The lowly barnacle is well-known. Most people can point to one and say, “that’s a barnacle.”

Well done, but how many people know exactly what a barnacle is?

“It’s this…thing…that lives on rocks at the beach.”

“Don’t they grow on whales?”

“Ships get covered in them.”

As an entomologist, I know a bit about barnacles, because they are Arthropods, just like insects, spiders, millipedes, crabs, and lots of other creepy crawlies. In fact, they are crustaceans, closely related to crabs and crayfish.

Wait, you say. Don’t Arthropods have ‘jointed legs’—that’s what the word means, after all—but barnacles don’t have legs.

Or do they?

Barnacles are perhaps the strangest of the Arthropods. The free-living larva is a weird, tiny, spiky creature with one eye. It goes through a metamorphosis, like insects do, in which it changes shape dramatically. In its last larval stage, it finds a nice place to spend the rest of its life, presses its forehead against it, and secretes a calcium-rich cement from near the base of its antennae that permanently affixes its head to the spot.

In its adult form, a barnacle grows a protective shell, complete with a clever two-part ‘door’ that it can snap shut to conserve water at low tide, or to protect itself from predators. Its legs grow long and feathery, and act as tentacles to waft particles of food to its mouth.

As you can imagine, adult barnacles don’t have much of a social life. Most species are hermaphroditic, meaning individuals are both male and female. Surprisingly, though, self-fertilisation is rare. Like other arthropods, most barnacles have what’s euphemistically known as ‘internal fertilisation’—that is, the male has a penis, and he deposits his sperm inside the female. How does an animal glued by its head to a rock get together with another to mate? The answer is a very long penis.

So the next time you feel like your life is rough, be thankful you’re not a barnacle.