If you are like us and enjoy fossicking on the beach, you may have found leathery looking cases washed onto the shore and wondered what they were. Well, they are shark egg cases. They can come in all sorts of shapes and we thought it would be interesting to do some research on the topic. So read on and next time you beachcomb, you can go on a great egg case hunt, knowing which egg case belongs to which shark or skate!
Oviparous Vs Viviparous Sharks?
Firstly we found that not all sharks lay eggs. Of the 400 or so species of sharks, only 40% do. This is called oviparity. The other 60% of shark species give birth to live young. This is called viviparity.
You might wonder why one type of shark would lay eggs rather than give birth to live young. Well apparently there may be a link between reproductive strategies and habitat. Littoral sharks tend to be oviparous, whereas pelagic sharks tend to be viviparous. Feeding ecology also has an influence. Large predatory sharks are typically viviparous. Smaller sharks and skates which feed on smaller fish and invertebrates are oviparous. I knew there had to be an explanation!
Shark species that lay eggs include:
- Carpet sharks
- Horn (bullhead) sharks
- Swell sharks
Those odd looking egg cases
Most oviparous sharks and skates lay their eggs in strangely shaped cases sometimes called a ‘mermaid’s purse.’ When the eggs are laid, they are in a thin, protective capsule made of collagen. They often are square or rectangular with stringy or pointy corner horns, but can come in a variety of odd shapes. The egg case has tendrils that allow it to attach to corals, seaweed or the ocean bottom. A few sharks, such as the Port Jackson shark, have egg cases in the form of a spiral which are secured into the sand like drill bits. Shark egg cases usually remain underwater until the baby sharks which they contain hatch. They then can be picked up in currents and wash up on our Southern beaches.
Which egg case goes with which shark?
With a bit of detective work, here is what we found:-
- The Port Jackson Shark, and its 10cm long spiral egg case:
- The Spotted Swellshark and its 8-9cm long washboard egg case:
- The Elephant Fish and its 16cm long finned capsule:
- The Catshark and its 4 to 6cm long egg case:
- The Melbourne Skate and its 10cm ragged-edges Mermaid Purse:
- The Thornback Skate and its Mermaid Purse:
- The Southern Round Skate and its 8cm long Mermaid Purse:
Most photos of the egg cases were taken on beaches of Wilson’s Promontory or along the Ninety Miles Beach off the Gippsland Lakes. The shark and skate photos are public domain images.
So there you have it, next time you take a walk along our Southern shores, keep an eye out for these fragile, lightweight egg cases. You will be able to identify them. Have fun!