Barnacles can swim at birth. When they reach adult stage, they attach themselves to objects in the water and grow a shell.
If you have ever visited a seacoast where there were rocks and piers you have almost certainly seen barnacles, for the “crush” you saw on the wharf’s pilings and the rocks was made up of millions of salt water shellfish called barnacles.
When barnacle is first hatched, it resembles a young water flea and can swim about in the water.
But when it reaches adult stage it can no longer swim, so it attaches itself to any convenient object, such as the hull of a ship, piling, rock, whale, or even a sting of seaweed.
Once attached, a hard lime-like shell forms around the barnacles. The barnacle eats by waving its feathery legs through an opening in the shell to pull tiny sea creatures and plants into its mouth.
In olden days, sailor of wooden sailing ships had to periodically pull their ships ashore to scrape off the masses of barnacles clinging to the hulls, because they reduced the ship’s speed and made steering difficult. Today, special paints, prevent growth of barnacles. – Dick Rogers