Aquatics - Anatomy

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Fins and Locomotion

  • movement in water is less costly for fish than it is for terrestrial animals on land
    • streamlined shape and mucoid secretions 
      • mucoid secretions lubricate the body surface and reduce friction
  • buoyant properties of water contribute to the efficiency of movement
  • aquatic animals spend less energy supporting their bodies against gravity
  • fins and body wall push against the incompressible surrounding water
  • Specialised muscles arranged in a zigzag pattern
    • these muscles extend posteriorly and anteriorly
      • the contraction of each muscle bundle affects a large portion of the body wall
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Fins

  • pectoral fin = steering
  • ventral/pelvic fin = balance, but is adapted in some species
  • anal/dorsal fin = balance
  • caudal fin = propulsion
    • forked in some fast swimming fish (e.g., tuna and mackerel)
      • reduced surface area for turbulence that could interfere with forward movement
  • adipose
    • only present in some species (e.g. trout and salmon)
    • ichthyologists unsure of the purpose
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Gills

  • oxygen and carbon dioxide
  • made of filaments covered in lamellae
  • covered in operculum
  • Counter Current Mechanism - blood flows in the opposite direction to the water flow 
    • higher concentration gradient
  • some fish have lungs (e.g., mudskippers and lungfish)
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Buoyancy

  • swim bladder - expandable sac
    • fish can control the amount of air in the sac
    • when gas is in the swim bladder of Osteichthyes the fish becomes less dense
      • when the swim bladder is deflated the fish will sink 
  • cartilaginous fish have an oil-filled liver
    • filled with squalene
    • can't control the amount of oil in the liver
    • the oil makes the fish less dense
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Heart & Circulation

  • closed circulatory system
    • evolution of lungs in fish paralleled by changes in the vertebrate circulatory system
      • associated with the loss of gills, delivery of blood to the lungs, and the separation of oxygenated and deoxygenated blood in the heart
  • vertebrate heart develops from four embryological enlargements of the ventral aorta
  • most fish have a single circulatory system
    • a few fish have lungs (e.g. lungfish)
      • presence of lungs changes the pathway of circulation
      • circulation to gills continues but a separate artery has developed that branches off the aortic arch VI
        • pulmonary artery
      • blood returns to the heart via the pulmonary veins
      • atrium and ventricle potentially divided
        • less oxygenated blood prevented from mixing with oxygenated
      • spiral valve in the conus arteriosus helps direct blood from the right side of the heart to the remaining aortic arches
      • lungfish show distinction between the pulmonary circuit and the systematic circuit
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Heart & Circulation : part 2

  • blood flows from the venous system through the thin-walled sinus venosus
  • from sinus venosus into thin-walled atrium
  • atrium to single ventricle
    • the ventricle is the primary pumping structure
  • anterior to ventricle is the conus arteriosus
    • connects to the ventral aorta
  • in teleosts, the conus arteriosus is replaced by an expansion of the ventral aorta (bulbus arteriosus)
  • ventral aorta to gills
  • efferent vessels collect oxygenated blood and deliver it to the dorsal aorta
  • blood is spread through the body before being returned to the heart by the venous system
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Digestion

  • earliest fish thought to have been filter feeders and scavengers
    • sifted through mud on the sea floor
    • most likely to have contained decaying organic matter, annelids, molluscs and other bottom-dwelling invertebrates
  • fish have changed dramatically since then
    • they are now efficient predators
    • modern fish feed on invertebrates and other vertebrates
  • teeth of most fish are simple cone-shaped structures that are uniform in length (homodont condition)
    • seated in a shallow depression at the summit of the jaw bone (acrodont condition) by a cement-like material
  • fish usually swallow prey whole 
    • teeth capture and hold prey 
  • some teeth are modified to crush the shells of molluscs and the exoskeletons of arthropods
  • to capture prey, fish close their opercula and rapidly open their mouth
    • causes a negative pressure
    • pulls prey into the fish's mouth
  • some fish are filter feeders (e.g. herring, paddlefish, whale sharks)
    • long gill processes (gill rakers) trap plankton while the fish swims with its mouth open
  • fish such as carp feed on a variety of plants and small mammals
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Digestion: pt 2

  • fish such as lampreys are ectoparasites for at least a portion of their lives
  • only a few fish are primarily herbivorous
  • fish stomachs (if they have one) store large and infrequent meals
  • carnivores have a large stomach and short gut
  • small intestine is the primary site for enzyme secretion and digestion
  • herbivorous fish have a small or no stomach
    • long intestine 
      • typically coiled
      • can be longer than the fish's body length
  • sharks and other elasmobranchs have a spiral valve in their intestines
  • bony fishes have out pockets on their intestines called pyloric ceca
  • pyloric ceca increase the absorptive and secretory surfaces
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Gonads

  • sperm is called milt 
  • spawn several times = iteroparous
  • spawn once they die = semiparous
  • sequential hermaphrodites = change sex during lifetime
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Eyes and Eyesight

  • eyesight in fish is varied
    • can see in colour, UV and sometimes polarised
  • Four-eyed Fish: eyes split horizontally (each section has an iris and a retina)
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