While one can imagine a young Ariel using one of these items as a purse, they are not well suited to this role on land. Commonly called mermaids purses, these items are in fact egg cases from a variety of species of sharks, skates and rays (elasmobranchs). Fertilisation of the egg occurs inside the female’s body, and the fertilised embryo is then enclosed in an egg case which is deposited in the environment. Egg cases come in a variety of shapes and sizes, with specialised tendrils and sticky filaments all aiding in the attachment of the egg to a structure (regularly seaweed) or substrate (often rocks) where the embryo will develop. Initially the egg cases are soft and pliable so they can be laid, but after a few hours in salt water they become harder and more leathery. The egg cases are primarily made of collagen. Embryos can develop within the egg case for a period as little as 8 weeks and as long as over a year before hatching! When they do hatch, the little animals are miniature replicates of their parents.
While walking on the beach, or out swimming, snorkeling, or diving, do keep an eye out for egg cases. You can find out more about the Elasmobranch Monitoring project being undertaken along our beaches cataloging egg cases by visiting http://elmoafrica.wix.com/elmo.
Written by: Minke Witteveen
For more reading:
• Bone, Q. and Moore, R.H. 2008. Biology of Fishes. 3rd edn. Taylor & Francis Group, New York.
• Carrier, J.C., Musick, J.A. and Heithaus, M.R. 2012. Biology of Sharks and Their Relatives. CRC Press, Florida.
• Hoar, W.S., Randall, D.J. and Donaldson, E.M. 1983. Fish Physiology. Academic Press Inc., Florida.