# Can a fish swim in incredibly viscous-dominated liquids?

• member 428835
In summary, the conversation discusses the possibility of a fish swimming in highly viscous liquids and whether time-symmetric motion can lead to net motion. It is mentioned that for low Reynolds numbers, time-symmetric motion in Newtonian fluids won't lead to position changes, and asymmetric motion is needed for swimming in these conditions. The concept of the Scallop theorem is introduced and sources are recommended for further understanding. The use of honey-like liquids for low Reynolds number swimming is also mentioned, with the arm stroke being a possible solution but the legs being problematic. It is clarified that honey and syrup have different levels of viscosity and the syrup used in a video is not as viscous as honey.
member 428835
I was wondering, can a fish swim in incredibly viscous-dominated liquids? I'm thinking it cannot. My first thought is to consider the Navier-Stokes equations and see that the inertial and transient terms are small, so flow is not accelerating, and thus the fish couldn't accelerate either.

Any thoughts?

You need a different swimming motion - one that is not time-symmetric. For single-celled organisms, water is a very viscous liquid, and they can swim (some of them). Something similar to the way snakes moved on the ground is an interesting option.

mfb said:
You need a different swimming motion - one that is not time-symmetric.
Could you elaborate why you need something different?

Scallop theorem
For low Reynolds numbers, time-symmetric motion in Newtonian fluids won't lead to position changes.

mfb said:
Scallop theorem
For low Reynolds numbers, time-symmetric motion in Newtonian fluids won't lead to position changes.
Could you prove this, or intuitively explain it, or direct me to a source where this is explained? I believe you but would like to understand why.

Alongside what you've said, what if the fish fins press back very fast and then reset forward very slow (not time-symmetric, right?)

The article has references, and you can search the literature for "scallop theorem".
If the motion is time-symmetric, and all equations describing the water are time-symmetric as well (low Reynolds number!), how could be there net motion? If you reverse the time direction, the motion looks the same, the reaction of the water is the same, but the swimming direction suddenly has to reverse? Moving faster or slower doesn't break the time symmetry enough. You need asymmetric motion.

Thanks for this! I didn't see the link before, but I'll check it out now.

That was still with large Reynolds numbers. You would need something honey-like to get low Reynolds numbers with humans.

The arm stroke would still work there, it is asymmetric in time. The legs are more problematic.

Syrup is very honey like it's sort of a defining characteristic of syrup
Honey is in fact a syrup
So I don't understand what you mean by that

The syrup they used in the video is certainly not as viscous as every honey I ever saw (at reasonable swimming temperatures).
Look how much it splashes around.

## 1. Can any type of fish swim in incredibly viscous-dominated liquids?

No, not all fish are adapted to swim in extremely viscous liquids. Some fish species have specialized adaptations to swim in these types of liquids, while others may struggle or even die in such environments.

## 2. How do fish swim in incredibly viscous-dominated liquids?

Fish that are adapted to swim in viscous liquids have unique features such as streamlined bodies, flexible fins, and mucus-covered skin that help them navigate through the thick liquid. They also use different swimming techniques, such as undulating their bodies or using their fins to push against the liquid, to move efficiently.

## 3. Can fish survive in incredibly viscous-dominated liquids?

It depends on the fish species and the specific type of liquid. Some fish have evolved to survive in extremely viscous environments, while others may struggle or even perish. Factors such as oxygen levels, temperature, and the fish's physical adaptations play a crucial role in their survival.

## 4. What are some examples of fish that can swim in incredibly viscous-dominated liquids?

The hagfish and the lamprey are two examples of fish that are adapted to swim in incredibly viscous liquids. Both of these species have mucus-covered skin, which reduces drag and helps them move through the liquid more efficiently. Some species of eels and catfish also have adaptations that allow them to swim in thick liquids.

## 5. How do scientists study fish swimming in incredibly viscous-dominated liquids?

Scientists use different techniques such as high-speed cameras, flow tanks, and computer simulations to study fish swimming in viscous liquids. They also collect data on the fish's physical features and behavior to understand how they are adapted to these environments. Additionally, some researchers also conduct experiments by altering the viscosity of the liquid to observe how it affects the fish's swimming abilities.

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