Big Skate Egg Case / Mermaid’s Purse
In a complex time, it made me smile today to receive two ID requests from the same area of the same thing.
This . . . .
And this . . .
I’ve got your attention don’t I? (Insert cheeky smile here).
These are the egg cases of the Big Skate (Beringraja binoculata) and there are babies growing inside. Such egg cases are known as “mermaids’ purses”. But, as cleverly commented by a friend, these “purses” are so big for this species they should be called “mermaids’ carry-on bags” (Thanks for that Erin Johns Gless).
My compilation below shows the various mermaids’ purses you can find on our coast.
The Big Skate is unique in that it can have more than one embryo growing inside each egg case. Up to a maximum of 7 embryos have been reported but more often it is 3 or 4 per egg case. Jared reported that he could feel movement in the case before he put it back in the ocean. Each embryo has its own yolk sac providing the nutrition for growth. See above for what the embryo of a Longnose Skate (different species) attached to a yolk sac.
Two Big Skate embryos developing inside an egg case. A “window” has been cut into the egg case to allow development to be observed. Photo Kelcie Chiquillo.
You can imagine that it should not be common to find fresh egg cases floating or washed up on the beach. That would not offer the best chance of survival for the babies. The various egg cases of rays, skates and sharks are “designed” to hopefully hook onto substrate / remain on the ocean bottom.
What is also so interesting about the reproduction of the Big Skate specifically is that research (Jang, 2019) supports that the embryos in one case can have different fathers (multiple paternity). Say what? Yep, research conducted on Big Skates in captivity have found that the females can store sperm for up to 3 months and then fertilize the eggs prior to laying the egg cases (they are oviparous).
And how long will the babies grow inside the egg case? For this species, it is around 9 months. At that time, they will “hatch” by releasing an enzyme that breaks down the binding of the case. They swim away and that’s when we more often find the egg cases, when they have done their job and are dried up on the beach.
The babies in the cases that Jared and Tina found today could survive to become up to 2.44 metres long (more commonly around 180 cm long). It’s big alright – the biggest skate species in North American waters.
Below are photos of adult Big Skates.
Source and photo: Joseph J. Bizzarro; Friday Harbour Laboratories; How Can There Be So Many Skate Species … and Why Should We Care?
Next two photos source: NOAA – Alaska Fisheries Science Center.
Related blog: “Sharks Among Us – The Brown Cat Shark” providing detail on how the Brown Cat Shark embryos can take up to TWO YEARS to hatch from the egg case.
Sources:
- Fishbase: Beringraja binoculata
- Jang, Jessica Ja-Jei, “Reproductive Strategies of the Big Skate (Beringraja Binoculata) with Evidence of Multiple Paternity” (2019). Capstone Projects and Master’s Theses. 727.
- Jang, Jessica Ja-Jei presentation “Reproductive Strategies of the Big Skate with evidence of multiple paternity” (video)
The Marine Detective 
4 Responses to “Big Skate Egg Case / Mermaid’s Purse”
Thank you for such an interesting tutorial on the mermaids purse. I’d never even heard of them before today.
Wonderful to know this was of use.
That’s so cool how they put a window in the egg! Observing their growth must be fascinating.
Thank you so much for the comment / feedback.