Barnacles feeding. Close-up macrophotography footage of barnacles feeding underwater. Barnacles are marine arthropods found in tidal waters. They attach themselves to a substrate. The rest of the body is a carapace or mantle of calcified plates. When the valves of the mantle open, the thoracic limbs hang out. These limbs, which are covered in fine bristles (cirri), are thrust in and out allowing the cirri to filter out small plankton which are then conveyed to the barnacle's mouth.