How Fish Breathe

By The Doctor

Breathing under water is not an easy task, so how do fish do it? Let’s have a little look

Differences

Fish respiration is the same is ours - oxygen in, carbon dioxide out. However, unlike us, fish have different ways of doing it.
First, what are the differences between air and water? Let’s compare:

Water cannot hold as much oxygen as air. Under normal conditions, a litre of air holds about 210cm3 of oxygen, but a litre of water at 10c can only hold around 7cm3 of oxygen, so anything living in water has to be good at extracting O2
Water is 800 times more dense than air - a cubic cm of air weighs far less than a cubic cm or water. Also, water is 50 times more viscous (or sticky) than air. Both these factors make it really hard to force water over gills than it is for us to force air in and out of our lungs.

To make things more complex, the O2 content of water varies throughout the world, depending on where it is - Warmer water holds less oxygen than cold, just as salt water holds less O2 than fresh. This makes it obvious than the most demanding water to breath in, is tropical saltwater. There is a big difference between a trout in a cold fresh water river, a catfish in a tropical swamp and an Angel on a tropical reef

Breathing

We all know that a fish uses gill to breath. They carry the blood into close contact with the water so that respiration can take place. They also help the fish get rid of waste products (ie ammonia) and maintain the salt concentration of the fishes body fluids (all fish have salt in their bodies just like we do). As has already been said, it is hard to breath underwater, and to do so takes a lot of  effort (approx 10% of the energy a fish gets from oxygen is used in respiration).

The gills are supported on arches, and onto these are the ’V’ shaped filaments are attached.  The filaments are supplied with blood for respiration the take place. Gill structures vary from fish to fish.

Unlike the way we breath in and out, fish do it differently. Water is drawn into the buccal cavity (or mouth) and the gill cavity. These cavities can be expanded and contracted by opening and closing the mouth and gill flap (the operculum). A ’breath’ is taken by opening the mouth to draw water in. The buccal cavity is then lifted to force the water into the gill cavity, and then in turn forced out of the gills through the operculum. These two ’pumps’ are slightly out of sync, so a constant flow of water is going

Different ways

With all the different types of fish, it is not surprising to learn that some fish have different ways of breathing to the ‘standard way’ described above, but obviously, the purpose of breathing is the same.

Some fish spend so much of their time swimming, they save energy by using ‘ram ventilation’. These fish swim with the mouth and gill flaps slightly open . The movement of swimming forces the water in the mouth and out the gill flaps. Mackerel and some types of sharks are the best example of this - if they stop swimming, they die.

Some species, like the Striped Bass, use the buccal pump method when at rest, but switch to ram ventilation when swimming at high speeds.

Sucker mouth fish, which most species live in fast flowing water, use the sucker mouth to hold onto the rocks, but allow a swift current of water to flow over the gills.

Skates, Rays and some sharks don’t have the muscular gill flap, and therefore don’t have the ‘second pump’. They have flexible flaps between each gill arch. The pressure of the buccal cavity raising and falling force the water through

Gills only?

Gills are not the only way fish get O2. If the fish is small enough, respiration can be done across the body surface. As the fish gets bigger, this type of respiration becomes not much use.

Larval fish use surface diffusion, but in adult fish, the skin may only account for 5-30% of respiration. Eels, mudskippers and some gobies use surface respiration when they are out of water.

Some fish have developed special breathing apparatus because they come from swampy areas where the O2 content is very low. Gouramis are a good example of this.

Some loaches and catfish use part of the gastrointestinal tract for air breathing. They come to the surface and take a mouthful of air and store it in this special ‘bag’.

The king of air breathers must be the lung fish. They have special folds inside the gas bladder creating a massive surface area for gas exchange. The lung tissue is very similar to our own lungs - African and South American lung fish even have a pair of lungs as apposed to only one in the Australian lungfish.

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