Studies were conducted to determine the cause of high mortalities of cultured half-smooth tongue sole Cynoglossus semilaevis. Gross signs of disease included loss of appetite, erratic swimming, and haemorrhages on the head, opercula and base of fins, dorsal fin rot, swollen abdomen filled with ascitic fluid and herniation of the intestine. Histological examination of the liver showed focal areas of necrosis and extensive haemorrhages. Virtually pure, dense bacterial cultures were obtained from liver, kidney and spleen tissues, and high pathogenicity of the isolates to tongue sole was confirmed. The phenotypic characteristics of the isolates including morphological, physiological and biochemical characteristics were determined. The 16S rRNA and gyrB genes of the isolates were sequenced, and the phylogenetic trees representing genetic relatedness between the isolate and publicly available 16S rRNA and gyrB gene sequences from GenBank were constructed. The results confirmed that the diseased tongue soles were infected with Vibrio aestuarianus. The sequenced 16S rRNA gene of strain TS1 (GenBank Accession No. GQ372983) was 1446bp, the gyrB gene of strain TS1 (GenBank Accession No. GQ372984) was 1200bp and the two genes exhibited high similarity (98~99%) to those of V. aestuarianus from GenBank. In addition, the activities of extracellular enzymes and haemolysin were also studied; the results showed that the isolates produced beta haemolysis on rabbit blood agar, lecithinase, proteinase, DNase and lipase, but gelatinase was not produced.
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