Fish

Question 1

What are teleosts?

Answer 1

Ray-finned fish with calcified skeletons, which make up around 96% of the 30,000 or so fish species. Enormous variety in form and adaptations

Question 2

What are elasmobranchs?

Answer 2

Rays and sharks. Subclass of the cartilaginous group of fish, which mainly differ from teleosts in having a non-calcified skeleton.

Question 3

Compare water to air as a respiratory media.

Answer 3

• Oxygen content of water is far lower than in air.
• Water is more variable than air and can get to very low levels
• Most terrestrial animals will be breathing air of a consistent 20.9% oxygen, unless they are burrow animals/live in enclosed spaces
• The diffusion rate of oxygen in water is far lower than in air so diffusion will be slower and will be effective over shorter distances.
• So oxygen can only be extracted from water that is a short distance from the gas exchange surface.

Question 4

How does the density of water affect respiration?

Answer 4

The density of water is about a thousand times greater than air and when coupled with the lower oxygen content means that over 20,000 times the mass of water is required compared to air to provide the same oxygen content.

Question 5

How does viscosity of water affect respiration?

Answer 5

• Viscosity of water is substantially greater than air, so more frictional forces need to be overcome to move water compared to air.
• So, fish will typically be consuming more than 10% of their overall energy expenditure on ventilation of their gills, compared with 1-2% in similarly sized animals.

Question 6

What 2 problems arise when temperature increases on energy expenditure on ventilation?

Answer 6

Metabolic rate increases as temperature increases. This increases the energy requirement, to be met by aerobic metabolism. But as temperature increases, the solubility of oxygen in water decreases, so the oxygen content is lower and increasingly more water needs to be ventilated to get enough oxygen to meet the increased metabolic requirement.

At high temperatures the fish has to cease all activity as all the oxygen being extracted is being used to ventilate the gills.

Question 7

Describe the 1st phase of gill ventilation.

Answer 7

Skeletal muscle pumps increase the volume of the buccal cavity and the opercular cavities, while the presence of valves ensure the unidirectional water flow. The operculum is sucked closed and the mouth is opened, so water flows across the gills from the buccal cavity into the opercular cavity and water flow into the buccal cavity through the open mouth.

Question 8

Describe the 2nd phase of gill ventilation.

Answer 8

Skeletal muscle pumps decrease the volume of the buccal cavity and opercular cavities. But this time the mouth is closed, so the water in the buccal cavity is forced across the gills into the opercular cavity and the water in the opercular cavity is forced out via the open operculum.

Question 9

What maintains unidirectional flow of water during gill ventilation.

Answer 9

Volume change in the buccal and opercula cavities are almost in phase and maintain unidirectional flow of water for almost the complete cycle.

Question 10

What is ram ventilation?

Answer 10

Some species can save energy by stopping ventilatory movements when swimming fast. By swimming with their mouth continuously open they can force water to flow continually across the gills in ram ventilation.

Question 11

What is the arrangement of branchial arches in the gills?

Answer 11

• There are 4 main branchial arches enclosed by a gill cavity.
• Each branchial arch supports 2 rows of gill filaments stacked in parallel and overlapping with the gill filaments from neighbouring branchial arches.

Question 12

What is the arrangement of lamellae in the gills?

Answer 12

• The lamellae are thin sheets of tissue, forming the gas exchange surface, which project from either side of the gill filaments.
• Together the lamellae from adjacent fill filaments in the stack from narrow channels, only 20-50mm wide and 0.2-1.6mm long through which water flows.

Question 13

What does the overall arrangement of the gills allow for?

Answer 13

Maximises the surface area available for gas exchange, while minimising the distance between the water in the channels and the gas exchange surface to overcome the limitation of the shorter diffusion distance in water.

Question 14

Describe counter current blood flow.

Answer 14

• The blood entering the gills is deoxygenated.
• Although the water is encounters has had most of the oxygen removed from it during its flow through the gills, there is a still a atrial pressure gradient to maintain a diffusion gradient for exchange of oxygen into the blood.
• Blood loads up with oxygen as it flows through the channels in the lamellae but it is always flowing past water with a higher partial pressure of oxygen, maintaining the diffusion gradient for exchange of oxygen.

Question 15

Why does counter current blood flow maximise oxygen uptake efficiency?

Answer 15

• So gills maximise the uptake of oxygen from water by their large surface area, small diffusion distance and the maintenance of a concentration gradient between the water and the blood.
• The concentration gradient is also maximised by the relative flow rates of water and blood.
• Water is flowing across the gills at 10 times the rate of blood flow.
• These flow rates are sufficient to maintain about a 50mmHg partial pressure gradient across the gas exchange surface.

Question 16

Compare the oxygen consumption rate of a teleost fish with a mammal.

