Mussel

The Mussel is a type of sea creature in introduced in the 1.3.0 Free Summer Update.

In
When donating to the museum, Blathers will say the following:

"Did you know that the mussel has a beard?! Hoo! It's true! Well, partially true, anyway. Mussels have a special gland that secretes a thing called "byssus" or "byssal thread." They use these tough beard-like fibers to anchor themselves to rocks and keep themselves in place. So, you see, the mussel's beard is functional rather than merely stylish. Then again, perhaps all beards should be this way."

the mussel is located in the coral reef tank in the museum