If you find one of these blacks pods, prepare to freak yourself out with this experiment. Skate egg casings are the perfect subjects for a simple biology demo.
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.
These black pods are actually the egg cases or capsules of a skate, a flat, funny-looking fish.
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:
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:
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.