Answer 16

• Resting oxygen uptake of the teleost will vary with temperature, as metabolic rate varies.
• But it will be typically less than half of the resting O2 uptake of the mammal.
• Despite this lower rate of oxygen utilisation, the teleost is extracting more than 80% of the available oxygen from the water compared with about 25% extracted by the human.
• Fish do not have a problem is getting rid of CO2 due to its high water solubility and high diffusion gradient.

Question 17

What is the basis of ventilatory rate in fish?

Answer 17

Blood PaO2 (not PaCO2)

Question 18

Why do fish base ventilatory rate on PaO2?

Answer 18

• Primarily due to the high solubility of CO2 in water, which means that most of the CO2 is lost as the blood crosses the gills and there are relatively low levels in the arterial blood.
• And oxygen levels are much more variable in aquatic environments than they normally are in terrestrial environments, with the exception of burrows.

Question 19

Describe fish in mild and severe hypoxic water.

Answer 19

Mild: fish will increase its activity to escape to either water with a higher PO2 or water of a lower temperature to limits its metabolic rate and so oxygen requirement.

Severe: gill ventilation is increased to increase the rate of oxygen delivery and non-essential activities, such as feeding and breeding cease.

Certain fish, like carp, are very tolerant of hypoxia and will happily overwinter in the muddy bottom of lakes in hypoxic conditions with minimal energy expenditure.

Question 20

Describe the causes and consequences of oxygen fluctuations in pond closed systems.

Answer 20

• Water oxygen levels are particularly likely to vary in closed systems such as garden ponds.
• A well-balanced pond will have lots of pondweed, aquatic plants and algae that release oxygen into the water during daytime.
• But at night, photosynthesis stops but respiration continues and the plants and algae will remove oxygen from the water.
• Particularly a problem for fish on a warm summer’s night when the atmospheric pressure is low.

Question 21

Describe the causes and consequences of oxygen fluctuations in river and lake closed systems.

Answer 21

• Receive farm runoff or when excessive nutrients are added to the water, causing eutrophication.
• There is also a potential problem with prolonged ice cover.
• This is a barrier to exchange of gases with the atmosphere, leading to reduced oxygen content and build-up of toxic gases, such as ammonia.

Question 22

Distinguish the osmolarities of freshwater and marine teleosts.

Answer 22

Freshwater teleosts will have a tissue osmolarity of around 250-350mOsm/l, but they live in water that is only 1mOsm/l, so there is a large osmotic gradient for water influx and they are hyperosmotic to the environment.

Marine teleosts have a tissue osmolarity of around 400mOsm/l, but they live in sea water that is around 1000mOsm/l so they have an even larger osmotic gradient driving water efflux and they are hypoosmotic compared to the environment.

Question 23

Compare marine elasmobranch osmolarity with teleosts.

Answer 23

Although both freshwater and marine teleosts are out of osmotic balance with their environments, marine elasmobranchs maintain a tissue osmolarity of around 1050mOsm/l so are essentially in osmotic balance with their marine environment and only slightly hyperosmotic.

Question 24

Do freshwater teleosts drink?

Answer 24

• They do not need to as they are continually absorbing water from their freshwater surroundings
• They overcome the continual influx of water across their gills by the continual production of very hypoosmotic urine

Question 25

How do freshwater teleosts balance salt content of the blood?

Answer 25

• Even though they have a very efficient proximal tubule reuptake mechanism, they are still losing salt via their urine.
• To replace this loss, absorb Na+ against a steep concentration gradient from the extremely low levels found in freshwater.
• This is achieved by the pavement cells that form a thin layer covering the gill lamellae.

Question 26

Do marine teleosts drink?

Answer 26

• Are continually losing water across their gills and have to drink seawater to replace the water that is lost.
• Although the gills are impermeable to salt, it will be absorbed via the GI tract, along with the 70-80% of water that is eliminated in the urine.

Question 27

How do marine teleosts balance salt content of the blood?

Answer 27

• Produce hyperosmotic urine, as their kidneys lack loop of Henle.
• Instead, they secrete excess sodium and chloride ions via specialised cells in the gills and opercular epithelium called chloride ions.

Question 28

Do marine elasmobranchs drink?

Answer 28

No

• Maintain a tissue osmolarity close to the surrounding sea water
• Marine invertebrates have a body fluid composition that is essentially identical to seawater in its ionic constituents.

Question 29

What do only marine elasmobranchs do to nitrogenous waste?

Answer 29

• Convert to urea, which is retained highly efficiently by their kidneys due to a specialised reabsorption mechanism, although some is still lost across their gills.
• High level of tissue urea increases the osmolarity of the tissue fluids.

Question 30

How can marine elasmobranchs retain such high levels of urea that would be harmful for most vertebrates?

Answer 30

They produce high levels of the organic osmolyte trimethyl amine oxide, which in addition to contributing to the osmolarity of the tissue fluid also protects against the protein denaturing effects of the high urea concentration.

