Abstract

Five hundred and fifty four freshwater crayfish of age 6 months or more, comprising 359 Cherax tenuimanus (marron) from 6 marron farms, 120 wildstock marron from two different locations (Warren River and Wellington Dam) and 75 Cherax destructor (yabbies) from two farm dams along with 211 farmed marron of less than 12 months in age (Q+ marron) were examined for the occurrence of Psorospermium sp. as part of a general health survey of crayfish populations in Western Australia. The study was conducted in 1988/89. Psorospermium sp. was present in marron from three farms and in one wildstock population. No Psorospermium sp. were observed in the 0+ marron or in the yabby samples. The prevalence in the three infected farm populations was 62%, 37% and 2% and in the wildstock marron 3%. A further study conducted in 1995/96 included 5 marron farms (N= 10 or 20/farm; all different farms to those studied in 1988/89) and 3 wildstock populations (Warren River, Donnelly River and Gardiner River; N=20/site). None of the farmed marron were infected. Two wildstock populations (Donnelly River and Gardiner River) were infected. Infection prevalence in the two populations was 100% and 70% respectively. Two parasite forms, presumably different life stages of the same organism, were seen in histological preparations, a mature spore-like form and an immature membrane bound form. Only the mature form was seen in wet mount preparations. The mature form was ovoid in shape, 93 x 40 x 40 µm in size and exhibited a prominent central ridge which encircled the organism. Parasites were observed in gills, thoracic and abdominal muscle, antenna! gland, heart, digestive tract organs, pleopods, ventral nerve and hepatopancreas with the highest concentration occurring in the dorsal thoracic muscle. Hemocytic encapsulation of the parasites was observed in histolog­ical sections in tissues from a number of animals but was not a common finding.

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