Snapper Gill System
Concept 5: Overview of the Snapper Gill System
I can briefly describe the ecological niche of snapper.
I can describe ventilation and gas exchange in the snapper gill system.
Do Now in your books:
From what you already know, describe the ways breathing and gas exchange are similar and different in fish and mammals.
Watch these videos to broaden your understanding.
Ecological Niche of Snapper (Fish)
A fish is a taxonomic group that is characterised by a gill gas exchange system (among other things). One example of a fish is a SNAPPER.
(Remember: Ecological niche includes where animals live (their habitat) and the resources that are obtained from that habitat (like their source of oxygen)).
Snapper live in the ocean, which means they are AQUATIC animals. Because they live in water, they obtain their oxygen from the dissolved oxygen in water. Like all fish, snapper have a gill system as their gas exchange system, that has adaptations to try to get as much of the 1% dissolved oxygen in water as possible.
Snapper have high metabolic demands, which means they need a lot of energy to carry out necessary activities to survive. And this is for a couple of reasons. The first is that water is a lot denser and more viscous/thicker than air, which means it’s a much harder medium to ventilate or move over the gills. Because of this, the snapper has to spend a lot of energy on ventilating their gills. Also, because water is dense and viscous/thick, it’s difficult to swim through. So snapper need to spend a lot of energy to swim hundreds of kilometres to spawn or release their eggs.
Because water is dense, things float in water, which means water is buoyant - gravity has less of an effect when objects are in water. And finally, like air, water also contains debris like rocks and sand.
What are the parts of the snapper gill system?
The snapper gill system consists of the mouth (not labelled in the picture above), and this mouth leads to the buccal cavity, which is like a space inside the mouth, roughly shown by the green box above. This buccal cavity is important for VENTILATION.
To either side of the fish head are bony flaps called the OPERCULUM - there is an operculum on the left side and an operculum on the right side of the fish’s head. The two operculum cover the two gills, protecting them. Just behind the each operculum is a space called the operculum cavity. So there are two operculum cavities, because there are two operculums. These operculum cavities are important for ventilation.
The gills are made up several structures: the GILL RAKERS, GILL ARCHES, GILL FILAMENTS, and LAMELLAE. Gill arches are curved bony structures, that have gill rakers attached on one side, the side closest to the mouth, and gill filaments attached on the other side, furthest from the mouth.
Gill rakers are also bony structures projecting out from the from the gill arches, and they serve to protect the gill filaments from any debris in water that could damage the gill filaments.
Gill filaments are the delicate, bright red, long thin filaments projecting out of the gill arches. They kind of look like a comb or feathers. They are bright red because they contain a large amount of blood-carrying blood vessels. Each gill filament is highly folded into many lamellae.
Lamellae are very thin and contain these tiny blood vessels called capillaries. Lamellae are very important because they are the specialised respiratory surface of the gill system - this is where GAS EXCHANGE happens.
What's the journey of water in and out of the gills like?
Basically, water flows through the mouth, past the buccal cavity. From the buccal cavity, water splits in two directions - some water flows over the gills on the left side of the head, and some water flows over the gills on the right side of the head. After flowing over the gills, water exits the fish’s head through the operculum cavities - but only if the operculum is open.
This is type of ventilation is called UNIDIRECTIONAL PUMPING, because water is pumped in one direction: from the mouth, through the buccal cavity, over the gills, through the operculum cavity, and out through the operculum.
How are the lungs ventilated?
Muscles work together to increase and decrease the pressure inside the buccal cavity and the operculum cavities, causing water to enter the mouth, flow over the gills, and exit through the operculum. When I say “muscles work together to change the pressure inside the cavities”, I am referring to the muscles the control the floor of the buccal cavity, and the muscles that open and close the operculum.
Remember that the buccal cavity is the hollow mouth cavity at the front of the fish’s head. The bottom part of the buccal cavity is called the floor, which is kind of like the floor of our mouth. There are muscles that control how high or how low the floor of the buccal cavity is. If muscles lower the floor of the buccal cavity, the pressure inside the buccal cavity decreases. If muscles raise the floor of the buccal cavity, the pressure inside the buccal cavity increases.
