Three mollusc species are cultured commercially in Australian tropical and warm temperate waters: the Sydney rock oyster Saccostrea commercialis , the pearl rock oyster is susceptible to two protozoans: Marteilia sydneyi , which is the cause of QX Disease and infects oysters during the summer; and Mikrocytos roughleyi which is associated with winter mortality. It also harbours mudworm, Polydora websteri a parasite farmers try to avoid by growing oysters on racks. Transported pearl oysters Pinctada maxima die with heavy infections of Vibrio harveyi . Cultured giant clams Tridacna gigas are pestered by pyramidellid snails and some carry Perkinsus sp., a protozoan common in reef bivalves. Deaths in mollusc hatcheries have been associated with Vibrio tubiashi Vibrio spp., and Alteromonas spp.