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Animals with lungs (such as most terrestrial mammals) cannot breathe if submerged in water, and soon suffocate.

Whereas, fish can variously breathe air (at least for few-hours, as some fish frequently need to get air periodically), including those that are bigger than a human (say sharks).

But why? Do fish require less oxygen than lunged terrestrial-animals, despite being active and variously engaging in activities like predation? (so that uptaking only dissolved oxygen, and probably less surface area of gill than lung-alveoli is enough for them?)

Tahlor
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Always Confused
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    at least for few-hours in case of some fishes those have frequent need to come to air for a while... They are not fishes; they are mammals (cetaceans). – WYSIWYG Aug 07 '16 at 19:26
  • @WYSIWYG Maybe the confusions are due to I don't know any terminology for the phenomenon where fishes comes-out towards upper-surface of the pond-water at a certain interval of time (hours). This is why I had used the phrase 'at-least few hours'. In my place (West bengal, India), a very hot place, fresh-water-ponds it happens. Probably it is not due to predation, because aquarium-fishes sometime do that. – Always Confused Sep 10 '16 at 15:17
  • @WYSIWYG ...contd... There was an experiment in class 5 and 6 textbook (though I never performed it)... "take a fish that has additional respiratory system, Such as Heteropneustes fossilis/ Clarias batrachus/ Anabas testudineus and lock them in water inside a wire grid and within several hours the fish will die due to lack of oxygen". – Always Confused Sep 10 '16 at 15:18
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    Perhaps you can add a picture (of the fish in West Bengal that you are referring to) and the explanations that you gave in the comments (along with relevant references) to the question. It may clarify the question a bit. Don't use comments for additional explanation. Nobody will go through your comments. Moreover, comments are not indexed and are therefore not searchable. – WYSIWYG Sep 10 '16 at 16:50
  • You probably know the colloquial name of the fish. Do some search and try to find some images online. Also, you can search the fishes described in your book and see if they indeed show such behaviour. – WYSIWYG Sep 10 '16 at 17:05
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    Please add these details in the question for the reasons already mentioned. Plus comments are not for extended discussion; this conversation has been moved to chat. – WYSIWYG Sep 10 '16 at 17:38
  • Go to this one – WYSIWYG Sep 10 '16 at 17:47
  • Gills usually require constant movement and need the support of water to work well. Lungs are usually inflated and deflated by muscles, which doesn't work in water due to the density of water. Also the amount of oxygen in water is much lower and insufficient in particular for warm-blooded animals. – martinkunev Apr 03 '19 at 18:10

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Misleading sentences in your question

fishes can breathe in water (at least for few-hours)

Fishes can breathe for much longer than a few hours under water as they spend their whole life underwater.

Animals-with-lungs (such as most terrestrial mammals)

All mammals, whether terrestrial (like a cow) or not (like a dolphin) have lungs. While all mammals have lungs, most fish (but not all) have gills.

Lungs vs Gills

To understand the answer, you need to understand two key organs involved in gathering oxygen from the environment: gills and lungs.

Lungs are organs that allow the transfer of oxygen into the blood when inhaling air, while gills are organs that allow the transfer of oxygen into blood when inhaling water. Lungs don't do their job properly in water and gills don't do their job properly in the air. You should have a look at the following wikipedia entries to further your understanding of these organs:

Energy consumption

You say

does fishes require much-much less-amount of oxygen than lunged terrestrial-animals, though they stay in physical activities including predation?

I am not sure I understand the meaning of this sentence but anyway.

Making generalization about fish metabolic rate can only be misleading as there are an immense diversity of fishes. For example some fishes are endotherm (see here to understand the definition of this term) while some are not and this will vastly affect the metabolic rate.

Although eventually too advanced for the OP, Chabot et al. (2016) is an interesting reading to get an general overview of the Metabolic rate in fishes.

Remi.b
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    Small addition: some salamanders have gills. Also, another important point is that deep diving cetaceans have high concentration of myoglobin (or perhaps high oxygen affinity; don't remember that now. There was an article about that perhaps a year or a two, back) so that their muscles can store oxygen for a long time. I am not sure if all huge aquatic animals have this adaptation. – WYSIWYG Aug 07 '16 at 19:27
  • Cetaceans means dolphins and allied groups, google says. They are mammals. So their lungs doesn't take oxygen from water. They breathe in air and hold their breathe when they're under water. – Always Confused Aug 07 '16 at 19:45
  • A fish maintains a baseline of respiration through-out body mass, plus do respiration to obtain energy for task like predation, fly-away etc. that probably needs a lot of energy (so a lot of respiration so lot of oxygen), and in case of a massive shark probably greater than human. – Always Confused Aug 07 '16 at 19:50
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    The only fruitful part of this answer (yet) is "Lungs don't do their job probably in water and gills don't do their job properly in the air" . However it is here not explained why gill would not work in air and lung would not work in water. – Always Confused Aug 07 '16 at 19:53
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    @AlwaysConfused read the linked Wikipedia articles, as suggested by this post's author. Lungs evolved to move oxygen from gaseous air to the blood, while gills evolved to move dissolved oxygen from water to the blood. Lungs don't work in water, because they didn't evolve to work in water, and vice versa for gills. There's nothing else to explain. It's like asking why fins on a fish don't work for flying through the air - they didn't evolve to do that. – MattDMo Aug 07 '16 at 20:54
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    @AlwaysConfused (1) as MattDMo said, please just read the wikipedia articles on gills and lungs to have a better understanding of their physiology. (2) if then you have any question that are specific to the physiology of gills and lungs (your current question is not), then please open a new post, explain what you understand about this physiology and what you don't. Good luck! – Remi.b Aug 07 '16 at 21:30
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    @ Remi.b "Lungs don't do their job properly in water and gills don't do their job properly in the air." . But why? also I didn't found yet in wikipedia articles. Wikipedia tells how lungs work on air ... but not why it fail work in water (or opposite for gills). – Always Confused Sep 10 '16 at 15:24
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    @AlwaysConfused The question was edited. The original question was not so focus how on the physiology of lungs and gills. A question should not be modified after an answer has been posted. My opinion is that it would be better to roll back your edit and ask your specific question about the physiology of lungs and gills on another post. – Remi.b Sep 10 '16 at 15:39
  • @Remi.b thanks very much for your response I'm trying to improve it. – Always Confused Sep 10 '16 at 15:51
  • Anything other suggestions are welcome – Always Confused Sep 10 '16 at 15:51
  • Some crabs breathe air using gills. The linked wikipedia articles don't explain well how these organs work (and I'm quite sure this is the expected response). Also, "read wikipedia" is not an answer, but avoiding to give one. "because they evolved so" is not an explanation, but a mere statement that natural selection is a fact. – martinkunev Apr 03 '19 at 18:18
  • @Remi.b Terribly worded and confusing questions should absolutely be edited. It seems like it is for this precise reason there exists the option to edit them. Answers should similarly be updated. StackExchange should not exist to archive questioners and answerers talking past each other. Pointing out that fish can survive more than a few hours breathing in water is funny but not altogether productive. – Tahlor Mar 19 '21 at 19:04