A Mermaid's Purse

... and the secret inside ...

As a child, I was fascinated by Mermaid’s Purses. We found them often on the oceanside beach here in Cape Cod, and brought them home along with all the other treasures we collected. Mysterious black pouches with a fancy curlicue spinning out from each corner, they definitely spelled mystery. I tried to imagine what a mermaid would carry in her purse. A few times I carefully opened the dried-up husks, thinking to find miniature gold coins or a string of tiny jewels inside. But they were always empty.

Eventually I learned that these interesting artifacts are eggcases in which embryos of the Clearnose Skate (Rostroraja eglanteria) develop for roughly 12 weeks until they have grown too large for the enclosure and break free. This excerpt from an excellent Wikipedia article explains everything about the eggs of the Clearnose Skate:

Clearnose skates are oviparous, and therefore they lay fertilized eggs, commonly referred to as Mermaid's purses. Each corner of the rectangular egg case has a small curved horn. The size of the egg case ranges from 6.4 to 7.7 cm (2.5 to 3.0 in) long and 3.7 to 4.7 cm (1.5 to 1.9 in) wide.

Egg deposition occurs in pairs, and up to 30 pairs may be laid by a female. As the female lays the egg, the longer anterior horns emerge first. The shorter posterior horns follow and enable the egg case to anchor to the substrate, as they are covered in a sticky substance.

At first, the embryo is completely enclosed within the egg case. As it develops, a small hole (called the respiratory canal) along the base of the horns opens, allowing seawater to enter the case. The flow of seawater is maintained by the tail beating of the embryo.

Embryonic clearnose skates demonstrate a ventilatory freeze response when a weak low-frequency electric field is imposed upon the egg capsule. This freeze behavior will cause the egg-encapsulated embryonic skates to stop ventilatory streaming. This will decrease the likelihood of sensory detection, and thus "cloak" embryos from searching egg predators.

Last year, on a walk the morning after a fierce storm, I found a particularly plump purse that had been washed up from the deep and deposited by the waves at high tide far from the water’s edge, where it was destined to shrivel and dry up when the sun came back out. I didn’t see a visible exit slit, so I decided to try discovering what was inside.

Back home in the kitchen, I proceeded to cut carefully along three edges so I could fold back a flap. That done, I tipped the open purse over a dish and let the contents fall out. Imagine my astonishment upon seeing a live embryo, perfect in every detail, as all of Nature’s creations are. I was horrified, actually — and immediately hastened to add tap water to the dish — water that recent tests had shown to have excessively high levels of salinity — in terms of drinking water at least. But that might have been lucky for the little creature that was now whipping its lively tail back and forth in a motion I find mesmerizing. I very hastily took the video clip below before running back to the sea, wading into the surf, and releasing the hatchling. I knew it didn’t stand a chance of surviving, but I’d like to think it died a better death in the salty cradle of its own sweet sea home than it would have baking in the heat of a relentless sun.