Question 31

How do marine elasmobranchs balance salt content of the blood?

Answer 31

• They have much lower levels of sodium and chloride ions than the surrounding seawater and will gain some sodium chloride across their gills and across the GI tract with ingested food.
• Cannot produce hyperosmotic urine but are bale to secrete their excess sodium chloride via specialised cells in tehri rectal gland that function in a similar way to the chloride cells in gills of marine teleosts.

Question 32

Distinguish teleosts and elasmobranchs in terms of nitrogenous excretion.

Answer 32

Elasmobranchs are ureotelic and convert nitrogenous waste ions urea, which is mainly retained by the kidneys with some excretion occurring via loss from the gills.

Teleosts are ammoniotelic and excrete nitrogenous waste in the form of ammonia. Not only does this require less energy than conversion to urea, it is an effective mechanism of excretion due to the solubility of ammonia in water results in rapid diffusion rate.

Question 33

How can nitrogenous waste be excreted via the gills in carp?

Answer 33

• Membranes are impermeable to the ammonium ion, but highly permeable to free ammonia.
• Diffusion of ammonia across the gills causes further dissociation of the ammonium ion to provide further ammonia to diffuse across the gills.
• 90% is in the from of ammonia
• 10% as the ammonium ion in exchange for Na+
• Ammonia that is diffused across the gills recombines with hydrogen ions transported across the gill epithelium.
• This traps the ammonium ion in the water and prevents free ammonia from building up.

Question 34

How does water pH affect ammonia build up.

Answer 34

If the pH of the water is too high, then there are insufficient hydrogen ions to combine with the ammonia and free ammonia builds up in the water. This decreases the diffusion gradient across the gills, limiting the ability of the fish to excrete the ammonia, which can build up to toxic levels.

Question 35

What is hydrodynamic lift?

Answer 35

Fish that are denser than the surrounding water/negatively buoyant, can generate hydrodynamic lift from their fins and body shape.

Question 36

How do fish maintain neutral buoyancy with lower density materials?

Answer 36

• To reduce their overall density of their body to match that of the water around them.
• Saves energy and is easier for fish with a non-calcified cartilaginous skeleton such as marine elasmobranchs, as their cartilaginous skeleton is not much more dense than the salt water surrounding them.
• Elasmobranchs store low density oils such as squalene in their liver and muscles.
• Fats and oils are incompressible, but a large volume is required to make much difference to overall buoyancy.

Question 37

How do swim bladders aid buoyancy in teleosts?

Answer 37

• Relatively small volume of gas, which has a much lower density to compensate for the weight of their skeleton.
• This gas is normally mostly oxygen and is accumulated into the swim bladder.
• Smaller volume is required compared to oils and fats.
• But volume varies with external pressure as gas is highly compressible.

Question 38

Why does the variation of swim bladder volume with depth make it an unstable system?

Answer 38

If a fish with a swim bladder migrates up or down in the water column it will need to either expend energy by swimming back to prevent an uncontrolled change in buoyancy, or change the total amount of gas in eth swim bladder to maintain the same swim bladder volume despite the change in external pressure.

Question 39

Name the 2 types of swim bladder.

Answer 39

Physoclist is closed
Physostome is open

Question 40

Describe the physostome swim bladder.

Answer 40

• If atmospheric pressure increases or they want to descend, these fish can gulp air at the surface to maintain the volume of gas in their swim bladder at the increased pressure.
• If they are unable to reach the surface, they add more gas to their swim bladder by transfer of oxygen from the circulatory system by means of the gas gland, but this takes time.
• If atmospheric pressure decreases or they ascend, these fish can vent gas form their bladder via the oesophagus and mouth to maintain the volume of their bladder as the pressure on it decreases.

Question 41

Describe the physoclist swim bladder.

Answer 41

• Rely on oxygen accumulation into the swim bladder from the circulatory system via the gas gland and gas reabsorption into the circulation, which is achieved at a vascular structure known as the oval.
• But this addition and absorption of gas takes time and limits speed of vertical migration.
• A rapid reduction of pressure by about a 3rd is sufficient to burst the swim bladder of these fish, so if they are trawled from depth they are unlikely to survive even if they are thrown back into the sea.

Question 42

What is a positively buoyant fish?

Answer 42

Float at the surface, often upside down as seen here in the goldfish. Due to too much gas in their swim bladder possibly by gulping too much air at the surface during greedy feeding or something blocking their ability to vent gas via the oesophagus.

Question 43

What is a negatively buoyant fish?

Answer 43

• Persistently remain on the bottom of the tank, often on their side.
• This may be due to a congenital condition in which their swim bladder is too small for tehri size, which occurs more frequently in fancy ornamental goldfish due to selective breeding.

Question 44

What could be another cause of buoyancy issues?

Answer 44

A build up of fluid in their swim bladder or impairment of their gas gland secretory activity due to infection, which can be seen on radiographs.