It’s a window into one of the most interesting things you’ll ever see: four squirming big skate (Raja binoculata) embryos complete with attached yolk sacs.

They’re contained in an egg case, also known as a “mermaid’s purse.” It looks like corn husk but feels like rubber. One is on public display at Windows on Research, the other is behind the scenes.

Mackenzie pulls back a piece of the egg case.

Mackenzie pulls back a piece of the egg case.

Big skates are found from Alaska to southern California with 11 species in British Columbia. Skates are related to rays and sharks, so they have cartilaginous skeletons as well, rather than having bony skeletons.

Before cutting into the egg case to make a window, Mackenzie Neale and Christine Martinello, biologists at the Vancouver Aquarium Marine Science Centre, first had to make sure it was developed enough so the incision wouldn’t affect the embryos. They squeezed the egg case to see if the pores at the corners were open to sea water, allowing for gas exchange and the removal of waste. They were, which meant the two biologists could go ahead and cut into the egg case because the embryos had already been exposed to sea water flowing in and out.

A scalpel was used to gently slice into the mermaid’s purse before pulling the piece back to expose the embryos. They then used super glue to stick a clear plastic sheet on to create the window.

Super glue lines the opening to keep the plastic sheet in place.

Super glue lines the opening to keep the plastic sheet in place.

The development period for these skates inside a mermaid’s purse is about the same as a human carrying a fetus – nine months. There are between two and seven big skates embryos inside a mermaid’s purse at any one time. The embryos produce an enzyme near the end of their term in the egg case that dissolves the binding of the case, allowing the juveniles to swim out. These ones are expected to emerge in September.

Vancouver Aquarium skate babies

A fully grown adult skate.

We currently don’t have any adult skates on display but look forward to watching these not-so-big skates grow. Stay tuned for more updates.

Written by Karen Horak, writer-editor, content and digital experience at the Vancouver Aquarium.

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4 Responses

  1. Isaac Chaves


    I find this fascinating. Firstly, because I though there should be only one embryo per egg case, since the oviducal gland is supposed to synthesize an egg case per fertilized egg (oocite), have you seen more than one embryo in other occasions? Another question I have is if the 4 embryos managed to fully develop despite having the egg case opened? And lastly, I wonder which enzyme is involved in dissolving the egg case? Do you have a reference for that info?

    Truly grateful if you could answer me these questions.

    • Ocean Wise

      Hi Issac,
      Here’s a response from Mackenzie Neale, Ocean Wise’s Assistant Curator of Propagation:

      Big skates are one of only two species that are known to have multiple embryos in their cases. They can have up to 7 embryos but 3-4 is the average.
      When the cases are laid, they have mucous plugs in the ends of the cases. After a while, these plugs dissolve and it allows sea water to enter. We wait until the mucous plugs have dissolved before we open up the case so that we don’t disturb the fluid inside that the embryos are developing in. The cases split at the seams when the babies exit the case, but I do not know the mechanism by which that happens or whether an enzyme is involved.

  2. Denis Calvès


    I also tried once to make an opening in a non-hatched washed up eggcase of skate (raja clavata). But unfortunately the yolk sac of the embryo ripped on the sharp edge of the opening.
    Can the embryo survive out of the water during the time you put a window on the eggcase?

    Have a great day,
    Yours sincerely,
    Denis, Brest, France

    • Vancouver Aquarium

      Hi Denis,

      I would hesitate to completely remove the embryo from the water while you are putting the window on the eggcase, just in case the gas exchange is interrupted to the embryo. We usually keep the egg case in a shallow dish so that there is still water in and around the egg case but we can gently dry the edges of the opening and glue the window in place. We use a scalpel to gently cut the opening and have never had the yolk sac rip on the sharp opening that way.


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