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Dorsal fin helps to stabilize the creature in water much like rocket with fins, how could such a cunning apex marine predator be missing a dorsal fin? Also how do they compensate for it during high speed chase?

Note: merfolk enjoys drive hunting which demands sudden burst of speed as well as making sharp turns.

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  • $\begingroup$ Some species of whales don't have dorsal fins either. How can you explain that? $\endgroup$
    – a4android
    Oct 11, 2017 at 4:10
  • $\begingroup$ @a4android: and how fast do they swim? $\endgroup$
    – user6760
    Oct 11, 2017 at 4:20
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    $\begingroup$ I am sorry but this question is silly, and displays a huge lack of understanding how evolution works. Traits do not pop into existence because they would be useful to future generations. That is not at all how evolution works. If evolution had done what is best for the future of a species, then humans would not have had a genetic deficiency that makes us susceptible to Scurvy from lack of vitamin C instead of creating it in our bodies like so many other species, nor would we have a retina that is backwards. This sort of thing does is not a problem that needs solving. It is already solved. $\endgroup$
    – MichaelK
    Oct 11, 2017 at 6:21
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    $\begingroup$ @user6760 Then you need to read up on evolution a little bit before you ask a question about evolution. Dogs have become a separate family from wolves though selective breeding by humans, yes. $\endgroup$
    – MichaelK
    Oct 11, 2017 at 6:53
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    $\begingroup$ @user6760 That comment was a complete Non Sequitur. $\endgroup$
    – MichaelK
    Oct 11, 2017 at 13:23

2 Answers 2

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Hands

Dorsal fins largely function to stabilize against rolling. A Merfolk's hands can accomplish that function and much more. Being on wrist joints, they can pivot to allow the creature whatever degree of control they wish. Additionally, as they're attached to the useful appendages called arms, they can also be used for propulsion and other tasks as well.

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  • $\begingroup$ to achieve streamline in water I thought it would be best for merfolk to keep their arms by the sides or do the superman pose... except they don't clench their fists. $\endgroup$
    – user6760
    Oct 11, 2017 at 3:49
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    $\begingroup$ @user6760 If you're going for speed, yes. A remarkable amount of control can be achieved via small hand movements, especially if there's webbing between the fingers. Just like rockets, where the control surfaces are tiny compared to the actual vehicle. $\endgroup$
    – Andon
    Oct 11, 2017 at 4:01
  • $\begingroup$ @user6760 might I recommend checking out people who swim with monofins and mermaid tails? They have their hands out and to a point. Like the A in YMCA, but with the palms downward and not together. $\endgroup$
    – Lot-Of-Malarkey
    Oct 11, 2017 at 5:12
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An issue I thought of was placement. A dolphin's dorsal fin is kinda in the space behind where the rib cage ends. On humans that would be the small of the back. If you ever watch someone try to do a dolphin kick or swim with a monofin, they need to have a good sense of flow throughout the body. Putting a dorsal fin there may make the back stiffer and impede that.

A dolphin is very hydrodynamically shaped and the dorsal fin is basically the highest part of its body. I don't know if a human would be optimally shaped for using a dorsal fin, unless maybe it was really large. I still think that would be detrimental since that could also increase water resistance.

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  • $\begingroup$ You may want to clarify/re-word your answer. I initially thought you were answering on how a human-shaped creature would have a dorsal fin, not how a human-shaped creature is bad for a dorsal fin. $\endgroup$
    – Andon
    Oct 11, 2017 at 5:31
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    $\begingroup$ @Andon Thats fair. I've elaborated on the first part as to why that placement isn't very good. $\endgroup$
    – Lot-Of-Malarkey
    Oct 11, 2017 at 13:29

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