Mussel beds are persistent, dense mussel populations. Aquacultured shellfish include various oyster, mussel, and clam species. The oyster larvae preferentially settle out on the mussel shells. It also hosts Atlantic salmon and the near extinct freshwater pearl mussel. The zebra mussel ("Dreissena polymorpha") is a small freshwater mussel. The name came from the word "Liboo", a kind of mussel clam. The quagga mussel is a filter feeder. Immediately three fish appear floating, carrying a mussel. The Tay also maintains flagship population of Freshwater pearl mussel ("Margaritifera margaritifera"). The zebra mussel, an invasive species, is expanding upstream in the Ebro's waters. Conchoidal fracture breakage that resembles the concentric ripples of a mussel shell. There are nationally important horse mussel and brittlestar beds in the sea lochs. Early developmental stages are protected from predation within the body of the mussel. In cases such as the zebra mussel, invasion of US waterways was unintentional. The lifespan of a zebra mussel is four to five years.