So how exactly does a fish breathe underwater?
It’s a good and valid question to ask. Since the beginning of time, humans with their land-adapted internal lungs have always been fascinated by a fish’s affinity for being able to take in oxygen underwater. The first obvious point to note, in this case, is that unlike humans, most fish do not have lungs.
But what about dolphins and whales?
First of all, they are not classified as fish, but fall under the Class Mammalia, Order Cetacea. All Cetaceans breathe through blowholes- holes at the top of their heads that are homologous with nostrils in other mammals. The existence of the blowhole demonstrates that dolphins and whales have no need for gills for oxygen uptake, but have small lungs located behind the liver and directly positioned in front of the spinal chord. While the Cetacean is diving, pressure builds up in its lungs. This pressure is then released once the animal rises to the surface of the water in order to expel water and air from its surroundings, as well as carbon dioxide and mucus; byproducts of metabolic processes.
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So why don't most fish have lungs?
The term ‘fish’ in itself is a common umbrella term for a wide variety of organism classified in the Animalia Kingdom and in the Chordata Phylum. In evolutionary terms, it is therefore difficult to track the evolution of fish because there is such a wide variety of organisms that are grouped together under that umbrella term. However, it can be concluded that most fish never had a need to develop lungs due to the sheer inconvenience of having gas exchange occur internally. Consider this: Not only does the fish take in oxygen through inhalation, but water also. The structure of human lungs is such that, if we were to breathe in water, it would be extremely difficult to then remove that water from the lungs. Not only would we suffer from Pleural Effusion at first, but over time, the rest of our lung capacity would fill with water to the point where we would die, essentially from drowning. This is the same for a lot of fish who live in environments where having lungs would severely decrease their chance of survival.
In that case, how does gas exchange occur?
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Fish are able to breathe underwater thanks to gills, positioned on either side of the pharynx. Contrary to popular opinion, the flaps of skin and scales that can be observed on a fish’s exterior, located near the head, are not the gills. These are simply the opercula (or ‘gill covers’), which complete the function of protecting the gills. The slits that can be observed at the ends of the opercula are also used to expel water, carbon dioxide and excess oxygen once the oxygen-rich water has been passed over the gills and gas exchange has occurred. The gills themselves are the structures that lie underneath the opercula. They are comprised of threadlike structures called filaments, supported by serrated branchial arches: calcium-based structures often arranged in a semi-circular shape in order to support the filaments. Often, the gills come in compressed sets of three and are a deep red in colour. This is because the filaments are comprised almost completely of capillaries, in order to maximise the rate of gas exchange and to have the largest surface area possible over which oxygen and carbon dioxide can diffuse. Much like human capillaries, the walls are extremely thin to allow for a short diffusion gradient. In many fish, the blood flow in the capillaries is opposite to the direction in which the water is flowing, and as a consequence, a countercurrent exchange is created. This is a naturally occurring mechanism whereby a property is transferred between two bodies, flowing in opposite directions to each other. In the case of gas exchange in fish, the two bodies are the blood flowing in one direction in the capillaries and the water flowing in another, and both gases (oxygen and carbon dioxide) and heat are the properties being transferred.
Are there exceptions to the 'no lungs' rule?
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Not all fish depend solely on gills for gas exchange, and some fish have evolved to the point where their gills are too small to be their only mechanism for gas exchange. There are around 12 genera of fish that are considered amphibious fish. Some of these, such as the American and European eels, have skin so permeable that oxygen can diffuse directly across the skin and into the bloodstream. Though amphibious fish mainly reside in water, they are able to stay out of water for extended periods of time, and they can be crudely split into two categories: facultative air breathers and obligate air breathers. While facultative air breathers rely primarily on their gills but are able to survive in air or oxygen-deprived water if they must, obligate air breathers must breathe in air periodically or risk death by suffocation. The African Lungfish is an example of an obligate air breather operates in a manner similar to a dolphin or a whale: while it has gills, it also has small internal lungs. Because of this, it must rise to the surface for inhalation, but will then pass carbon dioxide and excess oxygen out through the gills. Amphibious fish are thought to be examples of divergent evolution. Divergent evolution occurs when differences between groups of organisms accumulate, thereby leading to the formation of new species. This can be the result of migration of the same species to different and isolated environments, since the gene flow among the distinct populations will be restricted. Due to this, amphibious fish are an obvious link between solely water-based fish and land animals.
Why do sharks always look permanently surprised?
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We’ve all seen the shutterstock photos or the David Attenborough film footage: no matter what species of shark seems to be represented, it always seems to look surprised. This is because all sharks have their mouths open when moving. Unlike smaller fish such as Groupers (Epinephelus lanceolatus), Koi (Cyprinus carpio haematopterus) and Goldfish (Carassius auratus), sharks are unable to use suction power to draw water in and pump it over their gills. It isn’t hard to notice that Koi and Goldfish exhibit the jaw movement necessary to take in water, both while at rest and while moving. They use this to create a pressure difference in the surrounding water, so it is enters their mouths and passes out again over their gills. Not only are sharks’ gills located further away from their head than those of most fish, but in evolutionary terms, their jaws are unable to move as flexibly. They are able to provide this suction while at rest, but while moving, they become ram ventilators. They rely primarily on the flow of water in a certain direction, and by moving against that, they maximize their oxygen uptake. Thus, their mouths are open almost constantly while they move. A small number of shark species are obligate ram ventilators- they cannot take water in even while at rest, and thus need to move constantly in order to breathe and live. Having to move all the time in order to survive? Seems like a hellish existence to me.
For more information visit:
http://science.jrank.org/pages/5841/Respiratory-System-Respiratory-system-fish.html
http://aquariadise.com/what-is-the-labyrinth-organ/
https://en.wikipedia.org/wiki/Fish
http://www.darwinwasright.org/divergence.html
For more information visit:
http://science.jrank.org/pages/5841/Respiratory-System-Respiratory-system-fish.html
http://aquariadise.com/what-is-the-labyrinth-organ/
https://en.wikipedia.org/wiki/Fish
http://www.darwinwasright.org/divergence.html