A nursery bed of mussels, Mytilus galloprovincialis, sensu lato, on a calcareous sandstone rock exposed at low tide at Blairgowrie, Victoria, Australia. The picture shows massed mussels (purple) with adhering barnacles (white). Mussels are bivalves, and can move about underwater by means of a flattened foot. In this picture, exposed to the air, they are attached to the substrate by means of a byssus; a series of strong threads extruded from the foot that behave as guy-ropes. This allows flexible movement of the animal as the tide rises, without its becoming dislodged. Barnacles by contrast are sessile organisms, permanently cemented to their substrate. In this picture most of the barnacles have settled onto the shell of a mussel, and will remain attached to it for life. Both mussels and barnacles are filter-feeders.
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