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The largest extant fish species is the whale shark (Rhincodon typus). With a length of 13 meters and a mass of about 20-30 metric tons, the whale shark is relatively small compared to the largest mammals, blue whales (Balaenoptera musculus, 180 tons) and fin whales (Balaenoptera physalus, 120 tons).

Is there a physical, physiological, or anatomical explanation why fish never reached the size of marine mammals?

Bigger isn't necessarily better, but I was wondering if there is a simple explanation why fish species are so much smaller than marine mammals.

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up vote 5 down vote accepted

I'd suggest that one factor could be the difficulties of breathing water - it is highly inefficient. The following is a summary of this response to a TREE article.

Fish spend 10-30% of their energy on breathing because it is so inefficient to use gills because diffusion rates are much slower and oxygen is less abundant. Gills grow in proportionate scales to body size, generally for every 100% increase in body size there is a 50-90% increase in gill size. Thus the larger fish species are working on a lesser body mass/gill size ratio so they can't take up oxygen very easily.

From this I'd suggest it is difficult for fish to grow gill systems much larger than those of a whale shark as it would become to inefficient for them thrive. By comparison whales are air breathers so, despite "breathing" (collecting fresh air) less frequently they can get much more oxygen from it.

I'm sure there is more to it but at least this is a starting idea. If anyone has more then feel free to add to/edit this answer, I merely suggest this as one possible theory and it is not my field.

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That is very interesting and definitely a possible reason. I was not aware of the fact that extracting oxygen from water was such an energy drain for fish. This might mean that larger fish species did exist at some point when the atmosphere (and the oceans) had a higher oxygen level. –  Bart Jacobs Mar 26 '13 at 8:39
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that is quite plausible: news.bbc.co.uk/2/hi/uk_news/scotland/glasgow_and_west/… –  GriffinEvo Mar 26 '13 at 8:42
    
But the metabolic rate of fish is a fraction that of mammals, so they don't necessarily require as efficient an oxygen delivery system. –  kmm Mar 26 '13 at 15:56
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Moreover mammals have four-chambered heart which makes the cardio-vascular system more organized and efficient. Basically the mixing of oxygenated and de-oxygenated blood is prevented. –  WYSIWYG Apr 8 '13 at 19:13
    
A nice point @WYSIWYG –  GriffinEvo Apr 8 '13 at 19:52
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