Sporulation And Encystment Of Aphanomyces invadans, The Causative Agent Of Ulcerative Disease Syndrome (UDS) In Atlantic Menhaden, Brevoortia tyrannus     Yasunari Kiryu1, Jeffrey D. Shields1, Wolfgang K. Vogelbein1, David E. Zwerner1, Howard Kator1, and Vicki S. Blazer2   1Department of Environmental Sciences, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062; 2National Fish Health Research Laboratory, Biological Resources Division, U.S. Geological Survey, 1700 Leetown Road, Kearneysville, West Virginia 25430     Oomycete infections caused by Aphanomyces invadans occur in freshwater fishes from around the world.  However, the disease is also endemic in the Atlantic menhaden, Brevoortia tyrannus, along the East Coast of the USA.  To better understand transmission, we characterized sporulation and encystment of zoospores at different salinities.  Three isolates of Aphanomyces invadans were used in this study: (1) WIC (an endemic strain isolated from menhaden in Maryland), (2) PA7 (a strain isolated from striped snakehead, Channastriata, infected with epizootic ulcerative syndrome from Thailand), and (3) ATCC-62427 (a strain isolated from menhaden in North Carolina).  At room temperature, using artificial-sea-salt-augmented "pond water" from Beaver Dam Reservoir (Gloucester County, VA), WIC and PA7 produced free-swimming secondary zoospores at salinities of 0, 1, and 2 psu, but not at 4 psu or higher.  Secondary zoospores of ATCC-62427 were observed at 1, 2, 4, and 8 psu, but not at 0 and 12 psu.  Secondary zoospores in all three isolates, but especially WIC, were highly abundant and motile for 1 – 2 d post-sporulation.  Zoospore production of all three isolates ceased 6 d post-sporulation.  Single secondary zoospores of WIC remained motile for approximately 12 to 18 hr.  Secondary zoospores of WIC encysted immediately upon transfer to higher salinities (from 1 to 4, 6, and 12 psu) or after being shaken vigorously (Vortex).  Encysted spores excysted into motile zoospores when transferred back to 1 psu water within 2-3 hr of encystment.  Spores that had encysted for 24 h did not excyst when transferred back to 1 psu water.  Germination of these cysts on agar was not examined.  Secondary zoospores of WIC encysted quickly upon exposure to menhaden tissues.  Salinity requirements for sporulation indicate that juvenile menhaden must acquire infections in the relatively low oligohaline waters of Chesapeake Bay.