Strange-looking black pouches have been washing up on beaches along North Carolina's shores.

But despite how they might look, they're not plastic pollution, as officials have reminded the well-meaning public.

That's because they're actually something much cooler - the egg casings of sea skates. Sometimes known as mermaid's or devil's purses, based on their weird appearance.

"No these are not pieces of plastic; they are skate eggs!" Cape Hatteras National Seashore authorities wrote in a Facebook post.

"Skates are in the shark family but share a flat-like appearance similar to stingrays. These leathery pouches serve as an egg sack [sic] and are typically attached to a plant or other stationary object in the water. Most often, these egg sacks wash ashore after the skate has hatched."

Some species of sharks, skates and chimaeras all produce strange leathery, alien-looking egg casings.

What they're actually made out of is collagen, the main structural protein in animal connective tissue, such as tendons, ligaments and skin - so it's no wonder it feels leathery and looks so much like plastic!

There are several species of skate that swim the waters of the Atlantic Ocean off the coast of North Carolina, including winter skates, barndoor skates, thorny skates, clearnose skates and little skates.

Skates are the only rays that lay eggs - all the others give birth to live young. And each species of skate's eggs have unique features, which makes them really handy for species identification.

Typically, each contains a single embryo - but the larger ones can contain up to seven.

Based on the size of these casings, they probably were single-embryo eggs, although which skate laid them is a little harder to ascertain for sure without seeing the cases laid out flat, with all horns visible.

As for those weird horns, they actually have a really cool function: small slits along the horns extract oxygen from the surrounding water, and allow waste to escape.

If you find any eggs in the water, still anchored to seaweed and with an embryo inside, it bears repeating that it's best not to touch it, and that it should be left exactly where and how you found it.

However, any empty egg casings are fair game for collection, and you should feel free to participate in The Shark Trust's Great Eggcase Hunt to help identify and count shark and ray species around the world.

You can record your findings on their website here.