What are teleosts?
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
What are elasmobranchs?
Rays and sharks. Subclass of the cartilaginous group of fish, which mainly differ from teleosts in having a non-calcified skeleton.
Compare water to air as a respiratory media.
- 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.
How does the density of water affect respiration?
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.
How does viscosity of water affect respiration?
- 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.
What 2 problems arise when temperature increases on energy expenditure on ventilation?
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.
Describe the 1st phase of gill ventilation.
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.
Describe the 2nd phase of gill ventilation.
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.
What maintains unidirectional flow of water during gill ventilation.
Volume change in the buccal and opercula cavities are almost in phase and maintain unidirectional flow of water for almost the complete cycle.
What is ram ventilation?
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.
What is the arrangement of branchial arches in the gills?
- 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.
What is the arrangement of lamellae in the gills?
- 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.
What does the overall arrangement of the gills allow for?
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.
Describe counter current blood flow.
- 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.
Why does counter current blood flow maximise oxygen uptake efficiency?
- 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.
Compare the oxygen consumption rate of a teleost fish with a mammal.
- 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.
What is the basis of ventilatory rate in fish?
Blood PaO2 (not PaCO2)
Why do fish base ventilatory rate on PaO2?
- 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.
Describe fish in mild and severe hypoxic water.
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.
Describe the causes and consequences of oxygen fluctuations in pond closed systems.
- 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.
Describe the causes and consequences of oxygen fluctuations in river and lake closed systems.
- 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.
Distinguish the osmolarities of freshwater and marine teleosts.
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.
Compare marine elasmobranch osmolarity with teleosts.
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.
Do freshwater teleosts drink?
- 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
