Abstract

An internal parasite of crayfish has been found in Astacus leptodactylus and Pacifastacus leniusculus in France. This agent, ovoid, or 35 x 70 ?, has been identified as Psorospermium haeckeli Hilgdf. a poorly known parasite generally considered as a sporozoan. Histological and ultrastructural studies allowed to describe the complicated structure of its wall and its cytoplasmic content. Adults as well as juveniles of A. leptodactylus are susceptible to the disease. Characteristic orange spots appear on the integument of heavily infected animals. Psorospermium is extremely abundant in the peripheric connective tissue, but is also present in diverse other tissues. Experiments on the transmission or the disease by injection and ingestion have given only scarce positive results. Psorospermium has a pathogenic effect on the attacked crayfishes which finally die, especially during molting periods. Further studies are now going on concerning the taxonomic position or the parasite, its developmental cycle, and the importance of P. haeckeli as a danger for crayfish rearings.

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