Egg case (Chondrichthyes) facts for kids
An egg case is a special protective covering. It surrounds the eggs of certain fish that lay eggs. People often call them a mermaid's purse because of their unique look.
These cases are made by some sharks, skates, and chimaeras. Most egg cases hold one baby animal. However, some, like those from the big skate, can hold up to seven babies!
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What Are Egg Cases?
Egg cases are made from tough, leathery protein strands. They often feel rough to the touch. Some cases have a fuzzy material on the outside. This helps them stick to rocks or plants on the seafloor.
Other egg cases can be bumpy, smooth, or have lines. Most are shaped like a rectangle. They usually have pointy parts, called horns, at each corner. Some species also have long, string-like parts called tendrils.
The Amazing Life Cycle of an Egg Case
After an egg is fertilized inside the mother, it enters the egg case. The case then fully forms around the egg. Soon after, the mother lays the egg case outside her body. They are often found in places like kelp forests or on rocky seabeds.
Usually, egg cases are laid in pairs. Each case holds one fertilized baby. But remember, some species can have more than one baby per case!
The baby animal grows inside the case for many months, sometimes over a year. After a week or two, tiny slits open on the sides of the case. These slits help fresh water flow in and out. The baby inside gently waves its tail. This helps it get enough oxygen from the water.
Sharks That Lay Eggs
Some sharks lay their eggs soon after they are fertilized. Other sharks keep the eggs inside their bodies for a while. Then they lay the fully formed egg cases.
It's interesting that female sharks in aquariums sometimes lay eggs even without a male shark around. These eggs are not fertilized and won't hatch.
Bullhead Shark Eggs
Bullhead shark egg cases look like a corkscrew or a drill bit. They have two spiral ridges. This shape helps them wedge tightly into cracks in rocks. This protects the eggs from hungry predators. Some bullhead sharks also lay their eggs on sponges or seaweed.
Baby bullhead sharks are quite large when they hatch. They can be over 14 centimeters long! These eggs usually hatch after 7 to 12 months. Female Japanese bullhead sharks sometimes lay their eggs together in one spot. This is like a shared "nest." The Mexican hornshark has a tendril on its egg case. This helps it anchor to things instead of just wedging into cracks.
Carpet Shark Eggs
Bamboo sharks and zebra sharks lay their eggs on the ocean floor. Their egg cases are oval-shaped. They are covered with sticky fibers. These fibers help the cases attach firmly to the bottom. Other types of carpet sharks give birth to live young.
Ground Shark Eggs
Some catsharks are the only ground sharks that lay eggs. Their egg cases look like small purses. They have long tendrils at the corners. These tendrils help anchor the cases to structures on the sea floor.
The size of these egg cases can vary a lot. For example, the small-spotted catshark's case is about 5 centimeters long. The greater spotted dogfish has cases around 10 centimeters long. These measurements don't include the long tendrils!
Skates and Their Egg Cases
Skates are the only type of rays that lay eggs. Female skates lay their egg cases on the seafloor. The protective case forms around the baby inside the mother.
Each skate species has unique features on its egg case. These features help scientists identify the species. Two main features are the "keel" and whether it has a fibrous covering. A keel is a flexible ridge along the sides of the case. Some keels are wide, others are narrow. Many egg cases also have a layer of fibers on the outside.
Big Skate Egg Cases
Big skate egg cases are larger than most others. They can be 21 to 28 centimeters long and 11 to 18 centimeters wide. This is about 15% of the mother skate's total length! These cases are very smooth and have no outside fibers. They are easy to spot because they have a steep ridge, making them look somewhat rounded on top. Their keels are very wide.
Big skates are special because their egg cases can hold many babies. Up to 7 babies have been found in one case! Most big skate egg cases usually contain 3 or 4 babies.
Longnose Skate Egg Cases
The longnose skate is a large skate species. It can grow up to 145 centimeters long. Its egg cases are smaller than the big skate's, about 9 to 10 centimeters long. Each case holds just one baby.
Longnose skate egg cases found in the ocean are usually brown. The outside is covered with a fibrous material. This material is thicker on the top and thinner on the bottom. Underneath, the case is smooth.
Chimaeras: Deep-Sea Egg Layers
All known chimaeras lay egg cases. Their cases are shaped like a spindle or a bottle. They have flattened "fins" on their sides. Chimaeras lay these cases on the bottom of the deep sea.
Chimaera egg cases have a rounded middle part. Flat, tough tissue extends from the sides. This tissue joins at one end to form a tail-like shape. Sharp points on both ends help secure the case between rocks. These points also offer protection from predators.
Ancient Egg Cases: A Look Back in Time
Scientists have found fossilized egg cases from ancient times. These cases belonged to extinct shark-like fish. Two types, called Palaeoxyris and Fayolia, looked a bit like modern bullhead shark eggs. They had spiral ridges.
These ancient egg cases were often found in freshwater. This is different from most modern shark and skate eggs, which are found in saltwater.
Dangers to Egg Cases
Many things in the ocean can be dangerous for egg cases. Predators eating the eggs is a big problem. Marine fish eggs are often a tasty and easy meal for other animals.
Once the mother lays the egg case, her job is done. The tough, leathery outside of the case is the only protection for the baby inside. Some snails can drill into the cases to eat the eggs. Other sharks also sometimes eat egg cases.
