Abstract

In farmed populations of the European freshwater crayfish Astacus astacus L. we have found two morphotypes of the parasite "Psorospennium haeckeli" which may represent two species. Both differ in size and shape but are largely identical with respect to their internal architecture and histochemical composition. One type is ovoid and measures ca. 100 x 60 x 60 m (Psorospennium sp. 1 ). The second type is more elongate and narrower and measures ca. 120-180 x 50 x 50 m (Psorospennium sp. 2). Both types develop from smaller stages particularly within the adventitia of the thoracic arteries and the subepidermal connective tissue underneath the carapace. The earliest stage that could be clearly identified as Psorospennium is roundish or ovoid, has a diameter of ca. 25 m and is limited only by a membrane-like structure. It includes lipid globules, small nonlipid globules and one nucleus. Further differentiated stages are increased in size and show already the typical shape of their mature forms. They are characterized by a broad, elastin-positive periphery and an inner sac comprising the globules. The shell plates which develop in th e periphery of the elastin-positive zone stain additionally for collagen but lose this staining feature when complete. Mature spores are characterized by a three-layered shell including solid shell plates and by numerous lipid globules, some medium-sized nonlipid globules and two closely associated nuclei. The lipid globules are very heterogeneous in Psorospennium sp. 1 whereas they are rather homogeneous in Psorospennium sp. 2. Psorospennium sp. 2 is generally ensheathed by a broad envelope of connective tissue and is not attacked by the immune defense system of the host. Psorospennium sp. 1, in contrast, is only surrounded by a narrow envelope of connective tissue and is often encapsulated by haemocytes and melanized.

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