What If? Lab
What if fish had no swim bladder?
Many fish have a little bag of gas inside called a swim bladder. It helps them float in the water without sinking. What if it was gone? Let's test it in Olly's lab!
Most bony fish carry a gas-filled pouch called a swim bladder. It makes the fish almost exactly as heavy for its size as the water around it, so it can stay at one depth without sinking or floating up. Use a floating-and-sinking model to see what happens when the gas is taken away.
The swim bladder is a gas-filled organ that lowers a bony fish's average density toward that of the water, so it can hold its depth with little effort. In adult zebrafish it takes up about 5 percent of body volume. Explore the density balance with a buoyancy model, then compare fish that live without one, such as sharks.
The big question
Look at a fish in a tank. It can stay still in the middle of the water!
It does not sink to the bottom. It does not pop up to the top. A little bag of gas inside helps. What if the fish had no gas bag?
Watch a fish in an aquarium and you may see something clever. It can hang in the middle of the water, barely moving its fins, without sinking or bobbing up.
Most bony fish can do this because of a swim bladder, a pouch of gas inside the body. So what would happen to a fish with no gas at all? In this lab, a block in a tank of sea water stands in for a fish, and you can take the gas away.
Britannica describes the swim bladder as a buoyancy organ found in most bony fish. It sits in the body cavity, is derived from an outpocketing of the gut, and holds gas, usually oxygen. It works as a hydrostatic, or ballast, organ, letting the fish keep its depth without rising or sinking.
This lab asks what that gas is worth. If a fish's body had no gas-filled space, how would its density compare with sea water, and what would the balance of forces do?
Try the experiment
Make a prediction
Olly's pretend fish floats in sea water. Olly takes its gas away. What will it do?
A block standing in for a fish floats in sea water, almost all of it under. Its gas is taken away, so it gets denser. What happens?
A 1,020 kg/m³ block floats in 1,025 kg/m³ sea water. Its density is raised to 1,070 kg/m³. What does the model show?
- It floats higher
- It sinks to the bottom
- It stays exactly where it was
| Control | In our world | What if… | Range |
|---|---|---|---|
| How dense the fish is · How heavy the pretend fish is for its size. Gas in a swim bladder makes it lighter; with no gas it is heavier. | 1020 kg/m³ | 1070 kg/m³ | 900–1200 kg/m³ |
| The water · How heavy the water is for its size. Sea water (1,025) is a little denser than fresh water (1,000), so it pushes up a little harder. | 1025 kg/m³ | — | 500–2000 kg/m³ |
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Established fact
Most bony fish have a bag of gas called a swim bladder. It helps them stay at one depth.
Most bony fish have a gas-filled swim bladder that lets them keep their depth without floating up or sinking (Britannica).
The swim bladder is a hydrostatic organ in most bony fish, usually holding oxygen-rich gas, that lets the fish maintain depth without rising or sinking (Britannica).
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Established fact
Without its gas, a fish's body is heavier than water for its size.
In adult zebrafish, the swim bladder took up about 5 percent of the body; without that space, the fish's body would be denser than water (Robertson and colleagues, 2008).
Adult zebrafish: whole-body density 1.01 g/cm³, 1.05 g/cm³ without the swim bladder's volume, which was 5.1 ± 1.4 percent of body volume (Robertson et al., 2008).
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What the model shows
In the model, the block with gas floats. The block with no gas sinks.
In the model, a 1,020 block floats in sea water with the readout showing 100 percent under. At 1,070 it weighs more than the water can push up, so it sinks.
At 1,020 kg/m³ the model gives a 0.40 m draft and 1 kg spare capacity; at 1,070 kg/m³ the weight is 2,099 N against a maximum buoyant force of 2,011 N, so it sinks. The model cannot show mid-water hovering.
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Established fact
Sharks have no swim bladder. An oily liver helps them.
Sharks have no swim bladder and are slightly heavier than sea water; oil in their livers is lighter than sea water and helps make up for it (Hawaii Division of Aquatic Resources).
All sharks are slightly negatively buoyant; oil-rich livers and a cartilaginous skeleton lighter than bone partly offset this (Hawaii Division of Aquatic Resources).
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An idea scientists discuss
A fish with no gas bag might have to keep swimming, or rest on the bottom.
A bony fish with no swim bladder might have to keep swimming to stay off the bottom, using more energy, because the gas is what lets fish hover cheaply.
Since the bladder reduces the cost of holding depth, a bony fish without one might need extra swimming effort or a bottom-living habit; how much would vary between species, which no box model can show.
In our world
Water pushes up on everything in it.
Without its gas, a fish's body can be a tiny bit too heavy for its size. Its gas bag makes it lighter, close to the water. That helps the fish stay still.
Water pushes up on anything in it. The push is as big as the weight of the water that the object moves out of the way. If an object is heavier than that much water, it sinks. If it is lighter, it floats up.
