Abstract

Psorospermium haeckeli Hilgendorf was present in 22 of 29 wild Astacus astacus (L.) populations in central Finland. A total of 1546 specimens was examined with overall 64% infection rate, primarily mature adult crayfish. Five populations were studied intensively since 1988 or 1989. The prevalence of P. haeckeli remains constant in 4 of 5 of these populations. The prevalence in Lake Suur-Lauas was 82% in the autumn of 1988, declined rapidly over the winter and has remained approximately 20% since then. However, the prevalence has not changed in crayfish collected in 1988 and held in indoor tanks until 1993. High mortality of infected adults and replacement by non-infected younger crayfish is one possible explanation. Museum samples of A. astacus (32 specimens) from the years 1875-1968, were examined. The first infected specimen was from the year 1905. No Psorospermium sp. was found in three populations of the introduced astacid crayfish Pacifastacus leniusculus (Dana) (37 specimens). The typical Finnish P. haeckeli has multilayered, thick walls and numerous small globules inside. Overall size is 100 µm x 53 µm in most crayfish. Variations were observed. Smaller P. haeckeli (40-60 µm in diameter) have thin homogeneous cell walls with very few internal globules.

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