DAO 76:49-56 (2007)  -  doi:10.3354/dao076049

ABSTRACT: Laboratory-produced and -reared sub-adults of silver pomfret (zobaidy) Pampus argenteus affected by severe scuticociliatosis during April and May 2005 were investigated and the causative was a scuticociliate, presumptively identified as Uronema sp. The parasite was capable of producing highly necrotic skin lesions. Mortalities started with a rise in the seawater temperature from 20 to 21.5°C, coinciding with an increase in the total bacterial load of the rearing tank water from 10³ to 10⁵ CFU ml^–1. The parasite was successfully cultured in vitro (85 passages), using 10% brain heart infusion broth inoculated simultaneously with an aquatic Vibrio sp. The parasite reached peak cell density (1.82 × 10⁵ cells ml^–1) on the third day of inoculation. The parasite cell count was found to be inversely related (r = –0.45) to the bacterial count in the incubation medium. The ciliate body dimensions measured, on average, 34.05 ± 0.99 µm in length and 16.70 ± 0.74 µm in width (n = 300). Silver staining revealed morphological characteristics of Uronema sp., including a truncated anterior end, a buccal apparatus, caudal cilium and 3 oral polykinetids, with the first one appearing as a single row. Average number of kineties was 12 ± 3. There was an inverse relationship (R² = 0.84) between the proteolytic activity and the number of in vitro passages of the scuticociliate. However, the parasite from a freshly infected fish (using inoculum from the tenth passage) showed higher proteolytic activity (31.2%) compared with that produced by the inoculum from in vitro cultures (9.75%). The ciliate also showed a distinct histophagous capability when tested in vitro using host muscle tissue.

KEY WORDS: Silver pomfret · Zobaidy · Pampus argenteus · Scuticociliate · Uronema sp. · Growth kinetics · Protease activity · Histophagous ability