When you think of barnacles, you think of animals that are fixed in place. But they are not all like that...
The photographer tentatively identifies this floating barnacle as Lepas fascicularis, but searching around suggests that that name is no longer valid. It might be Dosima fascicularis, which are given the common name of buoy barnacles.
Diversity is wonderful.
Photo by Willapalens on Flickr. Used under a Creative Commons license.