Acorn Barnacle

Found in shallow, inter-tidal waters, on the surface of corals and sponges, over coral and rocky reefs, also on piers, in seaweed, hulls of ships, on turtles and even crocodiles etc.

Barnacles form part of the fouling community.
These barnacles attach to anything, even found on crocodiles, where the size of the barnacle shows how long the crocodile has been in salt water.