Study CommonsRead · reason · practise

Daily Read / Oceans

Oceans · Cause and effect

Why Don’t Sharks Sink?

Some sharks do sink when they stop. Find out how oil, cartilage and movement help them stay at depth.

Suggested Grades 3–4280 core-passage wordsUpdated 2026-09-18
What you’ll practiseExplain how several features work together, without confusing floating with breathing.

Device/browser voice. Pronunciation and availability vary.

Simplified vertical forces on a swimming shark: weight downward, buoyancy and an upward component of swimming lift upward.BuoyancySwimming liftWeightNot to scale
A simplified force model for maintaining depth. Arrow sizes do not represent measured forces.

Not every fish has the same equipment

A shark gliding through an aquarium seems to hang in the water. Yet many sharks would slowly sink if they stopped moving. Staying at the same depth takes more than a powerful tail.

Many bony fish have a swim bladder, an organ containing gas that helps control buoyancy. Sharks do not have this organ. Their bodies use other features instead.[1]

Oil and a lighter skeleton

A shark’s liver contains oil. Oil is less dense than water: the same volume has less mass. An oil-rich liver lowers the shark’s average density, reducing its tendency to sink. It does not turn the whole animal into a floating balloon.

Sharks also have skeletons made of cartilage rather than bone. Cartilage is firm but flexible. It is less dense than bone, so it helps reduce the weight of the skeleton. The liver and skeleton both help, but neither explains everything.[2]

Movement adds another force

Water flowing around a swimming shark’s body, tail and fins can generate lift. This motion-related force helps many sharks maintain depth. Its size and direction depend on how the animal moves and positions its body.

A leopard shark provides a useful example. It is a little denser than the surrounding water, and swimming helps keep it up. If it stops, it tends to sink. That is different from saying that it will die immediately.[3]

Staying up is not the same as breathing

Sharks need water to pass over their gills to obtain oxygen. Some species can pump water over their gills while resting. Others depend on swimming to keep water flowing.

So ask two separate questions: How does this shark maintain its depth? How does it move water over its gills? A claim about every shark should make you pause. The answer can depend on the species.[1]

Word Lab

Use the meaning, hear the word, then try it in your own sentence.

buoyancy

noun

The tendency to float or remain supported in a fluid.

The oily liver helps with buoyancy.

density

noun

Mass in a given volume.

For equal volumes, oil has less mass than water.

cartilage

noun

A firm, flexible supporting tissue.

Cartilage supports the shark’s body.

maintain

verb

To keep something at a particular level or condition.

The shark uses movement to maintain its depth.

generate

verb

To produce something.

Movement through water can generate lift.

species

noun

A biological group used to classify organisms.

Different shark species have different habits.

Check your understanding

Choose an answer, explain your choice, and then check. Hints and retries are welcome.

Cause and effect1. Why does oil in the liver help a shark?
Show answer and explanation

Answer: It lowers the shark’s average density.. Oil has less mass for the same volume. An oil-rich liver reduces average density; it does not change seawater or become a swim bladder.

Compare2. Which help depends on movement through water?
Show answer and explanation

Answer: Lift generated by the moving body and fins.. Motion-related lift is generated as water flows around the animal. Cartilage and liver oil are still present when it stops.

Inference3. You see a shark resting on the seabed. Which conclusion is best supported?
Show answer and explanation

Answer: The claim that every shark must always swim is too broad.. A resting shark challenges the universal claim. It does not tell us that all species use the same breathing method.

Try the questions here. Public practice does not save a learning record.

Put it in your own words

This draft is not saved when you leave or refresh the page.

Check your own explanation

This is self-review, not automatic marking.

Compare with a model response

Many sharks tend to sink when they stop, but that is a question about depth. Some species can pump water over their gills while resting, so stopping does not mean every shark dies.

Sources and revision notes

Source check: 2026-09-18. Grade guidance is an editorial suggestion, not a standardised reading score.

  1. Smithsonian Ocean — shark anatomy and swimming
  2. NOAA Fisheries — shark skeletons and oily livers
  3. Monterey Bay Aquarium — leopard shark buoyancy
What changed in this edition?

Clarified that some sharks do sink, separated breathing from buoyancy, and replaced estimated word counts with counts calculated from the passage.

Choose another reading