Possible impacts of zoosporic parasites in diseases of commercially important marine mollusc species: Part II. Labyrinthulomycota

The phylum Labyrinthulomycota comprises diverse marine fungus-like protists that are an abundant and widespread component of the marine microbiota. Despite their ubiquity in marine ecosystems, relatively little is known about the ecology of any of the pathogenic species in the Labyrinthulomycota. Most are thought to exist as saprobes, but many species have been documented as pathogens of marine metazoans and metaphytes. The best studied labyrinthulomycotan pathogen in molluscs is Quahog Parasite Unknown (QPX), which causes mortality events in both wild and cultured hard clams, Mercenaria mercenaria. Other labyrinthulomycotan pathogens affect a variety of other molluscs, including juvenile abalone, squid and octopus. Both the ectoplasmic network, a unique feature of the Labyrinthulomycota that attaches vegetative cells to their substrate, and their heterokont zoospores may play important roles in pathogenesis. Most labyrinthulomycotan pathogens appear to be opportunistic, rarely causing disease in healthy hosts but becoming pathogenic when changes in host immunity or other factors alter the host-parasite relationship. All reported labyrinthulomycotan pathogens appear to be host-specific, potentially making Labyrinthulomycota unusual in terms of pathogenesis and virulence, as they seem to be host-specific like obligate pathogens, but appear to behave and cause disease like opportunistic pathogens.