Eggcase Anatomy

2/15/2017

Egg case washed up at Long Reef. (Photo by myopixia)

If a Chondrichthyan is oviparous, then their fertilized eggs will pass out of the female body via the cloaca and vent. On contact with sea water, the shell material of the egg will harden into a tough protective coat. Here we will discuss the general anatomy of a Chondrichthyan egg and Shark Trust’s “The Great Egg Hunt” initiative.

Chondrichthyan egg cases, also known as mermaid’s purses and devil’s purses, are made out of collagen protein strands around fertilized eggs of some species of Chondrichthyans (note: all stingrays give birth to live young). The parts that make up the egg case can be seen in this diagram provided by the Shark Trust’s “Great Egg Hunt” guide.

The most distinguishable features on these egg cases are the keel and absence/presence of fibrous covering. The size and shape of these egg cases usually varies on whether it belongs to a shark, skate, or chimaera—and then it varies on the family! For example: a Port Jackson shark egg differs from a zebra shark egg which different from a catshark egg. All sharks, yet all have different egg shapes! These different egg case shapes will be looked at on the following post of this series: "Different egg cases around the world."

Once the pup exits the egg, they become lightweight and usually dislodged from wherever they were hidden or attached to. The ocean current takes over then, and they usually wash up on the beach by the strandline (the farthest point of the high tide). Most of these washed up egg cases are empty, though some may not be—do check before taking one home!

Shark Trust's “The Great Egg Hunt,” a UK-wide citizen science project that allows for beachgoers who find egg cases on the beach to identify the species of who the egg case belongs to, record their findings and help scientists gain knowledge about distribution and preferred habitat of these animals! You can find the latest published findings here.

have you ever seen a shark eggcase before?

glossary

Keel: The line along which the shell is closed.
Oviparous: The eggs hatch and the babies develop inside the female’s body, without a placenta to nourish the pups.
Pups: Baby sharks.
Strandline: The farthest point of the high tide.