# Sea creature moving via rotational motions

So we all know that the majority of sea creatures use fins that move either from side to side or up and down to propel themselves.

But our boats don't do that. Our boats use propellers - a rotational motion not based on sea life at all.

That begs the question: how plausible is it for a sea creature to evolve to "spin" in the water as a method of movement?

For example, imagine a fish that had a body containing the shape of a screw propeller (perhaps multiple screw propellers?). It would move through the water by "corkscrewing" itself forwards.

# What scenario would result in this sea creature evolving, and as a bonus, how efficient is the corkscrew fish movement compared to the current method of locomotion?

NOTE: Creature must be large enough to be visible to the naked eye (no microscopic organisms allowed) But if you want to say that the creature evolved from a microscopic organism that already uses said approach to movement if you can make it plausible, feel free.

EDIT for clarity: You may assume that there are an abundance of nutrients available. Optimal growing conditions, you can even assume that this guy has no predators.

Explanation for reality check tag: it says "if a concept is realistic in a given context" to which the context here is whatever it needs to be to make it happen

• There is a good vsauce video on why land animals don't have wheels, and I think it deals with the same issues – Orfby Apr 27 '15 at 20:51
• @orfby There's a million ways to say why something won't happen, based on the circumstances of the world it belongs to. But the whole point of this is to fathom up a circumstance where it IS possible. – Aify Apr 27 '15 at 20:53
• If so, why does the question have the 'reality-check' tag? I made that comment assuming the tag was indicative of the question, but I'm sorry anyway for the useless comment – Orfby Apr 27 '15 at 21:07
• @orfby Explanation for reality check tag: it says "if a concept is realistic in a given context" to which the context here is whatever it needs to be to make it happen - I apologize for the confusion (but I won't edit out the tag for fear of invalidating answers). I had the same issue with worldbuilding.stackexchange.com/questions/14807/… – Aify Apr 27 '15 at 21:10
• fish evolved streamline body and fins for a good reason, they use less energy to move through water and create less noise too. Your "corkscrew marine lifeforms" will need to drill into thick skin to reach the meaty parts of the animal or to confuse other predator or maybe they prefer to mate in an unusual way for reproduction. – user6760 Apr 28 '15 at 9:56

Symbiosis easily bypasses natures "no continous circular motion" law (i.e. nothing that requires a hub and wheel).

You've a scavenger animal type thing with at least with two feet. You've a vaguely propeller shaped coral/plant, that filter feeds. The scavenger/crab/mollusc/? lives in the hollow core of the plant, gains protection (like a hermit crab).

The crabs flight reflex sometimes spins the vaguely propeller shaped plant which moves them both away from predators, and faster than the scavenger could move on their own. We now have an evolutionary advantage.

Fast forward a million years. You've free floating plants with extremely well designed bodies for "spinning" locomotion through the water, each with at least one (possibly a colony?) of descendants of the crabs, running like mad inside.

The crabs can pop out to feed, everyone benefits with protection and locomotion for the crab, and locomotion for the plant.

So now you have swarms of sea life that propel them selves through the sea by spinning like propellers.

## Update

You could have several species of these evolve. I'm picturing a predator which extends its head and body out in front of the propeller part. It's many legs have evolved to fit inside the plant and spin it around. The whole assemblage is torpedo like.
Eyes and mouth are forward facing like a shark, and it steers by flexing its body.

• How are crabs living inside providing propulsion? They're running like a hamster wheel? Because I don't think that will work at all underwater. – Samuel May 2 '15 at 19:27
• @Samuel: I suppose it depends on the mass of the hamster v's the mass of the wheel. – Binary Worrier May 5 '15 at 8:46
• Not really. The hamster will just keep running around the inside of wheel. If the hamster is very dense then the whole thing will just sink in the water. – Samuel May 5 '15 at 15:24
• @Samuel: Are you milling a ":)" there? Lets say we have an water breathing hamster, with a neutrally buoyant wheel. The hamsters presence will cause the hamster+wheel combination to sink, unless the spinning wheel causes propulsion, where in it'll move through the water. The hamster can move towards the left of the wheel and provide lift. – Binary Worrier May 5 '15 at 15:29

Macro animals aren't going to do this.

Single-celled creatures can do this because they don't need to supply the rotating body component with nutrients, those come in from the surrounding medium. The flagellum used by some bacteria is essentially a biological rotary engine. It likely evolved from a type three secretion system, which has very similar components.

