There was a good question and answer here: Does mermaid evolution come with buoyancy control?

But I would like to expand the question. I got off talking with a friend, it got me thinking about mermaid buoyancy. It came up that perhaps mermaids should exhibit some synergy or combination of the methods used by fish, sharks and dolphins. An interesting example was that, if a mermaid ascends too quickly, the gas from their swim bladder might empty out into their lungs or stomach, leading them to feel sick.

Reason for This

For plausible reasons this might be the case, I'll point first to the unusual nature of the mermaid granting the possibility of unusual biology. They are a naturally composite creature, so being a further composite biologically seems to build on that narrative.

More importantly, I'll point to the fact they're not exactly a whale, seal, shark or dolphin. Mermaids look pretty light, not like fat-laden sea creatures nor dense sharks. Their presumably muscular tails indicate they will not be as buoyant as humans. So it leaves the question of how they can best control/maintain the preferred buoyancy.

Advantages of Each?

There is the question of which advantages each biological method has, and how they may not mesh well or how they might mesh well.

Swim Bladders: As was mentioned in the other question post, swim bladders are good for saving up energy, if you're not spending much. Mermaids probably won't be spending just about all their time patrolling and hunting like sharks, so ways to preserve energy for low energy activities seem handy.

Large oily livers... I do not know enough about the biological implications of that to talk about pros and cons. It is intended to increase the buoyancy of the creature, not really to reach neutral buoyancy in the case of a shark. A mermaid probably can't afford to have an enormous, oily liver.

This could actually serve as a good reason for a combination. If a mermaid can't have a large enough liver for desired buoyancy, they need other factors involved. That is, assuming a small oily liver isn't much more trouble than it is worth.

Lungs and Swallowing Air: Sharks sometimes swallow air to help them counteract their density. Dolphins and seals apparently control their breath and lung capacity to aid in buoyancy control. The likelihood of these may depend on what mermaids breathe, air, water, or both. The Mermaids I'm thinking of can breathe both. If they still surface frequently, I don't see the problem with them utilizing this techniques for better buoyancy control, in what limited capacity a human-like creature can utilize them (when I tried to swallow air, it hurt).

A point of importance, is that whales apparently don't get much use out of this, because their lungs are too small relative to their great weight. They rely more on their blubber.


The most synergy I can see currently, is mermaids probably have mammal lungs capable of operating in the air, so they are free to use those lungs as divers and mammals do. Since their lungs will probably be pretty large, as humans' are, that makes them more useful for this purpose.

The other point, is simply the ineffectiveness of any one method for the diverse activities mermaids are likely to perform, as human-like creatures of the sea. Adaptability is one of humanity's real strong points, and I would hope the same to be true of mermaids both biologically and technologically.

Anyone with biological/marine knowledge who would like to weigh in?

  • $\begingroup$ It's worth bearing in mind that from a biological point of view, mermaids are certainly not fish but mammals. Their tail fins tend to match the horizontal angle found on whales and dolphins, as opposed to fish and sharks which have vertical fins. From this it is reasonable to assume that they breathe only air, and must hold their breath like other seaborne mammals. $\endgroup$
    – Cooper
    Oct 27, 2016 at 15:13
  • $\begingroup$ @Cooper They surely possess mammalian traits. But I am intending these mermaids to have non mammalian traits as is commonly depicted, such as gills. $\endgroup$
    – J. Doe
    Oct 27, 2016 at 23:20
  • $\begingroup$ In "when I tried to swallow air, it hurt", did you mean water instead? Either way, it's interesting to know that humans can breath liquids! (Gasses are compressible, liquids are almost not - important for deep diving) $\endgroup$ May 7, 2019 at 5:49

2 Answers 2


It could be possible for them to have a duel system (both gills and lungs), like lungfish.

Some lungfish are able to run the two systems independently, so when they are breathing through their lungs then the gills are closed, and when they are breathing through their gills then the lungs are bypassed.

There could be a system in mermaids where some of the gas captured through the gill system is transferred to the lungs to increase buoyancy, and if they have to ascend quickly they'd be able to exhale and equalize pressure.

They could also have an oily liver or spermaceti organ like dwarf sperm whales to get them to mostly neutral, possibly combined with some level of body fat, and then you would only need small changes in buoyancy to make a difference.

  • $\begingroup$ This sounds like the best combination. Thank you Andy. $\endgroup$
    – J. Doe
    Oct 27, 2016 at 23:38

In order of increasing probability, I would arrange them as: swim bladder, oily liver, large lungs. As in, swim bladder is the least likely/advantageous while large lungs are the most likely/advantageous.

Reasons for choosing larger lungs are easy.

  • All marine mammals use large lungs to hold their breaths for extended periods of time. The best examples being dolphins and whales.

  • Being (probably) closely related to humans/mammals, the mermaids wouldn't have gills or swim bladders since those are more closely related to fish than mammals.

  • Breathing in air and holding breath during dive has been a very successful strategy for large marine animals in all evolutionary times. For example, the ichthyosaurs, plesiosaurs, nothosaurs, placodonts and turtles, all these lineages breathed in air and held their breaths during diving. All of them were quite successful in their times.


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