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How fish and other sea creatures breathe and move through the water.
Have you ever tried to run through the water in a swimming pool? It feels heavy and slow! Yet, fish zoom through the ocean like tiny rockets. How do they breathe and move so easily where we can't?
Just like humans, fish need oxygen to live. However, they don't have lungs to breathe air. Instead, they have a special adaptation called gills. Gills are usually located on the sides of a fish's head. As water flows over the gills, they take the oxygen out of the water and send it into the fish's blood. It is like having a built-in water filter that catches air! Without gills, a fish could not stay underwater for more than a few minutes. This is why fish constantly gulp water—they are actually 'breathing' it through their gills.
Quick Check
What is the name of the body part fish use to get oxygen from the water?
Answer
Gills
To move through the heavy water, fish have two main secrets: fins and a streamlined body. Fins act like oars on a boat or the steering wheel of a car. They help the fish push, turn, and stop. A streamlined body means the fish is shaped like a smooth torpedo—pointed at the front and narrow at the back. This shape helps the fish 'cut' through the water with very little resistance. Imagine trying to push a flat piece of cardboard through water versus pushing a sharp pencil. The pencil moves much faster because it is more streamlined!
Think about two different sea creatures: 1. A Tuna has a very pointy nose and a smooth, slippery body. It is highly streamlined. 2. A Pufferfish is round and bumpy like a balloon.
Because the Tuna is streamlined, it can swim at speeds of up to miles per hour! The Pufferfish is not streamlined, so it wobbles along slowly.
Quick Check
If a fish has a smooth, pointy shape that helps it go fast, what do we call that shape?
Answer
Streamlined
The ocean has strong currents and waves that could easily wash plants away. Unlike land plants that have deep roots to suck up water, sea plants like kelp use something called a holdfast. A holdfast looks like a bunch of tangled fingers that grip onto rocks on the ocean floor. It doesn't soak up nutrients like a tree root; its only job is to act like super-glue. This keeps the plant anchored in one spot so it can grow toward the sunlight at the surface without being swept out to sea.
Imagine a giant storm hits the coast. The waves are moving at feet per second. 1. A plant with no anchor would be pushed feet away in just one minute! 2. However, the kelp stays in place because its holdfast is gripped tightly to a heavy rock. The plant bends with the water but does not move from its home.
Which adaptation is most like a human's lungs?
Why is a torpedo shape (streamlined) helpful for a shark?
Sea plants use holdfasts to soak up food from the soil.
Review Tomorrow
Tomorrow, try to explain to a friend why a fish is shaped like a torpedo and how it breathes without coming up for air.
Practice Activity
Draw your own 'Super-Sea-Creature' and label its gills, fins, and streamlined body. Add a sea plant next to it with a holdfast!