Scientists who studied adult zebrafish, a small freshwater fish, found that without the swim bladder's space, a zebrafish's body was a little denser than water: 1.05 grams per cubic centimeter. The swim bladder took up about 5 percent of the body and brought the fish's average down to 1.01, close to fresh water's 1.00.
OpenStax states the rule: if the buoyant force exceeds an object's weight it rises, if it is smaller the object sinks, and if the two are equal the object stays suspended at that depth. The buoyant force equals the weight of fluid displaced (Archimedes' principle).
Robertson and colleagues (2008) measured adult zebrafish. Mean whole-body density was 1.01 g/cm³, but with the swim bladder's volume removed it was 1.05 g/cm³. The bladder occupied 5.1 ± 1.4 percent of total body volume, significantly reducing the fish's density. Without the gas-filled space, the zebrafish body is denser than fresh water.
Take the gas away
In Olly's lab, a block plays the part of a fish.
With its pretend gas, the block floats in sea water, almost all under. Take the gas away, and the block sinks to the bottom!
Our model is a tank of sea water and a block 1 meter long. We set the block's density to 1,020 kilograms per cubic meter, a touch lighter than the model's sea water at 1,025. That is our stand-in for a fish with its swim bladder. It floats, but only just: the readout shows 100 percent under water, with room for just 1 kilogram of cargo.
Now take the gas away by setting the block to 1,070, the lab's value for a fish with no gas. The block sinks to the bottom of the tank.
The buoyancy model places a 1 m × 0.5 m × 0.4 m solid block in liquid. At 1,020 kg/m³ in sea water of 1,025 kg/m³ it floats with a draft of 0.40 m; the part-under-water readout rounds to 100 percent and the spare cargo capacity is 1 kg. Weight and upward push are both 2,001 N.
Raise the density to 1,070 kg/m³, the lab's chosen value for a gas-free fish (not a measured species value). The block's weight becomes 2,099 N, but fully submerged it can displace only 2,011 N of sea water, so it sinks. A real zebrafish without its bladder volume measured 1.05 g/cm³, or 1,050 kg/m³; in the model that sinks too.
Why it happens
Gas is very, very light. A little bag of gas makes the whole fish lighter for its size.
Take the gas away, and the fish is heavier than the water it pushes aside. So it sinks.
The water's upward push depends on how much water an object moves aside. A fish with a swim bladder takes up more space for almost the same weight, because gas weighs very little. More space means more water pushed aside and a bigger upward push.
Our model has a limit: its block can only float at the top or sink to the bottom. A real fish matched to the water hangs in the middle instead. The model's block at 1,020, floating almost completely under, is its way of showing a fish that is almost exactly as dense as the water.
Average density is total mass over total volume. Gas adds volume at almost no mass, so a gas-filled bladder pulls the average down toward the water's density. Remove that volume and the same tissue is denser than the fluid; buoyant force falls below weight.
The model's limits: it is a level, rigid box with straight sides, so it either floats at the surface with a computed draft or sinks to the floor. It cannot represent a body suspended mid-water, it ignores fins, swimming and lift, and its gas-free density of 1,070 kg/m³ is a chosen value. Neutral buoyancy in real fish is a balance the fish keeps adjusting, which a fixed density cannot show.
Fish that live without one
Sharks have no swim bladder!
Sharks are a tiny bit heavier than sea water. Their liver holds lots of oil, and oil is lighter than sea water. That helps them stay up.
Some fish really do live without a swim bladder. Britannica notes that sharks, skates and rays have none, and some bony fish that live on the bottom or in the deep sea lack one too.
Hawaii's Division of Aquatic Resources explains that all sharks are slightly heavier than sea water, so they tend to sink. Their livers hold large amounts of oil that is less dense than sea water, and their skeletons are made of cartilage, which is lighter than bone. Open-ocean sharks generally have bigger livers with more and lighter oil than sharks that live near the bottom.
Britannica lists the swim bladder as absent in all cartilaginous fish (sharks, skates and rays) and in some bottom-dwelling and deep-sea teleosts; in some species it holds oil instead of gas. Hawaii's Division of Aquatic Resources reports that all sharks are slightly negatively buoyant and offset this with oil-rich livers and a cartilaginous skeleton lighter than bone; pelagic sharks have larger livers with more and lighter oil.
A gas bladder has its own problem. Bone and colleagues (1995) note that the swim bladders of almost all teleosts change volume with pressure, obeying Boyle's law, so a fish must secrete or absorb gas to stay neutrally buoyant at a new depth. Gas can be added from the blood through a gas gland in the bladder wall.
Amazing facts
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Some fish use their swim bladder to help make sounds.
A swim bladder is not only for floating. Britannica notes it can also work as a resonating chamber to help a fish make or pick up sounds.
