37 The Mermaid's Purse-Low

The Mermaid's Purse FOCUS Question How does an egg case help a young shark?

What Is That? Sam and her father walked along the beach. They came across a pile of seaweed. It was up near the dunes. Sam turned the pile over with a stick. She saw a few shells on the sand. Then she spotted something strange. It was shaped like a rectangle. But it had hooks on the corners. "What is that?" Sam asked her father. "Nice find! That's a mermaid's purse," Dad said. "It's an egg case from a shark."

A Different Kind of Fish There are over four hundred kinds of sharks in our oceans. Sharks are a kind of fish. Their skin is covered with special scales. Shark skin feels rough, like sandpaper. Most fish have skeletons made of bone. But a shark's skeleton is made of a material called cartilage. It is softer and more flexible than bone. Your outer ears are made of cartilage. Sharks live in every ocean on Earth, They hunt for fish and other sea animals.

Sharks, Skates, and Rays Most sharks have long, thin bodies. They swim by moving their strong tails. The biggest fish on Earth is the whale shark. It is longer than a school bus. The smallest sharks are only as big as a shoe! Skates and rays are closely related to sharks, but they look different. Their bodies are flat and shaped like kites. They swim by moving their wing-shaped fins up and down. Some rays have points on their tails. In stingrays, this point has venom in it.

Being Born Many sharks and rays give birth to live young. The eggs hatch inside the mother's body. The babies are born ready to swim. They look like their parents only smaller. They hunt as soon as they are born. Other sharks and some rays lay eggs. Skates also lay eggs. These eggs are different from other fish eggs. The babies grow inside a tough egg case. Do You Know? Newborn sharks are called pups!

The Mermaid's Purse The egg cases of sharks, skates, and rays are not shaped like other eggs. Some are long and flat (see cover.) Many have long hooks on the ends. The hooks catch on seaweed or rocks. They keep the egg in one place. The case hides the egg from animals that might eat it. The hooks look like the straps on a purse. For this reason, the egg case of a shark, skate, or ray is sometimes called a "mermaid's purse." One shark has an egg case shaped like a screw. The mother shark lays the eggs between underwater rocks. The screw shape of the egg case helps secure it between rocks or coral.