral superficial sheets under the skin
  • Highly vascularised with lots of myoglobin
  • High numbers of mitochondria in muscle fibres
  • Located as band along side of fish in most species

• Paired fins - evolved from paired hydrodynamic planes 
• Closer the fins are together, the more propulsive capabilities
• Provide thrust through figure of 8 sculling motion
• Osteichthyes - pelvic fin par to move forward until they are beneath or in front of the pectoral fins = more propulsive component and stability 

Rajiform mode - rays, skates and mantas

• Similar to flight of birds
• Large undulations along pectoral fins
• Amplitude envelope of undulations increases from anterior part to fin apex
• Higher the amplitude of motion the bigger the degree of thrust
• Oscillatory mode - fins are flapped up and down like wings of birds

Brook trout - median fin swimming has significant contribution to thrust when fish is in economy mode - tail only used for speed bursts

Bluegill sunfish - dorsal and pelvic fins supplement the caudal fin swimming ,creating mixed FCF and BCF swimming mode

'Stand' on bottom of seafloor w/ modified fins

Deep-sea tripod fish - modified pectoral fin rays and caudal fins to stand and potentially detect predators w/ sediment vibrations

Gurnard - bare fin rays w/ olfactory cells to detect buried invertebrates

Flying fish - glide above water as escape response

• Elongated lower tail lobe to help provide thrust in air

Amazonian hatchetfish - glide in air 

• Claims that pectoral fins can be flapped by muscles to achieve 'true' powered flight

Swimming is most metabolically form of locomtion but energy expended to overcome forces opposing forward movement:

• Inertial/pressure drag
• Viscous/frictional drag

Forces counter-balanced to prevent sinking

Fish are negatively buoyant - hydrodynamic lift accommodates some issues

Static lift (liver fats and swim bladders) = neutral buoyancy with little metabolic energy expenditure = musculature only provides thrust, not hydrodynamic lift

Aquatic animals perfectly supported by medium so little energy needed to overcome gravity

Achieved by minimising drag created by friction, turbulence and body form

Fusiform (torpedo) shape = hydrodynamically efficient for cruising at high speeds

• Takes 3 types of drag into account - surface drag, form drag and turbulent drag

Reynolds number (Re - unitless) quantify fluid flow over animals, useful in determining life-histories

• Vo = incident velocity, Lx = foil length, p = density, u = viscocity

Cicular shape = minimises surface drag

Long shape = minimises form drag

Bulbus shape with long body = minimises turbulent drag

Fusiform body shape - reduce form and turbulent drag

Not affected greatly by speed of swimming, but by smoothness of skin surface and skin surface area

Production of mucus - reduce viscous drag

Primarily reduced by type of scale making up fish external surface:

• Placoid - Chondrichthyes
• Cycloid and ctenoid - teletost fish

Present in Chondrichthyes

Reduce drag by absorbing turbulence between denticles

Sharp edges - surface vortices break away easily

Fast-swimming sharks have sharp-edged riblets on their skin, created by denticles

Placoid scales homologous with vertebrate teeth

Thin, flexible and arranged in overlapping rows (imbricate) = good strength/flexibility

Imbrication sets up boundary layer vortices between projecting scales = reduces drag turbulence

Ends of scales pointed = break away easily = fish leaves no turbulent wake

Scales made of bony surface layer (organic matrix impregnated with Ca salts) and enamel layer on edge

Circuli (growth rings) - useful in taxonomic and life history analysis

Median and paired fins can balance weight in water = neutral buoyancy

Hydrodynamic lift relies on forward movement - requires energy expenditure (advantageous if fish swims anyway)

Energetic costs of swimming reduced by passive buoyancy (static lift)

2 competing models explaining how forward locomotion in sharks is accomplished:

Classical model 

• Shape of heterocercal tail generating downward and backward thrust
• Resultant force (Fr) = upward and forward
• Fr = shark's head pushed downward - countered by planing surfaces of pectoral fins and head
• Now the tail is thought to only generate horizontal forward thrust

Alternative Thomson model

• Upper and lower lobes of tail provide counteracting forces that drive fish
• Generate 'independent' forces on upper and lower lobes 

Primary role of caudal fins - generating hydrodynamic lift - transmit forces generated by body musculature to produce forward thrust 

