Fish Functional Morphology: How Fish Swim - PDF

2. Fish functional morphology: How fish swim Part 1  PREFACE TO FUNCTIONAL MORPHOLOGY 1) Fish anatomy is the study of the morphological structure of fishes 2) Fish physiology is the study of how these parts function Structure and function are inseparable: there...

2. Fish functional morphology: How fish swim Part 1  PREFACE TO FUNCTIONAL MORPHOLOGY 1) Fish anatomy is the study of the morphological structure of fishes 2) Fish physiology is the study of how these parts function Structure and function are inseparable: therefore, anatomical descriptions of structures make sense when we understand their functions The study of how the body parts operate (function) in a given environment is referred to as functional morphology Thus: The goals of the next lessons (and of the course) are to explore the anatomical and physiological adaptations of fish for living in water Firstly, we will focus on the general task of the fish locomotions  How fish swim Background Body shape and locomotory behavior in fishes are determined by the extreme density/viscosity of the water, although, differently from terrestrial animals, fish have to not overcome the force of gravity, due to hydrostatic thrust (Archimedes' principle) Water is about: - Eight hundred times denser than air (density = fluid mass per unit volume) - Fifty times more viscous than air (viscosity = a measure of fluid resistance to deformation) Therefore, locomotion (swimming) through this dense and viscous medium is energetically expensive In the first part of this group of slides we are going to see how fish can swim The swimming of a fish is relying on: I) Muscles for power II) Skeleton for framework III) Fins for thrust and direction The swimming of a fish is relying on: I) Muscles for power II) Skeleton for framework III) Fins for thrust and direction I) Fish muscles are structurally like those of other vertebrates, and fishes possess the same three kinds of muscles: a) Skeletal or striated, voluntary, mainly connected with the locomotion (swimming) b) Smooth, nonskeletal, involuntary, and mostly associated with the gut c) Cardiac, nonskeletal, involuntary, but striated: it is found only in the heart Differently from the tetrapods, fish show a greater proportion of the muscolar mass (40–60%), that is (a) Skeletal locomotory muscles In fishes, two major masses of skeletal muscles lie on each side of the body, divided by the horizontal connective tissue septum (blue oval/dashed line) The epaxial muscles (red oval) are the upper pairs The hypaxials muscles (green oval) are the lower pairs  The muscles are arranged in multiple layers (W-shaped myotomes or myomeres) that allow the fish to move in any direction The shape of the myotome is similar to a helical spring: if it is compressed it releases energy to return to its initial shape Skeletal muscles provide the power for swimming salmon myomeres separated by myosepta (white)  Inset: electric organs in electric ray The electric rays (Genus: Torpedo) possess two large electric organs formed by muscles, on each side of their head, where current passes from the lower to the upper surface of the body The main nerves branch attach to each muscular plate in the “batteries”, which are composed of columns, in honeycomb formation, oriented dorso-ventrally With such a battery, an electric ray may electrocute prey with a current of a voltage of fifty to two-hundred volts, and stun prey half-meter away  Inset: electric organs in electric eel The electric eel (Electrophoridae) - not a true eel (Anguillidae) but a close relative of the South American knifefishes - can generate pulses of four-hundred volts, with its several electric organs; these organs are embedded in the fish’s lateral musculature The electric eel has three separate organs, which it uses for producing an electric charge: 1) The Main organ and 2) the Hunters' organs are the high voltage producers, used for protection and stunning prey 2) The Sachs' organ is capable only of producing low voltage pulses - its purpose is mainly the electro-communication with other individuals. The eel tends to remain straight whilst moving, using its anal fin to propel itself. This is necessary to maintain a uniform electric field around itself, as a more effective sensory mechanism. The swimming of a fish is relying on: I) Muscles for power II) Skeleton for framework III) Fins for thrust and direction II) Differently from musculature, the fish skeleton is more complicated than in other vertebrates because it is made up of many more bones Fortunately, from a functional perspective, is not important to know each name of each bone, but is important to know how