Adaptations and Limitations of the Snapper Gill System

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.3 Plant & Animal Adaptations

Learning Intentions

By the end of this topic, you should be able to:

Discuss specific adaptations snapper have for each of the four characteristics of an efficient gas exchange system.
Discuss specific limitations of the snapper gill system, and how they limit the efficiency of gas exchange in snapper.

Glossary

Key terms and definitions.

capillaries: Tiny blood vessels that form a network surrounding the alveoli, transporting oxygenated blood away from the lungs and deoxygenated blood toward the lungs.

concentration gradient: A difference in the concentration of a substance between two areas. This characteristic is the sole driver of diffusion.

countercurrent exchange: Describes the process where two fluids flow in opposite directions, allowing for efficient transfer of gases across a barrier, down the concentration gradient.

desiccation: Drying out

gas exchange: The process of obtaining oxygen from the environment and releasing carbon dioxide

gills: The respiratory organ of fish.

gill arches: Cartilaginous rods that support gill rakers and gill filaments

gill filaments: Finger-like structures that extend from each gill arch.

gill rakers: A hard, tooth like projection from the gill arch that prevents large particles of food & other materials from passing into the gill lamellae.

lamellae: Many folds of the gill filaments, where gas exchange occurs in fish.

large SA : V: Characteristic that increases the rate of diffusion because there are more sites for gases to enter and exit the respiratory surface.

moist respiratory surface: Characteristic that increases the rate of diffusion because gases must first dissolve before they can diffuse.

operculum (gill cover): A flap of bony plates and tissue that protects the gills.

thin respiratory surface: Characteristic that increases the rate of diffusion because the diffusion distance for O₂ / CO₂ is short.

tidal ventilation: Describes how air enters and exists the system through the same way, causing new oxygen-rich air breathed in to mix with oxygen-poor residual air trapped in the system.

unidirectional pumping / ventilation: The process of pumping water in through the mouth, over the gills and out through the operculum in one direction.

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 lamellae, 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 desiccating.

Adaptations for a Large SA : V

A large SA : V 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.

Many lamellae

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 (gill cover) 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 (epithelial) cell in blue, and the capillary (endothelial) 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.

Circulatory system

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 pumping / ventilation and countercurrent exchange between blood and water.

Continuous unidirectional ventilation

Continuous unidirectional pumping / 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.

Countercurrent exchange

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 desiccate 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.

How cold-blooded sharks achieved warm-bloodedness. Also a good video to understand how being cold-blooded is a limitation.

Opah, the first known warm-blooded fish! Good video to understand how being cold-blooded is a limitation.

These are the two best videos out there covering the adaptations and limitations of the gill system. It includes a good description of why the countercurrent exchange system is advantageous, and how taking fish out of water reduces SA : V.

More on Adaptations and Limitations of the Snapper Gill System

📕 article 📸 image 🎮 interactive 🎦 video

Meet the Comical Opah, the Only Truly Warm-Blooded Fish (National Geographic) 📕