The Mermaid’s Purse: An inanimate treasure that can be found on our North Island beaches

Written by Neil McGregor

We here on the North Island are fortunate in what nature has to offer us.

Whether it’s of the land, the air or the sea, our environment is rich in life. But it’s not just the living wonders that are so special, there are many inanimate treasures to be found on our beaches.

I was recently fortunate enough to find one such treasure, a mermaid’s purse. A mermaid’s purse is an egg capsule that’s been produced by a type of shark, a skate or a ratfish. In this case it was that of “raja binoculta” or what is commonly called a “big skate.”

A female big skate can store sperm from a male skate for several months before using it to fertilize eggs. As a result, the eggs in a capsule may just be fertilized by different male skates. When the time is right the female will fertilize eggs and deposit them in either of two egg capsules she’s developed for the event.

Each capsule can contain up to seven eggs. Both egg capsules will be deposited on the seabed.

The hooks, or tendrils of the purse, are designed to catch on seaweed or substrate of the seabed. They also serve to oxygenate the developing embryos as seawater is able to pass through them into the capsule.

The eggs will hatch in around nine months. Enzymes from the hatching process break down the binding in one end of the capsule, thus creating a slit for the hatchlings to escape from. Clearly, a lot of engineering goes into a mermaid’s purse.

The juveniles still carry their yolk sac under their belly. Besides their real eyes, they also sport an eyespot on the top of each wing. This eyespot will serve to ward off or confuse would-be predators. Big skate off the B.C. coast are known to live over 25 years, reach lengths over 2 meters and weights of over 100 kgs.

Pretty impressive considering their size when they leave the mermaid’s purse and debut into their vast new world.

A mermaid’s purse is just one of many treasures to be found on the beach. Next time you’re on a beach walk, look carefully, you may just find your own treasure.