Pathogenic and toxic effects of Fusarium oxysporum (Schlecht.) on survivaland osmoregulatory capacity of Penaeus japonicus (Bate)

A gill-blackening disease in Penaeus japonicus was caused by Fusarium oxysp orum, now considered for the first time to be a parasite of this shrimp. Tw o different isolates of a strain of F. oxysporum, I-1 and I-2, have been us ed in our experiments. In I-2 treated for 3 days with antibiotics, sporulat ion and growth were inhibited compared to I-1 treated for only 3 h. The pat hogenic effect of F. oxysporum is dose- and isolate-dependent. With isolate I-1, all inoculated animals died within 14 days and their gills were cover ed in black patches, although they showed no signs of reduced behavioural a ctivity. In contrast, with isolate I-2, all animals died later within 22 da ys and gill lesions produced were Limited but, nevertheless, the behaviour activity of the animals was significantly reduced. Moulting or exposure to low salinities increased animal mortality. In juvenile animals, infection b y F. oxysporum resulted in a significant decrease in their hypoosmoregulato ry capacity (hypo-OC) in seawater and in their hyper-osmoregulatory in dilu ted medium. Injections of crude filtrates from shake cultures of the fungus showed that molecules greater than 6-8 kDa caused a significant decrease i n the hypo-OC and are likely to be responsible for the toxic effects of thi s fungus on these animals. (C) 1999 Elsevier Science B.V. All rights reserv ed.