The operculum cavities are the cavities on the sides of the fish’s head; the hollow space between the operculum and the gills. There are muscles that control the opening and closing of the operculum, which affects the pressure inside the operculum cavity. If the operculum is closed, it increases the pressure inside the operculum cavity, and if the operculum is open, it decreases the pressure inside the operculum cavity.
To let water in, the mouth opens and the floor of the buccal cavity lowers. This decreases the pressure inside the buccal cavity, which causes water to get sucked into the buccal cavity.
Then the mouth closes and the floor of the buccal cavity rises, increasing the pressure inside of the buccal cavity. At the same time, muscles cause the operculum to open, decreasing the pressure inside the opercular cavity. This forces the water to flow from the buccal cavity, over the gills, through the operculum cavity, and out through the operculum.
Concept 5 Task 1: Complete one of these worksheets on OneNote.
Worksheet 1: Sky Level
Worksheet 2: Sun Level (SciPAD Internals)
Concept 5 Task 2: Snapper Gill Dissection
Concept 6: Snapper Gill System Adaptations & Limitations
I can explain how specific adaptations of the gill system enable snapper to survive in their aquatic niche.
I can discuss the advantages and limitations of the snapper gill system.
Do Now in your books:
List all of the gill system structures you would expect see with the naked eye during our snapper dissection today.
Task 3 - Watch this video to broaden your understanding.
Overview of Gill System Adaptations
The snapper gill system needs to make sure it posesses the 4 characteristics of an efficient gas exchange system, because the gill system needs to extract as much of the 1% dissolved oxygen available in water, and get rid of as much carbon dioxide as possible.
So the gill system has adaptations for maximising the SA:V ratio of lamelle, which includes adaptations that prevent damage to the gills. There are adaptations to keep the respiratory surfaces thin, to minimise the diffusion distance across lamellae, and there are adaptations to maximise the concentration gradient across lamellae.
Those are adaptations for 3 of the 4 characteristics of an efficient gas exchange system… that means there is one missing. Moisture. The fish gill system have no adaptations for keeping the respiratory surface moist. So let’s start with that - moisture.
Adaptations for Moisture
Remember that gases must first dissolve in water before they can diffuse across the specialised respiratory surface, therefore in general, the respiratory surface must be kept moist.
But for snapper, moisture is not a problem because there’s already oxygen dissolved in the the water surrounding the fish. Snapper do not need to produce a layer of moisture on their lamellae respiratory surface because dissolved oxygen in water is already ready for diffusion. Also, snapper don’t need to produce a layer of moisture on their lamellae because the water they swim in can keep their gills moist and prevent it from drying out.
Adaptations for a Large SA : V
A large SA:V ratio is a requirement for efficient gas exchange, because the more sites for gases to enter and exit the specialised respiratory surface, the faster the rate of diffusion.
The gill system has adapted to maximise the SA:V ratio through presence of many gill filaments per gill arch, and the extensive folding of each gill filament to create many lamellae.
If gill filaments were not folded into lamellae, then the specialised respiratory surface would have less surface area, and therefore less sites for gas exchange to happen.
Any structures that protect the gills from getting damaged also contribute to a large SA:V ratio. Because if the gills get damaged by predators or by debris in water, there would be less surface area available for gas exchange.
Operculum and gill rakers
Two adaptations that prevent the gills from getting damaged are the operculum and gill rakers.
Both operculum (the left one and the right one) cover the gills and protect them from predators. Whereas gill rakers protect the gill filaments and lamellae from getting damaged by debris in the water - like rocks, sand, shell fragments and so on.
Adaptations for a Short Diffusion Distance (Thin)
Thin respiratory surface
A thin respiratory surface is a requirement for efficient gas exchange, because the shorter the diffusion distance, the faster the rate of diffusion. The gill system achieves this by having a specialised respiratory surface that’s only 2 cells thick creating a short diffusion distance.
In this diagram, you can see that there are only two layers of cells that separate the dissolved oxygen from the blood inside the capillary cell - the lamella cell in blue, and the capillary cell in red. For oxygen to diffuse from blood to water, it only has to cross one capillary cell and then one lamella cell. For oxygen to diffuse from water to blood, it only has to cross one lamella cell, and then on capillary cell. This short diffusion distance helps maximise the rate of diffusion and gas exchange.
