Buoyancy

Buoyancy

1. an upward force exerted by a fluid that opposes the weight of an immersed object

   • neutral buoyancy = minimised energy cost of staying at a particular depth
     • fish can exert propulsion force > 25-50% of its body wieght for only a brief periods of time 
     • continuoes effort to support its body by muscular power alone would be energetically costly
     • reducing the wieght of the body makes this easier - and less costly 

2. Fr = Fb - Fw - Fd = Fnb - Fd

   • Fr = resulting force  Fw = wet weight  Fb = Buoyancy force  Fd = drag force  Fnd = Net buoyancy 

3. solutions

   • there are 4 main bouyancy adaptaions 
   • most species will adopt with a swim bladder of low-density compounds with either being combined with weight reduction measures and/or lift generation 
   • swim bladder
     • the weight of seawater is 1.026 and the weight of a fish with no swim bladder is 1.07
     • to achieve neutral buoyancy gas needs to occupy around 5% of body volume at any given depth - so the bigger you are the more you give up for a swim bladder 
     • fish remaining at one depth only need to maintain gas in swim bladder by topping up - nateral leaks 
     • the swim bladder wall - gas impermeable material  fat or guanine crystals 
     • greater depth = greater potential rate of diffusive loss  deeper living fishes have thicker layers  crystals give swim bladder characteristics silver colour 
     • there are two different types both allowing buoyancy control
       • fish swim bladders generally contain a mixture of gases including oxygen, nitrogen and carbon dioxide but in the deep sea they are predominantly filled with oxygen 
       • for fishes living near the surface, changes in the volume of the swim bladder with depth are a major difficulty 
       • this requires maximum vertical thrust from the fins in order to compensate for increased buoyancy
       • Physotomus
         • these fish have a connection to thier mouth - so have to stay near the surface and gulp in air - this is really easy and efficient however they are always linked to the surface 
       • Physoclistous
         • these do not have a Pneumatic duct - between the swimbladder and the gut 
         • over two- thirds have these types 
         • other advantages are great efficiency at higher pressures  disadvantages are they are slow to adjust
         • retrieval of fish from depth often result in the rapid pressure equilibrium of the gas causing it to expand 
         • they add in gas by chemical reactions which produces gas which then inflates the swimbladder via gas diffusion 
         • then to deflate the gas bladder the air deflates gas into the blood stream 
     • Theory
       • the density of oxygen at 0 degrees as a function of pressure calculated from boyles law, vander waals and amagats experimenal results 
       • this shows that the difference between the density of oxygen as a function of pressure calculated from amagats results and the density of seawater at the same pressure remains negative down to the maximum ocean depth of 11 km 
       • therefore an oxygen-filled swim bladder always provides positive buoyancy to fishes 
     • deep sea fish
       • because of the relative incompressibility of gases at high pressure, ascent from 6000m to 5000m for an abyssal fish causes only a 14% increase in buoyancy
       • it is probably that deep-sea fishes do not make major adjustments to their swim bladder buoyancy since minimal change in vertical thrust from the fins sufficient to maintain equilibrium 
       • great vertical excursion are trivial for these abyssal fishes 
       • an anomaly remains that it is not fully understood is how fish living at great depths can pump gas into their swim bladders. 
       • this suggests that deep sea fishes should have larger swim bladders 
       • however this isnt the case - due to lack of food and limited energy excursion they adapt other adaptations to assist the swim bladder 
       • Gas composition and skeletal reduction
         • the deep water fishes lose all excess muscules and bones just the amin backbone
         • Acellular gelatinous matrix
           • structural support and transparency one possible role proposed for gelatinous larval fishes and some deep-sea invertebrates is to allow growth to large size at low metabolic cost
           • fishes in the superorder elopomorpha have larvae called leptocephali in which most of the body consists of an acellular gelatinous matrix that provides structural support in the absence of a vertobral column and transparency for camouflage
   • low- density compounds
     • used by most sharks and some teleosts  sharks use large quantities of lipids and hydrocarbon squalene in the liver to bring the body mass to bouyant 
     • shark bouyancy is further aided by heterocercal tail fins and pectoral find with a postiive angle of attack on the leading edges 
     • teleost use low density oil mainly in the bones 
     • SECM
       • many deep - sea fish have gelatinous layer or subdermal extracellular matrix below the skin or around the spine 
       • gelatinous tissue are mostly extracellular fluid, which are inexpensive to grow. most of these secm tissues float in cold seawater - lower density than seawater and thus contribute to buoyancy in some speies 
       • gelatinous tissue can also increase swimming efficiency by fairing the transition from trunk to tAIL 
       • SECM
         • the layer of gelatinous tissue is originally quit stable - but then when taken out an exposed to air it leaks out of the skin 
         • osmolarity of 350-400 mOsm kg-1. thus less dense then seawater 
         • these low density tissues and fluids would be adaptive under the high hydrostatic pressures of the deep sea, where the inflation of a swimbladder becomes increasingly difficult
         • they have been noted in several species and mass of a fish 
         • gelatinous tissues could also act as fairing along the fishs tail, creating a better hydrofoil and improved swimming efficiency
         • inexpensive bulking up and streamlining - gelatinous tissues are found in some species with gas-filled swim bladders indicating that bouyancy may not always be their primary adaptive role
   • generation of lift
   • reduction of heavy tissues