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The phylum Perkinsozoa includes well-known parasites of commercially important species of molluscs in aquaculture, such as Perkinsus marinus, a virulent pathogen of the eastern oyster, Crassostrea virginica, and in wild populations in many marine ecosystems throughout the world. These parasites are responsible for significant losses in aquaculture production. For this reason, much research has already been conducted with these parasites. The zoospores of Perkinsus secrete several classes of proteases which facilitate the digestion of host tissues. The host can respond by producing protease inhibitors and by mounting cellular immune responses as defence strategies. The impacts of infectious diseases on mollusc populations may be mediated by a variety of factors, such as temperature increase, acidification, pollution, aquaculture and overfishing. Farmed and wild populations of molluscs are difficult to separate and therefore the presence of disease in one is likely to impact the other. Disease prevalence in wild and cultured conditions can differ considerably because of the different stresses that are likely to occur. In aquaculture, overcrowding can increase susceptibility to disease compared to the more dispersed wild populations. More research is necessary for biosecurity and protection of valuable marine food resources for a growing human population. [ABSTRACT FROM AUTHOR]

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 hostspecific like obligate pathogens, but appear to behave and cause disease like opportunistic pathogens. [ABSTRACT FROM AUTHOR]

Zoosporic parasites have received increased attention during the last years, but it is still largely unnoted that these parasites can themselves be infected by hyperparasites. Some members of the Chytridiomycota, Blastocladiomycota, Cryptomycota, Hyphochytriomycota, Labyrinthulomycota, Oomycota, and Phytomyxea are hyperparasites of zoosporic hosts. Because of sometimes complex tripartite interactions between hyperparasite, their parasite-host, and the primary host, hyperparasites can be difficult to detect and monitor. Some of these hyperparasites use similar mechanisms as their parasite-hosts to find and infect their target and to access food resources. The life cycle of zoosporic hyperparasites is usually shorter than the life cycle of their hosts, so hyperparasites may accelerate the turnaround times of nutrients within the ecosystem. Hyperparasites may increase the complexity of food webs and play significant roles in regulating population sizes and population dynamics of their hosts. We suggest that hyperparasites lengthen food chains but can also play a role in conducting or suppressing diseases of animals, plants, or algae. Hyperparasites can significantly impact ecosystems in various ways, therefore it is important to increase our understanding about these cryptic and diverse organisms. [ABSTRACT FROM AUTHOR]

Abstract: Some species of zoosporic fungi in the Phylum Blastocladiomycota are obligate parasites of invertebrate animals. Various stages in the life history of the host can be infected, including eggs, larvae and adults. Some parasites, such as some species of Coelomomyces, alternate between two hosts; while others, such as some species of Catenaria, require only one host for reproduction. Infection generally begins after zoospores attach to the outer surface of the host body or to the gut wall. Host specificity and parasite virulence vary broadly among species. Some parasites such as Coelomomyces and Catenaria can naturally regulate dipteran insect or nematode populations. New roles for Blastocladian parasites in food web dynamics have recently been discovered. For example, the parasite Polycaryum can reduce the quantity and quality of Daphnia hosts as a food resource for planktivorous fish. We propose that zoosporic fungal parasites contribute significantly to the biodiversity and the complexity of the food webs in freshwater and soil ecosystems. Further research is needed to highlight the importance of zoosporic fungal parasites in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: Chytrids are very important components of freshwater ecosystems, but the ecological significance of this group of fungi is not well understood. This review considers some of the significant environmental factors affecting growth and population composition of chytrids in aquatic habitats. The physical factors include primarily salinity, dissolved oxygen concentration and temperature. The biological factors include the role of chytrids as saprobes and parasites and methods of dispersal of propagules throughout the ecosystem. Dispersal depends upon both zoospores for short range and whole thalli for long range dispersal. Five roles for chytrids in food-web dynamics are proposed: (1) chytrid zoospores are a good food source for zooplankton, (2) chytrids decompose particulate organic matter, (3) chytrids are parasites of aquatic plants, (4) chytrids are parasites of aquatic animals and (5) chytrids convert inorganic compounds into organic compounds. New molecular methods for analysis of chytrid diversity in aquatic environments have the potential to provide accurate quantitative data necessary for better understanding of ecological processes in aquatic ecosystems. [Copyright &y& Elsevier]