Black seaweed-like pouches on beaches are actually fish eggs

A
skate egg case or capsule, also called a devil’s purse or
mermaid’s purse.

Martin
Alonso

• Black pouches with tendrils that wash up on beaches are not seaweed or kelp —
  they’re
  the egg cases of fish called
  skates.
• Some of these egg cases contain live fish embryos.
• Holding a light up to a fresh, unhatched case will
  reveal the fish embryo inside.

North Carolina’s beaches are reportedly awash in rubbery black
pouches with two pairs of tendrils. These aren’t bits of plastic
pollution, nor are they a form of seaweed: They’re egg casings
for a particularly weird-looking fish.

I grew up in Ohio, a state that does not border a salty sea.

So any summer that my parents crammed our family into a car and
drove to the Outer Banks of North Carolina, I was in heaven. Day
after day, my brother, sister, and I would play in the warm surf
of the Atlantic Ocean. We met a zoo’s worth of marine life during
those beach excursions, including clams, crabs, copepods, fish,
birds, dolphins, and jellyfish.

But there was one common yet fascinating animal we completely
overlooked: a weird, rectangular object with a pair of pokey
tendrils on either side.

For years I assumed these were kelp pods or some strange-looking
pieces of seaweed. But they belong to mother skates.

What skate fish are — and a cool experiment to try with their egg
cases

“I’m a skate,
look at me!”


Shutterstock

Skates are related to rays and sharks, and like both they have no
bones — only cartilage. As adults, skates have thorny fins on top
of their bodies and eat whatever small creatures they can find on
the sandy ocean bottom, including shrimp and tiny fish.

Should you find a fresh skate egg case on a beach, perform this
simple experiment — it will blow your mind (and likely freak out
squeamish friends and family).

First, find a skate egg that is not dried to a crisp. Your best
shot is to check the wrack line, where high-tide waves deposit
junk from the sea. Spring storms will often rip the egg cases
from the seaweed they attach to in deeper waters and wash them
ashore.

Since these egg
cases are made of collagen, a protein that takes forever to
break down, you’re likely to find many more empty ones than fresh
ones. As a general rule: the more wet, slimy, bubbly, pliable,
and translucent, the better.

Once you’ve got one in hand, grab a smartphone, turn on its LED
light, and then move the egg in front of the light. If you’re
lucky, you will see something like this:

A skate fish egg case with
a light shining through to reveal an embryo and yolk
sac.

Dave Mosher/Business
Insider

The pink mass is a skate embryo attached to its yolk sac.

Depending on the age of the skate egg you’ve found, you can
sometimes see its head or a devilish tail flicking around. You
might even see more than one embryo.

Dave Remsen, a bioinformaticist and scientist at the Marine
Biological Laboratory (MBL) in Woods Hole, Massachusetts, showed
me this little guy (or girl) when I visited the lab:

“I like to show these to kids and tell them they used to look
like that,” Remsen told Business Insider. (It’s a good
joke to a scientist, since all embryonic animals
look remarkably similar during early stages of development.)

Some shark species also lay egg cases, but those are much rarer
to find on a beach, since mother sharks lay them in the deeper
ocean.

If you don’t have an LED light handy, there’s a great substitute
in the sky: the sun. This video shows what a skate egg case looks
like when you hold it up to the light:

Some aquatic centers and aquariums go even further.

For example, the Aquarium of the Bay in San Francisco cuts out a
panel of the egg case and super-glues on a piece of transparent
plastic. That way, visitors can clearly see the embryos inside as
they develop:

A skate egg case with a
section removed and a transparent window glued in its
place.

Kai
Schreiber/Flickr (CC BY-SA 2.0)

It can take months for embryos like these to develop, so if
you’re fortunate enough to find an egg case, give it a good toss
back into the ocean.

Even if it doesn’t hatch, it will at least make a good meal for
another sea creature.

This story has been updated. It was originally published on
June 17, 2017.