STADIA / LIFE IN MOTION
A whole world beneath the surface.
Fish, drifting animals, ships and submarines.
Water supports a body while resisting its motion. Compare the ways animals swim or drift, then follow the same questions of support and control into sailing and diving machines.
Animal biology
Fish do not all solve buoyancy alike
Buoyancy: compare, do not generalise
- Many bony fish
The gold region symbolises a gas-filled swim bladder.
- Sharks
Sharks lack a swim bladder. Their anatomy offers a different buoyancy strategy.
- Observation
Body angle, fins and movement all deserve attention.
Conceptual fish silhouettes and organ symbol, not anatomical sections. Buoyancy strategies vary between species. [1]
Many bony fish use a gas-filled swim bladder to help regulate buoyancy. Sharks lack this organ. Their oily livers and relatively light cartilaginous skeletons contribute to buoyancy, while movement and body shape also matter. One model cannot explain every fish. [1]
When observing a fish, notice what changes and what stays still: tail, fins, body angle and position relative to the water. Describe these features without assuming that an animal which stays in one place is doing no work.
Which observations would help distinguish swimming forward from controlling depth?
More ways to move
Pulses, jets and arms
Pulsing, beating, jetting
- Jellyfish
Bell contractions contribute to movement.
- Comb jelly
Rows of beating cilia provide propulsion; this is a different animal group.
- Cephalopod
Water expelled through a funnel provides jet propulsion.
Simplified symbols. The jet arrow indicates expelled water; actual funnel direction can change. [2] [3]
Jellyfish and comb jellies are different groups. Jellyfish can move by contracting their bells; comb jellies use rows of beating cilia. Currents can carry both. “Drifting” therefore need not mean an animal has no capacity for active movement. [2]
Cephalopods include octopuses, squid, cuttlefish and nautiluses. Expelling water through a funnel can produce jet propulsion. Bottom-dwelling octopuses also use their arms to move across the seabed. Different actions can serve different circumstances. [3]
Why might one animal use a different form of movement when escaping than when exploring?
Human engineering
Travelling on—and under—the water
On the surface and below it
- Sailing
Hull, sails, rigging and crew form a working system.
- Taking in water
Water admitted to ballast tanks helps a submarine submerge.
- Displacing water
Compressed air can drive water out of the tanks and increase buoyancy.
Conceptual ballast comparison. Arrows indicate water entering or leaving ballast tanks, not the vessel’s speed. [4] [5]
A sailing ship brings together a hull, sails, rigging and the work of its crew. Cutty Sark’s preserved rig offers a way to investigate how a large sail area was managed. A ship is both a machine and a record of people’s skills. [4]
Submarines use ballast systems to change buoyancy. Taking water into ballast tanks helps a submarine submerge; displacing that water with compressed air helps it become more buoyant. This introductory principle is only one part of controlling a real vessel. [5]
What is different about staying afloat, changing depth and travelling forward?
A FEW USEFUL WORDS
Build your vocabulary.
Buoyancy
The upward force exerted by a fluid on an immersed body.
Jet propulsion
Movement produced by expelling a stream of material in the opposite direction.
Ballast
Mass used to adjust a vessel’s stability or buoyancy.
THE CONVERSATION CONTINUES
Which would you like to understand better: a fish holding its depth, an octopus moving, or a submarine diving?
Choose one example. Tell us what you noticed, why it matters to you, and what you would like to understand next. Share a source when making a factual claim. Contributions in your preferred language are welcome.
Discussion takes place in the existing Stadia Community. Sign in to contribute; its access and moderation rules apply. The questions on this page are editorial prompts.
FOLLOW THE EVIDENCE
Sources & Further Reading
Selected museum, university, scientific-society and institutional sources. Main reading consulted 20 October 2026; illustrations added 21 October 2026. Sources retain their original languages and may be updated by their publishers.
- Smithsonian OceanSharks (opens in a new tab)
Shark anatomy, swimming and buoyancy compared with bony fish.
- Smithsonian OceanJellyfish and Comb Jellies (opens in a new tab)
Different animal groups, movement and relationships with currents.
- Smithsonian OceanCephalopods: Octopus, Squid, Cuttlefish, and Nautilus (opens in a new tab)
Jet propulsion, movement with arms and buoyancy.
- Royal Museums GreenwichCutty Sark rigging (opens in a new tab)
Sails, rigging and the practical work of operating a sailing ship.
- US Naval Undersea MuseumBallast and Buoyancy (PDF) (opens in a new tab)
An introductory explanation of ballast tanks and buoyancy.
Original illustrations are educational or decorative schematics, not anatomical reconstructions or scale drawings. No external photographs are reproduced. Linked sources and their images retain their own rights. Suggest a correction in the Community, including a source.
CONNECTED EXPLORATIONS
Follow another route
Keep the question moving.
Across the Elements
What changes when the surroundings change?
Explore Across the Elements↗Nature and Human Ingenuity
Myth, evidence and the work of invention.
Explore Nature and Human Ingenuity↗Life in the Air
From drifting seeds to controlled flight.
Explore Life in the Air↗