Besides its hydrostatic role, the swim bladder serves as a resonating chamber for producing or receiving sound (Britannica).
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In a few fish, the swim bladder holds oil, not gas.
In some kinds of fish, the swim bladder is filled with oil instead of gas.
In some species the swim bladder contains oil rather than gas, and in certain primitive fish it works as a lung or breathing aid (Britannica).
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Sharks, skates and rays have no swim bladder at all.
Sharks, skates and rays have no swim bladder, and neither do some bony fish that live on the bottom or in the deep sea.
The swim bladder is absent in all cartilaginous fish and in some bottom-dwelling and deep-sea teleosts (Britannica).
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Sharks that live out in the open ocean have bigger livers with more oil.
Open-ocean sharks generally have bigger livers, with more and lighter oil, than sharks that live near the sea floor.
Pelagic sharks generally have larger livers, with more and lighter oil, than shallow-water or bottom-living sharks (Hawaii Division of Aquatic Resources).
Words to know
- swim bladder
- a pouch of gas inside most bony fish that helps them stay at one depth
- density
- how heavy something is for its size
- buoyant force
- the upward push of a liquid or gas on something in it
- neutral buoyancy
- being just as dense as the water, so you neither sink nor float up
- cartilage
- a firm, bendy material, lighter than bone, that makes up a shark's skeleton
- gas gland
- the part of a swim bladder's wall where gas from the blood enters the bladder
Discovery challenge
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What is inside a fish's swim bladder?
What fills the swim bladder of most bony fish?
In most bony fish, the swim bladder contains…
- Gas
- Sand
- Fresh water
Show the answer
Gas
It holds gas, which is very light.
It holds gas, usually oxygen, which makes the fish lighter for its size.
…gas, usually oxygen; in a few species it holds oil instead (Britannica).
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In the lab, the pretend fish loses its gas. What does it do?
In the model, the block's density goes from 1,020 to 1,070 in sea water. What does it do?
In the model, a 1,070 kg/m³ block is placed in 1,025 kg/m³ sea water. What happens?
- It floats high
- It sinks to the bottom
- It floats half under
Show the answer
It sinks to the bottom
With no gas, it is too heavy for its size, so it sinks.
It is now denser than the sea water, so the water cannot push up hard enough and it sinks.
Its weight, 2,099 N, exceeds the largest buoyant force the sea water can give, 2,011 N, so it sinks.
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In the lab, Olly moves the floating pretend fish (1,020) from sea water to fresh water. What happens?
In the model, the 1,020 block floats in sea water. What does it do in fresh water (1,000)?
In the model, a 1,020 kg/m³ block that floats in 1,025 kg/m³ sea water is moved to 1,000 kg/m³ fresh water. Result?
- It sinks
- It floats higher
- It floats the same
Show the answer
It sinks
Fresh water is a little lighter, so it pushes up a little less. The block sinks.
Fresh water is less dense than sea water, so it pushes up less. The 1,020 block is now denser than the water, and it sinks.
Maximum buoyant force in fresh water is 1,962 N, below the block's 2,001 N weight, so it sinks.
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Sharks have no swim bladder. What helps them stay up?
Sharks have no swim bladder. What helps make up for it?
Which feature helps offset a shark's slight negative buoyancy?
- An oily liver
- Gills full of air
- Heavy bones
Show the answer
An oily liver
Their liver holds lots of oil, which is lighter than sea water.
Their livers hold lots of oil that is lighter than sea water, and their skeletons are light cartilage.
An oil-rich liver, less dense than seawater, plus a cartilaginous skeleton lighter than bone (Hawaii Division of Aquatic Resources).
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In the lab, which is the heaviest pretend fish that still floats in sea water?
In the model, which is the densest block that still floats in sea water (1,025)?
In the model's 1,025 kg/m³ sea water, which is the highest block density that still floats?
- 1,000
- 1,025
- 1,070
Show the answer
1,025
A block just as heavy as the sea water still floats, right at the top. The 1,070 block sinks.
A block exactly as dense as the sea water, 1,025, still floats, with its top level with the surface. At 1,030 it sinks.
Floating requires block density ≤ fluid density; at 1,025 the model gives 100 percent under water and 0.00 m freeboard, and 1,030 sinks.
Experiments to try
- Watch Try it What If? Lab What if water were denser? Water pushes up on anything you put in it. A heavier, denser liquid pushes up harder. In our model, a block that sinks in water floats in a pretend liquid twice as dense, with only 60 percent of it under the surface.
- Watch Try it What If? Lab What if ice were heavier than water? Ice floats because it is about 9 percent less dense than liquid water, which is unusual: most solids sink in their own liquid. Make a block of pretend ice heavier than water in our model, watch it sink, and find out why floating ice matters for life in lakes.
