According to A-Z animals, barnacles are primarily filter feeders that collect microscopic particles of food from the water column. These food particles can be comprised of plankton or bits of organic debris.
Fully developed barnacles are sessile organisms that remain fixed in one place. This means they cannot move about to collect food, so they must survive on food that floats by them. Barnacles live in intertidal areas, feeding when water covered and closed to remain moist when the tide is out. Some barnacles even attach themselves to crabs, whales or other animals. While most barnacles are harmless, a few species are parasitic and can cause their host harm.