the main bones structures favour fishes locomotion, with particular regard to i) skull and ii) vertebral column i) The skull acts as a fulcrum, the relatively stable part of the fish ii) The vertebral column acts as a lever that operates for the movement of the fish Three forces are involved in the movement of the fish (see diagram): 1 2 3 Let's see how I) muscles and II) skeleton are involved in each of these forces. Vertical lift 1) Thrust - force in animal direction Most fishes swim by contracting the muscles on one side of the body, and relaxing the muscles on the other side, alternately, using the spinal column as lever The progressive passage of the sinusoidal wave of contractions from the head to the tail: - Pushes back the water - As a result, pushes forward the fish  2) Lift - force opposite in right (vertical) angle to the thrust The lift (or hydrodinamic lift, or vertical lift) is a force perpendicular (vertical) to the movement of the fluid The explanation of the phenomenon of vertical lift can be given by the variation of the fluid speed between the lower and upper surfaces, mainly of the paired fins of some fishes (that act like an aircraft wing) This change in speed causes a change in pressure, that generates vertical lift (upward or downward) In particular: 1) If the water flows with low speed (high pressure) on the lower surface, this lead to negative pressure difference, and the fish swims pointing upwards (see scheme below) Otherwise: 2) If we have equivalent speed between the lower and upper surfaces, there is no pressure difference, and the fish swims at the same depth 3) If we have high speed (low pressure) on the lower surface, there is a positive pressure difference, and the fish swims pointing downwards  Inset: lift, buoyancy and swim bladder Many teleosts have a swim (or gas) bladder, which to help them maintain buoyancy Swim bladder may voluntarily fill or empty of gas (e.g., oxygen), to regulate lift  Inset: lift, buoyancy and swim bladder Many teleosts have a swim (or gas) bladder, which to help them maintain buoyancy Swim bladder may voluntarily fill or empty of gas (e.g., oxygen), to regulate lift Physostomous and physoclistous fishes 1) Physostomous are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal (esophagus) (primitive teleosts) 2) Physoclistous are fishes that lack a connection between the gas bladder and the alimentary canal (advanced teleosts) Nota bene: the original function of the gas bladder was probably as a lung Physostomous and physoclistous fishes 1) Physostomous are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal (esophagus) (primitive teleosts) 2) Physoclistous are fishes that lack a connection between the gas bladder and the alimentary canal (advanced teleosts) Nota bene: the original function of the gas bladder was probably as a lung  Inset: the liver (and other organs) as a “swim bladders” Nota bene: sharks and rays, primitive bony fish, and some teleosts do not have swim bladder ! To float, sharks rely on i) a large liver (that occupies most parts of abdomen, 30% of total mass) filled with oil that contains squalene, lighter than water, and ii) their cartilage, which is about half the normal density of bone The effectiveness in buoyancy of liver is limited, so sharks employ dynamic lift (due mainly to the shape of pectorals, as seen) to maintain depth when swimming Nota bene: some sharks (e.g., the sand tiger sharks, Carcharias taurus) store (endogenous) gas in their stomach, using it as a form of “passive” swim bladder 3) Drag - force opposite to the direction of movement - The main cause of added energetic cost for a fish is the drag, due to the high density/viscosity of water - The drag has two components: a) The frictional (viscous) drag, caused by the friction between the fish’s body and the water b) The inertial drag (pressure + vortex drag), caused by the water displaced by the swimming action of the fish a) The frictional drag is mainly affected by the smoothness of the body surface and by the amount of the body surface area: the more a fish is hydrodynamic, the less is the frictional drag -Thus, frictional drag is linked to body and fins shape Nota bene: production of mucus reduces frictional drag, due to the lubricating power of the mucus itself Blue fin tuna (very hydrodynamic) Sun fish (not at all streamlined) b) The inertial drag (pressure drag ahead + vortex drag behind) is mainly affected by the cruise speed: the more the speed, the more the pressure acting on the movement direction (on front) and the vortexes produced (on back) - As frictional drag, inertial drag too is mainly linked to the body shape, but also to the caudal fin form and development