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I've always liked the idea of merfolk having extra fins(by this I mean fins like the dorsal and pectoral ones and not the main tail fins). After seeing this article about how fish use their many types of fins it only interested me more. To me it seemed like they would help merfolk swim more efficiently, but after coming across this article on dorsal fins, I began to question how useful they would be to merfolk. In the article they mentioned that their hands would serve the same purpose as the fins would while in the water, but both the question and answers did not give much information to go by. The merfolk in my world will mostly be the typical half-fish-half human type as of now but some different types will have variation in their anatomy. If some types of fins could be useful, then what types would be useful and which ones would not? It would be amazing for the merfolk in my world could have these fins, but I don't want them to do more harm than good.

I hope this question makes sense. Please let me know if I left anything out that should be considered. Thanks!

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  • $\begingroup$ I would suggest you clarify what kind of merfolk exist in your world. Typical half-fish half-humans, or something else? $\endgroup$
    – Alendyias
    Commented Jul 1, 2021 at 0:16
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    $\begingroup$ Can you narrow it to a single question please as per the help center? $\endgroup$ Commented Jul 1, 2021 at 0:16
  • $\begingroup$ What are you really asking? It's your world, so the obvious answer to the title question is "yes" because whether or not they do bad or good is up to you and the rules you set for your world. Maybe a better question to ask would be "what advantages would dorsal fins give my merfolk?" In that case we could cite the use of extraneous "fins" on the wings of airplanes and explain their advantages and disadvantages to rationalize how you want to use your merfolk. $\endgroup$
    – JBH
    Commented Jul 1, 2021 at 0:31
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    $\begingroup$ In my opinion, it would make for a better question fitting the way this site works. We deal with fictional worlds, meaning the answer to any "could X happen?" question is always "yes." We focus on helping you develop and consistently use the rules of your fictional world (no one else's, asking about 3rd party or commercial worlds is verboten). We're delighted to help you work out the rules surrounding your merfolk. The trick to this site is learning to focus your questions on those rules. So, "My merfolk have X, here's my world's rules, what would X do?" is our sweet spot. $\endgroup$
    – JBH
    Commented Jul 1, 2021 at 0:46
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    $\begingroup$ @JBH thanks for telling me this. I rephrased the question and I hope that I will be able to get the answers I need now. $\endgroup$
    – Parakeets
    Commented Jul 1, 2021 at 14:49

3 Answers 3

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Your merfolk do not use hands to swim.

Let us consider the Man from Atlantis. The 1977 TV one starring Patrick Duffy, who went on to have some fame as Scuzzlebutt's leg.

man from atlantis

https://www.youtube.com/watch?v=4EI6jDeRtAs

He swims using his back and legs in a butterfly kick. His hands stay at his sides, sculling. He never reaches forward. Here is a good gif of him swimming. This method of swimming makes a lot of sense for the mammal body plan and is what cetaceans use. Patrick Duffy fairly cooked along underwater with this stroke. I cannot say this was a great show but he was a great merman.

Fins to provide stability would help Patrick, I think, possibly at the expense of his considerable aesthetics. A dorsal fin at least but one could justify additional fins as your own aesthetics deem appropriate.

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    $\begingroup$ Correct me if I'm wrong, but isn't this the scenario contemplated by the links cited by the OP? "They mentioned that their hands would serve the same purpose as the fins would while in the water..." Just playing devil's advocate. $\endgroup$
    – JBH
    Commented Jul 1, 2021 at 0:33
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    $\begingroup$ @JBH - when I swim underwater I flail around vigorously with my hands and arms. If a human swam with arms spread out to the sides that would provide a lot of lateral stability; if used for propulsion you can correct with them. But if you are barely using them like Man here, that is where the fins would come in. $\endgroup$
    – Willk
    Commented Jul 1, 2021 at 0:40
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    $\begingroup$ Yeah, but I think that's the point the OP came to after reading the linked question. Just wondering. I'll delete my comments in a couple of minutes so you have a chance to read this one. $\endgroup$
    – JBH
    Commented Jul 1, 2021 at 0:48
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Fins just need to be seen as sexy

There are many reasons for evolutionary features to evolve other than pragmatic ones.

For instance something could evolve because it is sexy. As your merfolk culture is half human, one could expect a similar emphasis when choosing a partner to be on traits that are sexually desirable, perhaps there is great respect for other fish, they appear as strong swimmers, and are thus sexier, so eventually dorsal fins would appear in response to this desire.

This is why we have colourful fish, 'flowery' fish with lots of visual spectacle. It is actually damaging in terms of practicality and pragmatism, but useful for signalling to mates your prowess.

And in many ways, the impression of your prowess is more important than actual prowess. This is called signal theory, so for instance in our culture being 'strong' isn't necessary about actual strength, but just that we 'look' strong. So this is reinforced by cultural or psychological attitudes more than physical requirements.

This theory explains:

  • Why birds have so much plumage, even if it severely hampers their movement, such as peacocks
  • Why plants are willing to sacrifice themselves to create good looking flowers, in order to appeal to insects
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Each fin needs different considerations

Pectoral Fins

Pectoral fins have the major disadvantage of being impossible with normal anatomy. There are some solutions if you're willing to break from the vertebrates:

  • Humanoid shoulders with low fins. This would require few alterations; all that should be required would be for the arm-pectoral muscles to be split from eachother to provide space for the fins, which would therefore need to use less space. One way to achieve this could be achieved by splitting the sternum into two widely spaced bones, with some internal bone-scales for protection

  • Reptilian arms with high fins. This should be easier to explain, at the cost of being less humanoid. This would require only that the arm-raising muscles don't go all the way to the fins, and vice-versa

  • One shoulder. You could also have a single pair of humanoid (or reptilian, for that matter) shoulders, which would have both the fins and the arms. This would mean that the fins and upper-arms would be linked together in motion (though you might be able to explain a little bit of independance with extra shoulder muscles)

Even so, pectoral fins wouldn't have much utility alongside arms. It is entirely plausible to just leave them out

Dorsal Fins

Dorsal fins are useful for stabilization, and easy to add. There aren't many disadvantages for a mermaid

Adipose Fin

The adipose fin is a small fleshy fin behind the dorsal fin, which has some sensory function. There isn't any reason to add it or leave it out

Pelvic Fins

The utility of pelvic fins seems to depend on how your mermaids swim: If their tail moves sideways like in fish, then they would be quite useful for stabilization. However, if their tail moves up and down like marine mammals, they most likely won't benefit (and may even be detrimented) from pelvic fins

Anal Fins

Anal fins would aid in stability, like the dorsal fin. However, due to their positions, it's likely that these fins could interfere with reproduction, depending on how it happens. They should only have anal fins if they use external fertilisation like in fish

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