PHILASTERIDES-DICENTRARCHI, N-SP, (CILIOPHORA, SCUTICOCILIATIDA), A HISTOPHAGOUS OPPORTUNISTIC PARASITE OF DICENTRARCHUS-LABRAX (LINNAEUS, 1758), A REARED MARINE FISH

In sea bass Dicentrarchus labrax (L.) reared in the Mediterranean sea, mortality suddenly arose. The diseased fishes, noticeable when almost dead, were massively infested with a histophagous ciliate. Depending on the infected host organs, the ciliate showed in its food vacuoles various categories of ingested cells: erythrocytes, ovocytes, and others. The infecting mode of this presumed ''parasite'' is still unknown, all experimental infections having failed. Morphological, biometric and ultrastructural studies lead to the conclusion that, because of its morphological characteristics and its main stomatogenesis processes, this ciliate is a Scuticociliatida Philasterina. Compared to other known species, it differs by its small size, the small and regular number of its somatic kineties, the position of its buccal organelles (and, mainly, the bipartite paroral membrane), the presence of two types of extrusomes, and a perinuclear envelope. All these characteristics justify the creation of a new species: Philasterides dicentrarchi. This particular ciliate habitat is compared to that of other commensal or opportunistic parasitic ciliates of invertebrates and marine fishes.