Why do most fish wiggle their tails side to side, and not up-down?

Question

Is moving a vertical fin side to side more efficient than moving a horizontal fin up and down in water for fish locomotion? The answer might lie in how fish propel themselves using vortices, but I don't know the details.

Context: I wondered why most fish evolved to wiggle their tails side to side instead of up and down. If the ocean is an isotropic environment, I wouldn't expect any preference for any particular direction.

Could there be a physical reason?

I think there is more physics than biology in my question. Similar questions have been asked before, and the tag Biophysics exists. How can ants carry items much heavier than themselves? . Does physics explain why the laws and behaviors observed in biology are as they are?

Answer 1

As suggested in the question, symmetry tells us that the caudal (tail) fin of a balanced, neutrally buoyant, underwater animal or vehicle in an isotropic unbounded aquatic environment should work equally well at any angle. This is why studies of hydrodynamical caudal fin efficiency or the fluid mechanics of oscillatory swimming can ignore the orientation of the fin.

This isotropy argument fails, however, if the symmetry is broken because of, for example:

• a preferred (typically horizontal) plane of movement,
• deviations from neutral buoyancy,
• proximity to a water-air or water-bottom interface.

In such cases it may be advantageous to have better manoeuvrability in a particular plane. Caudal fins are more effective in driving acceleration in directions at right angles to the orientation of the tail, so a caudal fin orientation perpendicular to the preferred plane of movement would usually be more effective.

Vertical tails improve horizontal manoeuvrability

As noted in @Andrew-Steane's answer, fish often prefer to move in horizontal layers of water with specific salinity, temperature, and light levels, or distance from the surface or bottom. A horizontally flexible body with a strong vertical tail exerts its forces in the horizontal plane and hence can drive horizontal changes in direction (yaw), maximizing manoeuvrability in the preferred horizontal plane.

If, has been argued, fish first evolved in shallow intertidal-subtidal environments, it is possible that horizontal mobility and hence vertical tails gave an evolutionary advantage.

Horizontal tails provide more direct control of horizontal trim

Deviations from neutral buoyancy require corrections from horizontal (or at least non-vertical) control surfaces to correct ("trim") undesired deviations from horizontal motion. Aquatic animals are generally close enough to neutral buoyancy to be easily trimmed by relatively small ancillary control surfaces (e.g. pectoral fins) and body shape, but one advantage of horizontal caudal fins is that they can provide both propulsion and horizontal trim control. Most independently evolved aquatic thunniform animals such as sharks, tuna, and ichthyosaurs, all have (or in the last case had) vertical caudal fins and pelvic fins to help control trim. Extra fins increase drag, however. With horizontal caudal fins, evolving dolphins did not need their hind pelvic limbs/flukes for trim control and have completely lost them, reducing drag.

Horizontal tails improve vertical manoeuvrability

In contrast to fish, aquatic mammals need to breathe air so they are frequently moving vertically up to and down from the water surface, so their horizontal tail is well matched since it is optimal for driving changes in vertical (pitch). This can be particularly helpful when they are moving long distances at speed and need to breathe frequently. Simply swimming on the surface is less efficient because of surface-wave drag, so fast moving aquatic mammals typically swim below the surface coming up briefly for air, or engage in energy-saving porpoising where they spend a significant fraction of their time out of the water completely. Such movements are easier with their horizontal tail.

Biological evidence that horizontal fins are not better

Finally, it is fun to note that there is a strong biological evolution argument that horizontal fins are not better than vertical fins for propulsion. Flatfish with horizontal tails are closely related to the billfish (sailfish, blue marlin, sailfish,…) that are the fastest fish on Earth. Also, some fast sharks frequently swim rolled on their side by up to $60$° in order to take advantage of vertical lift provided by their large vertical dorsal fins. There is clearly no significant evolutionary barrier preventing fish from evolving to swim on their side, so if having tails in a horizontal orientation moving vertically was significantly more efficient, this would be used by at least some fast fish.

For additional information from a biology perspective on this interesting biophysics question, see "Best tail for swimming?" and "Why is there a difference in the rotation of the tail fin in fish compared to marine mammals?".

Answer 2

Such questions are about evolutionary biology, in part, but a central fact about evolution by natural selection is that it allows structure to get gradually adapted to physical facts, and the latter are about physics. In this case the main relevant physical fact is, I think, that pressure increases with depth, and fish are well-adapted to this via their swim bladder (a buoyancy-equalising device). There are then two related matters. First, fish swim mostly horizontally. I suppose this is because the whole ocean environment changes with depth and they have a preferred depth (at any particular time of day). So they are more likely to want to nose around in horizontal turns than vertical ones. This is one reason to prefer horizontal waggling. Another reason is a bit of I guess, but I am guessing that vertical waggling would make it harder to get the buoyancy just right.

Answer 3

It’s to do with their skeletal structure. Most fish have a vertical bone structure, they wag their tails horizontally.

When I say "vertical" I mean that the bones radiate from the spine vertically, up and down, this means that the natural movement of the spine is constrained in this direction and any flexing of the spine is going to be horizontally, left and right.

Flat fish and mammals have a horizontal skeletal structure - they wag their tails up and down.

Flat fish evolved from fish with a vertical bone structure. If you look at their eyes, one of which migrated from the other side of its body, they are not symmetric. Flat fish live on the sea floor, so their horizontal structure makes sense.

SO the real question is why do fish have a vertical bone structure and land based animals, which evolved from them, do not.

That's a question for biology.