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.
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.
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.
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.