Snapper are not as large as humans, but they are still relatively large animals compared to insects like crickets. So passive diffusion of oxygen from gills to other body cells would take far too long because the diffusion distance would be too great.
Snapper need a circulatory system to pump oxygenated blood to cells that need it for aerobic cellular respiration relatively quickly. And the opposite happens to carbon dioxide. If these gases were not pumped around the body by the heart, the diffusion distance from gills to cells would be too far and would take too much time. With the circulatory system, the diffusion distance is greatly reduced because passive diffusion would only have to be across very thin cell membranes.
Adaptations for Maintaining a Steep Concentration Gradient
A steep concentration gradient is a requirement for efficient gas exchange, because diffusion is solely driven by a concentration gradient. So the steeper the concentration gradient, the faster the diffusion, and the higher the rate of gas exchange.
There are two adaptations that increase the concentration gradient across the gill respiratory surface. The first is continuous unidirectional ventilation, and the second is countercurrent exchange. The effectiveness of gas exchange in some gills is increased by continuous unidirectional ventilation and countercurrent exchange between blood and water.
Continuous unidirectional ventilation
Continuous unidirectional ventilation describes the continuous one-way flow of water. Water is almost always flowing in through the mouth, out through the gills. It does not go in through the mouth and out through the mouth again. Unidirectional ventilation is the opposite of tidal ventilation we see in the human lung system, where air flows in through the mouth and out through the mouth again.
Continuous unidirectional ventilation is an advantage to fish, because it means that the gills always receive new oxygen-rich water, and that this new, oxygen-rich water does not mix with old stale oxygen-poor water. This means that the concentration of gases in the water that makes it to the respiratory surface remains the relatively high. This is contrasted with what we see with tidal ventilation, where new and old air do mix, and reduces the the concentration of gases in the air that makes it to the gas exchange surface.
Gills are incredibly efficient because they can extract around 80% of the dissolved oxygen in water. This is largely due to an adaptation called countercurrent exchange. Countercurrent exchange is where water flows through over the lamellae in one direction, but blood flows through inslide lamellae capillaries in the opposite direction.
The diagram on below clearly shows the advantage of countercurrent exchange. The blue part represents water, the pink part represents blood, the large arrows represent the direction of water and blood. Water and blood are flowing in opposite directions, with water flowing to the right, and blood flowing to the left - hence countercurrent exchange.
The numbers represent the oxygen level in either water or blood, and the small white arrows represent gas exchange.
As you can see, with countercurrent exchange, oxygen levels are always higher in the water than in the blood over the entire length of the lamella. A concentration gradient is maintained across the entire length of the lamella. This means that there is gas exchange over the entire length of the lamella, as shown by the white arrows spanning the entire length of the lamella. This in contrast to the white arrows stopping in the middle of the length of the lamella in this bottom diagram.
The bottom panel shows what happens without countercurrent exchange, if water and blood flowed in the same direction. Without countercurrent exchange, oxygen levels are only higher in the water than the blood for part of the lamella. A concentration gradient is only maintained for part of the lamella, and there is a portion of lamella where there is absolutely no concentration gradient. In this part of the lamella, there is nothing driving diffusion. Therefore gas exchange only happens for half the length of the lamella without countercurrent exchange.
Go to this website for a great animation on countercurrent exchange: http://www.kscience.co.uk/animations/anim_3.htm
Limitations of the Gill System
There are 2 limitations of the gill system:
1) Incompatibility with air
One of the most obvious limitations of the gill system is that it cannot exchange gases with air, because air is not dense or buoyant enough, and it is dry. The gill system is only efficient in water because the buoyancy of the water helps keep lamellae and gill filaments apart. In air, the gill filaments and lamellae would collapse due to gravity and stick together, drastically reducing the surface area available for gas exchange. In air, the gills would also dry out because fish do not have any adaptations to stay moist.
2) Cold bloodedness
Another limitation of the snapper gill system is that they are cold blooded. This is for 2 reasons. The first is that any warmth in the blood is lost to the cold water passing through the gills through heat conduction.
The second is that the limited oxygen concentration in water can’t support the high energy demands of a warm-blooded animal.
The consequence of being cold-blooded is that snapper cannot swim as fast and as far as other warm blooded fish or animals can. Check out these links below, to explore the discover the usefulness of warm-bloodedness in fish.