Macro scale is much harder to accomplish. Spinning parts don't exist on the macro scale. Having the entire body as the corkscrew doesn't work well because they still need some way to push to get spinning in the first place. You've probably seen octopuses doing something that looks like corkscrewing through the water, but they use jet-propulsion to get themselves going and sometimes spin while doing it.

To have a body part that spins freely is a problem. It's not well connected to the rest of the body. Blood vessels, tendons, muscles, and whatever can't be used between the main body and spinning portion.

It's not more efficient to rotate for macro animals. If you look at gymnoformes, they are sort of (from one frame of reference) partially rotating to achieve locomotion. But rotating oneself in order to propel forward is like adding an unnecessary energy conversion step.

However.

I can imagine some small animals that might use a external component, like a shell, which they would spin to propel themselves through the water. It wouldn't be very efficient and would probably only work in calm waters.

• The entire body doesn't have to be a corkscrew, I only said that it should use said mechanism to move - even if it just has a tiny propeller on the tail, if you can make it work then it's fine. Anyways, I've update the question to note that this specifically must be a macro scale creature (I already knew that microscopic ones used it, I want to know if it would be able to be scaled up). – Aify Apr 27 '15 at 17:37
• TIny = subcellular here. You can't do it with anything large. – Oldcat Apr 27 '15 at 19:07
• The part about using a shell counts as answering the question; +1 for that. – Aify Apr 27 '15 at 20:27
• @Aify, one can easily imagine a long muscular appendage that works like a flagella in water, simply by waving it around in a circular pattern. The thing is, that particular critter is probably going to figure out pretty quickly that he just has to wave it back and forth in a single plane for it to work, maybe undulating like an eel or a snake. – Sean Boddy May 2 '15 at 2:53

I can't see an evolutionary path to why this would happen, but I believe that physiologically, it is possible.

Consider the human arm. The ball and socket joint at the shoulder allows for full circumduction of the arm (imagine winding up and pitching a softball or bowling a cricket ball). If we have an underwater creature with a couple of appendages of the right shape and in the right places, and joints with the correct articulation, I don't see why it isn't possible.

However, I don't see what the evolutionary benefit of this would be. This seems to be something more like a mad scientist would piece together. I don't know what the thrust of such a mechanism would be, but I can imagine the energy requirements for sustained locomotion of the Frankenfish would be quite high.

### Edit:

I hate saying that something is not possible. After all, this is worldbuilding. So, I'll try to make a case for evolution of rotational locomotion in a macro-organism. Incidentally, the mechanism of movement is different than what I described above.

As sea levels have been steadily rising and land area has been steadily decreasing, land mammals, which had previously evolved onto land, are now evolving back into water. Some animals (amphibians, reptiles) are obviously better equipped to deal with this than others. But there is one interesting case to consider: the field mouse.

It started as field mice had to swim more often than they used to, from one piece of land to another. The mice that ended up surviving more often than not were the ones that were better swimmers - a trait that became primarily determined by their tails. Eventually, mice ended up spending more time in water than out of water. Their physical features changed over the centuries to reflect their aquatic lifestyles: their feet shrank to reduce drag, they could hold their breath for incredible amounts of time, they had special eyelids so they could see well underwater, and their tails grew powerful as a flagellum-like appendage for movement.

• @Aify, I've edited my answer to try to provide an evolutionary case for your question. – Seth Apr 27 '15 at 21:44

I'm sorry, but the answer is 'no'. Physics and scaling are important:

Flagella are quite unefficient: in order to rotate them, most of the force is applied perpendicularly to the direction of motion, and it costs work to compensate all that friction. For Seth: An evolved rat would swim, just like any other mammal, because it's way more efficient (and a minor evolutionary change, too). [I dont' know if this answers your bonus question. If I had more time I would throw you the relevant equations]

So, why do bacteria use cillia and flagella instead of swimming? The answer is the Reynolds number: a number that describes how important are inertial effects in fluids. Reynolds number depends on the size, so bacteria in water have lower Reynold number than rats in water. In other words, bacteria struggle to move in water as you would do to move in honey. Swimming strokes are symmetric if the time is reversed, and that means that no net motion is possible without inertia (Purcell called that the Scallop theorem). To overcome this, bacteria use movements that are not invariant under time reversal, like rotating flagella (seen in reverse, you would see the flagellum rotating the other way). The extra energy cost is not a big deal for bacteria (they live surrounded by enough resources), but is a problem for a rat or other macroscopic beings.

