The mosquito parasite, Coelomomyces psorophorae (Blastocladiales, Chytridiomycetes) alternates obligately between the larvae of Culiseta inornata and the copepod Cyclops vernalis. Isogametes, derived from heterothallic, wall-less gametangia which develop in the copepod, fuse to produce a diploid zygote that subsequently infects the mosquito host. Zoospores from the resistant sporangia which are produced in the haemocoel of the mosquito infect the copepod. A tentative lifehistory is proposed and implications of these discoveries for the biology, taxonomy, and possible role of Coelomomyces in biological control are discussed.The water mold Coelomomyces (Chytridiomycetes) is a hostspecific, obligate parasite of the larvae of mosquitoes, black flies, chironomids, and tabanids (1-5). The fungus develops in the haemocoel of the host as a weakly-branched coenocytic thallus. At maturity, this vegetative structure differentiates into a number of thick-walled resistant sporangia (RS) (6). Under appropriate conditions, each of these sporangia will give rise to several hundred posteriorly uniflagellate zoospores. The genus includes pathogens of the major genera of mosquitoes and may cause significant epizootics in mosquito populations. Preliminary field trials have suggested that Coelomomyces may have potential as a biological control agent of mosquitoes (7-11). Efforts to explore this possibility have encountered significant difficulties when workers attempted to establish infected colonies in the laboratory. Larvae bathed in high densities of zoospores from the RS did not become infected. Couch (12) has developed a technique for mass production of infected larvae but the conditions leading to infection are complex and include a number of living components other than the host and its parasite.Our studies have utilized C. psorophorae in Culiseta inornata. This species. of Coelomomyces is a common parasite of mosquitoes in the irrigated regions of southern Alberta (10). Establishment of an infected colony in the laboratory was achieved by use of the Couch procedure (12) along with additions of various components of the biota from the original habitat, e.g., microalgae, arthropods, etc. Attempts to standardize infection rates by controlling the environment, density of the RS-zoospores, or age and condition of the host were unsuccessful. Attempts to reduce the biotic complexity of our gross cultures typically resulted in loss of infection. The source of this problem was revealed when single components of the complex or "gross" system were added individually to "clean" pans containing only the fungus and host insects. When the copepod Cyclops vernalis was added to pans containing RS and larvae, infection was obtained.

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A strain of Pythium marinum (Peronosporales: Pythiaceae) from Puget Sound, Washington, was isolated from lesions of Porphyra nereocystis. The fungus grew on a modified Vishniac medium, from temperatures of 4 to 25 °C, although growth was slow at the lowest temperature. Sexual and asexual reproduction also occurred within this temperature range. Mycelium diluted in seawater initiated zoospore release within 16 h and continued to release zoospores for over 200 h at temperatures from 4 to 20 °C. Zoospore encystment on several species of marine red, brown, and green algae was readily monitored following staining with lactophenol – cotton blue. Pythium marinum zoospore encystment occurred on rhodophyceaen species, including Porphyra (gametophytes), Gigartina exasperata (tetrasporophyte), Mastocarpus papillatus (gametophyte), Prionitis lanceolata (nonfertile), and Iridaea heterocarpa (gametophyte and tetrasporophyte), but not on Nereocystis leutkeana or Ulva lactuca. Over 50% of zoospores held in half-strength seawater at 4 and 20 °C encysted within 24 h, whereas those held at 12 °C reached 50% encystment only after 32 h. For 4-mm diameter discs of Porphyra nereocystis and Porphyra perforata (formerly Porphyra sanjuanensis) blades, there was only a transient relationship between cell damage and number of encysted zoospores. Zoospores did not attach to the conchocelis phase of two species of Porphyra. Sequential extraction of carbohydrates from the blades of Porphyra perforata implicated separate chemical signals for zoospore encystment and appressorium formation prior to the initiation of blade invasion. Addition of diverse monosaccharides and polysaccharides to zoospore suspensions suggested that these chemical signals are specific, with the attachment–encystment signal chemically related to polysaccharides consisting of sulfated or nonsulfated galactose and 3,6-anhydrogalactose found in commercial agars and carrageenans. There was no consistent relationship between zoospore encystment and the amount of 3,6-anhydrogalactose present in the blade phase of several species of red algae. Key words: Pythium, Porphyra, zoospore, encystment, sulfated galactans, 3,6-anhydrogalactose.

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