CELLULAR DEFENSE-MECHANISM OF THE CLAM TAPES SEMIDECUSSATUS AGAINST INFECTION BY THE PROTOZOAN PERKINSUS SP

The cellular defence mechanism of the clam Tapes semidecussatus (Mollu sca, Bivalvia) against infection by the parasite protozoan Perkinsus s p. (Apicomplexa, Perkinsea) was studied in the gill filaments. The par asites, localized in the connective tissue, induced a cellular reactio n involving infiltrated granulocytes. These showed a secretory aspect, with the cytoplasm being filled by membrane-bound granules with inter nal membranes. The holocrine secretion, which was proteic and slightly glycosylated, by the granulocytes gave rise to the encapsulation of t he parasites. After incubation with lectins from Canavalia ensiformis, Triticum vulgar is, Helix pomatia, Glycine max, Arachis hypogaea, Ric inus communis (agglutinin), Ulex europeus I and Limax flavus, a lack o f specific and/or main sugars was observed in the plasma membrane of p arasite and granulocyte, and in the wall of the former. Furthermore, G alNac alpha 1,3GalNac and beta-D-gal residues were only detected in as sociation with the internal membranes and dense regions of both granul es and capsule, respectively. Blood granulocytes were observed at the periphery of the cellular reaction, close to blood vessels, and these appeared to re-differentiate to give the granulocytes of the cellular reaction. The data reported here suggest that this parasite induces th e infiltration and re-differentiation of specialized cells in the host mollusc. In addition, a polarized secretion of a specific defence pro duct is described for the first time.