• "Swimming strokes are symmetric if the time is reversed" - this is not true in general. The scallop is a special case, and even then there is no true time reversal symmetry. One half of its swimming stroke is the opposite motion to the other, but performed much faster. If it was the same speed, the animal would indeed make no progress through the water. – Hugh Allen Apr 28 '15 at 15:55
• @HughAllen Having no inertia implies that, for net motion, is irrelevant: if you eliminate inertial terms from the Navier-Stokes equation, it doesn't depends on time anymore. Slower or faster doesn't matter at all. Again, I recomend you Purcell's Life at low Reynolds number – Bosoneando May 1 '15 at 18:15
• Even if you ignore variations in speed, most swimming strokes are not what Purcell calls "reciprocal motion". He refers to it as "One special kind of swimming motion" - not the only kind, and not even the most popular kind. So your statement that "Swimming strokes are symmetric if the time is reversed" is false. It is only sort of true for the scallop, which has a particularly simple shape - two rigid pieces held together by one hinge. – Hugh Allen May 2 '15 at 2:07

An animal that lives inside of two shells, positioned end to end, each shaped like a corkscrew or propeller, could rotate the shells on their common axis to propel forward. The animal itself wouldn't need to have a rotating part, it could just have pseudopods that grip the inside of the shell and twist, like an octopus letting itself out of a jar.

Imagine if the lid to this jar was propeller shaped: https://www.youtube.com/watch?v=IvvjcQIJnLg

Some microscopic organisms use the corkscrew approach. Here's a page with some information and video about them. I don't know how practical that would be to scale up, considering the complexity of the brain, dizziness & sense of up and down, relative thickness of the material (water) they're moving through, etc. But corkscrew critters do exist.

• The issue is that for a tiny critter, feeding the corkscrew part of yourself is as simple as dumping nutrients into the gap for the cells to absorb. More complex, large critters need actual arteries and veins to move food in and waste out, and these will get twisted and torn by the spinning motion. – Oldcat May 5 '15 at 0:10
• If my memory (from a few decades ago) was that the corkscrew is a separate piece of the organism, dead like hair or a fingernail, residing in a socket. However, I see the problem now: that piece would most likely wear down over time, and without a circulatory system it wouldn't regenerate. So another option would be that the corkscrew is its own creature, working symbiotically with the main creature like a Portuguese Man 'o War (which is a colony of creatures working symbiotically, if memory serves!). – BrettFromLA May 5 '15 at 4:37

I'll give the 'no' answer to this...the most obvious answer is locomotion techniques that involve the entire body are going to be quicker than a little prop hanging off the creature.

1. The propeller isn't a terribly efficient method of transportation. Without engines to spin these props, they don't move too fast and accelerate horribly slow...a creature with a propeller would be pretty open game in any open water. The swimming motion creates much faster acceleration.

2. Propellers were designed specifically for boats that can concentrate a huge amount of power in a small area (incredibly small area if you consider how small the prop is to the boat it's powering). I can't think of any creature that could sustain the rotations a prop would require over the time needed, nor am I aware of any creature that dedicates such a tiny portion of it's body to locomotion

3. High speed spinning prop = damaging. Props are made of strong metals and even these frequently chip. A high speed propeller on a creature would be very prone to damage and any attacks that damage it would cripple it heavily.

One of those cases where I think it's quite likely feasible that it could exist, but it's so inefficient that they would easily as prey to species possessing better locomotion

• So what if the environment didn't have any predators, they had an abundance of nutrients, etc? Grr, I seem to have the same problem with answers on this question as I do on my other one about evolving 2 heads. I know it's not likely, I want a scenario where it IS likely. – Aify Apr 27 '15 at 20:43
• @Aify - your question is "That begs the question: how plausible is it for a sea creature to evolve to "spin" in the water as a method of movement?"...if you aren't looking for plausibility but instead looking for best scenario that this could come about, then state that in your question please. I did try, my first thought was the Cambrian explosion where a few odd locomotion forms came about before being over-run by more efficient methods...but there isn't any I can find, it's just not an efficient method of locomotion beyond microscopic. – Twelfth Apr 27 '15 at 20:59
• I apologize, the actual question was hidden in the paragraph after the "begs the question" statement. I've bolded and resized the question to be much more... blatant. – Aify Apr 27 '15 at 21:03
• @Aify - lol that works. I still can't get this to work in anything we'd consider natural...to get reliable movement, this corkscrew needs to turn faster than any sea creature could generate and I can't find any natural precedence to refer to. In hindsight, it's the same issues I saw trying to design a propellered bird. The fish swimming motion is far superior in every measure I could define and you have to get such a specialized and isolated environment for this to occur. The only real environment I can see this rising is in a laboratory unfortunately. – Twelfth Apr 27 '15 at 21:21