buoyancy

Question 1

what is buoyancy

Answer 1

a force exerted by a liquid, gas or other fluid, that opposes an object’s weight
- Archimedes’ Principle

Question 2

buoyancy characteristics in Early fish

Answer 2

Placoderms (“plate-skinned”) - Class Placodermi
- Early fossil fish had heavy armour - negatively buoyant so were bottom dwellers

Question 3

what 2 things did A reduction in submerged weight allow fish to do and how was this achieved

Answer 3

1. Exploit the midwater region
2. Reduce the cost of transport
   - Reduction of heavy, dense body parts
   - Buoyancy aids

Question 4

what is specific gravity

Answer 4

the ratio of the density of a substance : density of distilled water
- Freshwater density = 1.00
- Seawater density = 1.026
- Fish muscle density = 1.050
- Cartilage density = 1.100
- Bone density = 2.000
- Average lipid density = 0.900 - less dense
- Squalene density = 0.860 - less dense

Question 5

what is Counteracting lift

Answer 5

dynamic + static lift
**Not mutually exclusive

Question 6

what is dynamic lift

Answer 6

lift generated by motion (usually forward)
- More economical at high speed - Actively seeking out prey
- Sharks use it – negatively buoyant
- interplay between Submerged weight of fish (W), Hydrodynamic lift from pectoral fins (B), lift from caudal fin (C)
- Faster sharks have almost homocercal tails - don’t need to generate as much lift, pectoral fins will be enough
- slow sharks need to generate as much lift as possible so have much bigger upper lobe

Question 7

what determines if a shark will sink or not

Answer 7

If W = B + C, Neutral buoyancy
If W > B + C, Negative buoyancy = sink
i.e. if fish stops forward movement it will sink – stalling speed depends on density of the fish itself

Question 8

what is static lift

Answer 8

having a part of you that is less dense (buoyancy)
e.g. lipids in elasmobranch, gas in teleosts

Question 9

explain static lift by lipids in elasmobranchs

Answer 9

Elasmobranchs use lipid storage (in liver) - Liver can exceed 20% of body weight - Whereas teleost liver is 1-2% of body weight
- Advantages:
-Lipid = incompressible - buoyancy not affected by depth
- Disadvantages:
-Lipid only slightly less dense than water - So need a lot: ↑ bulk = ↑ drag
-Quantity of lipid cannot be adjusted quickly - So no ability to compensate for short-term changes
- More economical at low speed - Opportunistic prey encounter

Question 10

examples of sharks and their buoyancy method

Answer 10

• Benthic (dogfish/rays) 1.075 - don’t need to be very buoyant
• Fast swimmers (blue) 1.051 - go fast enough to generate enough dynamic lift (almost homocercal tails - pectoral fins = enough)
• Very slow sharks (basking/whale) 1.030 - don’t generate much dynamic lift – rely on lipid storage
• Deep sea squaloid sharks 1.026 - neutrally buoyant due to huge livers stuffed with squalene (lipids) - gives 50% greater buoyancy than normal fish oil - But makes them very fat and sluggish, with relatively tiny pectorals as no need for them

Question 11

explain static lift by gas in teleosts and its advantages and disadvantages

Answer 11

• Teleosts use a swim or gas bladder for buoyancy – density of air = 0.001204 (much less dense)
• Advantages:
  -Gas is ca. 750x less dense than lipid – much less is required for neutral buoyancy
  -Opens up new niches
  -Quantity of gas can be adjusted quickly - able to compensate for short-term changes
• Disadvantages = boyle’s law

Question 12

what is Boyles law and its equation. how does this relate to teleosts

Answer 12

“At a constant temp. the volume of a gas varies inversely with pressure”
- p1 V1 = p2 V2
- On descent, swim-bladder will compress - Fish becomes negatively buoyant
- On ascent, swim-bladder will expand – fish becomes positively buoyant
- To maintain neutral buoyancy: more gas into gas-bladder
- Changes in volume are greatest near surface

Question 13

what’s are the 2 types of gas bladders

Answer 13

Physostomatous (phytostomes) - lower teleost
Physoclistous - advanced teleosts

Question 14

explain the Physostomatous (phytostomes) gas bladder

Answer 14

pneumatic duct between swimbladder + gut open throughout life (lower teleosts, e.g. clupeids, anguilla)
- Swallow gas at the surface to get gas into the bladder
- Some species also have a special organ to inflate bladder: gas gland (anguillid eels)
- Problem = gas effected by hydrostatic pressure – have to gulp much more air to be equilibrium at that depth – limited to certain depths
- Expanding gas can be burped out

Question 15

explain Physoclistous gas bladder

Answer 15

pneumatic duct closes early in development (advanced teleosts)
- Gasbladder not connected to alimentary canal
Physoclist fish have a gas gland to inflate the bladder – imbedded in the wall of swim bladder – very well supplied with blood
- Gas gland uses passive diffusion to get oxygen into the gas bladder lumen - Relies on differential partial pressure

