Support and Locomotion

Lessons

Senior 4 · Biology

Support and Locomotion

5 lessons

Lesson 2 of 5

Support and locomotion in fish

25 min

Hanging still in the water

Watch a tilapia in a clear pond or fish-farm tank. It can hang perfectly still in mid-water, neither sinking nor rising, with only small movements of its fins. Then, with one flick of its body, it shoots forward. A swimmer has to work hard to do either. Water is about 800 times denser than air and much more viscous: it holds a fish up, but it also resists every movement. Everything about a fish's body, from its shape to its gas bladder, fits it to these two facts.

Definition

Myotome (myomere): one of the W-shaped blocks of muscle along each side of a fish, separated by sheets of connective tissue attached to the vertebral column.

Swim bladder: a gas-filled sac in bony fish whose volume can be adjusted to make the fish's density equal to that of the water (neutral buoyancy).

Pitch: the nose tipping up or down. Yaw: the nose swinging from side to side. Roll: the body rotating about its long axis.

Thrust: the forward force produced by a swimming or flying animal.

Support in water

Water gives an upthrust equal to the weight of water displaced, so a fish needs a much lighter skeleton than a land animal of the same mass. Its vertebral column is flexible from side to side but cannot be shortened, so it acts as a strut for the muscles to pull against. In bony fish, the swim bladder fine-tunes the density: gas is added from the blood when the fish goes deeper (where pressure squeezes the gas) and removed when it rises, so it floats at any depth without swimming. Sharks and rays have no swim bladder; they rely on a large oily liver (oil is less dense than water) and on stiff pectoral fins that act like wings to give lift as long as they keep swimming.

Figure 1. The external features of a bony fish and what they do (simplified; myotomes are shown as zigzag lines).

Feature	How it helps the fish move
Streamlined (spindle-shaped) body, no neck	Water flows smoothly past, so drag is small
Overlapping scales pointing backwards, and slimy mucus	Smooth surface: less friction with the water
Myotomes on both sides of the vertebral column	Contract alternately to bend the body and drive the tail
Caudal (tail) fin with a large surface	Pushes against a large mass of water: main thrust
Dorsal and anal fins (unpaired)	Act like a keel: stop rolling and yawing
Pectoral and pelvic fins (paired)	Control pitch, steer, brake and help the fish hover
Swim bladder	Neutral buoyancy, so no energy is spent staying up
Lateral line	Senses water movements and pressure, helping the fish keep position and avoid obstacles

A Nile tilapia (Oreochromis niloticus) photographed from the side against a black background: a deep body covered with overlapping scales, a long spiny dorsal fin, a broad tail fin, an anal fin and a pelvic fin below. Photo: Sahat Ratmuangkhwang, CC BY 3.0, via Wikimedia Commons

Worked example 1: how big a swim bladder?

A tilapia has a mass of 500 g. Its tissues (everything except the gas) have a density of $1070\,\mathrm{kg\,m^{-3}}$ , that is $1.07\,\mathrm{g\,cm^{-3}}$ ; fresh water has a density of $1.00\,\mathrm{g\,cm^{-3}}$ . Ignoring the tiny mass of the gas, what volume of gas must the swim bladder hold for neutral buoyancy, and what percentage of the body volume is this?

Volume of tissue $= \frac{500}{1.07} = 467\,\mathrm{cm^3}$ .
For neutral buoyancy the whole fish must have the density of water, so its total volume must be $\frac{500}{1.00} = 500\,\mathrm{cm^3}$ .
Gas volume $= 500 - 467 = 33\,\mathrm{cm^3}$ , which is $\frac{33}{500} \times 100 = 6.5\,\%$ of the body volume (2 s.f.).

How a fish swims

The myotomes on one side contract while those on the other side relax; then the opposite side contracts. The contractions pass one myotome after another from head to tail, so an S-shaped wave travels back along the body and grows larger towards the tail. Each part of the body, and especially the tail fin, sweeps sideways and pushes water backwards and to the side. By Newton's third law the water pushes back on the fish. That force can be split into two parts (Figure 2): a forward thrust and a sideways part. The sideways parts cancel out as the tail beats first one way, then the other, and the dorsal and anal fins resist the remaining tendency to yaw and roll.

Figure 2. The S-shaped wave and the force on the tail, split into forward and sideways components.

Worked example 2: the forward part of the push

At one moment the water pushes on a fish's tail with a force $F = 0.80\,\mathrm{N}$ at $\theta = 30^\circ$ to the direction of swimming. (a) Calculate the forward thrust and the sideways force. (b) Repeat for $\theta = 60^\circ$ and comment.

(a) Thrust $= F\cos\theta = 0.80 \times \cos 30^\circ = 0.80 \times 0.866 = 0.69\,\mathrm{N}$ . Sideways $= F\sin\theta = 0.80 \times 0.5 = 0.40\,\mathrm{N}$ .
(b) Thrust $= 0.80 \times \cos 60^\circ = 0.80 \times 0.5 = 0.40\,\mathrm{N}$ ; sideways $= 0.80 \times 0.866 = 0.69\,\mathrm{N}$ .
The more the push is angled sideways, the less of it drives the fish forward and the more energy is wasted in sideways movement that the fins must correct. Fast fish keep the head steady and confine most of the side-to-side movement to the tail.