Nota bene: most fast-swimming fishes have a fusiform shape and forked or lunate caudal fin, that minimize both inertial and viscous drag (e.g., Atlantic salmon, blue fin tuna) The swimming of a fish is relying on: I) Muscles for power II) Skeleton for framework III) Fins for thrust and direction III) The fins give to the fish the control over its movements, by e.g., directing thrust, supplying lift, acting as brakes - Fins are composed of hard rays (spines) and/or soft rays, protruding from the body - Skin covers and joins them together: a) Like a folding fan (e.g., teleosts): foldable on themselves, in b which rays are visible b) Similar to a paddle (e.g., sharks): a not foldable, in which rays are not evident Nota bene: the position of the paired fins in teleost fish may help in understanding their phylogenetic relationship. Generally speaking: a) In basal (more ancient) teleosts: -Pectoral fins are oriented horizontally and located in the thoracic position -Pelvic fins occur at mid-body in an abdominal position (see e.g., pike, trout) a b) In advanced teleosts: -Pectoral fins move on the sides of the body, and their base assumes a vertical orientation -Pelvic fins move in thoracic position (i.e., below the insertion of pectoral fins) or in jugular position (i.e., in front of the insertion of pectoral fins) (see e.g., perch, largemouth bass) b The relocations of pectoral and pelvic fins in advanced teleosts may have several functions: -Pectorals on the side can be used as a flap for precise swimming and positioning; -As these fins are within the body profile, their use in locomotion might be less obvious for a predator. -Pelvics placed forward help in braking when sudden erected; -Their location under a spinous dorsal, in combination with hard rays (spines) increases the effective body depth.  Locomotory types in fishes The chief characteristics of the different locomotory types in fish are: i) How much the body is involved in swimming ii) Which parts of the body are involved in propulsion iii) Whether the body and/or the fins undulate or oscillate Nota bene: - Undulation involves the body and/or the fins, crossed down by sinusoidal waves; - Oscillation involves mainly the fins, that move back and forth.  Locomotory types in fishes On the basis of the aforementioned points, a general classification (see table below) of swimming modes among fishes has been developed We distiguish two main types: 1. Locomotory types in fishes via trunk and tail  1a. Locomotory types in fishes via trunk and tail - undulation Within fishes that swim via trunk and tail, at least four types of swimming (with decrasing undulation) are recognized: a) Anguilliform swimming occurs by undulation of a very flexible trunk (eels, some sharks) The progression of types: b) Subcarangiform (trouts, cods) c) Carangiform (jacks, herrings) d) Thunniform (mackerels, tunas) sees increasing involvement of the tail and decreasing involvement of the trunk during swimming a) Anguilliform (e.g. moray eel) b) Subcarangiform (e.g. cod ) From a) to d) the trunk is less involved in swimming, and undulation of the body decreases. c) Carangiform (e.g. amberjack) d) Tunniform (e.g. tuna)  1b. Locomotory types in fishes via trunk and tail - oscillation Yellow boxfish - Ostracidae (Ostracion cubicus) Nota bene: boxfishes swim through oscillation of tail, but also of dorsal and anal fins  Locomotory types in fishes On the basis of the aforementioned points, a general classification (see table below) of swimming modes among fishes has been developed. 2. Locomotory types in fishes via fins  2a. Locomotory types in fishes via fins - oscillation Sunfish, triggerfish, and coelacanth swim through oscillation of median fins (unpaired) Sunfish Triggerfish - Oscillation involves a structure that moves back and forth. Coelacanth Sun fish (Mola mola) Filefish (Monacantidae)/Triggerfish (Balistidae)  Wrasse swim through oscillation of pectoral fins (paired) Wrasse - Oscillation involves a structure that moves back and forth. Napoleon fish - Labridae (Cheilinus undulatus)  2b. Locomotory types in fishes via fins - undulation Ray and knifefish swim through undulation of pectoral fins and median fins, respectively Ray - Undulation involves sinusoidal waves passing Knifefish down the body and/or fins. Stingray (Dasyatidae) (pectoral fins) Knife fish (Apteronotidae) (anal fin) Nota bene: WITH THE SAME SHAPE OF FISHES (e.g. shark vs. tuna) a different type of skeletons produces different ways of swimming Cartilaginous skeleton of e.g. sharks produces a very sinuous, “elastic” way of swimming Bony skeleton of e.g. tuna, produces a more rigid, “robotic” way of smimming  Great white shark Blue fin tunas