ASSATEAGUE ISLAND, Md. (WJZ) — Seeing a horseshoe crab along the seashore of Maryland and Delaware beaches isn’t out of the ordinary.

But have you ever seen a crab with a weird growth on it?

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The Assateague Island National Seashore posted an explainer on Facebook. Those growths are actually common clipper shells, scientifically called crepidula fornicata. They attached themselves to the carapace, or hard outer shell, of the crabs.

These marine snails actually pile onto each other to reproduce and use the horseshoe crabs’ shells to do so.

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The snails actually change from male to female for reproduction purposes.

“Some slipper shell stacks can be 10 or 15 snails high, with the larger females found toward the bottom of the stack and the males near the top,” the post reads.

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Read more about the phenomenon below.