Have you had problem explaining to people that you are not a fish even when they are hell bent that you are a fish because according to genetics everything with a backbone is a fish but then what other thing defines a fish. People who say a fish is anything that lives in the water, breath oxygen, does not have limbs rather it has fins, and is a vertebrate having a skeleton either bony or cartilaginous.
Looking at evolution, the first vertebrate was a fish, and so all vertebrates are fishes but then according to evolution, we share more ancestral origin with the bony fish than the cartilaginous ones like sharks. With this explanation, what happens to aquatic animals with the name fish but do not have a bone like Jelly fish, and Star fish? The answer is simple, they are not fishes. I guess that is a little funny right? Well, animals like Sea Horse and Sea Dragon are actually not horses and dragon, they are fishes. Let me crack you up! Have you heard of the animal Silverfish? Well it is not a fish, rather it is a tiny bug.
Now that we have justified that, another thing that amazes people is how fishes get oxygen in the water when humans cannot do the same. This has to do with the gills in fish but it is more complicated than that. In fishes, breathing has to do with diffusion. The gills of fishes are vascularized, thereby allowing much blood flow into them and with that, when water enters through the gills, oxygen enters into the bloodstream through diffusion where it is less concentrated.
Carbon dioxide from the gills diffuses out from higher concentration to the lower concentration in the environment and oxygen from the higher environment goes into the gills with lower concentration of oxygen. When fishes have gills with larger surface areas, then they will live in low oxygen environment so as to be able to get all the oxygen possible from the environment while if they have a smaller surface area gills, then they will live in high oxygen environment.
While some fish do not live for a long time, there are some that can live for a very long one, like the Greenland Shark which is the longest living fish and vertebrate specie known currently can live between 300 to 500 years and this is because their metabolism a well as growth is very slow compared to any other fish. Greenland Sharks grow 1cm yearly and this helps them outlive other fishes. The age of these fishes are known using carbon dating of protein which is found in their eyeballs. We can say that while we outlive a lot of fishes, there are a number of them that outlive us.
Scientist are trying to understand how a particular fresh water fish produce milk to feed its young after birth. Tha Arapaima is the largest fresh water fish with its early ancestor being of the Osteoglossomorpha super order who processed bony tongues. Arapaima gets 90% of its oxygen from the atmosphere because they possess a swim bladder which helps them to suck air in before it is diffused into its vessels.
Its no doubt that Fishes are fun, and while there are a lot of fishes we haven't studied, the ones we know of are very fascinating and there are a lot to learn about them. So the next time you are told that you are a fish, you should understand that they are referring to your ancestry genetically.
Post Reference
https://www.nps.gov/articles/blue-in-the-face-breathing-underwater.htm
https://journals.biologists.com/jeb/article/211/22/3519/17747
[https://oceanservice.noaa.gov/facts/greenland-shark.html](https://oceanservice.noaa.gov/facts/greenland-shark.html#:~:text=The slow metabolism could explain,re over a century old)
https://news.mongabay.com/2013/07/forgotten-species-the-arapaima-or-dinosaur-fish/
https://www.sciencedirect.com/science/article/abs/pii/S0968432812001023?via%3Dihub
https://www.sciencedirect.com/science/article/abs/pii/S1742706114001664?via%3Dihub milk
https://www.fws.gov/sites/default/files/documents/FOTW_Arapaima_transcript.pdf