- Watch Try it What If? Lab What if a boat changed shape? Drop a solid block of steel in water and it sinks straight down. Shape steel into a hollow boat hull and it floats. In our model, a steel hull the same outside size as the block bobs on the water with about a third of it showing, and can even carry cargo. Find out how shape, not just material, decides whether something floats.
- Watch Try it What If? Lab What if a boat carried too much cargo? A steel boat floats because its hollow shape pushes aside lots of water. Every kilogram of cargo pushes it a little deeper. Load Olly's model boat step by step, find the most it can carry, then see what happens with 400 kilograms on board.
- Watch What If? Lab What if you could dive to the deepest trench? The Challenger Deep, at the bottom of the Mariana Trench, is about 11 kilometers down: deeper than Mount Everest is tall. Imagine diving there. Discover how strong the water's squeeze becomes, what lives in the dark, and the explorers who have already made the trip.
- Watch What If? Lab What if dinosaurs still lived today? Most dinosaurs died out 66 million years ago, after a huge asteroid hit Earth. One group survived: birds, which scientists count as living dinosaurs. Imagine the other dinosaurs had survived too. Where could they live, what would they eat, and how would they fit into today's world?
Explore next
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The blacktip reef shark is a medium-sized shark of warm, shallow coral reefs in the Indian and Pacific Oceans. You can spot it by the black tips on all its fins. It hunts fish, squid and crabs, stays close to its home reef, and gives birth to just a few live pups at a time.
- Watch Try it What If? Lab What if water were denser? Water pushes up on anything you put in it. A heavier, denser liquid pushes up harder. In our model, a block that sinks in water floats in a pretend liquid twice as dense, with only 60 percent of it under the surface.
- Watch Try it What If? Lab What if ice were heavier than water? Ice floats because it is about 9 percent less dense than liquid water, which is unusual: most solids sink in their own liquid. Make a block of pretend ice heavier than water in our model, watch it sink, and find out why floating ice matters for life in lakes.
- Watch Try it What If? Lab What if a boat changed shape? Drop a solid block of steel in water and it sinks straight down. Shape steel into a hollow boat hull and it floats. In our model, a steel hull the same outside size as the block bobs on the water with about a third of it showing, and can even carry cargo. Find out how shape, not just material, decides whether something floats.
- Watch Try it What If? Lab What if a boat carried too much cargo? A steel boat floats because its hollow shape pushes aside lots of water. Every kilogram of cargo pushes it a little deeper. Load Olly's model boat step by step, find the most it can carry, then see what happens with 400 kilograms on board.
- Watch What If? Lab What if you could dive to the deepest trench? The Challenger Deep, at the bottom of the Mariana Trench, is about 11 kilometers down: deeper than Mount Everest is tall. Imagine diving there. Discover how strong the water's squeeze becomes, what lives in the dark, and the explorers who have already made the trip.
- Watch What If? Lab What if dinosaurs still lived today? Most dinosaurs died out 66 million years ago, after a huge asteroid hit Earth. One group survived: birds, which scientists count as living dinosaurs. Imagine the other dinosaurs had survived too. Where could they live, what would they eat, and how would they fit into today's world?
For parents & teachers
What children will learn
- Objects float or sink depending on whether they are less or more dense than the liquid around them.
- A swim bladder adds gas to a fish's body, lowering its average density close to the water's so it can hold its depth.
- Sharks have no swim bladder and use oily livers and light cartilage skeletons to help offset sinking.
- A model can show the balance of forces but has limits: this one cannot show a fish hovering in mid-water.
Recommended ages: 7 to 17 · About 15 minutes
Talk about it
- Why might a little bit of gas make such a big difference to whether a fish floats?
- Some fish that live on the sea floor have no swim bladder. Why might that suit them?
- Our model's block can only float at the top or sink. What would a better fish model need to show?
Try at home or in class
- In a sink or tub, put a small empty plastic bottle with its cap on into water. Then add water a little at a time, capping it each time, until it only just floats. How little air does it need to stay up?
- Try a fresh egg in a glass of tap water, then stir plenty of salt into another glass of water and try again. Does the salty water push up harder?
Key vocabulary: swim bladder, density, buoyant force, neutral buoyancy, cartilage, gas gland
Where these facts come from
- Swim bladder , Encyclopaedia Britannica
- The contribution of the swimbladder to buoyancy in the adult zebrafish (Danio rerio): a morphometric analysis (Robertson, Lindsey, Dumbarton, Croll and Smith, Journal of Morphology, 2008) , Zebrafish Information Network (ZFIN), University of Oregon
- Skeleton & Buoyancy , Hawaii Department of Land and Natural Resources, Division of Aquatic Resources
- Swim Bladder Helps Maintain Buoyancy (quoting Liem 1998 and Bone et al. 1995) , AskNature, Biomimicry Institute
- College Physics 2e, §11.7 Archimedes' Principle , OpenStax
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.