Worked example 3: bony fish against shark

Evaluate the swim bladder of a bony fish and the oily liver plus fins of a shark as ways of staying up in water.

Swim bladder: neutral buoyancy at any depth, so a bony fish can hover cheaply and manoeuvre precisely; but gas changes volume with depth, so rapid rising or diving needs time to adjust the gas (a fish pulled up fast from deep water may be harmed as the gas expands).
Shark: the oily liver does not change volume with depth, so a shark can change depth quickly; but it is not quite neutrally buoyant, so it must keep swimming to get lift from its pectoral fins, which costs energy.
Each suits its way of life: hovering among plants and rocks for many bony fish, continuous cruising in open water for many sharks.

Harvard Museum of Natural History: "Studying Fish Locomotion at Harvard's Robotics Lab". Watch how researchers film fish in flowing water to see how the body wave and the fins produce thrust.

Warning

Common mistakes

Saying the swim bladder is used for breathing. In most bony fish it controls buoyancy; gills are for gas exchange.
Saying the paired fins "push the fish forward". The main thrust comes from the body wave and tail; paired fins steer, brake and control pitch.
Writing that both sides' myotomes contract together. They contract alternately; together they would only stiffen the body.
Forgetting the vertebral column's job: it stops the body shortening, so contraction bends it instead.

Tip

Memory hook for the fins: "Unpaired stops the Unwanted" (dorsal and anal fins stop roll and yaw); "Paired for Pitch" (pectoral and pelvic fins: pitch, steering, braking); "Tail for Thrust".

✦

Practice: fish

State three features of a fish that reduce drag, and explain one.
Explain how the contraction of myotomes makes a fish move forward.
Name the fins that control (a) roll, (b) pitch, and say whether each is paired or unpaired.
A fish of mass 200 g has tissue density $1.08\,\mathrm{g\,cm^{-3}}$ . Calculate the gas volume needed for neutral buoyancy in fresh water ( $1.00\,\mathrm{g\,cm^{-3}}$ ).
The water pushes on a tail with 1.2 N at 40° to the direction of swimming. Calculate the forward thrust ( $\cos 40^\circ = 0.766$ ).
Explain why a bony fish must add gas to its swim bladder as it swims deeper.
Suggest why a fish's head moves from side to side much less than its tail.

Show answersHide answers

Any three: streamlined spindle shape, no neck, backward-pointing overlapping scales, mucus, fins that fold flat. For example, a streamlined shape lets water flow smoothly past with little turbulence, so drag is small.
Myotomes on one side contract while those on the other relax, then the reverse, passing from head to tail; the incompressible vertebral column makes the body bend, so an S-shaped wave runs back along it; the body and tail push water backwards and sideways, and the water pushes the fish forward (Newton's third law).
(a) Dorsal and anal fins, unpaired. (b) Pectoral and pelvic fins, paired.
Tissue volume $= \frac{200}{1.08} = 185\,\mathrm{cm^3}$ ; total volume needed $= 200\,\mathrm{cm^3}$ ; gas $= 200 - 185 = 15\,\mathrm{cm^3}$ (2 s.f.).
$1.2 \times 0.766 = 0.92\,\mathrm{N}$ .
Water pressure increases with depth and compresses the gas, so the swim bladder's volume falls, the fish becomes denser than water and would sink; adding gas restores its volume and neutral buoyancy.
The head carries the eyes and the lateral line, which work better when steady; and side-to-side movement of the front of the body would create drag and waste energy, so most bending is kept near the tail, where it produces thrust.

Quick check

Match each part of a fish to its main role in swimming.

Tap one on the left, then its partner on the right.

Key points

Water supports a fish (upthrust), so its skeleton can be light; a swim bladder gives neutral buoyancy in bony fish; sharks use an oily liver and fin lift.
Alternate contraction of myotomes against the vertebral column sends an S-shaped wave to the tail; the water's reaction gives thrust $F\cos\theta$ .
Streamlining, scales and mucus reduce drag; unpaired fins stop roll and yaw; paired fins control pitch, steering and braking.

Sources and credits

5.9.3: Fishes, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the features of bony and cartilaginous fish, the swim bladder and the lateral line. Adapted by Ganzaa.
"Fish locomotion" and "Swim bladder", Wikipedia contributors, licensed under CC BY-SA 4.0. Used for body-wave swimming and buoyancy control. Adapted by Ganzaa.
Figures 1 and 2, the table, worked examples (the masses, densities and forces are illustrative), practice and quick check: written by Ganzaa.
Photos: Sahat Ratmuangkhwang, CC BY 3.0; Warren Garst, CC BY-SA 4.0; both via Wikimedia Commons.
Video: "Studying Fish Locomotion at Harvard's Robotics Lab", Harvard Museum of Natural History, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Hanging still in the water

2Support in water

3How a fish swims

Hanging still in the water

Support in water

How a fish swims