Barnacles are very unusual crustaceans related to crabs and shrimp that are attached to rocky surfaces, docks or pilings. Barnacles can even be attached to marine mammals like the migrating grey whales of the Pacific Northwest. On dives I have noticed barnacles attached to the exoskeletons of crabs and even scallops.
These animals are protected by calcareous plates, which form a protective area resembling the shape of a volcano. The small invertebrate lives inside this space, and has the top entrance covered with another two plates. These top two plates can be opened and closed at will allowing the barnacle to extend its basket-like feeding limbs called cirri into the water to collect food. Basically barnacles are standing on their head and eating with their feet! There is also a cement gland located on the underside allowing the barnacle to attach itself to a solid surface.
It’s interesting how many barnacles live near the surface and are covered with water for only a few hours each day, relying on high tide to provide the necessary water flow with food particles. Most of their day is spent hidden in their calcareous home in the hot sun, waiting for the next cooling high tide to arrive. When exposed to the air during low tide, a barnacle has its two upper plates tightly closed and also has a small quantity of ocean water stored inside to keep it from drying out, techniques necessary for the animal’s survival.