Week 8 · 1-9 Effect of Water Pressure on Organisms: pressure inside a liquid, P = Pa + Pwater, swim bladders, skeletons, cell membranes; 1-10 Ecological Balance and the Role of Humans in Sustaining Aquatic Life · 3 lessons
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Living under pressure: swim bladders, skeletons and membranes
Integrated Science Sec 1 Term 1, Unit 1: pp. 56–66
GoalExplain the swim bladder, the difference between bony and cartilaginous fish, and how cell membranes help deep-sea fish.
New words
swim bladder · a gas-filled sac in bony fish that helps them control their buoyancybuoyancy · the tendency to float; an animal floats, sinks or hovers depending on its densitybony and cartilaginous fish · fish with a skeleton of bone (tilapia, mullet) and fish with a skeleton of cartilage (sharks, rays)unsaturated fatty acids · fats in cell membranes that keep them fluid and stable under pressure
Explanation
Bony fish, such as tilapia and mullet, have a skeleton of bone and a swim bladder, a gas-filled sac that helps them control buoyancy at middle depths (200 to 1000 m in the book). Gas is squeezed into a smaller volume as the pressure grows with depth, so the fish has to adjust the gas to keep its buoyancy. Cartilaginous fish, such as sharks and rays, have a skeleton of cartilage, which is lighter and more flexible than bone. They have no swim bladder at all; a shark stays buoyant with a large oily liver, because oil is lighter than water (extra). Deep-sea fish have cell membranes with more unsaturated fatty acids, which keep the membrane fluid and stable under high pressure. Deep-sea animals are not crushed, because they have no large gas-filled spaces and their body fluids are at the same pressure as the water around them. Do not say that their bodies are “compressed”.
Feature
Bony fish
Cartilaginous fish
skeleton
bone
cartilage, lighter and more flexible
swim bladder
yes
none
examples
tilapia, mullet
shark, ray
Worked examples
A bony fish dives from the surface to 30 m. The total pressure goes from 1 atm to 4 atm, so the gas in its swim bladder is squeezed to a smaller volume. To stay at the same depth it must adjust the amount of gas: the bladder helps it control buoyancy, it does not fix it.
Which statement is correct? “Sharks and rays have liquid-filled swim bladders.” This is wrong: cartilaginous fish have no swim bladder at all. A shark’s large oily liver helps it float.
Why does a fatty-acid change help a deep-sea fish? Cell membranes tend to stiffen under high pressure. More unsaturated fatty acids keep the membrane fluid, so the cell works normally.
Class activity
Oil floats, with a grown-up. Pour a little cooking oil onto water in a clear cup and describe where the oil goes. Link it to a shark’s oily liver. Then write a sorting list of four fish: tilapia, mullet, shark, ray. Mark each as bony or cartilaginous, and say which has a swim bladder.
Practice
1
Why does the gas in a swim bladder take up less volume when a fish dives deeper?
The pressure grows with depth and squeezes the gas into a smaller volume.
2
Name the skeleton material of sharks and say how it differs from bone.
Cartilage; it is lighter and more flexible than bone.
3
Do rays have swim bladders? How do sharks stay buoyant?
No, rays have none. A shark floats with a large oily liver.
4
Which of tilapia, mullet, shark and ray are cartilaginous fish?
