On page 32, Shubin describes the lungfish, specifically pointing out that it had a certain bone that we humans have, which is a humerus, an essential bone in the limbs. Shubin also mentions that "curiously, it is not just any fish; it is a fish that also has lungs." Since we just learned about the respiratory system and it's makeup in fish versus humans, why would a fish grow lungs? What is the biological or evolutionary advantage that lungs can give to an animal that gets oxygen to it's body in an already efficient way, specifically since it is always in the water? What other fish have this adaptation?
Alex Sapozhnikov marijio@gmail.com
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ReplyDeleteNormally, fish intake water through their mouth and pass water through their gills. Upon passing water through their gills, some fish contain a countercurrent exchange mechanism which is extremely effective as the flow of blood is in the opposite direction as the flow of water. Thus, the fish is easily able to allow for oxygen to pass through the barriers through diffusion straight into the bloodstream.
ReplyDeleteThe lungfish has a great adaptation. Because the fish has lungs, the fish must go to the surface in order to obtain oxygen, store oxygen within its lung, which is surrounded by veins, and then it can descend into great depths. Because it respired above water and received a good supply of oxygen, the lungfish is now able to swim and lower depths. In lower depths, the amount of oxygen is quite less than at higher depths. For example, at 100m below sea-level, there is more oxygen present than at 10000m below sea-level. With this in mind, seasons can also affect the amount of oxygen present at ocean floors. Seasonal variations cause different amount of oxygen at the bottom of an ocean because of the different climates above water, such as temperature and sometimes precipitation. Thus, because the lungfish obtains oxygen from above sea-level, it is able to descend to depths as low as the sea floor because it contains its oxygen and does not need to filter water through its gills in order to breathe. The gills alone on the lungfish are unable to support the fish at great depths. Thus, the lungs of the lungfish help the fish in a secondary action of breathing by allowing the fish to store oxygen in the lungs after obtaining it from above sea-level.
Other fish with lungs include mammals, such as dolphins and whales. Whales primarily use their lungs in order to dive to deep depths. The reason they are able to stay at deep depths for long periods of time is because their heart beats extremely slow at lower depths in order to conserve the oxygen present in the lungs.
To come to these conclusions, I have consulted the following reliable, factual sources:
http://en.wikipedia.org/wiki/Lungfish
http://www.lungfish.info/
Shreeraj Patel
shreeraj.patel1@gmail.com
Shubin says that there is one major thing that connects us to the fish. Both humans and lungfish have single bone that attach to the shoulder known as the humerus. This is what allows lungfish to actually posses what they are known for, their lungs.
ReplyDeleteShreeraj explains what the adaptation allows the lungfish to do but I believe there is a bigger picture that we must see.The fish did not tell their bodies to grow lungs. Instead, what happens is all by chance, and occurs over millions of years. Imagine a pond drying up, located near another pond. In the same way some people are stronger or taller than others, the fish with the strongest fins, and able to last longest out of water will be able to survive and drag themselves to the other pond. Here they will survive and have offspring. In the same way tall parents will often have tall children, and parents with dark hair will often have children with dark hair, these stronger fish, able to last longer out of water will have children able to do the same. When the time comes that the pond dries out again, only the strongest offspring with the ability to last longer out of water will survive.
This is how selective advantage works. A character or 'trait' is selected by the conditions of the environment, and it becomes common and exaggerated in the group of animals, because only those with the trait survive long enough to have children, and then the children inherit these traits.
Something along these lines must have occurred in order to have what we have as the modern day lungfish (http://en.wikipedia.org/wiki/Lungfish) which can breath by passing air over it's gills as opposed to water.
We can even take this a step further, evolution from fish created amphibians. Yes they were land animals, but they still need water to keep their skin moist and to breed in. Through selective advantage and evolution they were able to keep the moist skin and coexist in an aqueous environment but also further develop their lungs in order to survive on land. The step in between the fish and frog can be seen in Acanthostega (http://en.wikipedia.org/wiki/Acanthostega), in which the fins can be seen progressing into fingers and toes. So to answer to the main of your question, "what causes fish to grow lungs?" is due to chance, variation, and selective advantage caused by environmental conditions over time.
Here is an easy and good read on the evolution of lungfishes as well:
http://jmm.aaa.net.au/articles/17298.htm
Also to expand a little on Shreeraj’s ideas about whales and dolphins. I beleive that what you said about “The reason [whales] are able to stay at deep depths for long periods of time is because their heart beats extremely slow at lower depths in order to conserve the oxygen present in the lungs.” is true. People would believe that a whale’s lung capacity must be massive in order to sustain the animal for such a long time as it dives. However the truth is that the whale lung is not very large in proportion to its body. Extremely large lungs would actually cause the whale harm. The smaller lungs “minimize the the risk of problems caused by dissolved gases in the body, essentially nitrogen.” (http://www.thewildclassroom.com/cetaceans/adaptations.html)
-Adnan Jahan
(adnanjahan@gmail.com)