Apect ratio = (span of the fin)2/surface area of the fin

Low AR = rapid acceleration, high manoeuvrability (larger caudal fin)

High AR = fast and prolonged cruising (thinner caudal fin)

Cartilaginous skeleton - lighter than bone and only marginally denser than seawater

High levels of low-density lipids in greatly enlarged livers (20-30% body mass)

Compressibility of stored lipids is poor - individuals are little affected by ambient pressure associated with depth changes

Problems:

• Density of lipids not much less than seawater = neutral buoyancy only obtained by large deposits of lipids
• Extra bulk of body increases swimming drag and energy to reduce it
• Metabolic cost of synthesising lipids is high and increases with age/size
• Hunting due to use of lipids

Reduce body density by reducing ossification of bone and increasing water content of tissue - common in deep sea species

Utilises lipid deposits in skin, muscle and bone

Uses gas in swim bladder in pelagic species

Not all species posses swim bladder - absent in deep sea/benthic/fast-swimming pelagic species

Bottom-dwelling/swift-stream species e.g. gobies, sculpins

Species that continously swim over wide depth range e.g. tuna, mackrel

• Rapid change in depth = swim-baldder unable to adjust quick enough

Vertical migrators - 100s metres diel migrations

Bathypelagic species - sparse food so energetically expensive (reduce tissues to near neutral buoyancy)

Originated from out-pocket of oesophagus

Used as accessory respiratory structure 'lung' in swampy habitats

Two forms of development - physostomatous and physocilstous

Bladder wall poorly vascularised and lined with guanine crystals = impermeable to gases

Adjusting volume of gas in bladder achieves neutral buoyancy = fish remains suspended in water column

Metabolic costs modest to acieve neutral buoyancy

Gas-filled spaces have inherent vertical instability

Fish decends = bladder compressed = fish heavier and sinks

Fish ascends = bladder gases expand = fish light

If gas is not removed, fish will increase rate of ascent in positive feedback loop

= Swim-bladder overexpansion

Both types of swim-bladder require gas secreted into bladder from blood, against a concentration gradient

Process = gradient diffusion (NOT ACTIVE TRANSPORT) = low energy cost

High [O2] blood must be produced to move through bladder cell wall into bladder, down a concentration gradient

Gas gland - highly vascularised area of swim bladder

Uses:

• Bohr effect
• Root effect
• Salting out
• Rete mirabile and counter current exchange

Primative physostomous bladder - penumatic duct connecting to oesophagous

• Fish relaxes sphincter muscle = expels oxygen through duct into alimentary tract

Physoclistous bladder - duct in juveniles but in adults gas secreted into blood from vacularised ovale

• Ovale seperated from rest of bladder by sphincter muscle = secretion moving down a concentration gradient
• Facilitated by respiratory pigments

Gas gland - secretes lactic acid from anaerobic respiration into blood = localised high acidity

CO2 produced at same time - contributes to Chloride shift

Bohr effect - acificication changes how tightly O2 bound to haemoglobin = reduction in affinity

Root effect - haemoglobin cannot bind as much O2 due to increase in acidity

= unloading of O2 from blood into solution

Lactate increases = reduces solubility of O2 = salting out

Salting out = gasous O2 less soluble at high salt concentrations 

• Used by Salmonids (swim bladder 80% N2)

CO2 generated in gas gland - diffuses into RBC

= H+ and HCO3- - exchanged for Cl- 

Enters RBCs and reduces affinity of haemoglobin for O2

In gills reverse takes place - restoring O2 affinity = Haldane effect

Complex equilibrium of affinity for O2 by haemoglobin caused by shift of Cl- ions

Lactate and hydrogen ions produced by gas gland = reduce solubility of gases in aqueous solution. eases trasnport into gas bladder

Combined effects fill a gas bladder down to 25m, after which gas stays in blood

= rete mirablile counter-current multiplier crucial to gas gland functionability

Capillary vessels take blood to (afferent) and from (efferent) gas gland run counter current and stack in large bondles

High [O2] and lactic acid - maintained in immediate area of gas bladder wall

Diffuse across thin capillary wall from efferent Rete to afferent Rete 

Passive exchange