Acorn barnacle

Acorn barnacle is a stalkless barnacle, a creature related to shrimp that hide its identity in snail-like shell.

It starts its life as larva and after some time it sticks to hard surfaces like rocks, ships, marine animals etc. Large number of acorn barnacles sticking up at the bottom of a ship form a heavy encrustation that slows down or badly affect the speed and the movement of the ship.