Egg Case Identification

About Egg Cases                                                     


All sharks, skates and rays reproduce by physically mating, unlike bony fish which release eggs and sperm into the water to be fertilised. Once the eggs are fertilised they develop in one of three different ways:

  1. In a tough egg case (often called a mermaid’s purse) which is laid in areas of sea grass or in rock crevices to develop until the pup is ready to hatch and swim away. This process is known as “Oviparity”.
  2. Within a much thinner egg case which stays protected within the female and develops, gaining nutrition from the yolk sac within the egg case until it hatches, the pup is then born fully developed at the same time as its other siblings. This process is known as “Oviviviparity”.
  3. Within the egg case, as with oviviviparity, developing a placental type connection to the mother which allows additional nutrients through uterine milk to be shared from the mother to the developing pup. The pup is born fully developed. This process is known as “Viviparity”.

During heavy seas and strong winds empty egg cases (mermaid’s purses) are often washed ashore, the egg cases being now hollow and empty are light and get blown up onto the back of beaches and onto rocks. They are not so easy to identify because they are well camouflaged with seaweed, sand and gravel but if you are lucky enough to find them this could indicate that a potential nursery area could be close by.

Eggcases can give us some very important information.

Firstly, it can confirm the presence of certain species in our local waters.

Secondly, if a significant number of eggcases from the same species are found, it can indicate that nursery areas could be in the vicinity.

Finally, if no eggcases are found in certain areas, we can investigate the differences between areas where they are found and areas where they are not.

The guide and pictures below should help you to identify any egg cases that you find.


Eggcases come in all different shapes & sizes, even from the same species. So don’t worry if you can’t identify your eggcase species  exactly.

We have attached a few more photos below to help you see how they vary – hope this helps! 

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