#cirriped

For years I was mystified. Aren't barnacles cemented to a surface? I see dead barnacles all the time, like little shelly empty cones still glued to rocks. Those plates seem stuck good and fast to the rock along with the basal plate.

The basal plate is attached to the surface with cement, and as the the basal plate expands outwards a series of cement glands glue it down to the substrate.

But those plates are never shed, so how does it grow? The answer:

Image source:

THE SIDE PLATES OF THE BARNACLE ARE NOT GLUED TO THE BASAL PLATE OR THE SUBSTRATE!

At the periphery of the basal plate, there is a rim of cuticle. Like any other crustacean, a barnacle develops a new, bigger cuticle under the old, and it's sorta scrunched up like an accordion or paper fan.

When the barnacle molts, its old cuticle splits along the rim and expands outwards. Soon, new cement glands glue down this new expanse of fresh cuticle to the substrate. So a barnacle's side plates (parietal plates) are not glued directly to substrate or its own basal plate. This is why it can continue to grow and expand So when you look at the empty shell of a dead barnacle, despite appearances the parietal plates are actually stuck to a thin ring of remaining cuticle around the periphery of the basal plate.