Question 16

what’s the problem with The primary gas in the swim-bladder being Oxygen (O2)

Answer 16

O2 has relatively low pp
- Pp of O2 increases with depth
- Challenge at the surface = getting rid of gas + retaining gas
- Challenge at depth = obtaining substantial partial pressures to pump enough O2 into swim bladder to give neutral buoyancy
- When O2 is bound to Hb it does NOT contribute to the pp of O2 in blood
- If O2 is released from Hb it goes into solution, raising the pp of O2 in the blood

Question 17

what’s the Gas gland

Answer 17

high density of blood capillaries against SB wall

Question 18

what is Rete mirabil

Answer 18

a counter-current exchange system with large numbers of arterial and venous capillaries running counter to and in very close proximity to one another

Question 19

how does O2 Enter the gas gland to inflate gas bladder

Answer 19

1.Blood enters gas-gland through arterial capillary with oxygen primarily bound to haemoglobin (hb)
2.O2 needs to disassociate from hb and go into solution
3.Increase in pp leads to passive diffusion
4.Cells surrounding the gas gland are rich in glycogen, secreting lactic acid (anaerobic metabolism)
5.Lactic acid enters gas gland – lowers ph – haemoglobin loses O2 and gives high partial pressure that crosses into the swim bladder lumen
*It is increasingly difficult to move gas into the gas- bladder with increasing depth – need to pump more and more O2 molecules into gas bladder to maintain same volume

Question 20

4 Implications of increased acidity in gas glands that lead to diffusion of O2 into swim bladder

Answer 20

• Bohr effect: A reduction in O2-Hb affinity – causes Hb to offload O2
• Root effect: a reduction in the O2 carrying capacity of Hb
• Salting out effect: Increase in ionic content of blood reduces carrying capacity of all gases, i.e. they are less soluble – ability of liquid to hold gases in solution reduces
• Dissociation of CO2 from bicarbonates↓ pH, results in ↑ CO2 in blood - CO2 reacts with Hb + plasma proteins to produce Carbamino compounds, further reduce O2-Hb affinity

Question 21

what’s the multiplication effect of O2

Answer 21

• Not all oxygen going into solution diffuses into the swim bladder
• Excess O2 in solution is transferred to arterial capillary from venous capillary
• The longer the rete mirabile, the greater the multiplication effect

Question 22

explain how Gas gets out of the bladder

Answer 22

• Crucial for any physoclist wanting to undertake vertical movement
• Ascent increases swim bladder volume, increases positive buoyancy…. vicious cycle – can push other organs out of the fish
• The oval is a bed of venous capillaries; i.e. low pp of O2 - gas can passively diffuse out of the swim bladder into the blood – problem = diffusion is rate limited

Question 23

explain Retaining gas in the bladder

Answer 23

• cells containing sheets of guanine crystals in wall of SB reduces permeability 1000-fold (applies to physoclist and physostome fish)
• Restrict access of O2 in swim bladder to blood supply:
• Reduce blood supply in oval
• Restrict contact of oval capillaries to swim bladder gases by constriction of sphincter muscle

Question 24

why is it hard to achieve Neutral buoyancy

Answer 24

Fish with gas bladders probably rarely achieve optimal bladder volume for neutral buoyancy:
1. Any vertical movement affects the volume
2. It is potentially dangerous – too close to positively buoyant
- Vertical movement of a cod and modelled swim-bladder volume
- The fish only comes close to neutral buoyancy at the top of its vertical range

Question 25

Adaptations for vertical movement

Answer 25

• Large oval for size of swim bladder
• Large blood vessels supplying oval
• Reduce diffusion distance between swim bladder and capillaries <1μm
• (small swim bladder for size of fish)
• (partially fill swim bladder with fat)
• (surround reduced swim bladder with fat)
• Lose swim bladder altogether

Question 26

Fish that benefit from not having a swim bladder

Answer 26

Flatfish – benthic
Freshwater stream fish + intertidal fish – like to stay low and hide under rocks

Question 27

example of a group of fish that don’t have a swim bladders and its benefits

Answer 27

Scombrids (mackrels + tunas)
- allows rapid vertical movement to evade predators and capture prey
- To avoid detection by toothed whales (swim-bladders are estimated to contribute >90% of backscatter)

Question 28

Adaptations for buoyancy in deep sea fish

Answer 28

• Quantity and speed of gas secretion into swim bladder depends on the size of capillary bed - so many deep sea fish have multiple gas glands
• Deep water fish tend to have larger oval per volume of swim bladder
• Lightly ossified skeleton
• Few, if any scales
• Reduced musculature (important if weak bones)