Have you ever considered acorn barnacles other than when one skins your knees or makes you dance painfully, barefoot across intertidal rocks?

What are acorn barnacles?

Acorn barnacles are little crustaceans get ignored by most of us. If we see them at all, they’re out of the water and closed up – a small, mysterious capsule of calcium that gives away few clues as to what actually lives inside.

Within each tiny turret is an intertidal survival specialist able to tough out the harshest of conditions, by trapping a bubble of seawater in its fortress of perfectly interconnecting plates when the tide retreats. Superficially, a barnacle looks like a limpet; many mistakenly think that they are molluscs. But look closely at barnacles underwater and you’ll see something that no mollusc has: jointed legs.

Next time you’re at the seaside, take a small barnacle-encrusted object, immerse it in a jar of seawater and hold it up to the light – you’ll see the two perfectly fitting pairs of valves in the middle slide open enough to reveal six pairs of feeding legs, called cirri. These seem to grab rhythmically at the nothingness of the water and retreat, but what they’re in fact doing is snatching fragments of microscopic life from the water and pulling them into the animal’s hidden mouth.

The 19th-century biologist Louis Agassiz nicely summed barnacles up as: “nothing more than a little shrimp-like animal standing on its head in a limestone house kicking food into its mouth”. Which is pretty much what they are. Barnacles start life looking much more like shrimps than the familiar adult stage.

More like this

What are barnacle larvae called?

Barnacles develop as eggs inside their parent’s shell, then are ejected into the wide world as soon as they hatch. Each free-swimming planktonic larva is called a nautilus.

Barnacle lifecycle

The larvae make the most of the seasonal abundance of plant plankton and, as Darwin pointed out in his seminal work on barnacles, this stage is analogous to the caterpillar stage of a butterfly’s lifecycle. The larvae feed and grow over the next few weeks, moulting as they do so.

Sometime in April, the larvae stop feeding and moult into another form, the cypris larva, and each has 13 days to find a spot to settle for life. When it does, it immediately glues itself upside down using glands on its antenna, and starts to concentrate calcium salt from the seawater to built its house.

Barnacles continue to grow once settled, which is why, if you look at a colony on a rock, they come in different sizes. By adding calcium and proteins to the base and inside, the whole structure of the shell slides outwards to accommodate the growing barnacle within.

Acorn barnacle penis

If you study your barnacles long enough, you might see another claim to fame – as the the proportionally longest penis in the animal kingdom, 8–9 times the length of its owner. If you need to mate and are glued to the spot, then you need a fairly neat solution. The human equivalent would be a man impregnating his neighbour via the letterbox.

Intrigued about seashells? Check out our seashells expert guide, which explains what are they, where they come from, and how they are made?

Main image: Getty Images


Nick Baker headshot
Nick BakerTV presenter and naturalist