Buoyancy

  1. General considerations
    1. Advantage of depth regulation.

      2. Vertical structuring of: food, predation, temperature, oxygen, light

    2. Depth regulation = generating lift

      3. fish will tend to sink

      1. Bone: specific gravity about 2
      2. Cartilage: specific gravity 1.2
      3. Proteins also denser than water
    3. Bottom dwellers: good to be dense. Friction will keep in place on bottom
  2. One way to stay up: dynamic lift
    1. Definition: lift (positive buoyancy) generated during movement
    2. How is it done?
      1. Hold your hand out the window of driving car
      2. Pectoral fins of sharks and tuna: lifting foils
      3. heterocercal tail of sharks: lift at caudal end
      4. some fish hover by driving water down with pectoral fins
    3. Considerations
      1. High energy expenditure
      2. Have to maintain a certain speed for it to work
      3. Best for:
        1. fish that have to swim anyway
        2. bottom dwellers that don’t have to swim often
  3. Another way to stay up: static lift
    1. Definition

      2. positive buoyancy generated without muscular effort.

    2. How is it done?
      1. Density of bone, cartilage and protein close enough to water that storage of low-density materials might balance and produce neutral buoyancy
      2. Storage of gas
        1. Swim bladder (or more properly, gas bladder)
        2. To be neutral
          1. Freshwater: swim bladders need to be 7% of body volume
          2. Marine: swim bladders need to be 5% of body volume
        3. Boyle’s Law
          1. in fixed amount of gas, volume varies inversely with pressure
          2. that means that a fixed amount of gas will provide less buoyancy at greater depth
        4. Which fish have swim bladders? Teleosts
      3. Storage of fats and oils
        1. squalene
          1. mostly in squaloid sharks

            2. very large livers

            hover over bottom in v. deep waters

          2. squalene contributes buoyancy in other fishes: ratfishes, some bony fishes
        2. wax esters in coelacanths. Latimeria swim bladder has lipids, no gas!
        3. Lipids in bone
        4. Mostly a strategy used by sharks, rare in bony fishes
        5. Advantage: lift doesn’t vary with depth
        6. Disadvantage: regulation of buoyancy linked with metabolism, use of fuels. Regulation of lipid metabolism may be complex.
      4. Reduction of dense materials
        1. Reduced calcification of bones
        2. Reduced protein in muscles
        3. Deep-sea fishes
        4. Advantage: lift doesn’t vary with depth
        5. Disadvantage: restricts activity
  4. Structure and function of swim bladder
    1. Ancestral character: physostomous swim bladder (handout; also fig. 5.8, Helfman et al.)
      1. Pneumatic duct between swim bladder and gut
      2. The gulp ‘n burp strategy
        1. gulp air at surface and get down to depth
        2. burp when need to reduce buoyancy (i.e., when shallower depth desired)
      3. Limitations of physostomes
        1. difficult to maintain neutral buoyancy at any depth, bkz would have to gulp v. large quantity of air at surface
        2. some don't gulp (gas secreted into swim bladder physiologically, explained below), so you can see physostomous bladders in deep dwelling fishes; see conger eel on handout.
    2. Derived condition: physoclistous swim bladders
      1. Larval swim bladders initially physostomous, for first inflation
      2. Thereafter, inflation by secretion at gas gland, resorption at oval window. Gas is usually oxygen.
      3. How does gas gland work?
        1. secretion of gas against a pressure gradient: high concentration and partial pressure of gas inside bladder
        2. blood leading to/from gland runs through rete mirabile. Afferent and efferent flows in opposition and close together: counter-current multiplier. (handout illustrating Anguilla rete).
        3. oxygen in blood: carried by hemoglobin
          1. cells in gas gland release lactic acid; countercurrent maintaining gradient of high acid and low pH towards gland
          2. low pH: oxygen unloaded into plasma by Hb, increasing partial pressures. (handout of blood composition)
          3. Diffusion into swim bladder (slight drop in oxygen concentration expected, not seen in these data)
          4. if this expt run at higher depth/pressures, there would also be an oxygen concentration gradient maintained by countercurrent
        4. amazing facts. Eel rete 100 m.sq. area for diffusion.
      4. Resorption
        1. most of bladder lined with guanine, preventing diffusion back out of swim bladder
        2. oval window: passive diffusion back into blood here, controlled by bloodflow.
      5. Limitation of physoclists
        1. Powerful constraint on upward movement. Swim bladder will rupture if ambient pressure reduced too rapidly. Most fish can compensate at rates of about 1 meter/hour, depending on depth and temperature. Faster fish (billfish, bluefish) about 3 meters/hr.
        2. This may explain physostomous bladders in deep-dwelling fishes, who rise to surface at night to feed