The Snapper Gill System (+ snapper gill dissection)

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:

Explain aspects of the snappers ecological niche, that is relevant to gas exchange.
Describe the parts of the snapper gill system and where gas exchange takes place.
Explain how the snapper gill system is ventilated.

Glossary

Key terms and definitions.

aquatic: To live in water.

breathing / ventilation: Movement of air/water in and out of the gas exchange system.

buccal cavity: The space inside the mouth and throat area of a fish.

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

ecological niche: The functional position of an organism in its environment (the environment being the habitat and the resources available in this habitat).

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.

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

operculum cavity: The space just behind each gill cover of a fish.

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

Ecological Niche of Snapper (Fish)

Fish are a taxonomic group characterised by a gill gas exchange system (among other things). One example of a fish is the 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 that could potentially damage the gills.

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 (gill cover) - 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.

Above: This is an excellent gill dissection video. It talks about ventilation, and the structures that water passes through. It identifies most gill structures, and clearly demonstrates how gill filaments have the greatest surface area in water.

Above: This is a silent video animation taking you through a 3D model of the fish gill system, including the circulatory system.

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.

More on The Snapper Gill System

📕 article 📸 image 🎮 interactive 🎦 video